Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Prasad, Anil
Doctor of Philosophy - PhD
University of British Columbia
Nuclear fission energy is playing a vital role in providing many countries with carbon-free energy. However, nuclear accidents such as the Fukushima, Chernobyl, and Three Mile Island, have raised questions about the accident tolerance of nuclear power plants around the world. As a result, nuclear energy scientists and engineers are continuously working towards making nuclear fission energy safer. As a part of this effort, the concept of Accident Tolerant Fuel (ATF) is being investigated globally by optimising geometric or material properties of nuclear fuel. The global CANada Deuterium Uranium (CANDU) fleet of reactors are capable of utilising both Uranium dioxide (UO₂) and Thorium dioxide (ThO₂) as fuel without enrichment. In this study, development of ATF fuel was investigated by both geometric and material approaches for both Uranium dioxide and Thorium dioxide fuel materials targeted for CANDU reactor application. Material additives, such as SiC, diamond, CNTs, graphene etc. decompose under the extreme thermal conditions used during conventional sintering of nuclear fuels. In this study, Spark Plasma Sintering (SPS) was used to synthesize nuclear fuel materials and their composites and to study their densificiation mechanisms during sintering. The study covered the two different forms of ATF: ThO₂ based composite fuels (ThO₂-SiC, ThO₂-diamond and (Th,U)O₂) and Internally and eXternally cooled Annular Fuel (IXAF). The first part of the dissertation focused on studying the densification kinetics of ThO₂ and its composites, which enabled the construction of Master Sintering Curves (MSCs) that could be used as roadmaps for future SPS research on these fuel materials. The second part of the dissertation focused on the comparison of IXAF fabricated via SPS and conventional sintering.
Characterization of Spark PlasmaSintered Nuclear Fuel MaterialsbyAnil PrasadM.A.Sc, The University Of British Columbia, Canada, 2017B.Tech, The Indian Institute Of Technology Madras, India, 2015A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHYinThe College Of Graduate Studies(Mechanical Engineering)THE UNIVERSITY OF BRITISH COLUMBIA(Okanagan)July 2020© Anil Prasad 2020The undersigned certify that they have read, and recommend to the College of GraduateStudies for acceptance, a dissertation titled: "Characterization of Spark Plasma SinteredNuclear Fuel Materials" submitted by Anil Prasad in partial fullment of the requirements ofthe degree of Doctor of Philosophy.Dr. Lukas Bichler, Mechanical Engineering, School of EngineeringSupervisor, ProfessorDr. Dimitry Sediako, Mechanical Engineering, School of EngineeringSupervisory Committee Member, ProfessorDr. Jian Liu, Mechanical Engineering, School of EngineeringSupervisory Committee Member, ProfessorDr. Stephen O’Leary, Electrical Engineering, School of EngineeringUniversity Examiner, ProfessorDr. Mathieu Brochu, Materials Engineering, McGill UniversityExternal Examiner, ProfessorJuly 20, 2020(Date Submitted to College of Graduate Studies)iiAbstractNuclear ssion energy is playing a vital role in providing many countries withcarbon-free energy. However, nuclear accidents such as the Fukushima, Chernobyl andThree Mile Island, have raised questions about the accident tolerance of nuclear powerplants around the world. As a result, nuclear energy scientists and engineers arecontinuously working towards making nuclear ssion energy safer. As a part of this e ort,the concept of Accident Tolerant Fuel (ATF) is being investigated globally by optimisinggeometric or material properties of nuclear fuel. The global CANada Deuterium Uranium(CANDU) eet of reactors are capable of utilising both Uranium dioxide (UO2) and Thoriumdioxide (ThO2) as fuel without enrichment. In this study, development of ATF fuel wasinvestigated by both geometric and material approaches for both Uranium dioxide andThorium dioxide fuel materials targeted for CANDU reactor application.Material additives, such as SiC, diamond, CNTs, graphene etc. decompose underthe extreme thermal conditions used during conventional sintering of nuclear fuels. In thisstudy, Spark Plasma Sintering (SPS) was used to synthesize nuclear fuel materials and theircomposites and to study their densiciation mechanisms during sintering. The study coveredthe two di erent forms of ATF: ThO2 based composite fuels (ThO2-SiC, ThO2-diamond and(Th,U)O2) and Internally and eXternally cooled Annular Fuel (IXAF). The rst part of thedissertation focused on studying the densication kinetics of ThO2 and its composites, whichenabled the construction of Master Sintering Curves (MSCs) that could be used as roadmapsfor future SPS research on these fuel materials. The second part of the dissertation focusedon the comparison of IXAF fabricated via SPS and conventional sintering.iiiLay SummaryThe scientic consensus on climate change is that more than 97% of climate scientistsagree that greenhouse gas emissions from human activities are the primary cause forclimate change. Hence, there is an ever rising need to utilise carbon-free energy sources,such as nuclear energy. Nuclear accidents such as Fukushima, Chernobyl and Three MileIsland question the safety of currently operational commercial nuclear reactors. Hence,there is a need for exploring alternative forms of nuclear fuel materials that could makenuclear reactors safer. The currently used nuclear fuel manufacturing technique is slow andthe introduction of additives that could make the nuclear fuel safer is problematic. SPS is anovel sintering technique that has been researched for prototyping nuclear fuel materialsthe last decade. In this research, SPS was used to fabricate novel nuclear fuel materials forthe current and the next generation of nuclear reactors.ivPrefaceThis dissertation titled "Spark Plasma Sintering of Novel Nuclear Fuel Materials" was writtenfor the partial fulllment of the Doctor of Philosophy (Ph.D) degree at the University of BritishColumbia.The following research articles were prepared from selected parts of this dissertation• L. Malakkal, A. Prasad, E. Jossou, J. Ranasinghe, B. Szpunar, L. Bichler and J. Szpunar;"Thermal conductivity of bulk and porous ThO2: atomistic and experimental study";published in Journal of Alloys and Compounds, vol. 798, pp 507-516 (2019).• A. Prasad, L. Malakkal, L. Bichler and J. Szpunar; "Optimising processing conditionsfor thorium dioxide using Spark Plasma Sintering"; published in Ceramic Transactionsvol. 266 (2019).• L. Malakkal, A. Prasad, E. Jossou, J. Ranasinghe, B. Szpunar, L. Bichler and J. Szpunar;"Atomistic and experimental study on thermal conductivity of bulk and porous ceriumdioxide"; published in Nature Scientic Reports, vol. 9 6326 (2019).• L. Malakkal, A. Prasad, E. Jossou, J. Ranasinghe, B. Szpunar, L. Bichler and J. Szpunar;"The e ect of SPS processing parameters on the microstructure and thermalconductivity of ThO2"; under review in Journal of Nuclear Materials.• A. Prasad, A. Bergeron and L. Bichler; "Spark Plasma Sintering of annular nuclear fuelpellets I: Conventional Sintering vs Spark Plasma Sintering"; under preparation.vPreface• A. Prasad, A. Bergeron and L. Bichler; "Spark Plasma Sintering of annular nuclear fuelpellets II: A coupled electro-thermal nite elemental study"; under preparation.Some parts of this dissertation were also presented at the following conference:• A. Prasad, L. Malakkal, L. Bichler and J. Szpunar; Material Science and Technology2018 (International), Columbus, Ohio; October 2018; presentation titled “Optimisingprocessing conditions for thorium dioxide using Spark Plasma Sintering”.The following copyrighted materials were used in this dissertation:1. Figure 1.1 Source: IEA (2017) Key World Energy Statistics. All rights reserved ©.2. Figure 1.2 Source: IEA (2017) Key World Energy Statistics. All rights reserved ©.3. Figure 1.3 Based on IEA data from the IEA (2016) Headline Global Energy Data 2016,www.iea.org/statistics, © as modied by Anil Prasad R.4. Figure 2.1 Republished with permission of Taylor & Francis Group LLC, fromSintering of Ceramics, M. N. Rahman, 1st Edition and 2007; © permission conveyedthrough Copyright Clearance Center, Inc.5. Figure 2.2 Reprinted by permission from Springer Nature Customer Service CentreGmbH: Springer, Encyclopedia of Nanotechnology edited by B. Bhushan, © (2012).6. Figure 2.4 Reprinted from V. Tyrpekl, C. Berkmann, M. Holzhäuser, F. Köpp, M.Cologna, T. Wangle and J. Somers, “Implementation of A Spark Plasma SinteringFacility in A Hermetic Glovebox for Compaction of Toxic, Radiotoxic and AirSensitive Materials,” Review of Scientic Instruments, vol. 86, pp 23904 (2015), © withthe permission of AIP Publishing.viPreface7. Figure 2.5 Reprinted from T. Ironman, J. Tulenko and G. Subhash, “Exploration ofViability of Spark Plasma Sintering for Commercial Fabrication of Nuclear FuelPellets,” Nuclear Technology, vol. 200, pp 144–158 (2017), © with the permission ofTaylor & Francis.8. Figure 2.6 Reprinted from Ceramics International, vol. 45 no. 6, B. Nili and G.Subhash, “In uence of Porosity and Pellet Dimensions on Temperature and StressInhomogeneities During Spark Plasma Sintering of Ceramic Fuel,” pp 7376-7384, ©(2019), with permission from Elsevier.9. Figure 2.7a Republished with permission of John Wiley & Sons, Inc., from"Preparation of Bulk-Nanostructured UO2 Pellets Using High-Pressure Spark PlasmaSintering for LWR Fuel Safety Assessment", Journal of American Ceramic Society, V.Tyrpekl, M. Cologna, J.F. Vigier, A. Cambriani, D. W. Wim and J. Somers, vol. 100, pp1269-1274 and 2017; © permission conveyed through Copyright Clearance Center, Inc.10. Figure 2.7bReprinted from ScriptaMaterialia, vol. 131, Y. Miao, T. Yao, J. Lian, J. S. Park,J. Almer, S. Bhattacharya, A. M. Yacout and K. Mo, "In situ Synchrotron Investigationof Grain Growth Behavior of Nano-Grained UO2", pp 29-32, (2017), © with permissionfrom Elsevier.11. Figure 2.8a Reprinted from Journal of Nuclear Materials, vol. 442, S. Yeo, R. Baney, G.Subhash and J. Tulenko, “The In uence of SiC Particle Size and Volume Fraction onthe Thermal Conductivity of Spark Plasma Sintered UO2–SiC Composites," pp 245-252,(2013), © with permission from Elsevier.12. Figure 2.8b Reprinted from Ph.D thesis, University of Florida, S. Yeo "UO2-SiCcomposite reactor fuels with enhanced thermal and mechanical properties preparedviiPrefaceby spark plasma sintering" pp 105 (2013)13. Figure 2.9 Reprinted from Ceramic International, vol. 44, B. Li, Z. Yang, M. Chu, Q.Huang, Z. Wang, R. Gao, Y. Zhong, X. Liu, L. Duan, and P. Zhang, "Ti3SiC2/UO2Composite Pellets with Superior High-Temperature Thermal Conductivity", pp19846-19850, © (2018), with permission from Elsevier.14. Figure 2.10a Reprinted from H. Muta, Y. Murakami, M. Uno, K. Kurosaki, and S.Yamanaka, “Thermophysical Properties of Th1-xUxO2 Pellets Prepared by SparkPlasma Sintering technique,” Journal of Nuclear Science and Technology, vol. 50, pp181-187 (2013), © with the permission of Taylor & Francis.15. Figure 2.10b Reprinted from Journal of Nuclear Materials, vol. 500, M. Saoudi, D.Staicu, J. Mouris, A. Bergeron, H. Hamilton, M. Naji, D. Freis, and M. Cologna,"Thermal Di usivity and Conductivity of Thorium-Uranium Mixed Oxides,", pp181-187, © (2018), with permission from Elsevier.16. Figure 2.11 Reprinted from Journal of Nuclear Materials, vol. 483, H. Muta, H. Kado,Y. Ohishi, K. Kurosaki and S. Yamanaka, "Thermal Di usivity and Conductivity ofThorium-Uranium Mixed Oxides," pp 192-198, © (2017), with permission from Elsevier.17. Table 2.1 Reprinted from Journal of European Ceramic Society, vol. 34, L. Ge, G.Subhash, R. H. Baney, and J. S. Tulenko, "In uence of Processing Parameters onThermal Conductivity of Uranium Dioxide Pellets Prepared by Spark PlasmaSintering," pp 1791–1801, © (2014), with permission from Elsevier.18. Figure 3.2 Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of CeriumDioxide and Its Composites". MASc thesis, University of British Columbia, pp 24, © bypermission from author.viiiPreface19. Figure 3.3 Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of CeriumDioxide and Its Composites". MASc thesis, University of British Columbia, pp 25, © bypermission from author.20. Figure B.1 Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of CeriumDioxide and Its Composites". MASc thesis, University of British Columbia, pp 26, © bypermission from author.21. Figure B.2 Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of CeriumDioxide and Its Composites". MASc thesis, University of British Columbia, pp 27, © bypermission from author.22. Figure B.3 Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of CeriumDioxide and Its Composites". MASc thesis, University of British Columbia, pp 27, © bypermission from author.23. Figure B.4 Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of CeriumDioxide and Its Composites". MASc thesis, University of British Columbia, pp 28, © bypermission from author.24. Figure 3.4 Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of CeriumDioxide and Its Composites". MASc thesis, University of British Columbia, pp 34, © bypermission from author.25. Figure 4.12 Reprinted from Powder Metallurgy Science, 2nd Ed., German, Randall M.,(1994) ISBN 1-878954-42-3 Metal Powder Industries Federation, 105 College Road East,Princeton, New Jersey, USA.26. Figure 4.20 Reprinted from Journal of the European Ceramic Society, vol. 34, L. Ge, G.Subhash, R. H. Baney, and J. S. Tulenko, "In uence of Processing Parameters on ThermalixPrefaceConductivity of Uranium Dioxide Pellets Prepared by Spark Plasma Sintering," pp 1791-1801, © (2014), with permission from Elsevier.27. Figure 2.13 Reprinted from Y. W. Rhee, D. J. Kim, J. H. Kim, J. H. Yang, K. S. Kim,and J. H. Yang, “Rod-Inserted Sintering of Annular Pellets for Dual-Cooled AnnularFuel,” Nuclear Technology, vol. 184, pp 54–62 (2013), © with the permission of Taylor &Francis.28. Figure 5.4b Reprinted from Materials Transactions, vol. 49, no. 12, : S. Grasso, Y. Sakka,and G. Maizz, "E ects of Initial Punch-Die Clearance in Spark Plasma Sintering Process,"pp 2899-2906, (2008), © with permission from Elsevier.29. Figure 5.4c Reprinted from Scripta Materialia, vol. 116, C. Manière, L. Durand, and C.Estournè, "Powder/die friction in the Spark Plasma Sintering process: Modelling andexperimental identication," pp 139-142 (2016), © with permission from Elsevier.30. Figure 5.19c Republished with permission of John Wiley & Sons, Inc., from "GrainGrowth in Sintered Uranium Dioxide: II, Columnar Grain Growth," Journal of AmericanCeramic Society, J. R. MacEwan and V. B. Lawson, vol. 45, no. 1 and pp 42-46 (1962); ©permission conveyed through Copyright Clearance Center, Inc.31. Figure A.3 Reprinted from R. L. Coble, “Sintering Crystalline Solids. I. Intermediateand Final State Di usion Models,” Journal of Applied Physics, vol. 32, pp 787–792 (1961),© with the permission of AIP Publishing.32. Figure A.4 Reprinted from Acta Metallurgica, vol. 116, M. F. Ashby, "A First Report onSintering Diagrams", pp 275-289 (1974), © with permission from Elsevier.33. Figure A.5a Republished with permission of John Wiley and Sons, Inc., from "Sinteringof Low-Density Glasses: I, Theory,", Journal of American Ceramic Society, G. W. Scherer,xPrefacevol. 60, no. 5-6 and pp 236-239 (1977); © permission conveyed through CopyrightClearance Center, Inc.34. Figure A.5b Republished with permission of John Wiley and Sons, Inc., from "Sinteringof Low-Density Glasses: II, Experimental Study,", Journal of American Ceramic Society,G. W. Scherer and D. Bachman, vol. 60, no. 5-6 and pp 239-243 (1977); © permissionconveyed through Copyright Clearance Center, Inc.35. Figure A.8 Reprinted from Ceramics International, vol. 45, no. 6, D. Salvato, J. F. Vigier,O. Dieste Blanco, L. Martel, L. Luzzi, J. Somers and V. Tyrpekl, “Innovative PreparationRoute for Uranium Carbide Using Citric Acid As A Carbon Source,” pp 1661-1675, ©(2006), with permission from Elsevier.36. Figure A.9 Reprinted from Journal of Nuclear Materials, vol. 496, no. 6, D. A. Lopes,A. Benarosch, S. Middleburgh and K. D. Johnson, “Spark Plasma Sintering andMicrostructural Analysis of Pure and Mo Doped U3Si2 Pellets,” pp 234-271, © (2017),with permission from Elsevier.37. Figure A.6 Reprinted from Journal of Nuclear Materials, vol. 389, H. Muta, K. Kurosaki,M. Uno and S. Yamanaka, “Thermophysical Properties of Several Nitrides Prepared bySpark Plasma Sintering,” , pp 186-190, © (2009), with permission from Elsevier.38. Figure A.7a Reprinted from A. Raftery, (2015) "Fabrication and Characterization ofUN-USix Nuclear Fuel". Master thesis, KTH Stockholm, pp 37, © by permission fromauthor.39. Figure A.7b Reprinted from K. Johnson, (2015) "High Performance Fuels for Water-Cooled Reactor Systems". Ph.D thesis, KTH Stockholm, pp 81 and 84, © by permissionfrom author.xiPreface40. Figure A.10 Reprinted from Journal of Nuclear Materials, vol. 433, R. C. O’Brien andN. D. Jerred, “Spark Plasma Sintering of W–UO2 Cermets,” , pp 50-54, © (2013), withpermission from Elsevier.41. Figure A.11 Reprinted from Journal of Nuclear Materials, vol. 505, J. A. Valdez, D. D.Byler, E. Kardoulaki, J. S. C. Francis, K. J. McClellan, “Flash Sintering of Stoichiometricand Hyper-stoichiometric Urania,” pp 85-93, © (2018), with permission from Elsevier.42. Figure A.12 Reprinted from Journal of Nuclear Materials, vol. 517, F. Cappia, J. M.Harp, and K. McCoy, “Post-irradiation Examinations of UO2 Composites As Part of TheAccident Tolerant Fuels Campaign,” pp 97-105, © (2019), with permission from Elsevier.43. Figure D.1 Republished with permission of John Wiley and Sons, Inc., from "MasterSintering Curve: A Practical Approach to Sintering", Journal of American CeramicSociety, D. L. Johnson and H. Su, vol. 79 no. 12 (2005); © permission conveyed throughCopyright Clearance Center, Inc.The contributions from the authors and other collaborators are listed below: The SparkPlasma Sintering experiments were carried out at the SPS facility for nuclear fuel materialsat the University of British Columbia Okanagan, Kelowna, BC, Canada by A. Prasad underthe supervision of Dr. L. Bichler. The characterisation precursor and sintered fuel materialswere carried out at University of Saskatchewan, Saskatoon, Saskatchewan, Canada andCanadian Nuclear Laboratories (CNL), Chalk River, Ontario, Canada. X-Ray Di raction(XRD), Scanning Electron Microscopy (SEM) and Di erential Scanning Calorimetry (DSC)experiments were carried out at the University of Saskatchewan, Saskatoon, Canada by L.Malakkal, M.K. Tummalapalli and J. Ranasinghe under the supervision of Dr. J. A. Szpunar.The stereomicroscopy and optical microscopy experiments was carried out at CNL by A.Prasad under the supervision of A. Bergeron with the support provided by T. Studdart and J.xiiPrefaceCrigger. The SPS tooling for IXAF was prepared by T. Studdart and J. Crigger under thesupervision of A. Bergeron. The dissertation was written and edited by A. Prasad under thesupervision of Dr. L. Bichler. The thesis was written in a latex template that was developedby Dr. McNeil Forbes and was updated to College of Graduate Studies guidelines.xiiiTable of ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiLay Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ivPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vTable of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xivList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiList of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiiiList of Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xliiiList of Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xlviList of Elements and Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . xlixList of Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . liiAcknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . livDedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lviChapter 1: Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Global Energy Crisis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Nuclear Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Research Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4 Thesis Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.5 Novelty and Originality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7xivTABLE OF CONTENTSChapter 2: Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.1 Powder Metallurgy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.1.1 Sintering Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.1.1.1 Sintering Mechanisms . . . . . . . . . . . . . . . . . . . . . 122.1.2 Sintering Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.1.2.1 Conventional Sintering . . . . . . . . . . . . . . . . . . . . 142.1.2.2 Hot Pressing . . . . . . . . . . . . . . . . . . . . . . . . . . 152.1.2.3 Microwave Sintering . . . . . . . . . . . . . . . . . . . . . . 152.1.2.4 Electric Current Assisted Sintering . . . . . . . . . . . . . . 152.2 SPS of Nuclear Fuel Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2.1 Mitigation of Radiotoxicity . . . . . . . . . . . . . . . . . . . . . . . . 182.2.2 SPS of Pure UO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2.2.1 Commercial Fabrication of Nuclear Fuel Pellets . . . . . . . 222.2.2.2 Simulation of Irradiated Fuel . . . . . . . . . . . . . . . . . 282.2.3 SPS of Accident Tolerant Fuels . . . . . . . . . . . . . . . . . . . . . . 322.2.3.1 UO2-Silicon Carbide (SiC) Composite ATFs . . . . . . . . . 322.2.3.2 UO2-Carbon Composite ATFs . . . . . . . . . . . . . . . . . 352.2.3.3 UO2-Titanium Dioxide (TiO2) Composite ATFs . . . . . . . 382.2.3.4 UO2-Beryllium Oxide (BeO) Composite ATFs . . . . . . . . 392.2.3.5 UO2-Titanium Silicon Carbide (Ti3SiC2) composite ATFs . . 392.2.4 SPS of ThO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402.2.5 Discussion on ECAS of Nuclear Fuel Materials . . . . . . . . . . . . . 452.3 Background on Internally and eXternally cooled Annular Fuel . . . . . . . . 48Chapter 3: Experimental Procedure . . . . . . . . . . . . . . . . . . . . . . . . 533.1 Precursor Powders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53xvTABLE OF CONTENTS3.1.1 ThO2 Powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543.1.2 UO2 Powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543.1.3 SiC Powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.1.4 Diamond Powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.2 Material Processing Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 563.2.1 Planetary Ball Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.2.2 Spark Plasma Sintering . . . . . . . . . . . . . . . . . . . . . . . . . . 573.3 Sample Preparation Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 603.4 Material Characterisation Techniques . . . . . . . . . . . . . . . . . . . . . . 613.4.1 Density Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . 613.4.2 X-Ray Di raction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623.4.3 Light Stereomicroscopy and Microscopy . . . . . . . . . . . . . . . . 643.4.4 Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . 643.5 Experimental Setup for IXAF . . . . . . . . . . . . . . . . . . . . . . . . . . . 653.6 FEM Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.6.1 Coupled Thermo-Mechanical Model . . . . . . . . . . . . . . . . . . 683.6.2 Coupled Electro-Thermal Model . . . . . . . . . . . . . . . . . . . . . 693.6.3 Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70Chapter 4: Part I: Densication Studies on ThO2 Based Fuels . . . . . . . . . 724.1 Powder Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724.1.1 ThO2 Powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724.1.2 UO2 Powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734.1.3 Diamond Powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.1.4 SiC Powders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.2 Tooling Deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76xviTABLE OF CONTENTS4.3 Densication of ThO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804.3.1 E ect of Sintering Temperature . . . . . . . . . . . . . . . . . . . . . 804.3.1.1 E ect of Peak Sintering Temperature . . . . . . . . . . . . 804.3.1.2 E ect of Heating Rate . . . . . . . . . . . . . . . . . . . . . 854.3.1.3 E ect of Dwelling Time . . . . . . . . . . . . . . . . . . . . 864.3.2 E ect of Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874.3.2.1 E ect of Pressure at Room Temperature . . . . . . . . . . . 874.3.2.2 E ect of Pressure During High Temperature . . . . . . . . 894.3.3 Characterisation of SPS processed ThO2 Pellets . . . . . . . . . . . . 904.3.3.1 Visual Observation . . . . . . . . . . . . . . . . . . . . . . . 904.3.3.2 Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 914.3.3.3 XRD Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 994.3.4 E ect of Additives on Densication of ThO2 . . . . . . . . . . . . . . 1034.3.4.1 E ect of SiC on the Densication of ThO2 . . . . . . . . . . 1034.3.4.2 E ect of Diamond on Densication of ThO2 . . . . . . . . . 1084.3.4.3 E ect of U Doping on Densication of ThO2 . . . . . . . . 1114.3.4.4 Summary on the E ect of Additives . . . . . . . . . . . . . 114Chapter 5: Part II: Comparison of SPS and CS for IXAF . . . . . . . . . . . . 1205.1 Preliminary Results from SPS of Surrogate IXAF . . . . . . . . . . . . . . . . 1205.2 Density Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1245.2.1 E ect of Tooling Clearance on The Densication During SPS of IXAF 1255.3 Visual Observation of As-sintered Pellets . . . . . . . . . . . . . . . . . . . . 1325.3.1 Modelling of Residual Stress Distribution During SPS of IXAF . . . . 1345.3.2 Dimensional Comparison of CS-ed and SPS-ed IXAF . . . . . . . . . 1375.4 Stereomicroscopy Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138xviiTABLE OF CONTENTS5.4.1 Stereomicroscopy of CS-ed IXAF . . . . . . . . . . . . . . . . . . . . 1395.4.2 Stereomicroscopy of SPS-ed IXAF . . . . . . . . . . . . . . . . . . . . 1415.5 Optical Microscopy Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1445.5.1 Optical Microscopy of CS-ed IXAF . . . . . . . . . . . . . . . . . . . 1455.5.2 Optical Microscopy of SPS-ed IXAF . . . . . . . . . . . . . . . . . . . 1465.6 Grain Size Analysis of CS-ed and SPS-ed IXAF . . . . . . . . . . . . . . . . . 1505.6.1 Grain Size Analysis of CS-ed IXAF . . . . . . . . . . . . . . . . . . . 1505.6.2 Grain Size Analysis of SPS-ed IXAF . . . . . . . . . . . . . . . . . . . 1515.6.2.1 Modelling of Thermal Field During SPS of IXAF . . . . . . 1545.7 Summary on CS and SPS of IXAF . . . . . . . . . . . . . . . . . . . . . . . . 161Chapter 6: Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1636.1 Densication Studies on ThO2 Based Fuel . . . . . . . . . . . . . . . . . . . . 1636.2 Comparison of SPS and CS for IXAF . . . . . . . . . . . . . . . . . . . . . . . 1646.3 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167AppendicesAppendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199A.1 History of Sintering Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . 199A.2 Nuclear Fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205A.2.1 Nuclear Fuel Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . 206A.2.2 Fabrication of Nuclear Fuel Pellets . . . . . . . . . . . . . . . . . . . . 207A.3 Additional Literature on ECAS of Nuclear Fuel Materials . . . . . . . . . . . 208A.3.1 SPS of Uranium Nitride . . . . . . . . . . . . . . . . . . . . . . . . . . 208A.3.2 SPS of Uranium Carbide (UC) . . . . . . . . . . . . . . . . . . . . . . 213xviiiTABLE OF CONTENTSA.3.3 SPS of Uranium Silicides . . . . . . . . . . . . . . . . . . . . . . . . . 214A.3.4 SPS of CERMET Fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . 216A.3.5 EDC and FS of Nuclear Fuel Materials . . . . . . . . . . . . . . . . . . 218A.3.6 Application of SPS for Fuel Waste Management . . . . . . . . . . . . 220A.3.7 Post Irradiation Examination of SPS-ed Fuel . . . . . . . . . . . . . . 220A.3.8 Summary of ECAS of Nuclear Fuel Materials . . . . . . . . . . . . . . 221Appendix B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233B.1 SPS Tooling Preparation Procedure . . . . . . . . . . . . . . . . . . . . . . . 233B.1.0.1 SPS Program . . . . . . . . . . . . . . . . . . . . . . . . . . 236B.2 Repeatability Test SPS Program . . . . . . . . . . . . . . . . . . . . . . . . . 238B.3 Theoretical Density Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240B.4 Consolidated List of SPS Trials . . . . . . . . . . . . . . . . . . . . . . . . . . 242Appendix C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247C.1 Stereo Microscopy of SPS-ed ThO2 . . . . . . . . . . . . . . . . . . . . . . . . 247C.2 Electron Back Scatter Di raction of SPS-ed ThO2 . . . . . . . . . . . . . . . . 249Appendix D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250D.1 Master Sintering Curve Theory . . . . . . . . . . . . . . . . . . . . . . . . . . 250D.1.0.1 Limitations of Master Sintering Curve Theory . . . . . . . 254D.2 SPS Conditions for MSC Experiments . . . . . . . . . . . . . . . . . . . . . . 255D.3 MSC of ThO2 and Its Composites . . . . . . . . . . . . . . . . . . . . . . . . . 258D.3.1 MSC of UO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258D.3.1.1 Step by Step Construction of MSC of UO2 . . . . . . . . . . 260D.3.2 MSC of ThO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265D.3.3 MSC of ThO2-5 wt% 40 µm SiC . . . . . . . . . . . . . . . . . . . . . 266D.3.4 MSC of ThO2-5 wt% 1 µm SiC . . . . . . . . . . . . . . . . . . . . . . 267xixTABLE OF CONTENTSD.3.5 MSC of ThO2-1 wt% 15 nm SiC . . . . . . . . . . . . . . . . . . . . . 268D.3.6 MSC of ThO2-5 wt% Diamond . . . . . . . . . . . . . . . . . . . . . . 269D.3.7 MSC of Th0.9U0.1O2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269D.3.8 MSC of Th0.95U0.05O2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 270D.3.9 Summary on MSC of ThO2 and Its Composites . . . . . . . . . . . . . 271D.3.10 MSC Database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272D.4 Master Sintering Curve Data . . . . . . . . . . . . . . . . . . . . . . . . . . . 274D.4.1 Master Sintering Curve Data of UO2 . . . . . . . . . . . . . . . . . . 274D.4.2 Master Sintering Curve Data of ThO2 . . . . . . . . . . . . . . . . . . 278D.4.3 Master Sintering Curve Data of ThO2-5 wt% 40 µm SiC . . . . . . . . 284D.4.4 Master Sintering Curve Data of ThO2-5 wt% 1 µm SiC . . . . . . . . . 292D.4.5 Master Sintering Curve Data of ThO2-1 wt% 15 nm SiC . . . . . . . . 298D.4.6 Master Sintering Curve Data of ThO2-5 wt% Diamond . . . . . . . . 305D.4.7 Master Sintering Curve Data of Th0.9U0.1O2 . . . . . . . . . . . . . . . 312D.4.8 Master Sintering Curve Data of Th0.95U0.05O2 . . . . . . . . . . . . . . 315Appendix E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322E.1 Engineering Drawings of "Tight tight" Tooling Components . . . . . . . . . 322E.2 Engineering Drawings of "Loose tight" Tooling Components . . . . . . . . . 326E.3 Engineering Drawings of "Tight loose" Tooling Components . . . . . . . . . 330E.4 Engineering Drawings of "Loose loose" Tooling Components . . . . . . . . . 334xxList of Tables2.1 SPS vs. conventional processing of UO2 (Reprinted from Journal of EuropeanCeramic Society, vol. 34, L. Ge, G. Subhash, R. H. Baney, and J. S. Tulenko,"In uence of Processing Parameters on Thermal Conductivity of UraniumDioxide Pellets Prepared by Spark Plasma Sintering," pp 1791–1801, © (2014),with permission from Elsevier) [67][96] . . . . . . . . . . . . . . . . . . . . . 523.1 Chemical composition in precursor ThO2 powder. . . . . . . . . . . . . . . . 543.2 Chemical composition in precursor UO2 powder. . . . . . . . . . . . . . . . . 553.3 Cross-sectional area of sintering cavity of various SPS tooling used in thiswork. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593.4 Variation in density measurement of indentical pellets. . . . . . . . . . . . . 623.5 XRD machines used in this study at University of Saskatchewan. . . . . . . . 633.6 Axisymmetric schematic of di erent tooling geometries for SPS IXAF . . . . 673.7 Variations in SPS tooling for IXAF fabrication for xed green pelletdimensions (OD: 15.75 mm and ID: 6.08 mm). . . . . . . . . . . . . . . . . . . 683.8 Physical properties used for FEM (T is temperature values in Kelvin, unlessspecied otherwise). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.1 PLP of precursor SiC additives. . . . . . . . . . . . . . . . . . . . . . . . . . . 76xxiLIST OF TABLES4.2 Stereoscopic examination of SPS processed and thermally etched ThO2-SiCcomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1174.3 Stereoscopic examination of SPS processed and thermally etched ThO2-3 wt%diamond composite and Th0.95U0.05O2 . . . . . . . . . . . . . . . . . . . . . . 1184.4 Stereoscopic examination of SPS processed and thermally etched Th0.95U0.05O2 1195.1 As-sintered density results from comparison of CS and SPS for IXAF. . . . . 1255.2 Change in dimensions of IXAF during conventional sintering. . . . . . . . . 1375.3 Comparison of grain size values of CS-ed and SPS-ed IXAF. . . . . . . . . . . 1535.4 Comparison of conventional and spark plasma sintering for IXAF. . . . . . . 162A.1 Summary of materials and processing conditions for ECAS processing ofnuclear materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222B.1 Theoretical density of materials used in this dissertation [246] . . . . . . . . 241B.2 Consolidated list of SPS experiments performed in this study. . . . . . . . . . 242D.1 MSC experimental conditions used in this study. . . . . . . . . . . . . . . . . 257D.2 Simplied MSC database generated using binomial dose response equation. . 273D.3 Master Sintering Curve Data of UO2. . . . . . . . . . . . . . . . . . . . . . . . 274D.4 Master Sintering Curve Data of ThO2. . . . . . . . . . . . . . . . . . . . . . . 278D.5 Master Sintering Curve Data of ThO2-5 wt% 40 µm SiC . . . . . . . . . . . . 284D.6 Master Sintering Curve Data of ThO2-5 wt% 1 µm SiC. . . . . . . . . . . . . . 292D.7 Master Sintering Curve Data of ThO2-1 wt% 15 nm SiC. . . . . . . . . . . . . 298D.8 Master Sintering Curve Data of ThO2-5 wt% Diamond . . . . . . . . . . . . . 305D.9 Master Sintering Curve Data of Th0.9U0.1O2. . . . . . . . . . . . . . . . . . . . 312D.10 Master Sintering Curve Data of Th0.95U0.05O2. . . . . . . . . . . . . . . . . . . 315xxiiList of Figures1.1 World total energy consumption from 1971 to 2015 in million tonne oilequivalent (mtoe) [10] (Source: IEA (2017) Key World Energy Statistics. Allrights reserved ©). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Electricity production in 1971 and 2015 [10] (Source: IEA (2017) Key WorldEnergy Statistics. All rights reserved ©). . . . . . . . . . . . . . . . . . . . . . 31.3 Total global nuclear energy supply from 1971 to 2015 in TeraWatt hour (TWh)[14] (Based on IEA data from the IEA (2016) Headline Global Energy Data 2016,www.iea.org/statistics, © as modied by Anil Prasad). . . . . . . . . . . . . 41.4 Research owchart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.1 Various mechanisms active during sintering (Republished with permission ofTaylor & Francis Group LLC, from Sintering of Ceramics, M. N. Rahman, 1stEdition and 2007; © permission conveyed through Copyright ClearanceCenter, Inc.) [40]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.2 a) SPS schematic of SPS die (Reprinted by permission from Springer NatureCustomer Service Centre GmbH: Springer, Encyclopedia of Nanotechnologyedited by B. Bhushan, © (2012)) [56] and b) SPS die during heating. . . . . . 172.3 Total annual publications on ECAS of nuclear fuel materials. . . . . . . . . . 18xxiiiLIST OF FIGURES2.4 Modied SPS setup implemented by Tyrpekl et al.: "The small scale SPS facilityintegrated in a glovebox (a); sintering chamber (b); 6 mm graphite die andpunch (c)" (Reprinted from V. Tyrpekl, C. Berkmann, M. Holzhäuser, F. Köpp,M. Cologna, T. Wangle and J. Somers, “Implementation of A Spark PlasmaSintering Facility in A Hermetic Glovebox for Compaction of Toxic, Radiotoxicand Air Sensitive Materials,” Review of Scientic Instruments, vol. 86, pp 23904(2015), © with the permission of AIP Publishing) [63]. . . . . . . . . . . . . . 202.5 Ironman et al.’s study on viability of SPS for commercial fuel production: a)Commercial specication pellets prepared by SPS, b) Surface roughnessmeasurement across central ring on SPS-ed commercial pellet, c)Multiple-pellet SPS die setup (Reprinted from T. Ironman, J. Tulenko and G.Subhash, “Exploration of Viability of Spark Plasma Sintering for CommercialFabrication of Nuclear Fuel Pellets,” Nuclear Technology, vol. 200, pp 144–158(2017), © with the permission of Taylor & Francis) [20] . . . . . . . . . . . . . 242.6 "Comparison of the maximum a) temperature and b) Hydrostatic stressdi erence between points 1 & 3 and 2 & 4 respectively within a pellet duringSPS for three pellet geometries" (Reprinted from Ceramics International, vol.45 no. 6, B. Nili and G. Subhash, “In uence of Porosity and Pellet Dimensionson Temperature and Stress Inhomogeneities During Spark Plasma Sinteringof Ceramic Fuel,” pp 7376-7384, © (2019), with permission from Elsevier) [76]. 26xxivLIST OF FIGURES2.7 a) High pressure SPS die setup used by Tyrpekl et al. (Republished withpermission of John Wiley & Sons, Inc., from "Preparation ofBulk-Nanostructured UO2 Pellets Using High-Pressure Spark PlasmaSintering for LWR Fuel Safety Assessment", Journal of American CeramicSociety, V. Tyrpekl, M. Cologna, J.F. Vigier, A. Cambriani, D. W. Wim and J.Somers, vol. 100, pp 1269-1274 and 2017; © permission conveyed throughCopyright Clearance Center, Inc.) [72] and b) Miao et al.’s in situ synchrotronWAXS experimental setup (Reprinted from Scripta Materialia, vol. 131, Y.Miao, T. Yao, J. Lian, J. S. Park, J. Almer, S. Bhattacharya, A. M. Yacout and K.Mo, "In situ Synchrotron Investigation of Grain Growth Behavior ofNano-Grained UO2", pp 29-32, (2017), © with permission from Elsevier) [82]. 302.8 a) E ect of SiC particle size on the thermal conductivity of UO2-5 vol% SiC(Reprinted from Journal of Nuclear Materials, vol. 442, S. Yeo, R. Baney, G.Subhash and J. Tulenko, “The In uence of SiC Particle Size and VolumeFraction on the Thermal Conductivity of Spark Plasma Sintered UO2–SiCComposites," pp 245-252, (2013), © with permission from Elsevier) [89] andb) Residual stress measured by micro Raman spectroscopy along SiCparticles [90] (Reprinted from Ph.D thesis, University of Florida, S. Yeo"UO2-SiC composite reactor fuels with enhanced thermal and mechanicalproperties prepared by spark plasma sintering" pp 105 (2013). . . . . . . . . . 34xxvLIST OF FIGURES2.9 a) Thermal conductivity and b) Thermal conductivity increase evaluation ofATF candidates by Li et al.: UO2-10 vol% BeO, UO2-10 vol% SiC and UO2-10vol% Ti3SiC2 (Reprinted from Ceramic International, vol. 44, B. Li, Z. Yang, M.Chu, Q. Huang, Z. Wang, R. Gao, Y. Zhong, X. Liu, L. Duan, and P. Zhang,"Ti3SiC2/UO2 Composite Pellets with Superior High-Temperature ThermalConductivity", pp 19846-19850, © (2018), with permission from Elsevier)[103][28][94]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402.10 Thermal conductivity of ThxU1-xO2: a) Muta et al.’s work (Reprinted from H.Muta, Y. Murakami, M. Uno, K. Kurosaki, and S. Yamanaka, “ThermophysicalProperties of Th1-xUxO2 Pellets Prepared by Spark Plasma Sinteringtechnique,” Journal of Nuclear Science and Technology, vol. 50, pp 181-187(2013), © with the permission of Taylor & Francis) [106] and b) Saoudi et al.’swork (Reprinted from Journal of Nuclear Materials, vol. 500, M. Saoudi, D.Staicu, J. Mouris, A. Bergeron, H. Hamilton, M. Naji, D. Freis, and M.Cologna, "Thermal Di usivity and Conductivity of Thorium-Uranium MixedOxides,", pp 181-187, © (2018), with permission from Elsevier) [107]. . . . . . 422.11 Muta et al.’s averaged atomic radius model, where a combination of smallerCe3+ and larger Ce4+ behaves similar to Ce3,x+ which has same atomic radiias Th4+: a) Model and b) Predicted and observed thermal conductivity values(Reprinted from Journal of Nuclear Materials, vol. 483, H. Muta, H. Kado, Y.Ohishi, K. Kurosaki and S. Yamanaka, "Thermal Di usivity and Conductivityof Thorium-Uranium Mixed Oxides," pp 192-198, © (2017), with permissionfrom Elsevier) [109]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.12 Key statistics from literature on ECAS of nuclear fuel material. . . . . . . . . 46xxviLIST OF FIGURES2.13 Rod-inserted sintering approach by Rhee et al.: "Cross section of ZrO2rod–inserted annular pellet sintered at 1700◦C for 4 hours in H2 atmosphere"[134] (Reprinted from Y. W. Rhee, D. J. Kim, J. H. Kim, J. H. Yang, K. S. Kim,and J. H. Yang, “Rod-Inserted Sintering of Annular Pellets for Dual-CooledAnnular Fuel,” Nuclear Technology, vol. 184, pp 54–62 (2013), © with thepermission of Taylor & Francis). . . . . . . . . . . . . . . . . . . . . . . . . . 513.1 Planetary ball mill at University of Saskatchewan used for preparingcomposite and mixed oxide fuel powder blends. . . . . . . . . . . . . . . . . 573.2 Thermal Technologies® 10-3 SPS system (Reprinted from A. Prasad, (2017)"Spark Plasma Sintering of Cerium Dioxide and Its Composites". MASc thesis,University of British Columbia, pp 24 © by permission from author) [136]. . 583.3 Modules within SPS equipment: a) SPS sintering chamber with loaded die andb) Process control module (Reprinted from A. Prasad, (2017) "Spark PlasmaSintering of Cerium Dioxide and Its Composites". MASc thesis, University ofBritish Columbia, pp 25 © by permission from author) [136]. . . . . . . . . . 593.4 X-Ray di raction equipment Bruker® D8 (Source: © Anil Prasad (2017)). . . . 633.5 X-Ray di raction equipment Rigaku® Ultima IV. . . . . . . . . . . . . . . . . 633.6 Nelson Riley function plotted against the lattice parameter to estimatePrecision Lattice Parameter (PLP) of SPS-ed Th0.95U0.05O2 . . . . . . . . . . . 643.7 Axisymmetric schematic of ’Tight tight’ conguration. . . . . . . . . . . . . 673.8 Axisymmetric schematic of ’Loose tight’ conguration. . . . . . . . . . . . . 673.9 Axisymmetric schematic of ’Tight loose’ conguration. . . . . . . . . . . . . 673.10 Axisymmetric schematic of ’Loose loose’ conguration. . . . . . . . . . . . . 67xxviiLIST OF FIGURES4.1 Characterisation of precursor powder: a) SE image and b) XRD scan of ThO2(Unit cell shown in inset was built using VETSA© software). . . . . . . . . . 734.2 Characterisation of precursor powder: a) SE image and b) XRD scan of UO2(Unit cell shown in inset was built using VETSA© software). . . . . . . . . . 744.3 Characterisation of precursor powder: a) SE image and b) XRD scan ofdiamond powder (Unit cell shown in inset was built using VETSA© software). 754.4 Characterisation of precursor powder: a) SE image of 1 µm SiC particles andb) XRD scans of 15 nm, 1 µm and 40 µm SiC (Unit cell shown in inset was builtusing VETSA© software). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764.5 Tooling deformation during SPS, measured using: a) Thermocouple controlled(low temperature) experiment and b) Pyrometer controlled (high temperature)experiment (negative displacement means an increase in the length of tooling) . 774.6 a) Displacement during empty tooling test when thermal and pressureprole was coupled and b) Displacement during 2nd run when temperatureand pressure was decoupled. . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.7 E ect of peak sintering temperature on the densication: a) As-sintered RDvs. peak sintering temperature (for various pressure and dwelling timeconditions) and b) Densication rate vs. instantaneous RD for 1700◦C - 50MPa - 10 min sample. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.8 Comparison of densication and rst derivative curves based on peaksintering temperature for 50 MPa pressure and 10 min: a) 1500◦C-1700◦Cand b)1700◦C-1800◦C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824.9 Tooling deformation from SPS run at 1800◦C - 50 MPa - 10 min: a) Polynomialcurve t of tooling deformation during empty die test and b) Displacementdata before and after subtraction of tooling deformation. . . . . . . . . . . . 85xxviiiLIST OF FIGURES4.10 E ect of heating rate on the densication of ThO2 during SPS: a) As a functionof time and b) As a function of temperature SPS processed at ’1800◦C - 40 MPa- 10 min’ with heating rates 50◦C/min, 75◦C/min and 100◦C/min. . . . . . . . 864.11 E ect of dwelling time on the densication of ThO2 during SPS: a) As-sinteredRD vs. dwelling time for various dwelling times and b) Instantaneous RD curveduring dwelling time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874.12 E ect of cold compaction pressure on fuel powders used in this study: a)Displacement during cold pressing and b) Stages of cold compaction (Source:Powder Metallurgy Science, 2nd Ed., German, Randall M., (1994) ISBN1-878954-42-3 Metal Powder Industries Federation, 105 College Road East,Princeton, New Jersey, USA.) [151]. . . . . . . . . . . . . . . . . . . . . . . . 884.13 E ect of pressure on the densication of ThO2 during SPS: a) Pressure vs.as-sintered RD (the error bars in 1800◦C - 0.5 min represent sample set ofvarying heating rates: 50◦C/min, 75◦C/min and 100◦C/min) and b)Densication curves for di erent peak sintering temperatures as a functionof instantaneous pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 894.14 SPS processed ’1700◦C - 50 MPa - 10 min’ ThO2 showing greyish discolorationon the surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914.15 Optical microscopy results from ThO2 SPS processed at ’1600◦C - 30 MPa -10 min’ macrographs (stitched from 4x images) from: a) Unetched specimenand b) Etched specimen and micrographs from: c) Etched from a region awayfrom edge and d) Unetched micrograph from edge of sample showing facetedboundaries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92xxixLIST OF FIGURES4.16 Secondary electron micrographs from fractured ThO2 SPS processed pellets at’50 MPa - 10 min’ and peak sintering temperature of: a) 1500 ◦C and b) 1600◦C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 934.17 SEM micrographs from ThO2 SPS processed at ’1800 ◦C - 50 MPa - 10 min’:a) Fractured surface and b) Polished surface (inset showing a triple grainboundary). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 944.18 Optical micrograph from ThO2 SPS processed at ’1800◦C 50 MPa - 10 min’:a) Stitched image from 97 micrographs showing a sample wide intergranularfracture and b) Intragranular pores in unetched specimen. . . . . . . . . . . . 954.19 SEM micrographs from ThO2 SPS processed at ’1800◦C - 50 MPa - 10 min’: a)Polished surface showing grain growth and b) An entrapped pore impinginggrain growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 964.20 Grain size vs. RD: a) Results of this work and b) Ge et al.’s SPS study on UO2(Reprinted from Journal of the European Ceramic Society, vol. 34, L. Ge, G.Subhash, R. H. Baney, and J. S. Tulenko, "In uence of Processing Parameterson Thermal Conductivity of Uranium Dioxide Pellets Prepared by SparkPlasma Sintering," pp 1791-1801, © (2014), with permission from Elsevier)[67]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 974.21 Grain size measurements along radial direction for ThO2 SPS processed at’1800◦C - 50 MPa - 10 min’ and ’1600◦C - 30 MPa - 10 min’ (connecting linesincluded for convenience). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 984.22 XRD analyses of SPS processed ThO2: a) XRD patterns, b) PLP measurementsand c) Carbonaceous compounds detected in crevice between grains on thesurface of ThO2 SPS processed at ’1700◦C - 70 MPa - 10 min’. . . . . . . . . . 101xxxLIST OF FIGURES4.23 E ect of natural aging, polishing and thermal etching on ThO2 pellet SPSprocessed at ’1800◦C - 50 MPa - 10 min’: a) XRD patterns and b) PLP. . . . . 1034.24 E ect of 3 wt% SiC addition to ThO2 during SPS at ’1800◦C - 60 MPa - 0.5 min’:a) Average as-sintered RD and b) Instantaneous RD and derivative curves from75◦C/min heating rate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1044.25 E ect of 5 wt% SiC addition to ThO2 during SPS at ’1800◦C - 60 MPa - 0.5 min’:a) Average as-sintered RD and b) Instantaneous RD and derivative curves from75◦C/min heating rate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1054.26 E ect of SiC concentration on densication of ThO2 - SiC: a) As-sintered RDand b) Instantaneous RD from SPS at ’1800◦C - 60 MPa - 0.5 min’. . . . . . . 1074.27 a) XRD analysis and b) SEM microstructure of SPS processed ThO2- 5 wt% 1µm SiC ’1800◦C - 60 MPa - 0.5 min’. . . . . . . . . . . . . . . . . . . . . . . . 1084.28 E ect of diamond addition to ThO2: a) As-sintered RD versus diamond wt%and b) Instantaneous RD curves of pellets SPS processed at ’1800◦C - 60 MPa- 0.5 min’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1094.29 Characterisation of SPS processed ThO2-diamond composites: a) XRDanalysis and b) Optical micrograph of ThO2-3 wt% diamond SPS processed at’1500◦C - 60 MPa - 5 min’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1104.30 Analyses of densication results from SPS of (Th,U)O2: a) Instantaneous RDcurves and b) E ect of U doping on the temperature of peak densication rate(connecting line is for convenience). . . . . . . . . . . . . . . . . . . . . . . . 1124.31 Analyses of densication results from SPS of (Th,U)O2 samples and peaksintering temperature: a) Densication curves and b) E ect of U doping onthe peak densication rate temperature (connecting line is for convenience). 113xxxiLIST OF FIGURES4.32 Optical micrograph of Th0.95U0.05O2 SPS processed at ’1700◦C - (75◦C/min) -60 MPa - 0.5 min’: a) Secondary oxide with higher Th content and b) Grainsize prole along radial direction of pellet. . . . . . . . . . . . . . . . . . . . 1144.33 E ect of additives on the as-sintered RD of ThO2 . . . . . . . . . . . . . . . . 1154.34 Macrograph of unetched ThO2-5 wt% 1 µm SiC ’1800◦C (100◦C/min) - 60 MPa- 0.5 min’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1174.35 Macrograph of etched ThO2-5 wt% 1 µm SiC ’1800◦C (100◦C/min) - 60 MPa -0.5 min’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1174.36 Macrograph of unetched ThO2-1 wt% 15 nm SiC ’1800◦C (50◦C/min) - 60 MPa- 0.5 min’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1174.37 Macrograph of etched ThO2-1 wt% 15 nm SiC ’1800◦C (50◦C/min) - 60 MPa -0.5 min’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1174.38 Macrograph of unetched ThO2-3 wt% diamond ’1500◦C (100◦C/min) - 60 MPa- 5 min’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1184.39 Macrograph of etched ThO2-3 wt% diamond ’1500◦C (100◦C/min) - 60 MPa -5 min’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1184.40 Macrograph of unetched Th0.95U0.05O2 ’1300◦C (50◦C/min) - 60 MPa - 0.5 min’. 1184.41 Macrograph of etched Th0.95U0.05O2 ’1300◦C (50◦C/min) - 60 MPa - 0.5 min’. . 1184.42 Macrograph of unetched Th0.95U0.05O2 ’1700◦C (75◦C/min) - 60 MPa - 0.5 min’. 1194.43 Macrograph of etched Th0.95U0.05O2 ’1700◦C (75◦C/min) - 60 MPa - 0.5 min’. . 1195.1 SPS of IXAF: a) Cross-section view of nal tooling and its various componentsand b) Surrogate pellet attached to core rod. . . . . . . . . . . . . . . . . . . 121xxxiiLIST OF FIGURES5.2 Selected samples from tooling design study using 8-YSZ: a) Solid core-rod andone wrap of grafoil, b) Solid core-rod and two wraps of grafoil, c) Annularcore-rod and one wrap of grafoil, d) Annular core-rod and two wraps of grafoil. 1235.3 E ect of a centre hole in core rod on residual stress distribution within SPS-edIXAF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1245.4 a) Correlation between (tooling-green pellet) clearance and as-sintered RDand b) Grasso et al.’s study on the e ect of die-punch clearance on pellettemperature (Reprinted from Materials Transactions, vol. 49, no. 12, S.Grasso, Y. Sakka, and G. Maizz, "E ects of Initial Punch-Die Clearance inSpark Plasma Sintering Process," pp 2899-2906, (2008), © with permissionfrom Elsevier) [172] c) Manière et al.’s study on e ect of die wall frictionduring SPS (Reprinted from Scripta Materialia, vol. 116, C. Manière, L.Durand, and C. Estournè, "Powder/die friction in the Spark Plasma Sinteringprocess: Modelling and experimental identication," pp 139-142 (2016), ©with permission from Elsevier) [171]. . . . . . . . . . . . . . . . . . . . . . . 1265.5 Instantaneous RD curves from SPS of IXAF: a) Comparison of time basedinstantaneous RD curves and b) Instantaneous RD and densication ratecurves of "Loose tight" specimen plotted against temperature. . . . . . . . . 1295.6 In situ power and total circuit resistance of data from SPS of IXAF. . . . . . . 1305.7 E ect of pellet tooling clearance on electric current path during SPS. . . . . 1315.8 Conventional sintering of UO2 IXAF: a) As-sintered samples and b) Surfacedefect on the corner of a pellet. . . . . . . . . . . . . . . . . . . . . . . . . . . 1335.9 As sintered SPS-ed IXAF pellets of varying clearances: a) "Tight Tight", b)"Loose tight", c) "Tight loose" and d) "Loose loose". . . . . . . . . . . . . . . . 134xxxiiiLIST OF FIGURES5.10 Residual stress distribution within IXAF samples modelled using ABAQUS: a)Stress build-up around inner edge of SPS-ed IXAF and b) Max. residual stressvs. C.S.A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1365.11 Side by side (left to right) comparison of green pellet, CS-ed and SPS-ed IXAF. 1385.12 Stereomicrograph of surface specimen of CS-ed IXAF: a) Stereo micrographstitched from 66 images taken at 4X magnication and b) One of the imagesused for stitching showing a radial crack at 4X magnication. . . . . . . . . 1395.13 Stereomicrograph of cross-section specimen of CS-ed IXAF: a) Stereomicrograph stitched from 153 4X images and b) Higher magnication imageof regions where axial cracks connect radial cracks. . . . . . . . . . . . . . . 1405.14 An exaggerated schematic showing the e ect of green density gradient onradial cracks observed in CS-ed IXAF. . . . . . . . . . . . . . . . . . . . . . . 1415.15 Stereomicrograph of SPS-ed IXAF: a) Stereo micrograph stitched from 129 4Xmagnication images (the yellow squares represent locations of micrographslater in Figure 5.18), b) Higher magnication image of the crevice type defectand c) Exaggerated schematic of evolution surface defects during SPS of IXAF. 1435.16 Stereomicrograph of SPS-ed IXAF: a) Stitched image of polished surface (theyellow squares represent locations of micrographs in Figure 5.18) and b)Higher magnication image of "neutral zone". . . . . . . . . . . . . . . . . . 1445.17 Etched microstructure of CS-ed IXAF: a) Surface, b) Cross-section, c) Image ofa crack showing crack healing and d) Intergranular and transgranular modeof fracture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145xxxivLIST OF FIGURES5.18 Unetched microstructure of SPS-ed "Tight tight" IXAF: a) Dense regions atthe periphery of the surface of pellet, b) Crevices in the mid section, c)Porosity surrounding the core rod, d) Representative high density region inthe cross-section sample, e) Interface between dense and porous region inthe cross section and f) High magnication image of porous region (thelocation of micrographs are identied in Figures 5.15 and 5.16). . . . . . . . . 1475.19 Microstructure of SPS-ed IXAF: a) Cross-sectional microstructure at a regionthat was in contact with core-rod, b) Equiaxed grains at a region in theinterior of the pellet and c) MacEwan et al.’s in-service fuel simulationexperiment (Republished with permission of John Wiley & Sons, Inc., from"Grain Growth in Sintered Uranium Dioxide: II, Columnar Grain Growth,"Journal of American Ceramic Society, J. R. MacEwan and V. B. Lawson, vol.45, no. 1 and pp 42-46 (1962); © permission conveyed through CopyrightClearance Center, Inc.) [181]. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495.20 Grain size analyses of CS-ed IXAF: a) Line proles used for grain sizemeasurement, b) Grain size along various lines indicated in rst gure. . . . 1515.21 Grain size analysis of surface specimen of "Tight tight" SPS-ed IXAF: a) Radialline prole used for selecting grain size measurement locations, b) Aggregatedgrain size vs. radial distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1525.22 Grain size analysis of cross-section specimen of SPS-ed "Tight tight" IXAF: a)Locations used for generating grain size map and b) Grain size map of samplecross-section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1535.23 Temperature of pyrometer point modelled using coupled electro-thermal FEMplotted with experimentally measured temperature data for "Loose tight" SPStrial. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154xxxvLIST OF FIGURES5.24 FEA model of SPS of "Loose tight" IXAF: c) Electric potential distribution att=11.00 min, a) Temperature distribution at t=3.33 min and b) Temperaturedistribution at t=11.00 min (t=3.33 min corresponds to the ramping stage andt=11 min corresponds to dwelling stage of SPS program). . . . . . . . . . . . 1555.25 Temperature di erence between four edges, center and pyrometer point of"Loose tight" IXAF during SPS: a) Thermal prole for various locations and b)Temperature of corners with reference to the temperature of the centre of thepellet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1565.26 Evolution of temperature distribution within as-sintered pellet: a) t=11.00min, b) t=19.85 min, c) t=21.06 min, d) t=21.13 min, e) t=21.46 min and f) t=22.10 min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1585.27 Evolution of electric eld during SPS within IXAF cross-section at: a) t=11min and b) t=20.81 min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161A.1 Surface area reduction by particle coalescence. . . . . . . . . . . . . . . . . . 199A.2 Two spherical particles necking during sintering. . . . . . . . . . . . . . . . . 200A.3 Coble’s sintering stages [197][198] (Reprinted from R. L. Coble, “SinteringCrystalline Solids. I. Intermediate and Final State Di usion Models,” Journalof Applied Physics, vol. 32, pp 787–792 (1961), © with the permission of AIPPublishing). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202A.4 Sintering diagram of spherical UO2 particles of diameter 10µm (Reprintedfrom Acta Metallurgica, vol. 116, M. F. Ashby, "A First Report on SinteringDiagrams", pp 275-289 (1974), © with permission from Elsevier) [38]. . . . . . 203xxxviLIST OF FIGURESA.5 Scherer’s continuum based sintering model: a) Model of a cell element(Republished with permission of John Wiley and Sons, Inc., from "Sinteringof Low-Density Glasses: I, Theory,", Journal of American Ceramic Society, G.W. Scherer, vol. 60, no. 5-6 and pp 236-239 (1977); © permission conveyedthrough Copyright Clearance Center, Inc.) [200] and b) Experimental dataobtained from sintering of SiO2 plotted against Scherer’s relation(Republished with permission of John Wiley and Sons, Inc., from "Sinteringof Low-Density Glasses: II, Experimental Study,", Journal of AmericanCeramic Society, G. W. Scherer and D. Bachman, vol. 60, no. 5-6 and pp239-243 (1977); © permission conveyed through Copyright Clearance Center,Inc.) [201]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205A.6 Thermal conductivity of nuclear fuel materials prepared by Muta et al. (UN,UN-60 mol%TiN and Zr0.6Ti0.4N) using SPS compared to previous literature[215] (Reprinted from Journal of Nuclear Materials, vol. 389, H. Muta, K.Kurosaki, M. Uno and S. Yamanaka, “Thermophysical Properties of SeveralNitrides Prepared by Spark Plasma Sintering,” , pp 186-190, © (2009), withpermission from Elsevier). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209A.7 a) SE image of UN-10 wt% U3Si2 showing UN in encapsulated in U3Si2 phase(Reprinted from A. Raftery, (2015) "Fabrication and Characterization of UN-USix Nuclear Fuel". Master thesis, KTH Stockholm, pp 37, © by permissionfrom author) [220] and b) Oxidation mechanism of UN (inter-granular) andUN-10 wt% U3Si2 [62] (Reprinted from K. Johnson, (2015) "High PerformanceFuels for Water-Cooled Reactor Systems". Ph.D thesis, KTH Stockholm, pp 81and 84, © by permission from author). . . . . . . . . . . . . . . . . . . . . . . 212xxxviiLIST OF FIGURESA.8 Salvato et al.’s die setup for CTR synthesis of UC (Reprinted from CeramicsInternational, vol. 45, no. 6, D. Salvato, J. F. Vigier, O. Dieste Blanco, L. Martel,L. Luzzi, J. Somers and V. Tyrpekl, “Innovative Preparation Route for UraniumCarbide Using Citric Acid As A Carbon Source,” pp 1661-1675, © (2006), withpermission from Elsevier) [225]. . . . . . . . . . . . . . . . . . . . . . . . . . 214A.9 Lopes et al.’s observations from SPS of U3Si2 and U3Si2- 1.5 at% Mo: a)Nanoscale porosities in U3Si2 resembling HBS and b) U-Mo-Si intermetallicsin U3Si2-1.5 at% Mo (Reprinted from Journal of Nuclear Materials, vol. 496,no. 6, D. A. Lopes, A. Benarosch, S. Middleburgh and K. D. Johnson, “SparkPlasma Sintering and Microstructural Analysis of Pure and Mo Doped U3Si2Pellets,” pp 234-271, © (2017), with permission from Elsevier) [226]. . . . . . 215A.10 Elemental mapping of UO2-50 vo% W-25Re material for space nuclear fuelapplications (Reprinted from Journal of Nuclear Materials, vol. 433, R. C.O’Brien and N. D. Jerred, “Spark Plasma Sintering of W–UO2 Cermets,” , pp50-54, © (2013), with permission from Elsevier) [230]. . . . . . . . . . . . . . 217A.11 Impact of electric eld on densication of: a) UO2.16 and b) UO2.00 (Reprintedfrom Journal of Nuclear Materials, vol. 505, J. A. Valdez, D. D. Byler, E.Kardoulaki, J. S. C. Francis, K. J. McClellan, “Flash Sintering ofStoichiometric and Hyper-stoichiometric Urania,” pp 85-93, © (2018), withpermission from Elsevier) [239] . . . . . . . . . . . . . . . . . . . . . . . . . . 219A.12 Post irradiation ceramography of SPS-ed: a) UO2 b) UO2-10 vol% SiC c)UO2-10 vol% diamond (Reprinted from Journal of Nuclear Materials, vol. 517,F. Cappia, J. M. Harp, and K. McCoy, “Post-irradiation Examinations of UO2Composites As Part of The Accident Tolerant Fuels Campaign,” pp 97-105, ©(2019), with permission from Elsevier) [91]. . . . . . . . . . . . . . . . . . . . 221xxxviiiLIST OF FIGURESB.1 Tooling components used for SPS process: a) Graphite die, b) Grafoil liningof the die, c) Grafoil disks and d) Graphite punches (20 mm and 12.5 mm)(Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of Cerium Dioxideand Its Composites". MASc thesis, University of British Columbia, pp 26, © bypermission from author) [136]. . . . . . . . . . . . . . . . . . . . . . . . . . . 233B.2 Stages of SPS die preparation: a) Die ready for lling powder inside theweighing balance, b) Fully prepared tooling for SPS and c) Static gloveboxused for preparing tooling for SPS (Reprinted from A. Prasad, (2017) "SparkPlasma Sintering of Cerium Dioxide and Its Composites". MASc thesis,University of British Columbia, pp 27, © by permission from author) [136]. . 235B.3 Focusing of pyrometer on the die surface: a) Not focused and b) Focused(Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of Cerium Dioxideand Its Composites". MASc thesis, University of British Columbia, pp 27, ©by permission from author) [136]. . . . . . . . . . . . . . . . . . . . . . . . . 236B.4 A screenshot of the iTools® software (the red box indicates the rst step of theprogram) (Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of CeriumDioxide and Its Composites". MASc thesis, University of British Columbia, pp28, © by permission from author) [136]. . . . . . . . . . . . . . . . . . . . . . 237B.5 SPS program used for SPS-ing Al powder for repeat-ability test. . . . . . . . 238B.6 Comparison of SPS data obtained from repeatability test: a) Temperature, b)Pressure, c) Electric potential, d) Current, e) Input power and f) Ramdisplacement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239C.1 Stereomicrographs of SPS-ed (’1600◦C - 30 MPa - 10 min’) ThO2 pellet: a)Unetched, b) Etched. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247xxxixLIST OF FIGURESC.2 Stereomicrographs of SPS-ed (’1800◦C - 50 MPa - 10 min’) ThO2 pellet: a)Unetched, b) Etched. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248C.3 EBSD maps from pellets sintered at varying temperatures: a) ’1500◦C 50 MPa10 min’, b) ’1600◦C 50 MPa 10 min’, c) ’1700◦C - 50 MPa - 10 min’, d) ’1800◦C- 50 MPa - 10 min’, e) ’1700◦C - 50 MPa - 5 min’, f) ’1700◦C - 50 MPa - 15 min’,g) ’1700◦C - 30 MPa - 10 min’, h) ’1700◦C - 70 MPa - 10 min’, i) Grain size vs.peak sintering temperature, j) Grain size vs. peak sintering pressure, i) Grainsize vs. sintering holding time. . . . . . . . . . . . . . . . . . . . . . . . . . . 249D.1 Example of MSC: a) MSC of alumina with activation energy value of 488kJ/mol and b) Mean residual square method used for calculation of best tactivation energy Q from a study by Cho et al. (Republished with permissionof John Wiley and Sons, Inc., from "Master Sintering Curve: A PracticalApproach to Sintering", Journal of American Ceramic Society, D. L. Johnsonand H. Su, vol. 79 no. 12 (2005); © permission conveyed through CopyrightClearance Center, Inc.) [252][253]. . . . . . . . . . . . . . . . . . . . . . . . . 253D.2 Unaltered displacement data obtained from SPS of UO2 at 1300◦C (75◦C/min)- 60 MPa - 0.5 min plotted alongside temperature and pressure. . . . . . . . . 259D.3 Removal of thermal expansion contribution from sintering data: a) Thermalexpansion data obtained from the sintering of UO2 at 1300◦C (75◦C/min) - 60MPa - 0.5 min and b) Corrected ram displacement plotted with actualdisplacement data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261D.4 Densication curves extracted from sintering runs at 50◦C/min, 75◦C/min and100◦C/min for UO2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263xlLIST OF FIGURESD.5 MSC calculation by MPCD method: a) MPCD minimisation for Q and b)Averaged MSC curve obtained at Q value 640 kJ/mol plotted against thesintering curves from various heating rates. . . . . . . . . . . . . . . . . . . 264D.6 Validation of MSC by comparison of calculated and actual RD: a) As a functionof time and b) As a function of temperature. . . . . . . . . . . . . . . . . . . 265D.7 a) MSC of ThO2 and b) Instantaneous RD predicted by MSC for SPS of ThO2at ’1500◦C - 60 MPa - 10 min’. . . . . . . . . . . . . . . . . . . . . . . . . . . . 266D.8 a) MSC of ThO2-5 wt% (40 µm) SiC and b) Instantaneous RD predicted by MSCfor SPS of ThO2-5 wt% (40 µm) SiC at ’1500◦C - 60 MPa - 0.5 min’. . . . . . . 267D.9 a) MSC of ThO2-5 wt% (1 µm) SiC and b) Instantaneous RD predicted by MSCfor SPS of ThO2-5 wt% (1 µm) SiC at ’1500◦C - 60 MPa - 5 min’. . . . . . . . . 268D.10 a) MSC of ThO2-1 wt% (15 nm) SiC and b) Instantaneous RD predicted by MSCfor SPS of ThO2-1 wt% (15 nm) SiC at ’1500◦C - 60 MPa - 5 min’. . . . . . . . 268D.11 a) MSC of ThO2-5 wt% Diamond and b) Instantaneous RD predicted by MSCfor SPS of ThO2-5 wt% Diamond at ’1500◦C - 60 MPa - 5 min’. . . . . . . . . 269D.12 a) MSC of Th0.9U0.1O2 and b) Instantaneous RD predicted by MSC for SPS ofTh0.9U0.1O2 at ’1000◦C - 60 MPa - 5 min’. . . . . . . . . . . . . . . . . . . . . . 270D.13 a) MSC of Th0.9U0.1O2 and b) Instantaneous RD predicted by MSC for SPS ofTh0.9U0.1O2 at ’1000◦C - 60 MPa - 5 min’. . . . . . . . . . . . . . . . . . . . . . 271D.14 Summary of MSC results: a) Activation energy, b) MPCD, c) Variance and d)MPCD vs. variance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272E.1 Engineering drawing of the bottom punch for "Tight tight" IXAF SPS setup(Dimensions in mm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322E.2 Engineering drawing of the die for "Tight tight" IXAF SPS setup (Dimensionsin mm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323xliLIST OF FIGURESE.3 Engineering drawing of the core rod for "Tight tight" IXAF SPS setup(Dimensions in mm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324E.4 Engineering drawing of the top punch for "Tight tight" IXAF SPS setup(Dimensions in mm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325E.5 Engineering drawing of the bottom punch for "Loose tight" IXAF SPS setup(Dimensions in mm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326E.6 Engineering drawing of the die for "Loose tight" IXAF SPS setup (Dimensionsin mm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327E.7 Engineering drawing of the core rod for "Loose tight" IXAF SPS setup(Dimensions in mm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328E.8 Engineering drawing of the top punch for "Loose tight" IXAF SPS setup(Dimensions in mm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329E.9 Engineering drawing of the bottom punch for "Tight loose" IXAF SPS setup(Dimensions in mm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330E.10 Engineering drawing of the die for "Tight loose" IXAF SPS setup (Dimensionsin mm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331E.11 Engineering drawing of the core rod for "Tight loose" IXAF SPS setup. . . . . 332E.12 Engineering drawing of the top punch for "Tight loose" IXAF SPS setup. . . . 333E.13 Engineering drawing of the bottom punch for "Loose loose" IXAF SPS setup. 334E.14 Engineering drawing of the die for "Loose loose" IXAF SPS setup. . . . . . . 335E.15 Engineering drawing of the core rod for "Loose loose" IXAF SPS setup. . . . 336E.16 Engineering drawing of the top punch for "Loose loose" IXAF SPS setup. . . 337xliiList of AbbreviationsATF Accident Tolerant FuelBWR Boiling Water ReactorCANDU CANada Deuterium UraniumCerMet Ceramic in Metallic matrixCOD Coe cient Of DeterminationCNL Canadian Nuclear LaboratoriesCNT Carbon NanoTubeCS Conventional SinteringCTE Coe cient of Thermal Expansionconc. concentratedet al. and othersECAS Electric Current Assisted/Activated SinteringECR Electrical Contact ResistanceEDC Electric Discharge CompactionxliiiList of AbbreviationsEDS Electric Discharge SinteringFCC Face Centered CubicFEM Finite Element MethodFS Flash SinteringGNP Graphite Nano PlateletsHBS High Burn-up StructureIXAF Internally and eXternally cooled Annular FuelLOCA Loss Of Coolant AccidentMOX Mixed OXideMSC Master Sintering CurvePCI Pellet Cladding InteractionPFT Peak Fuel TemperaturePID Proportional Integral DerivativePIE Post Irradiation ExaminationPLP Precision Lattice ParameterPM Powder MetallurgyxlivList of AbbreviationsPWR Pressurized Water ReactorRD Relative DensitySEM Scanning Electron MicroscopySIMFUEL SIMulated FUELSPS Spark Plasma SinteringSPS-ed Spark Plasma SinteredSPS-ing SPS processingTCR Thermal Contact ResistanceTMI Three Mile IslandU.Sask. University of SasketchewanXRD X-Ray Di ractionX-EDS X-Ray Energy Dispersive SpectroscopyYSZ Yttria Stabilized ZirconiaxlvList of Symbolsc speed of light∆L thermal expansion width of grain boundaryd recorded displacementDt coe cient of bulk di usionduorr corrected displacementd6(( corrected displacement during cooling at 600◦CDv coe cient of volume di usion− vLLvtlinear shrinkage rateE energy viscosity coe cientG grain diameterh height of pellethxinsl measured nal height of pelletxlviList of Symbolsi sum of series counterk Boltzmann’s constantL length of the sintered pelletm massN number of sintering trialsΩ atomic volumeØ diameterPD perpendicular distance between curves. Pi (/) MSC/instL instantaneous density/initL initial/green density/x nal density/thworL theoretcal densityQ Activation energyr radiusxlviiList of Symbolst time(tP i (t)) predicted Master Sintering Curvei initial (tP i (t))svy average (tP i (t))lpowvwr weight of precursor powderx sintering neck radius surface energyΓv scaling parameter for volume di usionΓt scaling parameter for grain boundary di usionxlviiiList of Elements and CompoundsAr Argon -SiC Hexagonal Silicon Carbide -SiC Cubic Silicon CarbideBeO Beryllium OxideC CarbonCeO2 Cerium DioxideCr ChromiumCs CaesiumCsI Caesium IodideCO Carbon MonoxideCu CopperH2 HydrogenH2SO4 Sulphuric AcidH2O2 Hydrogen PeroxidexlixList of Elements and CompoundsI IodineMo MolybdenumPuO2 Plutonium DioxideRe RheniumThO2 Thorium DioxideTi3SiC2 Titanium Silicon CarbideTlO2 Thalium(IV) OxideTl2O3 Thalium(III) OxideUC Uranium CarbideUCxOy Uranium OxycarbidesUN Uranium NitrideUO2 Uranium DioxideUO2+x Hyperstoichiometric Uranium DioxideU3Si2 Uranium SilicideW TungstenWC Tungsten CarbidelList of Elements and CompoundsXe XenonliList of Unitscm centimetre◦C degree Celsiusequiv. equivalenteV electron VoltgU gram UraniumGWd Giga Watt dayGWh Giga Watt hourK KelvinkA kilo AmpereskJ kilo Joulemg milligrammin. minutemm millimetremol moleliiList of UnitsMPa Mega Pascalnm nanometreO/M Oxygen to MetalO/U Oxygen to Uraniumppm parts per millionrpm rotations per minutes secondt timetHM tonne Heavy MetalV Voltvol% volume percentageW WattliiiAcknowledgementsA project like this would be incomplete without the help and support from an entireresearch team.First and foremost, I would like to thank and acknowledge Dr. Bichler for his continuedsupport over the years. Dr. Bichler was patient and understanding with his supervision andalways encouraged me to innovate rather than conforming to his ideas. In addition tointellectual support, Dr. Bichler never hesitated to get his hands dirty whenever I neededphysical help, be it lifting 100 pound concrete blocks to assemble the radioactive materialstorage bunker or to put on Hazmat clothing and clean up radioactive powder. Once again,thank you sir, for all your time and energy spend in making this research possible.The research team at University of Saskatchewan played a vital role in providing the rawmaterials and characterisation support. Dr. Jerzy Szpunar was always willing to get back tome at the soonest, despite his busy schedule. I would like to thank and acknowledge hissupport and time. I would also like to acknowledge the support provided by Dr. LinuMalakkal, without whom this project would be incomplete. Dr. Malakkal was alwayssupportive with my requests and I could not ask for a better research collaborator. I wouldalso like to acknowledge Jayangani Ranasinghe and Murali Krishna Tummalapalli for theircontributions to this research.A large portion of this thesis research was carried out at the historic Canadian NuclearLaboratories (CNL). The team at CNL was very welcoming and accommodating to all myimpossible requests. I would like to thank Andrew Bergeron for being a great host, duringmy four months stay at CNL. Andrew was sel ess in making sure my research objectiveslivAcknowledgementswere accomplished, despite it not serving any purpose to him. I would like to thank the FuelDevelopment branch manager, Dr. Rosaura Hamsura for allowing me to be onsite and lettingme be a part of the Fuel Development family. I would also like to thank the highly skilledand experienced technicians, James Crigger and Tim Studdart for their valuable support andtime. I would also like to acknowledge the support of the other members of the Fuel Science& Technology section for their support. I would like to thank Dr. Daniel Clu for all thetime spent coaching me on winter biking and the deep discussions during our commute. Iwould like to thank Dr. Stuart Bell and Dr. Raymond Dickson for allowing me to be a regularhitchhiker, whenever I needed a ride to the site.I was fortunate enough to work in one of the best research groups in UBC. Despite beingreally diverse, in expertise and background, there was always a helping hand when youneeded one. I would like to thank Somi Doja for all the moral and physical support over thelast ve years. Thank you for the countless discussions and being patient audience to mythoughts. I would like to thank the self proclaimed "Prince of the Lab" Kristian Mackowiakfor his support. Thank you for all the car rides to the school and for entertaining us with thebackyard BBQ, thanksgiving, sushi and even Haggis parties. I would also like to thank thegroup alumni Tyler Davis, Levi Lafortune, Sebastian Lemus Fonseca and Justin Mok fortheir support in the beginning of the project. I would also like to thank Mr. Kyle and LavaKumar for their help in the ongoing decommissioning activities. I would like to thank Rafaelfor his support in making sure the SPS machine remained well maintained. I would like tothank the technicians at UBC-O for their support in making this research happen.Lastly, I would like to thank anyone that I may have missed or forgot to mention here.lvDedicationTc Science, Faailm and FriendslviChapter 1: IntroductionThis chapter summarises the relevance of this dissertation to the scientic research communityand the nuclear industry. The chapter also lays out the research objectives and novelty of thiswork.1.1. Global Energy CrisisAccess to energy has been an indispensable resource for the function of civilizationsand humanity over the years. Global energy consumption can be indirectly used as a metricof tracking the progress and development of industrial countries, as illustrated in the annualenergy consumption trend shown in Figure 1.1. The fossil fuel resources (i.e oil, natural gasand coal) account for∼57% of the total energy currently consumed. In recent years, concernsover the sustainability of fossil fuels and their irrefutable e ect on the climate change havegone under extensive scrutiny. For example, according to BP’s World Energy Reserve, oil willlast until∼2066, natural gas until∼2068 and coal until∼2169 [1]. Therefore, a shift from fossilfuels towards alternative energy sources is necessary to ensure that technological progressof our societies is not compromised. Reducing fossil fuel consumption comes with an addedbenet, in the form of reduced carbon emissions. The scientic consensus on climate changeis that human activities that cause greenhouse emissions have resulted in irreversible climatechange that has manifested itself in the form of global warming, melting of polar icecaps,increased forest res, rising sea levels, etc. [2][3][4][5][6][7][7][8][9].11.1. Global Energy CrisisFigure 1.1: World total energy consumption from 1971 to 2015 in million tonne oilequivalent (mtoe) [10] (Source: IEA (2017) Key World Energy Statistics. All rights reserved©).Figure 1.2 shows the contribution of the various energy sources to electricitygeneration. In 1973, almost three quarters of electricity were generated from fossil fuelsources, whereas, 20.9% and 3.3% from hydro and nuclear sources, respectively. Althoughthe contribution from non-fossil fuel sources has increased by another ∼10% in 2015, thedependence on fossil fuel for electricity generation is still signicant. Hence, to steer awayfrom climate change, there should be more research and development in cleaner energysources such as nuclear, hydro, solar and wind energy.21.2. Nuclear PowerFigure 1.2: Electricity production in 1971 and 2015 [10] (Source: IEA (2017) Key WorldEnergy Statistics. All rights reserved ©).1.2. Nuclear PowerNuclear power, although not a renewable energy source, is a major contributor to cleanelectricity generation, due to its low carbon footprint. Surprisingly, the carbon foot print ofnuclear energy (29 tonne CO2 equiv./GWh) is lower than Photo Voltaic (PV) cell based solarenergy (45 tonne CO2 equiv./GWh) and only slightly higher than hydroelectric and windenergy (26 tonne CO2 equiv./GWh) [11]. Additionally, the extremely high energy densityof nuclear fuel compared to conventional hydrocarbon fuels attracted much interest due tolower demands for refuelling [12]. Therefore, the overall nuclear energy supplied per capitahas been increasing until the last decade, as shown in Figure 1.3. Although there was a minordrop in nuclear energy generated in 2007, due to some Japanese, German and French reactorsthat were taken o grid for maintenance or safety related reasons, the Fukushima incident of2011 caused the biggest drop in nuclear energy consumption between 2010-12 [13]. The publicopinion on the safety of commercial nuclear energy was severely a ected by the incident, to31.2. Nuclear Powerwhich governments, such as Japan and Germany, responded by shutting down their nuclearreactors. Nuclear accidents such as Fukushima, Chernobyl and Three Mile Island (TMI) haveshrouded the future of nuclear energy, thus applying more pressure on other clean energysources that are still under development.Figure 1.3: Total global nuclear energy supply from 1971 to 2015 in TeraWatt hour(TWh) [14] (Based on IEA data from the IEA (2016) Headline Global Energy Data 2016,www.iea.org/statistics, © as modied by Anil Prasad).The Chernobyl (1986) and Fukushima (2011) nuclear power plant accidents areconsidered to be two of the largest industrial catastrophes and were classied asInternational Nuclear Event Scale (INES) 7 accidents, i.e. the highest level. The twoaccidents have severely reduced the public condence in nuclear power, especially since theFukushima incident triggered questions on safety of aging eets of nuclear reactors aroundthe globe. As of 2016, Japan has shutdown 42 out of its 43 reactors and was accompanied byGermany, Belgium and Italy announcing their own nuclear energy phase-out plans, whichresulted in a drop in global nuclear energy production, as shown in Figure 1.3. Japan bridgedtheir energy demand by returning to coal powered plants, whereas Germany and Italy reliedmostly on energy import from their neighbours [15][16]. The "no nuclear energy" policy ofthese nations has severely limited their ability to meet carbon emission goals, hence there is41.2. Nuclear Powera rising need to develop and enforce technologies that could make nuclear power moreaccident tolerant to avoid future incidents.The incidents to-date fall under the category of Loss Of Coolant Accident (LOCA),which is characterised by the undesirable events (hydrogen generation, cladding rupture,etc.) that follow the loss of primary coolant in the reactor [17]. The development of AccidentTolerant Fuels (ATFs) is driven by the desire of creating high thermal conductivity and highmelting point fuels that could increase the coping time in the event of LOCA [18]. A shifttowards thorium based fuel could also increase the safety of nuclear fuel, due to the highermelting point and thermal conductivity of Thorium dioxide (ThO2)) compared to the currentlyused Uranium dioxide (UO2). Apart from changing the material composition of nuclear fuelpellet, the geometry of it could also be modied to increase the accident tolerance [19]. TheInternally and eXternally cooled Annular Fuel (IXAF) concept deviates from the conventionalexternally cooled solid cylinder fuel due to its inner bore that could be used for heat extraction[19].Currently, all commercial reactors around the world use natural or enriched UO2as a fuel. The nuclear industry has established strict specications for fuel pelletcharacteristics such as: pellet diameter, relative density, oxygen stoichiometry, surfacedefects, etc. [20][21]. The industrial practice for fabrication of nuclear fuel pellets iscurrently a multi-step process of granulation, pressing, sintering followed by centerlessgrinding to achieve nal specications. Although the exact process conditions vary betweenmanufacturers, the sintering process is usually carried out at high temperature(1600◦C-1700◦C) for ∼9 hours [22]. Commercial fuel fabrication is highly optimised from aneconomic and production standpoints. For example, the fabrication cost is as low as 275 $/kgU and the production rate is as high as 5000 pellets per hour [23][24][25]. However, newchallenges emerging for the nuclear industry such as reactor safety and fuel performance,51.3. Research Objectivesrequire advanced fuel types, which are di cult to process using the conventional fuelmanufacturing route. As an example, the high processing time and temperature ofConventional Sintering (CS) would be undesirable for the fabrication of Accident TolerantFuels (ATFs) based on UO2-Carbon nanotubes (CNTs), UO2-Silicon Carbide (SiC),UO2-diamond, etc. [26][27][28][29]. Hence, there is a rising need for advanced consolidationprocesses with lower process temperature and shorter processing time. Spark PlasmaSintering (SPS) is one such consolidation technique that has become popular among nuclearfuel fabrication scientists over the last decade.1.3. Research ObjectivesThis dissertation focuses on the utilisation of SPS to fabricate and characterise novelATFs. The objectives of this research were identied as follows:1. Formulate and synthesize powder blends of potential ATF materials.2. Identify the optimal processing conditions (relative density >90%, no physical andchemical change) for each fuel material using SPS.3. Select and perform crystallographic and microstructural characterization on select SPSprocessed (SPS-ed) ATF materials.4. Synthesize and compare IXAF fabricated from SPS and CS technique.1.4. Thesis StructureThe scope of the research is illustrated in Figure 1.4. The development of ATFs wasresearched by two routes (geometric and material), with the case studies of IXAF and ThO261.5. Novelty and Originalitybased composites for each of these two routes. The analysis performed on both of thesestudies could be divided into two types: characterisation of as-sintered materials and analysisof in situ densication data obtained from the data acquistion system.This thesis has been divided into the following chapters:• Chapter 1: Introduction• Chapter 2: Literature Review: A concise literature review of relevant publicationsfrom Electric Current Assisted Sintering of nuclear fuel materials.• Chapter 3: Experimental Procedure: This chapter outlines the details of allexperiments, materials and methods used in this dissertation.• Chapter 4: Part I: Densication studies on ThO2 based fuels: This chapterpresents the results on densication of ThO2 and its composites.• Chapter 5: Part II: Comparison of SPS and CS for IXAF: This chapter presents theresults obtained from the comparison of SPS and CS for processing of IXAFs.• Chapter 6: Conclusions and FutureWork: A summary of key results and directionsfor future work are outlined.1.5. Novelty and OriginalityThe following list summarizes the novel contributions from this dissertation to the nuclearfuel research community and industry:1. SPS and characterization of ThO2-SiC composites was carried out for the rst time.2. SPS and characterization of ThO2-diamond composites was carried out for the rst time.71.5. Novelty and Originality3. The e ect of particle size of SiC on the density and the microstructure of ThO2-SiCcomposites was investigated for rst time.4. IXAF was fabricated and characterised by SPS for the rst time.5. A one to one comparison of any nuclear fuel processed by SPS and commercialfabrication technique was performed for the rst time.81.5. Novelty and OriginalityFigure 1.4: Research owchart.9Chapter 2: Literature ReviewThis chapter summarises most recent literature on processing of nuclear fuel materials with anaim to identify knowledge gaps in this research area.2.1. Powder MetallurgyMaterials and their processing has dictated the progress of technology over manydecades. In fact, the role of materials is so vital, that epochs in human history are dividedbased on the most prominent material during that era: Stone Age, Iron Age, Bronze Age, etc.With the advent of technology, the complexity and accuracy of material processing alsoincreased. Techniques such as casting and forging are used extensively for manufacturingparts made of metallic and polymer materials. However, the processing of ceramics ischallenging due to their high melting point and nearly zero ductility. Thus, PowderMetallurgy (PM) has been the processing route of choice for these materials.PM is a modern technology compared to other materials processing techniques. Duringthe 1920s, when the industrial revolution was at its peak, Sumitomo Electric started thecommercial production of cemented carbide inserts by powder compaction [30]. Presently,PM is used to fabricate parts for the automobile, aerospace, oil to the nuclear industries[31][32][33][34]. PM has several advantages and disadvantages: [35][32][33][36].1. Processing temperature: PM (unlike casting) does not require the raw materials tobe melted and liqueed to create the desired product geometry. Although, the exactPM processing temperature depends on the material and the properties of the nal102.1. Powder Metallurgyproduct, it is typically between 50%-70% of the melting point of the material. As aresult, the lower processing temperature is one of the major advantages of PM sinceit allows fabrication at a lower temperature than casting or any other solidicationbased technique. A lower processing temperature allows to process thermally sensitivematerials, limits grain growth and enhances mechanical properties by the Hall-Petche ect, due to well controlled grain growth kinetics.2. Unique applications: PM products are used in several applications subjected tounique conditions that require unique properties. For example, with the rising needfor rapid prototyping and 3D printing, there have been several attempts at 3D printingceramics and metals as well. However, the high melting point of metals and ceramicsmakes it di cult to utilize traditional extrusion based 3D printing techniques. The 3Dprinting of ceramics and high melting points metals are typically carried out by PMprocess such as stereo-lithography or Selective Laser Melting (SLM) techniques.Despite the advantages, there remain several disadvantages:1. Availability of precursor powders: Any PM process requires the raw constitutivematerials to be available in a powder form. The preparation and processing of powdersrepresents an additional manufacturing step, such as milling or atomisation, and mayincrease the cost of PM parts.2. Risks associated with handling of powders: A powder particle is loosely denedas any particle with highest dimension less than a millimeter. As a result, powders forPM often pose a respiratory hazard. Several studies suggest that even powders frommaterials without any toxicological risks could cause physical damage to lung tissue[37]. Also, based on the toxicological properties of the material itself, its nal impact112.1. Powder Metallurgyon the human body could vary. For example, radioactive and hazardous materials aremuch more hazardous in powder form than in a solid form.2.1.1. Sintering TheorySintering is a sub category of PM, and is by denition the joining of powder particles bythe application of heat and pressure. A review of historical evolution of sintering technologies,mechanisms and theories was carried out as a part of a comprehensive literature review.However, for the sake of brevity, this literature review was added to Appendix A.2.1.1.1. Sintering MechanismsIn 1974, the British scientist M.F.Ashby wrote a paper titled "A rst report on sinteringdiagrams", where he introduced a new concept called sintering diagrams (also called Ashbydiagrams) [38][39]. Ashby also summarized the six major mechanisms that are active duringa sintering process, as illustrated in Figure 2.1:Figure 2.1: Various mechanisms active during sintering (Republished with permission ofTaylor & Francis Group LLC, from Sintering of Ceramics, M. N. Rahman, 1st Edition and2007; © permission conveyed through Copyright Clearance Center, Inc.) [40].1. Surface di usion: Mass transport happens during sintering by the di usion of atoms122.1. Powder Metallurgyfrom the surface of a particle towards the neck region. Surface di usion has no netcontribution to densication, but results in the change of grain morphology.2. Lattice di usion: Lattice di usion mechanism is similar to surface di usion, butatoms are transported by di usion through the particle rather than along its surface.This mechanism also doesn’t cause any densication, but causes microstructuralchange.3. Vapour transport: Since sintering is carried out at elevated temperatures, someatoms may evaporate at the surface and re-condense at a di erent point in thepowder, causing mass transport. Like surface and lattice di usion, vapour transport isalso non-densifying mechanism.4. Grain boundary di usion: Atoms di use through grain boundaries and start ll thepores between particles to cause densication. Grain boundary di usion results in thetransport of atoms towards pores, thus increasing the density of the sintered material.5. Lattice di usion at pores: Similar to grain boundary di usion, except that atomsdi use through the lattice rather than along the grain boundaries. Lattice di usion atpores results in an in ow of atoms towards pores, hence contributing to a material’sdensication.6. Plastic ow: In the case of crystalline materials, plastic ow is achieved by themovement of dislocations. During sintering, the applied temperature and pressureactivates dislocation motion, which contributes to a material’s densication.Any mechanism that causes mass transport towards the pores contributes to densication,(i.e mechanisms 4, 5 and 6). In contrast, mechanisms 1, 2 and 3 mainly cause a change inthe particle morphology, thus dictating the sintered material’s microstructure. However, all132.1. Powder Metallurgysix mechanisms contribute to the neck growth rate between two powder particles during asintering process.2.1.2. Sintering ProcessesThere are several di erent types of sintering technologies used in manufacturingtoday, based on the underlying principle [41]. Each sintering technique has its own uniquefeatures, which could be benecial for a specic material processing application. However,discussing each and every sintering technique that is used commercially or on a lab scalewould be beyond the scope of this study, hence the discussion is limited to the commonsintering techniques for nuclear fuel fabrication.2.1.2.1. Conventional SinteringThe history of Conventional Sintering (CS) techniques dates back to the Indus ValleyCivilization, where baked clay artifacts were fabricated [42]. The rst step in CS is creatinga green compact, by the application of pressure or by making a slurry/paste that could bemoulded into the required shape. The next step in CS is to bake the green compact in a furnaceat a temperature close to 70% of the material’s melting point. CS is often preferred due to itssimplicity in terms of instrumentation and cost e ciency. However, compared to other novelsintering techniques, CS requires a higher process time and temperature. Hence, materialfabrication via CS often relies on batch processing, requiring extensive capital investment forlarge furnaces. The typical process time of CS is a few hours to an entire day.142.1. Powder Metallurgy2.1.2.2. Hot PressingIn Hot Pressing (HP), the powder is pressed under high pressure, while being subjectedto high temperature. The coupling of temperature and pressure reduces the processing time.There are a few di erent ways to generate the heat required for hot pressing. For example, thesintering process could be carried out inside a furnace or it can be surrounded by an inductioncoil [43][44][45].2.1.2.3. Microwave SinteringMicrowave sintering is similar to hot pressing, however microwaves are used as heatsource for the process. Unlike hot pressing, microwave sintering only heats up the surface ofthe powder particles. Also, microwave heating is faster, which is desirable for processing ofnanocrystalline materials, where grain growth must be retarded [46][47]. However, productsize limitation, expensive instrumentation, and the formation of hotspots limits wider use ofthis technology.2.1.2.4. Electric Current Assisted SinteringElectric Current Assisted Sintering (ECAS) is similar to microwave sintering, exceptthat instead of microwaves, an electric current is used to generate heat. ECAS techniquesare broadly divided into two categories: Electric Discharge Compaction (EDC) or ResistanceSintering (RS) [48][49][50]. EDS is a rapid sintering process wherein a high voltage (on theorder of kV) capacitor is discharged on a material to be sintered [51]. In the case of aconductive material, the electric current passes through the material with low electricresistance, whereas in the case of non-conductive materials, the high electric eld causesdielectric break down.152.1. Powder MetallurgyRS techniques employ relatively low voltage (<10 V) over a longer duration of timecompared to EDS [52]. There are several techniques that fall under ECAS-RS such as SparkPlasma Sintering (SPS), Pulsed Electric Current Sintering (PECS), Plasma ActivatedSintering (PAS), Pulsed Discharge Sintering (PDS), Pulsed Current Sintering (PCS), PlasmaPowder Compaction (PPC), Field Activated Sintering Technique (FAST), etc. As pointed outby Orru et al., these sintering techniques are not necessarily unique, but a mere acronymthat was used by researchers or equipment manufacturers [51]. In some cases, there arelong standing debates on whether the names itself are misnomers. For example, there arecontradictory reports on the existence of "spark" and "plasma" during SPS [53][54]. Thename "Spark Plasma Sintering" was coined by Masao Tokita in 1993 for an ECAS techniquethat typically uses pulsed DC current in the vicinity of kiloAmperes (kA) and voltage lessthan or equal to 10 V. SPS has become the most widely used technique covering nearly twothird’s of the 6000 publications on ECAS, spanning ceramics, metals and polymers [51][55].Figure 2.2a shows the schematic of an SPS die, with a pulsed Direct Current (DC)passing through a graphite tooling that encompasses the precursor powder [56]. Dependingon the conductivity of the powder material, the electric current distribution in the toolingvaries. In the case of electrically conductive materials, the electric current ows through thematerial, as indicated by the red arrow in Figure 2.2a. In the case of electrically insulatingmaterials, the electric current bypasses the material and ows through the die, asrepresented by yellow arrows. However, due to the contact resistance between the die andthe punch, the electric current cuts through the corner of pellet irrespective of its resistivity.The Joule heat generated due to the electric current causes the material and tooling toheat-up, as shown in Figure 2.2b.162.2. SPS of Nuclear Fuel Materialsa bFigure 2.2: a) SPS schematic of SPS die (Reprinted by permission from Springer NatureCustomer Service Centre GmbH: Springer, Encyclopedia of Nanotechnology edited by B.Bhushan, © (2012)) [56] and b) SPS die during heating.2.2. SPS of Nuclear Fuel MaterialsOver the last decade, ECAS has been explored (on a laboratory scale) for processing ofnuclear fuel materials. A total of 83 publications on ECAS of nuclear fuel materials wereidentied since 2009. Figure 2.3 shows the yearly publications on the topic with an overallincreasing trend. A detailed tabular summary of sintering conditions and nuclear fuelmaterials processed by ECAS can be found in Table A.1.172.2. SPS of Nuclear Fuel MaterialsFigure 2.3: Total annual publications on ECAS of nuclear fuel materials.Although, nearly 90% of the publications on ECAS of nuclear fuel materials were on SPS,a review of FS and EDC of fuel materials were also added to Appendix A. Additionally, thematerial used for SPS was limited to only UO2 and ThO2 or their composites. Literaturereview of ECAS of Uranium Nitride (UN), Uranium Carbide (UC), Uranium Silicide (U3Si2)and Ceramic in Metallic matrix (CERMET) fuels can be found in Appendix A.This section details research in the eld of SPS of nuclear fuel materials. Due to the largevolume of literature available on SPS of nuclear fuel materials, this section has been sub-divided based on the type of material processed: UO2, ATFs and ThO2. SPS has been alsoconsidered for commercial fuel fabrication and for simulating in-service fuel material, hencearticles pertaining to this area have been also discussed in respective subsections.2.2.1. Mitigation of RadiotoxicityPowder Metallurgy of nuclear fuel materials is challenging due to the hazardsassociated with the handling of radioactive powders, hence several preliminary SPS studieson nuclear fuel materials used a surrogate material such as Cerium dioxide (CeO2)182.2. SPS of Nuclear Fuel Materials[57][58][59][60]. However, the high temperature ionic conductivity and chemical expansionof CeO2 made it a challenging material to process by SPS [61]. Hence, various researchgroups have developed unique ways of managing possible system contamination andcontainment of radiotoxic powder [62][63][64]. For example, as shown in Figure 2.4, acombined SPS and glove box facility was developed to contain radiotoxic powder spills byTyrpekl et al. [63]. Additionally, the typical hydraulic unit in a SPS was replaced with anelectromagnetic actuator to create a smaller unit, as well as reduce any risk associated withthe contamination of the hydraulic uid. Johnson et al. also used a similar approach,however their glove box was also used to prevent oxidation of pyrophoric UN and U3Si2powders [62]. The conventional sol-gel method was also employed to convert fuel powderinto bigger powder agglomerates called "beads", especially in the case of nano-sized powderparticles [65].192.2. SPS of Nuclear Fuel MaterialsFigure 2.4: Modied SPS setup implemented by Tyrpekl et al.: "The small scale SPS facilityintegrated in a glovebox (a); sintering chamber (b); 6 mm graphite die and punch (c)"(Reprinted from V. Tyrpekl, C. Berkmann, M. Holzhäuser, F. Köpp, M. Cologna, T. Wangleand J. Somers, “Implementation of A Spark Plasma Sintering Facility in A HermeticGlovebox for Compaction of Toxic, Radiotoxic and Air Sensitive Materials,” Review ofScientic Instruments, vol. 86, pp 23904 (2015), © with the permission of AIP Publishing)[63].2.2.2. SPS of Pure UO2Ge et al. investigated the e ect of SPS process parameters on the RD and the grain size ofpure UO2 in the temperature range from 850◦C to 1525◦C and holding times ranging from 0.5to 20 min [66]. The consolidation of UO2 was identied to occur between 720◦C-1000◦C (fromin situ displacement data during SPS). An increase in temperature above 1000◦C resulted innegligible increase in RD. Commercial nuclear fuel fabricated at 1650◦C and 9 hours typicallyhas a RD ∼96.5%. In Ge et al.’s study, UO2 SPS-ed at 1050◦C and 0.5 min had a 96.3% RD [66].Ge et al.’s study was critical in identifying that nuclear fuel could be sintered using SPS at202.2. SPS of Nuclear Fuel Materialslower temperature and holding times compared to commercial fuel fabrication routes [66],hence triggering more studies on the application of SPS for nuclear fuel materials.Ge et al. also observed that higher sintering temperature and longer holding time wouldlead to lower Oxygen to Metal (O/M) ratio during SPS of UO2, which was justied by atemperature-time-driven reduction reaction happening during SPS [67]. Tyrpekl et al.studied this phenomenon further by investigating the e ect of thermal and electric eldduring SPS on the grain size and O/M ratio across the pellet by using Raman spectroscopymapping, X-Ray Absorption Near Edge Structure (XANES) and XRD [68]. An elevated O/Mratio and grain size was observed in the pellet’s surface that was in contact with the anodeof the SPS machine, hence revealing the role played by the incident electric eld. Theincident electric eld was proposed to enable oxygen ions to move towards the positive SPSelectrode, causing an increase in O/M ratio near the anode compared to the cathode. Thesubsequent enhanced grain growth near the cathode was justied by citing the increasingself di usion of uranium cations with increasing O/M ratio. Tyrpekl et al.’s study showedthat despite its several advantages, the application of SPS for UO2, might need mitigation ofmicrostructural inhomogeneity.According to Tyrpekl et al., one of the factors that could be in uencing the oxygen ionmigration and thereby the microstructural inhomogeneity during SPS of UO2, was thereaction between graphite and UO2+x according to Equation 2.1:Ud2+x + xC → Ud2 + xCd (2.1)Papynov et al. compared SPS processing of UO2 in graphite and Molybdenum (Mo) dies, toeliminate some of the disadvantages of using graphite tooling during SPS (e.g., reductionreaction, carbon contamination, rapid wear, etc.) [69][70]. A supplementary advantage of212.2. SPS of Nuclear Fuel Materialsthe Mo tooling was the ability to apply higher loads compared to graphite. In Papynov et al.’sstudy, a higher load (141.5 MPa) and lower sintering temperature (1150◦C) were used duringSPS of UO2 in Mo die compared to experiments conducted with graphite tooling (1250◦C,1450◦C and 1650◦C; 24.5 MPa). Although, a lower carbon contamination was observed forpellets sintered in Mo tooling compared to graphite, the quantication of carboncontamination was questionable since X-EDS was used for quantitative chemical analysis. Itwas also concluded that the Mo die could be used for fabricating high RD pellets (97-98%),which were comparable to the pellets from graphite die (98-99%). Hence in theory, graingrowth could be minimised by lowering the processing temperature without compromisingRD, using the higher load bearing Mo tooling. This principle was exploited by researcherssuch as Gong et al., Yao et al. and Tyrpekl et al. to regulate grain growth during SPS ofnanometric UO2 using high pressure SPS dies for simulation of High Burn-up Structure(HBS) [71][72][73].2.2.2.1. Commercial Fabrication of Nuclear Fuel PelletsPreliminary studies on SPS of UO2 suggested that it is viable for fabrication of highdensity fuel pellets at laboratory scale; however, factors such as post sintering machining,economics, tooling wear, scalability, repeatability, quality control, homogeneity, etc. requiredfurther investigation for determining commercial viability. Several researchers focused onfabricating a near net-shaped fuel pellet using SPS [20][25][74][71]. Ironman et al. was therst to investigate the technical feasibility of SPS for commercial production of nuclear fuelpellets [20][25]. In a sample size of 20 individually SPS-ed pellets (refer to Figure 2.5a), all 20were within the dimensional (length, diameter and dish dimensions), surface crack and RDtolerance specications for commercial nuclear fuel pellets. Although a "ring" like feature wasobserved along the middle of the pellets, as seen in Figure 2.5a, according to prolometry this222.2. SPS of Nuclear Fuel Materialswas only a variation in surface roughness, as shown in Figure 2.5b. These rings were probablycaused due to die wall friction resulting from the double action pressing during SPS; however,their presence might not hinder the application of SPS-ed fuel pellets as there are no standardspertaining to it. The dissuading factor for the application of SPS might be the durability ofthe graphite tooling, since the fth pellet SPS-ed using the same tooling was not within thedimensional specication of the dish prole. Additionally, fuel powder residue was found toadhere to graphite tooling. However, studies by Papynov et al., Gong et al., Yao et al. andTyrpekl et al. had demonstrated that alternative tooling materials, such as Mo, SiC or WC ,which are harder than graphite, could be used to overcome the limitations of graphite tooling[69][70][71][72][73].232.2. SPS of Nuclear Fuel Materialsab cFigure 2.5: Ironman et al.’s study on viability of SPS for commercial fuel production: a)Commercial specication pellets prepared by SPS, b) Surface roughness measurementacross central ring on SPS-ed commercial pellet, c) Multiple-pellet SPS die setup (Reprintedfrom T. Ironman, J. Tulenko and G. Subhash, “Exploration of Viability of Spark PlasmaSintering for Commercial Fabrication of Nuclear Fuel Pellets,” Nuclear Technology, vol. 200,pp 144–158 (2017), © with the permission of Taylor & Francis) [20] .Another challenge for commercial fuel production using SPS is its low production rate,since apart from a few industrial applications, it is most widely used in laboratory settings[75]. Nuclear fuel production requires a high volume process at 5000 pellets per hour,whereas SPS is usually carried out for a single pellet with a typical sintering run lasting30-45 mins. Hence, to overcome this challenge Ironman et al. recommended a re-designedtooling to facilitate production of multiple-pellets (4 or 8) per sintering trial, as shown inFigure 2.5c. However, the pellets from various multiple-pellet tooling designs had a higherstandard deviation (1.1-2.3%) for RD compared to single pellet SPS (0.3%), which caused242.2. SPS of Nuclear Fuel Materialssome of the pellets to be out of RD specications. A possible explanation for the di erencein consolidation of fuel pellets from multiple-pellet SPS die would be the thermalinhomogeneity in the die setup, which was investigated further by Nili et al. using anelectro-thermo-mechanical Finite Element Model (FEM) [74]. The analyses compared powerconsumption, temperature gradient and stress concentration for single, two, four and eightpellet die setup. The study showed that having multiple cavities in the die, such as in case offour and eight pellet fabrication, the temperature gradient increased across the pellets.Thereby, the thermal gradient could have caused the pellets to have di erent consolidationduring SPS as was reported by Ironman et al. [20]. Although the scalability from one to eightpellet was demonstrated in this study, further scalability to 5000 pellets/hour productionand thermal inhomogeneity are concerning for the viability of multiple pellet setup.The thermal inhomogeneity during SPS was investigated further using FEM, to study thee ect of pellet dimensions on the thermal gradient. Nili et al. studied the evolution of thermaland stress gradients within SPS-ed UO2 pellets of varying dimensions using a coupled electro-thermo-mechanical FEM, with supporting experiments [76]. The analysis was carried out onthree di erent pellet dimensions (Ø=5.0 mm, 8.2 mm and 11.5 mm) with the same aspect ratio(length/diameter=1.21), to evaluate the scalability of SPS. As shown in Figure 2.6, the resultsshowed that the maximum di erence in temperature was across the axial direction betweenthe centre of the pellet and the top/bottom surface of the pellet (locations 1 & 3 marked inFigure 2.6), whereas the maximum hydrostatic stress di erence was between the mid diameterand top/bottom rim (locations 2 & 4 marked in Figure 2.6). For example, in the case of the11.5 mm diameter pellet, the centre of the pellet was 20◦C colder than the top/bottom edge,whereas the hydrostatic stress along the mid-section diameter was 100 MPa lower than thatat the top/bottom rim.252.2. SPS of Nuclear Fuel MaterialsabFigure 2.6: "Comparison of the maximum a) temperature and b) Hydrostatic stressdi erence between points 1 & 3 and 2 & 4 respectively within a pellet during SPS forthree pellet geometries" (Reprinted from Ceramics International, vol. 45 no. 6, B. Nili andG. Subhash, “In uence of Porosity and Pellet Dimensions on Temperature and StressInhomogeneities During Spark Plasma Sintering of Ceramic Fuel,” pp 7376-7384, © (2019),with permission from Elsevier) [76].The low electro-thermal homogeneity during SPS might also be in uencing othercharacteristics (O/M ratio, grain size, etc.) of UO2 pellets. Gong and Yao et al. studied thespatial homogeneity of O/M ratio and grain size across a commercial dimension SPS pellet262.2. SPS of Nuclear Fuel Materials[71]. An increase in the O/M ratio was observed near the positive terminal, similar toTyrpekl et al.’s observation [68]. The grain size distribution across the cross-section of thepellet was not uniform, with coarser grains observed along the centerline near the anode,while ner grains were observed near the rim and cathodic surface of the pellet. Asmentioned by Tyrpekl et al., the self-di usion of U ions would be enhanced near the anodedue to higher O2- ion concentration [68]. Although the grain size variation was within thespecications for commercial fuel pellets, it might not be negligible for a scaled up pellet ormultiple-pellet SPS tooling, as identied by Nili et al. [74][76]. The thermal eld within theSPS pellet is in uenced by electric current and density, since hot spots in electric currentdistribution become thermal hotspots. The increase in the thermal inhomogeneity withheating rate reported by Nili et al. could be due to the change in the electric current density[76].Cartas performed an economic analysis to explore the commercialisation of SPS processfor nuclear fuel manufacture [25]. The average fabrication cost for nuclear fuel materialprepared via SPS is less than by conventional sintering, primarily due to the reduced powerconsumption of SPS, which is "one-third or one-fth" of conventional sintering, howeverwithout considering tooling turnover [75]. The study also showed that the additional costfor incorporation of ATF additives such as SiC, diamond, Beryllium Oxide (BeO) would bebalanced out by the improved thermal e ciency of nuclear reactors. However, compositesinvolving additives, such as CNTs, require a complex blending process. The study failed toincorporate additional costs incurred due to tooling wear or repair, emphasizing that SPSmight not be economically viable unless a more resilient tooling material was deployed.In summary, the feasibility of SPS for commercial fuel production seemed promising withthe need for more studies on multiple-pellet fabrication, batch processing, tooling materialand thermal inhomogeneity mitigation. Economic concerns of fuel production by SPS also272.2. SPS of Nuclear Fuel Materialsrequire advanced research to measure the return period, rate of return, etc. for capital costincurred to switch to a new technique. It is important to note that theoretically the switchfrom CS to SPS only saves 20% of production cost, without considering the tooling turnover.This analysis should also consider immeasurable factors such as the decline in fuel demandfor the aging eet of conventional nuclear reactors. The outcomes of this analysis could behelpful in attracting the attention of entrepreneurs or fuel manufacturers towards commercialfabrication of nuclear fuel using SPS.2.2.2.2. Simulation of Irradiated FuelThe simulation of irradiated fuel using SPS was performed by two approaches:1. SIMulated FUEL (SIMFUEL): mimicking the chemical composition of irradiated fuelby adding elements or compounds that are commonly detected in irradiated fuel [77].2. Mock-HighBurn-up Structure (HBS): replicating the physical structure of irradiatedfuel by matching its grain size [65][72][78][79][80].During service, nuclear fuel produces a variety of ssion products, which a ect thethermomechanical properties and chemistry of fuel material. These ssion products (forexample, 137Cs and 131I, two common ssion products of 235U) and their decay products aremajor contributors to the intense radioactivity of irradiated fuel. Thus, the study ofirradiated fuel material is very challenging. Material handling issues can be partiallyovercome by using SIMFUEL, where the highly radioactive isotopes (131I, 137Cs, etc.) arereplaced by stable naturally occurring isotopes (127I, 133Cs, etc.) without a ecting thechemical and physical properties of irradiated fuel [81]. Wangle et al. sintered SIMFUEL (amixture of Caesium Iodide (CsI) and UO2) using SPS for the rst time [77]. CsI has a lowboiling point of 1277◦C, hence during conventional sintering there is a risk of evaporation of282.2. SPS of Nuclear Fuel Materialstoxic CsI fumes. The as-sintered pellets had good RD (S90%) and most of the CsI wasretained during sintering, proving the viability of SPS for SIMFUEL fabrication [81].The grain size of HBS was simulated by using the inherently high densication rate ofSPS due to its high heating rate, lower SPS temperature and a shorter processing time [65].Additionally, the grain growth could be further limited by carrying out the SPS process at aneven lower temperature using a high pressure tooling [72][78][79][80]. Cologna et al. sinterednanocrystalline UO2, with the objective of mimicking a HBS in irradiated fuel. The authorswere successful in retaining nanocrystallinity with a nal grain size of 300 nm, for startingpowder particle size of 100 nm. The use of nanocrystalline powder was found to reduce thesintering onset temperature of UO2 to 450◦C compared to 600◦C for microcrystalline UO2powder, likely due to the higher surface energy of nanocrystalline materials [65].Tyrpekl et al. used a "nested" high pressure die set-up to restrict grain growth further, asshown in Figure 2.7a [72]. The limitations associated with the poor fracture strength ofgraphite was overcome by replacing load-bearing tooling components with SiC or WCcomponents; however, since SiC is electrically insulating, the preliminary die containingfuel powder had to be placed inside another die. Using this setup, Tyrpekl et al. sinterednano structured UO2 at low temperatures (800◦C-900◦C) and 500 MPa pressure compared tothe commonly used S1000◦C and Q80 MPa conditions. The pellets were found to have a RDclose to 90% and the nal grain size of the pellets was as low as 34 nm for a starting powderof grain size 11 nm. Yao et al. used a similar HP-SPS die setup to sinter nanocrystalline UO2at an even lower 600◦C-700◦C temperature and 750 MPa pressure [73]. The pellet sintered at600◦C and 20 min had a nal grain size of 66 nm (initial grain size: 30 nm) with a RD of 95%.Thermal conductivity measurement carried out in this study showed that UO2.11 had a lowerthermal conductivity than UO2.03. The authors linked this observation to the presence ofWillis clusters in hypostoichiometric UO2, that caused more phonon scattering.292.2. SPS of Nuclear Fuel Materialsa bFigure 2.7: a) High pressure SPS die setup used by Tyrpekl et al. (Republished withpermission of John Wiley & Sons, Inc., from "Preparation of Bulk-Nanostructured UO2Pellets Using High-Pressure Spark Plasma Sintering for LWR Fuel Safety Assessment",Journal of American Ceramic Society, V. Tyrpekl, M. Cologna, J.F. Vigier, A. Cambriani, D.W. Wim and J. Somers, vol. 100, pp 1269-1274 and 2017; © permission conveyed throughCopyright Clearance Center, Inc.) [72] and b) Miao et al.’s in situ synchrotron WAXSexperimental setup (Reprinted from Scripta Materialia, vol. 131, Y. Miao, T. Yao, J. Lian, J. S.Park, J. Almer, S. Bhattacharya, A. M. Yacout and K. Mo, "In situ Synchrotron Investigationof Grain Growth Behavior of Nano-Grained UO2", pp 29-32, (2017), © with permission fromElsevier) [82].The application of SPS for simulating irradiated fuel was a successful frontier for thenuclear fuel research community [78][79][82][83][80]. Yao et al. followed up the HP-SPSstudy with a grain growth analysis of mock HBS in UO2+x (x=0.01 and 0.11) [78]. The pelletssintered using a high pressure SPS die setup were subjected to various heat treatments(700◦C-800◦C and 0.5h to 8h) to evaluate the grain growth exponent and activation energy.The results indicated that UO2.01 had sluggish grain growth compared to UO2.11. This wasjustied by the presence of interstitial O2- ions that increased vacancy di usion in UO2.11.302.2. SPS of Nuclear Fuel MaterialsIn a more detailed investigation of grain growth in nanocrystalline UO2, Miao et al. studieda pellet sintered by SPS at 700◦C using in situ Wide Angle X-Ray Scattering (WAXS), as seen inFigure 2.7b [82]. The focus of this study was to quantitatively evaluate grain growth kineticsof high burnup fuel. The grain growth measurement indicated that grain growth followedthe relation D2L5 − D2L5o = kt until ∼2.5 hours, after which grain growth slowed down andremained constant. This was justied by the Holm and Foiles model, where every grain sizehad a characteristic temperature above which grain growth started [83].Gong et al. carried out nano and micro-indentation studies at room and hightemperatures (200◦C and 500◦C) on nanocrystalline and microcrystalline UO2 sintered usingSPS. The researchers observed that the hardness of the as-sintered materials increased withdecreasing grain size, in accordance with the Hall-Petch e ect. Also, the fracture toughnessand microhardness were found to be higher for nanocrystalline UO2.11 than nanocrystallineUO2. This was justied by the hardening e ect provided by the additional interstitialoxygen atoms in the lattice of UO2.11 [80].The application of SPS for simulation of irradiated fuel material seemed to be morepromising than the previously identied commercial fuel manufacture application.Currently, the area of nuclear waste management is an important topic, as scientists areexploring techniques to store irradiated fuel material. Any post processing techniqueinvolving irradiated fuel could be simulated using SPS. Additionally, SPS is e ective inproducing mock-HBS to study its microstructural evolution and mechanical properties.Future research in this avenue could be mimicking more features of irradiated fuel such asthe ssion gas pores, columnar grain formation, central void formation, etc.312.2. SPS of Nuclear Fuel Materials2.2.3. SPS of Accident Tolerant FuelsThe development of ATFs was driven by the desire of fabricating high thermalconductivity and high melting point fuels that could increase the coping time in the event ofan Loss Of Coolant Accident (LOCA) [18][84]. In a letter to the United States Congress, theUS Department of Energy had identied four critical factors that could improve the accidenttolerance of nuclear fuel or materials [85]:1. Reduced hydrogen gas generation rate,2. Improved ssion product retention,3. Controlled Pellet Cladding Interaction (PCI), and4. Minimal cladding oxidation.The rst three factors are directly related to the thermal conductivity of the fuel. Anincrease in thermal conductivity would result in a cooler fuel in the event of LOCA, henceenhancing ssion product retention. Thermal conductivity could be enhanced by judiciousincorporation of a high thermal conductivity additives, such as SiC, diamond, BeO ornanotubes [26][27][28][29]. However, all these materials may oxidize in the presence ofUO2+x or self-decompose if enough time and temperature are available (such as in the caseof CS).2.2.3.1. UO2-Silicon Carbide (SiC) Composite ATFsSiC is a popular additive for ATF candidate materials, due to its high thermalconductivity, high melting point and good neutron transparency [18]. Yeo et al. comparedUO2-10 vol% SiC composite fuel material processed by SPS and by oxidative sintering [86].322.2. SPS of Nuclear Fuel MaterialsThe pellets sintered using SPS had higher RD than the pellets sintered by the oxidativesintering process. XRD analysis revealed that there was no reaction between UO2 and SiC inSPS, whereas a Uranium Silicide (USi1.88) phase formed during the oxidative sinteringprocess. Despite a lower grain size, the SPS-ed pellets had a higher thermal conductivitythan oxidative sintered UO2-10 vol% SiC. Microstructural investigation showed goodinterfacial contact between the SiC particles and the UO2 matrix, compared to the oxidativesintered samples, where undesirable voids and porosity were frequently observed at theinterfaces. The comparison of SPS to oxidative sintering seemed irrelevant as CS of nuclearfuel is carried out in a reductive H2 atmosphere. The SPS-ed UO2-10 vol% SiC pellet alsoexhibited a 54% higher thermal conductivity compared to pure UO2, thus making thecomposite a good candidate for ATFs [87][88]. Contrary to a claim made by the authors,that there would be no change in the thermal conductivity due to fuel burn-up, the PostIrradiation Examination (PIE) of SPS-ed UO2-SiC showed a highly disrupted microstructurecompared to pure UO2 (refer to subsection A.3.7) [89][90][91].A second study on UO2-10 vol% SiC by Yeo et al. focused on the e ect of SiC particlemorphology (whiskers and polyhedral particles) on SPS processing [92]. The results wereinconclusive in identifying an observable di erence between powder morphology and RDand the thermal conductivity of the as-sintered materials. Another publication by Yeo et al.investigated the e ect of particle size and volume fraction of SiC (polyhedral particles) onthe consolidation and thermal conductivity of UO2-SiC composites [89]. The e ect of particlesize was studied on UO2-5 vol% SiC, whereas the e ect of vol% SiC addition was studied using1µm SiC powders. Although there was no statistically signicant variation in the RD with SiCparticle size for UO2-5 vol% SiC, there was a consistent decrease in the RD with increasing SiCcontent for 1 µm SiC, due to the SiC impinging contact between the UO2 particles. An increasein SiC particle size resulted in a decrease of thermal conductivity of the SPS-ed UO2-5 vol%332.2. SPS of Nuclear Fuel MaterialsSiC such that above 9 µm the thermal conductivity of composites dropped below the value forpure UO2, as seen in Figure 2.8a. The deterioration of thermal conductivity was attributed tothe formation of micro-cracks and debonding of interfaces between the SiC and the matrix,which was supposedly driven by the di erence in the Coe cient of Thermal Expansion (CTE)between SiC and UO2. Despite the deterioration of densication, the increase in SiC vol%resulted in a proportionate increase in thermal conductivity for composites with 1 µm SiCaddition.a bFigure 2.8: a) E ect of SiC particle size on the thermal conductivity of UO2-5 vol% SiC(Reprinted from Journal of Nuclear Materials, vol. 442, S. Yeo, R. Baney, G. Subhash andJ. Tulenko, “The In uence of SiC Particle Size and Volume Fraction on the ThermalConductivity of Spark Plasma Sintered UO2–SiC Composites," pp 245-252, (2013), ©with permission from Elsevier) [89] and b) Residual stress measured by micro Ramanspectroscopy along SiC particles [90] (Reprinted from Ph.D thesis, University of Florida, S.Yeo "UO2-SiC composite reactor fuels with enhanced thermal and mechanical propertiesprepared by spark plasma sintering" pp 105 (2013).The e ect of SiC particle size on the thermal conductivity of UO2 was furtherinvestigated by Yeo using micro-Raman spectroscopy to measure the residual stress on theSiC particles [90]. The di erence in CTE between UO2 (9O9 × 10−6◦C−)) and SiC(4O4 × 10−6◦C−)) would cause the UO2 matrix to shrink more during a cooling cycle,resulting in a compressive stress within the SiC particles [89]. As shown in Figure 2.8b, the342.2. SPS of Nuclear Fuel Materialscompressive residual stress in SiC decreased with increasing particle size, hence the authorssuspected stress relief by the formation of micro-cracks around bigger SiC particles. The 55µm SiC particles were nearly stress free, and were seen to be surrounded by severalmicro-cracks according to microstructural investigation, proving the stress reliefhypothesis. Conversely, the compressive stress and absence of micro-cracks surroundingner SiC particles might be improving interfacial contact between the SiC particles and theUO2 matrix. Despite the drop in thermal conductivity, the thermally aged UO2-SiC had ahigher thermal conductivity than pure UO2.The study carried out by Yeo et al. was on -SiC. An independent study by Li et al.performed on -SiC, validated that [93][89][92][94]:1. SiC particles deteriorated consolidation of UO2-SiC composite, as a drop in RD for 10vol% addition of -SiC was observed.2. The vol% of SiC addition resulted in a proportionate increase in thermal conductivityof UO2- -SiC composites.Additionally, Transmission Electron Microscopy (TEM) at the interface between SiC and UO2showed the presence of a U-Si phase [94].2.2.3.2. UO2-Carbon Composite ATFsVarious allotropes of carbon, such as Carbon Nanotubes (CNTs), diamond andGraphite Nano Platelets (GNP) were also considered as additives for ATF development[95][25][84][96][97][27][98][83]. The concept of UO2-diamond composites was introducedby Yeo et al. in 2013, when a preliminary study on diamond addition showed a 55% increasein thermal conductivity for UO2, but a thermal aging study showed a reaction betweendiamond and UO2 to form undesirable uranium oxycarbides (UCxOy) [89]. Subsequently,352.2. SPS of Nuclear Fuel MaterialsChen et al. studied the e ect of diamond particle size on the RD and thermal conductivity ofUO2-diamond composite. The increase in the size of the diamond particles from 0.25µm,3µm, 12µm and 25 µm increased the RD of SPS-ed UO2-diamond, due to the agglomerationof ner diamond particles [97]. The e ect of the diamond particle size on the thermalconductivity was analogous to the SiC particle size, as it decreased with increasing particlesize except for 0.25 µm which had a lower thermal conductivity than the 5 µm, supposedlydue to increased agglomeration. Similar to SiC, microscopic cracks were seen around largerdiamond particles, which was cited as the reason for the reduction of thermal conductivitywith increasing particle size [90]. Microcrack formation caused by CTE mismatch would beexaggerated in the case of diamond, due to its low CTE (1 × 10−6◦C−)) than that of UO2. Aconcentrated UO2-70 vol% diamond composite powder was SPS-ed at 1300◦C, 1400◦C,1500◦C and 1600◦C to study the compatibility of diamond and UO2 during SPS. The resultsshowed graphitisation of diamond at 1400◦C and the formation of UC at 1500◦C at theinterface between diamond and UO2, which could deteriorate the interfacial thermal contact.The graphitisation temperature of diamond was reported in the literature to be above1500◦C; however, graphite was detected in pellets SPS-ed at 1400◦C [98]. The authorsclaimed the enhancement of graphitisation was potentially caused by the incident electriceld, however any potential impact of a reaction with UO2 or inaccuracy in temperaturemeasurement during SPS due to thermal gradient were not ruled out. The graphitisation andbuild-up of compressive residual stress within diamond particles was proven by Ramanspectroscopy of UO2-5 vol% diamond by Chen et al. in a separate study [99]. In a series ofmeasurements taken along the axial direction of a pellet, it was found that 20% of thediamond on the surface was graphitised compared to 10% in the interior. The thermalgradient in SPS (hotter edges and cooler interior) was cited as the reason for this behaviour,however it was also possible that the graphite surrounding the pellet during sintering may362.2. SPS of Nuclear Fuel Materialshave acted as seeding sites for the graphitisation in the pellet. A thermal aging treatment ofUO2-diamond composite at 1400◦C for 10 hours showed a 1% increase in RD, 0.8% decreasein mass and 1% decrease in thermal di usivity, suggesting a reaction between diamond andUO2.Cartas et al. incorporated CNTs into UO2 for ATF application, due to CNT’s highdirectional thermal conductivity value (∼6000 W/mK) [95]. CNTs are unstable at hightemperatures and not immune to the processing conditions used in conventional sinteringof nuclear fuel, however several studies have shown them to survive SPS processing[100][101][102]. In Cartas et al.’s study, 5 and 10 vol% of Single Walled (SW) and MultiWalled (MW) CNTs blended with UO2 in OrthoDiChloroBenzene (ODCB) and SPS-ed at1300◦C, 40 MPa and 5 minutes with RD ∼95%. It was demonstrated that ODCB was a moree ective dispersant than ethanol, yielding a sintered composite with a higher thermalconductivity. Fractography of an as-SPS-ed specimen showed that CNTs survived the SPSprocess, however surprisingly only the 5 vol% SWCNT- UO2 fuel material showed a slightincrease (+2 W/mK at 100◦C) in thermal conductivity. However, since thermal conductivitywas measured only in the through plane direction, an anisotropic thermal conductivityenhancement of these composites cannot be ruled out, since CNTs are anisotropic.GNPs (2D) are similar to CNTs (1D) and have a very high directional thermalconductivity (3000 W/mK) for in plane direction. Hence, GNPs were considered as anotherpotential additive for ATF development by Yao et al. Microstructural examination ofUO2-GNP composites (1 and 5 wt%, TSPS=1600◦C & RD∼98%) showed that the GNPssurvived the process and were aligned perpendicular to the load direction. The thermalconductivity measurements on UO2-5 wt% GNP showed a slight decrease for through planedirection (7.6 W/mK to 7 W/mK), whereas the in-plane thermal conductivity nearly tripled(7.28 W/mK to 19.1 W/mK) with the GNP addition. Hence, the negligible thermal372.2. SPS of Nuclear Fuel Materialsconductivity enhancement observed by Cartas et al. in previous study might be due to theanisotropy of UO2-CNT/GNP composites. The anisotropy in thermal conductivityenhancement might be benecial for ATF application, since the radial thermal conductivityof nuclear fuel pellet is more critical than in the axial direction. Hence, this study highlightsa unique opportunity for SPS using its lower processing temperature, time and anisotropy ofSPS-ed pellets for ATF application.2.2.3.3. UO2-Titanium Dioxide (TiO2) Composite ATFsA large grain size in a nuclear fuel material is expected to improve the capability of thefuel to retain ssion products, thereby increasing accident tolerance. Yao et al. studied thee ect of Titanium dioxide (TiO2) doping in UO2 to enhance the grain growth during SPS [83].However, a comparison of grain size between pure UO2 (13 µm ± 5 µm) and UO2-0.5 wt%TiO2 (17 µm ± 5 µm) SPS-ed at 1500◦C showed insignicant increase in grain size as a resultof the TiO2 doping. The authors claimed that TiO2 doping did coarsen grain size by citingUO2-TiO2 samples SPS-ed at 1600◦C and 1700◦C with 20 minutes holding time that had nalgrain sizes of 28±4 µm and 81±10 µm, respectively. There was no equivalent pure UO2 SPS-ed at same condition for a proper comparison. It is likely that the grain growth in the TiO2doped UO2 sintered at higher temperature and holding time was due to increased sinteringtemperature and holding time rather than the TiO2 doping. Further, a secondary (Ti,U)O2eutectic phase was reported to form along the grain boundaries during sintering at 1700◦C.The authors speculated that the presence of this phase was benecial for the consolidationand thermal conductivity of UO2 at high temperatures.382.2. SPS of Nuclear Fuel Materials2.2.3.4. UO2-Beryllium Oxide (BeO) Composite ATFsLi et al. studied the fabrication of 10 vol% BeO-UO2 using SPS [28]. The powders weremixed in a ball mill for 24 h followed by SPS processing at 1400◦C, 25 MPa and 10 minutesholding time. Microscopic examination showed BeO particles at the grain boundaries.Intragranular pores were observed, and were attributed to the rapid sintering during SPS.The presence of these intragranular pores was claimed to be benecial for both ssion gasstorage and mechanical properties of the fuel. As seen in Figure 2.9, the 10 vol% BeO-UO2sample was found to have a higher thermal conductivity than UO2 sample, making it apossible candidate for ATF, since the network of BeO particles at the grain boundaries wasconsidered to act as a heat sink.2.2.3.5. UO2-Titanium Silicon Carbide (Ti3SiC2) composite ATFsLi et al. studied Titanium silicon carbide UO2-10 vol % Ti3SiC2 composite using SPS asa potential ATF material [103]. The Ti3SiC2 was found to encapsulate the UO2 particlesduring powder blending. A comparison between thermal conductivity increase obtainedfrom three studies on 10 vol% BeO, SiC and Ti3SiC2 showed that BeO addition had a higherthermal conductivity at room temperature, whereas 10 vol% Ti3SiC2-UO2 had a higherthermal conductivity at ∼ 800◦C (refer to Figure 2.9). This phenomenon was attributed tothe higher CTE mismatch between UO2 and SiC or BeO compared to Ti3SiC2, which causedmicro cracking between these phases which hindered thermal contact. The thermalconductivity increase at this temperature range (800◦C-1000◦C) is more signicant than theroom temperature enhancement, since it is closer to the working temperatures of nuclearfuel.392.2. SPS of Nuclear Fuel MaterialsFigure 2.9: a) Thermal conductivity and b) Thermal conductivity increase evaluation ofATF candidates by Li et al.: UO2-10 vol% BeO, UO2-10 vol% SiC and UO2-10 vol% Ti3SiC2(Reprinted from Ceramic International, vol. 44, B. Li, Z. Yang, M. Chu, Q. Huang, Z. Wang, R.Gao, Y. Zhong, X. Liu, L. Duan, and P. Zhang, "Ti3SiC2/UO2 Composite Pellets with SuperiorHigh-Temperature Thermal Conductivity", pp 19846-19850, © (2018), with permission fromElsevier) [103][28][94].2.2.4. SPS of ThO2In the recent years, the possibility of utilising thorium-based nuclear fuel, instead ofuranium has been gaining traction [104]. ThO2 as a fuel o ers signicant advantages overUO2, such as higher melting point, higher thermal conductivity and better oxidationresistance [104]. As stated by Bergeron et al.: "Thorium fuels have been considered forbreeder fuel cycles whereby a net surplus of 233U is produced during irradiation, asalternative fuels for light water and heavy water reactors, and for disposition of civilian andweapons-grade plutonium, to name a few applications" [104].Muta et al. were the rst to process ThO2 by SPS, while studying mixed oxide fuel materials(ThxM1-xO2; M= Y, Ce, Nd, Gd and U; x=0.05 and 0.1) [105]. The addition of these elementswere either to act as a surrogate material for plutonium or other ssion products. The solidsolutions were prepared by ball milling for 12h followed by cold pressing and sintering at402.2. SPS of Nuclear Fuel Materials1600◦C for 72h. The conventionally sintered pellets were crushed in a ball mill prior to SPSprocessing at 1600◦C and 50 MPa. The lattice parameters of as-sintered pellets were found tobe related to the ionic radius and oxygen defect according to Shannon’s data correlation [105].The thermal conductivity values decreased with increasing dopant levels, with La doping andY doping having most reduction and U doping the least reduction in thermal conductivity.This was attributed to phonon scattering caused by solute atoms in the ThO2 lattice.Muta et al. followed this work with a study dedicated to ThxU1-xO2 (with x varying from0 to 1 in 0.1 increments) [106]. The SPS pellets were prepared by the same technique as intheir prior study; however, conventionally sintered pellets were also prepared by pressing at150 MPa and sintering at 1600◦C for 10h [105]. Despite the higher holding time forconventional sintering (10 hours) compared to SPS (10 mins), the SPS-ed pellets had higherRDs (S90%) than conventionally sintered pellets (Q87%). Although pure ThO2 had a higherthermal conductivity than UO2, the thermal conductivity of all solid solutions was lowerthan that of UO2, as seen in Figure 2.10a. This phenomenon was credited to additionalphonon scattering in substitutional solid solutions. The Young’s modulus and Debyetemperature were found to decrease with increasing uranium content, when measuredusing ultrasonic method.412.2. SPS of Nuclear Fuel Materialsa bFigure 2.10: Thermal conductivity of ThxU1-xO2: a) Muta et al.’s work (Reprinted from H.Muta, Y. Murakami, M. Uno, K. Kurosaki, and S. Yamanaka, “Thermophysical Properties ofTh1-xUxO2 Pellets Prepared by Spark Plasma Sintering technique,” Journal of Nuclear Scienceand Technology, vol. 50, pp 181-187 (2013), © with the permission of Taylor & Francis) [106]and b) Saoudi et al.’s work (Reprinted from Journal of Nuclear Materials, vol. 500, M. Saoudi,D. Staicu, J. Mouris, A. Bergeron, H. Hamilton, M. Naji, D. Freis, and M. Cologna, "ThermalDi usivity and Conductivity of Thorium-Uranium Mixed Oxides,", pp 181-187, © (2018),with permission from Elsevier) [107].Saoudi et al. studied the thermal conductivity of (Th,U)O2 solid solution as a function of Ucontent [107]. Six of the mixed oxides were in a solid solution form (1.5, 3, 8, 13, 30, 60 and 60wt% UO2 in ThO2) prepared by vibrartory milling followed by conventional sintering, whereas79 and 93 wt% UO2-ThO2 composites were prepared by manual mixing followed by SPS. Asshown in Figure 2.10b, small additions of UO2 (13 and 30 wt%) were found to reduce thermalconductivity due to additional phonon scattering at substitutional sites. The 79 and 93 wt%composites had a thermal conductivity value slightly higher than pure UO2. This observationwas contrary to Muta et al.’s work in which additions of ThO2 decreased thermal conductivityof UO2 [106]. However, according to Ge et al., the thermal conductivity of composites ishighly dependent on the homogeneity and characteristics of the additives [89]. Hence, thedi erence in the thermal conductivity of these two independent studies might be related tothe homogeneity, size and morphology of the powders used in the composites.422.2. SPS of Nuclear Fuel MaterialsTyrpekl et al. studied the sintering kinetics of nanocrystalline ThO2 by SPS processed at1600◦C [108]. The study showed that the nanocrystalline powder had a lower sinteringonset temperature (∼500◦C) than microcrystalline powder (∼1300◦C). The displacementdata obtained from nanocrystalline powders of three grain sizes (13 nm, 19 nm and 34 nm)showed an increasing sintering onset temperature with increasing grain size, showing thatsinterability of ThO2 improves with decreasing crystallite size. ThO2 is typically sintered athigh temperatures (S1700◦C), hence the sintering temperature of ThO2 could be lowered byusing nanocrystalline powders.Muta et al. studied SPS processing of Th0.8Ce0.2O2 as a surrogate material for Th0.8Pu0.2O2[109]. The as-sintered samples had a slight reduction in O/M ratio (Th0.8Ce0.2O1.97) due to alikely reaction with graphite. The e ect of O/M ratio on the thermal conductivity wasstudied by comparing the measurements for as-sintered samples, a reduced sample(Th0.8Ce0.2O1.91) and an oxidised (Th0.8Ce0.2O2) sample. Th0.8Ce0.2O1.91 had a lower thermalconductivity compared to the Th0.8Ce0.2O1.97 and Th0.8Ce0.2O2 samples, which was explainedby the additional phonon scattering caused by the presence of oxygen vacancies. However,Th0.8Ce0.2O1.97 had a higher thermal conductivity than Th0.8Ce0.2O2 at high temperatures,which was explained by an average atomic size model. As shown in Figure 2.11a, anon-stoichiometric (Th,Ce)O2- would consist of three cationic, species namely: Ce4+, Th4+and Ce3+. The ionic radius of these species decrease in the order: Ce3+, Th4+ and Ce4+ thusthe averaged atomic radius of Ce3,x+ would be close to that of Th4+. Any additional phononscattering caused by Ce doping would be cancelled out by this e ect. Figure 2.11b showscalculated thermal conductivity using averaged atomic radius model as a function of O/Mratio. As shown here, the calculated values were very closely related to the observedexperimental values.Scott et al. sintered nanocrystalline ThO2 and Th1-xLaxO2-0.5x (x=0.09 and 0.23) using SPS432.2. SPS of Nuclear Fuel MaterialsabFigure 2.11: Muta et al.’s averaged atomic radius model, where a combination of smallerCe3+ and larger Ce4+ behaves similar to Ce3,x+ which has same atomic radii as Th4+: a)Model and b) Predicted and observed thermal conductivity values (Reprinted from Journalof Nuclear Materials, vol. 483, H. Muta, H. Kado, Y. Ohishi, K. Kurosaki and S. Yamanaka,"Thermal Di usivity and Conductivity of Thorium-Uranium Mixed Oxides," pp 192-198, ©(2017), with permission from Elsevier) [109].in the temperature range 1300◦C-1700◦C [110]. The purpose of La doping was to improvedi usion and lower the processing conditions. The authors reported the Th1-xLaxO2-0.5x hadlower densities than ThO2. Microstructural examination showed La segregation during SPSleading to the formation of La2O3. The segregation was claimed to have occurred atundissolved La2O3 sites present before sintering. Although, the authors claim thenon-equillibrium nature of SPS to be the reason for the formation of these phases, there wasinsu cient evidence to support this claim. The study also compared thermal di usivityvalues of SPS-ed as-received ThO2 to that of ball milled ThO2 and Th1-xLaxO2-0.5x. Thethermal di usivity of ball milled ThO2 was lower than that of the as-received ThO2, due tothe increase in phonon scattering contribution from grain boundaries. In the case ofTh1-xLaxO2-0.5x, the La segregation caused it to have a thermal di usivity value lower thanthat of ThO2.Balice et al. synthesised ThxU1-xO2 (x= 1, 0.78, 0.6, 0.5, 0.41, 0.3, 0.15 and 0) byco-precipitation of oxalates followed by thermal decomposition [111]. Th0.5U0.5O2 and442.2. SPS of Nuclear Fuel MaterialsTh0.15U0.85O2 were sintered by SPS in the temperature range from 1300◦C to 1700◦C. Asecondary sintering peak was observed in the case of U0.85Th0.15O2, whereas in the case ofU0.5Th0.5O2, there was just one densication peak. Grain growth analysis of sinteredmaterial showed a progressive increase in grain size for higher U content at any giventemperature. It was suggested that the higher melting point of ThO2 made its grain growthkinetics sluggish compared to U-doped ThO2.2.2.5. Discussion on ECAS of Nuclear Fuel MaterialsFigure 2.12 provides some of the key statistics from the publications on ECAS ofnuclear fuel materials. A pie chart distribution of primary motivation for the publicationsshow that synthesising an ATF fuel material was key motivation to several authors. Thiscould be directly related to sudden spike in publications in 2013 (refer to Figure 2.3), postFukushima disaster in 2011. The next biggest motivation was using ECAS as either analternative processing route or for synthesising alternative fuel material. A few publicationsalso focused on using ECAS for simulating irradiated fuel material either in chemical(addition of Mo, CsI, etc.) or physical form (nanostructuring). There were also somepublications exploring ECAS for fabricating nuclear fuel for thermal propulsion systems andpower generation in the space industry. The avenue of nuclear waste management was alsoresearched in one article by Popa et al. for Ca(ThPO4)2 [112]. Although it is important tonote that the use of ECAS for nuclear waste management was researched for sinteringiodoapatite (a common form of disposing radio-iodine) earlier than this [113]. However,since the experiments were done on non-radioactive materials, this was excluded from thisreview. It is key to understand that in most of these cases, the motivations overlap to acertain extent, for example, UN is identied as a alternative fuel and an ATF. However, thethe basis of most of these motivations stemmed from the inherent properties of ECAS. For452.2. SPS of Nuclear Fuel Materialsexample, less extreme processing conditions were crucial for preserving additives such asSiC, CNTs, GNPs and CsI.Figure 2.12: Key statistics from literature on ECAS of nuclear fuel material.A pie chart indicating country of origin for publications was created by identifying thecountry of a liation of the primary author. The most number of publications were fromU.S.A, mostly due to the contributions from the research groups at University of Florida (24)and Rensselaer Polytechnic Institute (11). Both these projects were funded by the nuclearenergy university program (NEUP). The second and third highest contributions werea liated to Germany and Sweden. The combined contributions from U.S.A, Germany andSweden account for nearly three quarters of the total publications. The highest contributorfrom Sweden was KTH Royal Institute of Technology, whereas all the German contributionswere a liated to the Joint Research Center of European Commission based in Karlsruhe.The contributions from Japan were by Muta et al. from Osaka University.UO2 was the most researched fuel material, followed by UN and ThO2. Although notidentied in the pie chart, some of these articles were on mixed oxides of ThO2 and UO2.Another interesting observation was that most articles from U.S.A seemed to be based onUO2, whereas most articles from the Swedish group focused on UN and U3Si2. Although, theGerman research group worked primarily on UO2, there was also some interest in UC and462.2. SPS of Nuclear Fuel MaterialsThO2. As for the Japanese contributions, three were on ThO2 and one each on UN and U3Si2.It was evident from the above literature review that most of the publications were onSPS processing (78 out of 83). The versatility and simplicity of SPS makes it a more populartechnique than other ECAS techniques. For example, FS and EDC require very high electricelds, making them too complicated to implement even on a laboratory scale. However, thequestion that has to be answered is whether SPS is comparable to conventional sintering,which is the current commercial practice for sintering nuclear fuel materials. In order tounderstand this, a tabular summary of the major di erence between SPS and conventionalis presented in Table 2.1. This table was reproduced with modications from Subhash et al.’swork [96].Most of the listed factors suggest that SPS might be a better technique for sinteringnuclear fuel materials. The lower sintering time and temperature makes it more energye cient process than conventional sintering. Due to the same reason, temperature sensitivematerials (eg., SiC, CNTs, GNPs, CsI, etc.) could be added to the fuel material withoutreaction or degradation. These advantages make SPS a very important tool for prototypingnuclear fuel materials. However, commercial implementation of SPS is highly demanding ina high throughput industry, such as nuclear fuel fabrication. Although there were some highoutput designs proposed by Nili et al. and Cartas, bulk production of parts by SPS is yetrealised e ectively for any material. Another critical factor that was not in favour of SPSwas the high tool wear. Hence, in future it might be possible that with the help of a moreresilient and high production rate tooling, SPS might be used in commercial scalemanufacture of nuclear fuel.The sudden rise in interest for SPS as fuel manufacturing technique was fueled by theneed for ATFs after Fukushima incident, however the lack of a viable SPS based process forfuel commercial production technique over the last decade of research might indicate a dead472.3. Background on Internally and eXternally cooled Annular Fuelend for this route of research. Despite this, several inroads has been created for fabricationof ATFs using SPS on materials, that would not otherwise survive CS.2.3. Background on Internally and eXternally cooledAnnular FuelDuring reactor service, the Peak Fuel Temperature (PFT) is experienced in the fuelpellet’s centerline, and may reach over 1000◦C more than at the periphery of the fuel pellet,mostly due to the poor thermal conductivity (∼ 8 W/m◦C at RT) of UO2 [115][116].The thermal e ciency of a nuclear reactor is related to the coolant temperature, whichmust be controlled to keep PFT below safety margins. Apart from thermal e ciencylimitation, there are several other problems related to high PFT, such as ssion gas release,thermal stress cracking, Pellet Cladding Interaction (PCI), etc. [19]. The accident toleranceof nuclear fuel is also related to the poor heat transfer between the fuel and the surroundingcoolant [117].One of the strategies to improve the outward heat transfer is by increasing fuel’s thermalconductivity either by using fuel materials such as ThO2, UN, U3Si2, etc. or by adding highthermal conductivity materials such as SiC and diamond [116][118][119]. Alternatively, thegeometry of the fuel pellet could be modied, such as in Internally and eXternally cooledAnnular Fuel (IXAF) design. This design concept essentially replaces the hottest part (core)of the fuel pellet with an internal cooling channel [120].The concept of using fuel pellets with central holes was explored in the past with a viewof decreasing fuel swelling in Pu-bearing fuel pellets for Fast Breeder Reactors (FBRs).However, this design only required a central hole in the pellet (∼ 2 mm diameter) withoutany internal cooling channel [121][122] [123]. IXAF is a novel concept that was explored for482.3. Background on Internally and eXternally cooled Annular FuelPressurised Water Reactors (PWRs) and Boiling Water Reactors (BWRs) by a NuclearEnergy Research Initiative (NERI) project in early 2000s, which showed promising resultsfrom preliminary thermohydraulics, neutronics, economics and safety analyses[124][120][19][125][126][127][128]. An investigation by Feng et al. determined a 13×13IXAF array to perform better than a 17×17 solid fuel array with high inner and outerMinimum Departure from Nucleate Boiling Ratios (MDNBRs) and lower pressure drop[125]. An assumed 50% increase in power generation for IXAF resulted in a PFT of ∼900◦Ccompared to 2300◦C for solid fuel. Fuel safety analyses found that IXAF (at 150% power)performed with higher MDNBR than solid fuel in the event of Rod Ejection Accident (REA),Loss of Flow Accident (LofA) and Main Steamline Break Accident (MSBA) [126]. In the caseof Loss of Coolant Accident (LoCA), the PFT was higher for IXAF (927◦C) compared to solidfuel (∼ 777◦C), however still below the mandated limit. Neutronics study by Xu et al.showed that IXAF are viable with an 8-9% enriched fuel for existing PWRs and BWRs, butwith necessary changes to burnable absorber geometry [127]. A comprehensive economicfeasibility study showed that the rate of return on refuelling cost was higher for IXAF((11.5%) compared to solid fuel (6.3%)), due to 50% higher power generation [128].Lahoda et al. demonstrated that IXAF could be manufactured by either conventionalsintering or by VIbratory comPACtion (VIPAC) process [129]. However, several studies haveindicated that the fuel-sheath gap tolerances of IXAF pellets played a key role in itsperformance [130][131]. For example, Yuan et al. proved that a lower inner diametertolerance would be required for reducing strain on the outer cladding by fuel swelling andexpansion during service [131]. A higher inner diametric tolerance could also lead to alower gap conductance in the inner cladding [130]. Post Irradiation Examination (PIE) ofIXAF by gamma scanning showed good axial structural integrity, whereas macroscopicexamination of cross-section showed radial cracks. The di erence in gap conductance (inner492.3. Background on Internally and eXternally cooled Annular Fueland outer) was claimed to have caused thermal stress to develop in the pellet and resulted inradial cracking. Hence, the ability to control the dimensions of IXAF during fabricationprocess is considered critical for its in-reactor performance.Unfortunately, previous attempts at fabricating IXAF showed that precise control of theinner diameter was challenging [132][133][134]. Conventionally Sintered (CS-ed)commercial solid fuel pellets have an outer diameter that resembled an ’hour-glass’ shapeafter sintering, due to the green density gradient imposed on it during cold compaction. The’hour-glass’ shape of fuel was typically corrected by a post sinter center-less grindingprocess, in case of solid fuel pellets. However, in the case of IXAF, the ’hour-glassing’ posedmore challenges, as the brittle nature of fuel would make any grinding or machining on theinner bore problematic. Hence, there were several attempts at rectifying this duringsintering, such as rod-inserted sintering approach, wherein IXAF pellets were sintered withdense UO2 or ZrO2 rod inside the inner cavity, which resulted in an inner diameter toleranceof ∼0.006 mm [134]. However, there were challenges related to the removal of the core rod,as some pellets cracked due to the formation of a lip, as seen in Figure 2.13. Also, anindependent repetition of rod inserted sintering approach by Bergeron et al. did not resolvethe hourglass shape of the inner diameter [132]. Rhee et al. proposed another approach forbetter diametric tolerance by cold pressing a trapezoidal/conical pellet using single actionpressing to reverse the distortion while sintering [135].502.3. Background on Internally and eXternally cooled Annular FuelFigure 2.13: Rod-inserted sintering approach by Rhee et al.: "Cross section of ZrO2rod–inserted annular pellet sintered at 1700◦C for 4 hours in H2 atmosphere" [134](Reprinted from Y. W. Rhee, D. J. Kim, J. H. Kim, J. H. Yang, K. S. Kim, and J. H. Yang, “Rod-Inserted Sintering of Annular Pellets for Dual-Cooled Annular Fuel,” Nuclear Technology,vol. 184, pp 54–62 (2013), © with the permission of Taylor & Francis).512.3. Background on Internally and eXternally cooled Annular FuelTable 2.1: SPS vs. conventional processing of UO2 (Reprinted from Journal of EuropeanCeramic Society, vol. 34, L. Ge, G. Subhash, R. H. Baney, and J. S. Tulenko, "In uence ofProcessing Parameters on Thermal Conductivity of Uranium Dioxide Pellets Prepared bySpark Plasma Sintering," pp 1791–1801, © (2014), with permission from Elsevier) [67][96]Spark PlasmaSinteringConventionalSinteringHolding Time Q20min 1-10hHeating Rate (◦C/min) 50-200 2-10Sintering temperature (◦C) 750-1450 1600-1700Sintering Pressure (MPa) 20-80 N/APre-sintering cold pressing Not required RequiredNear Net Shaping Can be achievedSintered pelletsundergo post sinteringmachining or grindingto nal dimensionsFlexibility for prototyping Greater due to shortersintering times LimitedIncorporation ofmetastable/volatile/temperaturesensitive additives [114][77]Can be achieved dueto lower sinteringtemperatureLimitedMass production andcommercial feasibility Needs improvementUsed commercially forlargescale productionGrain growth suppression Can be limited byusing HP-SPS Not feasibleTooling Wear High LowReaction between tooling andmaterial Likely Unlikely52Chapter 3: Experimental ProcedureThis chapter summarises the experimental procedures used in this research. The source andcharacteristics of all precursor powders are documented in the "Precursor Powders" section. The"Material Processing Techniques" section outlines the details of processing powders into pellets.The characterisation techniques used for the precursor powders and pellets are summarised in"Materials Characterisation Techniques" section. The general preparation and handlingprocedures are also summarised in "Sample Preparation" section. FEM was used for modellingSPS of IXAF, the details of which are summarised in the "FEM Methodology" section.3.1. Precursor PowdersLaboratory grade powders were used for all experiments, unless specied otherwise.The fuel powders used in this study had natural isotopic composition of Th and U. Thefollowing text describes the source of all the precursor powder materials used in this study,as well as their purity levels as quoted by the manufacturers. There was no additionalcharacterisation performed for validating the impurity levels claimed by the manufacturers.This section focuses on the information provided by the manufacturers, a separate sectionoutlining the nding from characterisation of precursor powders has been included inChapter 4.Certain parts of this chapter has been quoted from the author’s M.A.Sc thesis, which was part of thepreliminary work done for this dissertation [136]533.1. Precursor Powders3.1.1. ThO2 PowderNatural ThO2 powder was obtained from International Bio-analytical Industries, FloridaU.S.A. The powder was 99.8% pure (Lot No. IBI051716) with the following trace impurities(Table 3.1):Table 3.1: Chemical composition in precursor ThO2 powder.Impurity Concentration(X 10-4 wt%) ImpurityConcentration(X 10-4 wt%) ImpurityConcentration(X 10-4 wt%)CeO2 1050 TlO2 600 CaO 560ZrO2 545 La2O3 340 SiO2 140Al2O3 Q100 V2O5 Q100 Y2O3 Q100Fe2O3 33 Yb2O3 Q20 MnO2 Q30MgO Q15 ThO2 balance3.1.2. UO2 PowderNatural UO2 powder was obtained from Cameco Fuel Services, Ontario, Canada. Thepowder classied as Cameco Specication-1 (CS-1) grade purity fuel powder, (Lab No. 302356,Marked: 200L021) with trace impurities as specied in Table 3.2. The UO2 powder had aOxygen/Uranium (O/U) ratio of 2.12 at the time of certication.543.1. Precursor PowdersTable 3.2: Chemical composition in precursor UO2 powder.Impurity Concentration(X 10-4 wt%) ImpurityConcentration(X 10-4 wt%) ImpurityConcentration((X 10-4 wt%)Al Q57 B Q1 C 308Ca Q57 Cd Q2 Cr Q57Cu Q9 Dy Q1 F Q57Fe 88 Gd Q1 K Q88Mg Q9 Mn Q9 Mo 4Na Q57 Ni Q4 P 132Si Q88 Th Q43.1.3. SiC PowderSiC powders, for fabricating ThO2-SiC composites, were obtained from various sources.The three di erent powders had di erent particles sizes:1. 40 µm SiC: The 40 µm SiC powder was obtained from Sigma Aldrich® (Product no:378097-250G; Lot no: MKBD4417V). The powder was specied to have a 200-450 mesh.2. 1 µm SiC: The 1 µm SiC powder was obtained from Alfa Aesar® (Product no: 14165;Lot no: N10B008). The manufacturer specied the powder to be phase and 99.8% purein metal basis. The powder had a 1 µm particle size and had a surface area of 19 m2/g.3. 15 nm SiC: The nanometric SiC powder was obtained from Alfa Aesar® (Product no:43884), and was specied to be phase and 95% pure in metal basis.553.2. Material Processing Techniques3.1.4. Diamond PowderThe diamond powder was obtained from Sigma Aldrich® (Product no: 483591-5G; Lotno: MKBS2084V). The powder was composed of monocrystalline synthetic diamond particles∼ 1 µm in diameter. No other impurities were reported in the diamond powder.3.2. Material Processing TechniquesThe pellets prepared in this research were either as-received powders (ThO2 and UO2)or were blended in a planetary ball mill to prepare mixed oxide or composite powder blends.3.2.1. Planetary Ball MillingThe mixed oxide and composite powder blends were prepared in a planetary ball(Manufacturer: Torrey Hills Technologies, LLC, U.S.A; Model: 2L), as shown in Figure 3.1, atthe University of Saskatchewan. Ethyl alcohol was used as a Process Control Agent (PCA)to dissipate localised heat generated during the milling process. Additionally, the use ofPCA also ensured minimal powder loss by sticking to the milling media and crosscontamination of milling media. The milling media used was 10 mm diameter stainless steelballs in a stainless steel jar.563.2. Material Processing TechniquesFigure 3.1: Planetary ball mill at University of Saskatchewan used for preparing compositeand mixed oxide fuel powder blends.Unless mentioned otherwise, the ball to powder weight ratio was maintained at 10:1 andthe milling speed was 400 rpm. After milling, the powder-PCA slurry was separated from theballs and dried in a fume hood under ambient conditions [136]. The milling time was set at 1hour and 12 hours, respectively, for composite and mixed oxide blends.3.2.2. Spark Plasma SinteringThe SPS system used in this study was the 10-3 machine manufactured by ThermalTechnologies®. Figure 3.2 shows the SPS machine at the University British ColumbiaOkanagan, with its four major modules:1. Sintering chamber: The SPS process was carried out inside this chamber. Figure 3.3aThe section titled "Spark Plasma Sintering" has been re-adapted from the author’s M.A.Sc thesis, with minoralterations.573.2. Material Processing TechniquesFigure 3.2: Thermal Technologies® 10-3 SPS system (Reprinted from A. Prasad, (2017)"Spark Plasma Sintering of Cerium Dioxide and Its Composites". MASc thesis, University ofBritish Columbia, pp 24 © by permission from author) [136].shows the sintering chamber with a fully set-up die ready for sintering. The chamberwalls were made of stainless steel with built in coolant channels. The temperature oftop and bottom rams were monitored using by a Eurotherm® 3116 controller, with anover temperature set-point of 600◦C.583.2. Material Processing Techniquesa bFigure 3.3: Modules within SPS equipment: a) SPS sintering chamber with loaded die andb) Process control module (Reprinted from A. Prasad, (2017) "Spark Plasma Sintering ofCerium Dioxide and Its Composites". MASc thesis, University of British Columbia, pp 25 ©by permission from author) [136].2. Hydraulic system: The module comprising of a hydraulic pump and piston controlledthe pressure applied on the die-punch assembly during SPS processing. The hydraulicsystem was rated for a maximum uniaxial force of 10 tonnes. The pressure appliedon the material during SPS was regulated by specifying the cross-sectional area of thegraphite punches (or the die cavity), as documented in Table 3.3.Table 3.3: Cross-sectional area of sintering cavity of various SPS tooling used in this work.Die type Cross-sectional area of sintering cavity (mm2)0.5" tooling 126.7IXAF tooling "Tight tight" 182.9IXAF tooling "Loose tight" 185.1IXAF tooling "Tight loose" 189.4IXAF tooling "Loose loose" 191.53. Vacuum system: The SPS process was carried out at high temperatures (∼1800◦C),hence to avoid oxidation of the work material and graphite tooling, an inert atmosphere593.3. Sample Preparation Techniqueswas maintained inside the chamber. This was achieved using the vacuum module. Thechamber was evacuated to 2O0 × 10−2 Torr, after which it was lled with Ultra HighPurity 99.999% Ar gas.4. Process control and data acquisition system: Figure 3.3b shows the processparameters monitored during sintering: sample temperature and pressure, ramtemperatures, chamber pressure and electric current characteristics. A closed loopEurotherm® 2704 PID controller regulated the temperature and pressure usingprogrammed set-points. The power system of the SPS operated at maximum voltageof 10 V and maximum current capacity of 3000 A. The SPS program was compiledusing iTools® software in a computer connected to the SPS machine and thensynchronised with the process control module. The Data Acquisition (DAQ) software(SpecView®) was used for recording the SPS data (temperature, pressure, vacuum,displacement, current and voltage).A detailed step by step description of SPS process can be found in Appendix B.3.3. Sample Preparation TechniquesAs-sintered samples were prepared for analysis at the University of Saskatchewan andCNL. Metallography samples were prepared by using the standard metallography practice.Rough polishing was carried out sequentially with 120, 240, 400 and 600 grit silicon carbidepolishing papers. Following this, the nal polishing was carried out with the polishing clothswith 9, 6, 3 and 1 µm diamond paste, respectively. The nal step was polishing in colloidalalumina solution in hydrogen peroxide. In cases where the sample size was too small, thesamples were mounted by cold mounting technique. A resin-adhesive mixture was poured603.4. Material Characterisation Techniquesonto a plastic mold with the sample placed inside the mold. The mounted sample was removedout of the die after 30 min of curing time.The pure UO2 fuel samples were chemically etched using a solution of 1:10 conc. sulphuricacid (H2SO4) and hydrogen peroxide (H2O2). The ThO2 containing fuel samples were etchedthermally at 1300◦C for 0.5 hrs. Grain size measurements were performed as per Abrams’sthree circle intercept procedure from ASTM E112 standard using a macro built for ImageJsoftware.3.4. Material Characterisation TechniquesThe following text summarizes the various materials characterisation techniques usedin this dissertation. All geometric dimensions reported in this study were measured usingMitutoyo digital calipers with accuracy of 0.01 mm.3.4.1. Density MeasurementArchimedes’ principle was used to measure the density of as-sintered pellets. Theweight of pellet in air, lsir, and the weight in water, lwstwr, was used to calculate thedensity (/) of the pellet using Equation 3.1. The density of water (/wstwr) was factored in byreferring to a standard chart relating ambient temperature of water to the density of water:/ =lsirlsir −lwstwr /wstwr (3.1)The density measurements were carried out in University of Saskatchewan and CNL tostudy the error in measurement of immersion density. The average variation for four SPSpellets were ±0.104 g/cc, hence any variation within 1% of RD could be considered error foras-sintered ThO2.613.4. Material Characterisation TechniquesTable 3.4: Variation in density measurement of indentical pellets.SampleNo MaterialDensity measuredat University ofSasketchawan (g/cm3)Density measured atCNL (g/cm3)Variation(g/cm3& %)1 ThO2 9.983 9.7970.186 &1.82 ThO2 9.714 9.728-0.014 &0.13 ThO2 9.401 9.584-0.183 &1.94 ThO2 9.805 9.7610.044 &0.53.4.2. X-Ray Di ractionX-Ray Di raction (XRD) experiments were carried out using two di erent machines atthe University of Saskatchewan, as mentioned in Table 3.5. Although, the characterisationwas done in both Cr K and Cu K modes, all the 2 values were converted to Cr K to avoidany confusion. The Precision Lattice Parameter (PLP) was calculated by using the NelsonRiley (NR) technique, which is a statistical model used to measure crystallographic latticeparameter from XRD peak positions [137]. In this method, the lattice parameter calculatedfrom Bragg’s law using 2 is plotted against theNgxunution (Equation 3.2) as shown in Figure3.6 [137]. The intercept of this plot with Y -axis was the PLP, which had a standard deviationof 0.00005 nm [137].Ngxunution =cos2 sin +cos2 (3.2)623.4. Material Characterisation TechniquesTable 3.5: XRD machines used in this study at University of Saskatchewan.Bruker® D8 Rigaku® Ultima IVFigure 3.4: X-Ray di raction equipmentBruker® D8 (Source: © Anil Prasad (2017)).Figure 3.5: X-Ray di raction equipmentRigaku® Ultima IV.Wavelength: 2.2898 Å Wavelength=1.5406 ÅRadiation: Cr K Radiation: Cu K Step size: 0.01° Step size: 0.01°Time per step: 600s Time per step: 1s633.4. Material Characterisation TechniquesFigure 3.6: Nelson Riley function plotted against the lattice parameter to estimate PrecisionLattice Parameter (PLP) of SPS-ed Th0.95U0.05O23.4.3. Light Stereomicroscopy and MicroscopyStereomicroscopy was carried out in CNL using Olympus® SZ61 stereomicroscope witha Lumenera® Innity 2 colour camera. Microscopy was carried out using Nikon® Optiphotwith a Moticam® 1300 camera. Grain size measurements were carried out as per ASTM E112standard using Abram’s three circle intercept method.3.4.4. Scanning Electron MicroscopyA Hitachi® SU 6600 eld emission Scanning Electron Microscope was used for allimaging and chemical analyses. This microscope had an X-Ray Energy DispersiveSpectroscopy (X-EDS) attachment and an Electron Back Scatter Di raction (EBSD)attachment for chemical analysis and grain structure analysis.643.5. Experimental Setup for IXAF3.5. Experimental Setup for IXAFNatural uranium dioxide (O/M=2.27) obtained from CAMECO was used for all pelletsfabricated in this study. The powder was pre-pressed into∼ 50g slugs of∼ 35% green densityunder 41 MPa. The slugs were then passed through a sieve to produce granulated powder.The granulated material was pressed into 10 g annular green pellets (Outer Diameter (OD):15.75 mm, Inner Diameter (ID) 6.0 mm and green density ∼ 50%) in a zinc stearate coateddie at 48.6 MPa. These green pellets were used as starting material for both CS and SPS toenable a comparison of nal sintered fuel material. The green pellet dimensions were chosensuch that a volumetric shrinkage during CS would result in nal pellet of OD∼13 mm andID∼4.5 mm. These dimensions were chosen based on the preliminary thermal hydraulics,stress and heat transfer studies on IXAF CANDU bundle [133]. A dish and chamfer similar tocommercial nuclear fuel pellets were also incorporated into the green pellets.The Conventional Sintering (CS) conditions were based on the industrially acceptedconditions used at Canadian Nuclear Laboratories. CS was carried out in a molybdenumresistance furnace at 1650◦C for 2 hours with a ramp rate of 8.33◦C/min for a batch of eightsamples. The furnace was evacuated prior to sintering to remove any air and then lledwith hydrogen. A hydrogen ow rate of 20 L/min was maintained during sintering.SPS was carried out at 1300◦C for 10 minutes under 50 MPa pressure and Argonatmosphere. Heating and cooling rates were programmed at 100◦C/min and 200◦C/minrespectively. Four separate sintering experiments were carried out using SPS using di erenttooling and green pellet dimensions. The conventional sintering conditions used for IXAFwere based on the industrially accepted program used at CNL. Although the CS and SPSconditions (temperature, heating rate, cooling rate and sintering atmosphere) were di erentfor the IXAF pellets, the resulting relative densities of CS-ed and SPS-ed materials were653.5. Experimental Setup for IXAFcomparable. Thus, the comparison focused on the pellets rather than the technique itself.Table 3.7 (Complete engineering drawings with exact dimensions of the nal tooling usedfor SPS of IXAF can be found in Appendix E.) shows the dimensions of the four types oftooling used for SPS of IXAF. The ID and OD were changed to study the e ect of green pellet- tooling clearance on the SPS of IXAF using green pellets of xed dimensions (OD: 15.75 mmand ID: 6.08 mm). In order to di erentiate the samples, they were named by combining thequalitative descriptors of green pellet-die and green pellet-core rod clearances. The changein green-pellet tooling tolerance should in theory a ect factors such as the pellet-die friction,TCR and ECR in SPS of IXAF. A secondary purpose of changing the tooling tolerances was toidentify the e ect of tooling wear on SPS of IXAF. Ironman et al. had proven that tooling madeout of graphite only lasted upto ve trials, before losing dimensional tolerance [20]. Hence astudy on the e ect of tooling wear on the in situ electro-thermal characteristics during SPS,would be benecial for future research on SPS of fuel materials.663.5. Experimental Setup for IXAFToolingdimensionDie IDCore rod OD 16.25 mm 16.5 mm5.33 mmFigure 3.7: Axisymmetric schematicof ’Tight tight’ conguration.Figure 3.8: Axisymmetric schematicof ’Loose tight’ conguration.5.58 mmFigure 3.9: Axisymmetric schematicof ’Tight loose’ conguration.Figure 3.10: Axisymmetricschematic of ’Loose loose’conguration.Table 3.6: Axisymmetric schematic of di erent tooling geometries for SPS IXAF673.6. FEM MethodologyTable 3.7: Variations in SPS tooling for IXAF fabrication for xed green pellet dimensions(OD: 15.75 mm and ID: 6.08 mm).Sample Identier(OD ID)Die ID(mm)Core OD(mm)Clearance(OD+ID, mm)Tight tight 16.25 5.58 0Loose tight 16.5 5.58 0.25Tight loose 16.25 5.33 0.25Loose loose 16.5 5.33 0.53.6. FEM MethodologyThe details of the FEM models used in this study are summarised in the followingsection. Two di erent models were used in Chapter 5 for calculation of the residual stressand thermal gradient, respectively.3.6.1. Coupled Thermo-Mechanical ModelThe coupled thermo-mechanical model was used to calculate residual stress within theSPS-ed IXAF during cooling. The residual stress was calculated on a 2D quadrant sectionof the core rod-IXAF cross-section. The model was built with the objective of qualitativelycomparing the residual stress for the di erent IXAF tooling types used in this study. The built-in solver for thermal stress computation for "Static, general" step in Abaqus/CAE StudentEdition 2018 was used for analysing the models. The element type for the nodes was set asCPS4R. The total number of nodes were limited to below 1000, since the student version ofthe software was used. The boundary conditions for temperature was taken from the data683.6. FEM Methodologyobtained during cooling. The average runtime of the model was under a minute in a AcerDestop with Intel i5 CPU (3.2 GHz) and 8 GB RAM.3.6.2. Coupled Electro-Thermal ModelThe coupled electro-thermal model was created to model the thermal and electricaleld within the cross-section during SPS of IXAF. The model was created on an axisymmetricsection of the "Loose tight" tooling with spacers. The total number of nodes were 68227 (die:23885, core rod: 2552, top punch: 3926, bottom punch: 4176, pellet: 10201 and spacers: 23487)and the element type was set as DCAX4E. A mechanically static model was used for simplicity,hence the dimensions of IXAF were xed as those measured after sintering. However, in orderto account for the change in the resistance during consolidation, the voltage data was adjustedas per the following formula:ksvjL =kinstL ×gxinslginstL(3.3)where, ksvjL is the adjusted voltage, kinstL is the recorded in situ voltage, ginstL is the in situresistance recorded shown in Figure 5.6. Also, the clearance between the pellet and toolingwas assumed to be zero. The modelling was carried out on the "Loose-tight" trial, since ithad the least perturbed voltage data. The analysis was carried out with the built-in solver for"Transient, thermal-electrical" analysis. The boundary conditions were 0 V for the bottompunch and ksvjL for every time step (1 second) for 3146 (52.43 min.) time steps. An additionalboundary condition was set for the top ram and bottom ram temperature that was recordedduring SPS. The total runtime of the model was∼8 hours. in a Dell Desktop with Intel i5 CPU(3.2 GHz) and 8 GB RAM.693.6. FEM Methodology3.6.3. Material PropertiesThe material properties used for FEM are summarised in Table 3.8. Most of the propertyvalues were adapted from Nili et al.’s study on modelling temperature and stress distributionin UO2 [76]. Since the as-sintered RD of SPS-ed IXAF pellet was 98%, the contribution fromporosity was neglected.703.6. FEM MethodologyTable 3.8: Physical properties used for FEM (T is temperature values in Kelvin, unlessspecied otherwise).PhysicalProperties UO2 GraphiteSpecicheat(J/kgK)52O1743 + 87O95 − 84O2411 2 +31O542 +−2O6334 4+0O71391 −2; = iR1000 [76]10−6−0O0046i 2+6O6069i−790O4[76]Thermalconductivity(W/mK)(V+Wi + Ci 2)−) +Die−EOkfV = 6O8337× 10−2mKl−)W = 1O6693× 10−4ml−)C = 3O1886× 10−0ml−)K−)D = 1O2783× 10−)lm−)K−2E = 1O1608ek [138]−10−0i ++3×10−5i 2−0O0462i+80O133 [76]Density(kg/m3)10960K−+iK) = 0O99734 + 9O802× 10−6i −2O705×10−)(i 2+4O291×10−)+i +;273 ≥ i ≤ 923K2 = 0O99672 + 1O179× 10−5i −2O429×10−1i 2+1O219×10−)2i +;923 ≥ i ≤ 2000 [76]1904− 0O01414i [76]Electricalresistivity(Ωm)Log( ))L1(45−0+(L60Of ) [76]−2 × 10−)6i + + 3 × 10−)2i 2 −10−0i + 2× 10−5 [76]Joule heatfraction 1 1Elasticmodulus(GPa)233O4(1− 1O0915× 10−4i ) [139] 14.7Poisson’sratio 0.316 [139] 0.23 [76]71Chapter 4: Part I: Densication Studieson ThO2 Based FuelsThe results of research on studying the densication of ThO2 and its composites with ATF additives(SiC and diamond) are discussed in this chapter. The densication curves obtained during SPSof these materials were analysed to identify the densication regimes and how the additivesin uenced them. A similar analysis was carried out for UO2 doped ThO2 fuel, aimed atmimicking(Th,Pu)O2 MOX fuel.4.1. Powder CharacterisationSince the densication kinetics of a powder during SPS processing could be in uencedby the characteristics of the powder, especially particle morphology and size. Thecharacterisation of the various powders used in this study was carried out using XRD andSEM.4.1.1. ThO2 PowderFigure 4.1 shows the general morphology of the ThO2 powder. The particles appearedto be faceted with sharp edges, which resembled powder processed by oxalate decompositionroute [140]. The particles appeared to be ∼ 5µm in size. The XRD scan from the precursorpowder identied peaks corresponding to uorite type FCC crystal structure, as seen in FigureParts of this chapter has been published as an article in Ceramic Transactions Vol. 266724.1. Powder Characterisation4.1b. Thorium atoms occupied the corners and face centers, while the Oxygen atoms occupiedthe tetrahedral voids of a cubic unit cell with PLP of 0.5606 ± 0.0006 nm. Additionally, twopeaks corresponding to Thallium oxides (TlO2 and Tl2O3), an impurity (600 ppm) conrmedby the manufacturer, were also detected in the XRD pattern of precursor ThO2 powder. Since,only one peak for each of these phases were detected, they were indexed as impurities due tothe reduced condence in this analysis.a bFigure 4.1: Characterisation of precursor powder: a) SE image and b) XRD scan of ThO2(Unit cell shown in inset was built using VETSA© software).4.1.2. UO2 PowderThe UO2 powder also had a uorite type crystal structure similar to the ThO2 powder,with U atoms occupying the position of Th atoms, as seen in Figure 4.2b. The PLP wasmeasured to be 0.5453 ± 0.0009 nm, which corresponded to hyperstoichiometric UO2+x,where x S 0O025 [141]. The most intense peaks of UO2 (i.e (111), (311) and (220)) hadcorresponding shoulder peaks towards lower 2 angle, hence suggesting the presence ofsecondary oxide phase with higher lattice parameter and lower O/M ratio.734.1. Powder Characterisationa bFigure 4.2: Characterisation of precursor powder: a) SE image and b) XRD scan of UO2 (Unitcell shown in inset was built using VETSA© software).4.1.3. Diamond PowderThe diamond particles had a sharp faceted morphology and a particle size of ∼2 µm,as seen in Figure 4.3a. The lower lattice parameter of diamond resulted in the appearance ofonly the (111) peak. The lattice parameter of diamond is smaller in comparison to other othermaterials studied here, hence only one peak was detected in the angular range of detection.The "diamond" cubic crystal structure had C atoms at the unit cell corners, face centers andalternating tetrahedral voids, as shown in the inset of Figure 4.3b.744.1. Powder Characterisationa bFigure 4.3: Characterisation of precursor powder: a) SE image and b) XRD scan of diamondpowder (Unit cell shown in inset was built using VETSA© software).4.1.4. SiC PowdersThree di erent particle sizes of -SiC were utilised in this study: 15nm, 1µm and 40µm. The particles from the 1 µm SiC powder had an inhomogeneous size distribution, asseen in Figure 4.4a. The major peaks corresponding to the -SiC phase were detected in allthree powders, as seen in Figure 4.4b. The -SiC phase also had a "diamond"-cubic crystalstructure, with Si atoms occupying the corners and the face centers of the lattice, whereas Catoms occupied the alternate tetrahedral voids. Minor peaks corresponding to hexagonal SiCwere distinctly evident in nanometric SiC, and upon closer examination were also present inthe 1 and 40 µm SiC powders as shoulder peaks. The broadening of cubic peaks associatedwith nanocrystallinity could have made the hexagonal-SiC peaks more evident in the caseof nanometric SiC powder. The grain size related broadening of XRD peaks would cause thenon-nanometric peaks to appear more evidently. The width of the -SiC peaks were usedin a Williamson-Hall plot to obtain a crystallite size of 10 nm, which was close to the 15 nm754.2. Tooling Deformationspecied by the manufacturer. The PLPs of all three powders was∼ 0.436 nm, as seen in Table4.1, hence lattice parameter seemed to be independent of particle size for precursor -SiC.a bFigure 4.4: Characterisation of precursor powder: a) SE image of 1 µm SiC particles and b)XRD scans of 15 nm, 1 µm and 40 µm SiC (Unit cell shown in inset was built using VETSA©software).Table 4.1: PLP of precursor SiC additives.SiC particle size (µm) Lattice parameter (nm)0.015 0.4360±0.00031 0.4360±0.000140 0.4361±0.00074.2. Tooling DeformationAll the densication curves constructed in this research were based on the displacementdata from the Data AcQuisition (DAQ) unit during SPS processing. The displacement wasrecorded by tracking the position of the upper steel ram during the SPS process.764.2. Tooling DeformationIn addition the deformation of the of sample during sintering, the tooling (graphite dies,punches and spacers) deformation (thermal expansion, elastic and plastic deformation) alsocontributed to the ram displacement. It was critical to measure and subtract this toolingdeformation from the densication data to ensure that there were no tooling deformationartifacts in the subsequent densication curves. Control experiments (referred to as "emptydie tests") were carried out (with no powder material), in order to record tooling deformationduring a low temperature (1000◦C) and high temperature (1800◦C) controlled experiments.The displacement during empty die tests mimicked the thermal and pressure prole of anactual sintering experiment, as seen in Figure 4.5. The negative displacement of the rams wasthe result of the thermal expansion of graphite tooling, while a positive displacement wasrelated to the elastic and plastic deformation of the tooling.a bFigure 4.5: Tooling deformation during SPS, measured using: a) Thermocouple controlled(low temperature) experiment and b) Pyrometer controlled (high temperature) experiment(negative displacement means an increase in the length of tooling) .The net displacement of the rams at the end of the empty die tests negligible at 0.1 mmand 0.5 mm for thermocouple and pyrometer modes, respectively. However, the maximumdisplacement during the empty die tests was∼0.75 mm and∼2.5 mm in the thermocouple and774.2. Tooling Deformationpyrometer modes, respectively. A typical nal sintering shrinkage of ∼10 mm was observedduring SPS of ThO2 and UO2, hence the tooling deformation alone could contribute upto∼20%of the sintering displacement. In addition to the tooling deformation observed during theempty die tests, there was also thermal expansion caused during regular SPS experiments thatneeded to be accounted for. Hence, if these displacement contributions (tooling deformationand sample’s thermal expansion) were neglected, the densication curves would appear lesssteep or move the position of peaks in densication rate curves. Additionally, the thermalshrinkage of the tooling during cooling could also appear as additional densication of thesintered pellet. Therefore, it was necessary to separate the contribution of thermal expansionof the tooling and material from the ram displacement data.The e ect of pressure on empty tooling displacement can be extracted from Figure 4.6a.The di erence in tooling deformation when temperature and pressure was coupled wasevident during the heating part of the program, where the tooling deformation seemed to bedependent on both temperature and pressure ramp rates. The displacement during heatingappeared to be parabolic in nature, due to the coupled ramping of temperature and pressure.During cooling, the pressure was unloaded faster than the temperature decrease, and thedisplacement data upon unloading had a linear relationship with temperature. This regionwhich could then be used to calculate displacement due to thermal expansion (∆L) of thetooling. These observations also proved that it is essential to keep the pressure constantduring the entire SPS trial, to remove the e ect of pressure on the displacement. As a result,for the Master Sintering Curve (refer Appendix C.2) sintering experiments, the pressure wasapplied before the start of temperature variation.784.2. Tooling Deformationa bFigure 4.6: a) Displacement during empty tooling test when thermal and pressure prolewas coupled and b) Displacement during 2nd run when temperature and pressure wasdecoupled.The contribution from thermal expansion could be resolved from the ram displacementusing two approaches:1. Use the shrinkage data (∆L = f(i )) during cooling to predict and subtract thermalexpansion during heating (i.e., sintering phase) (dtrue = d−∆L).2. Once the sintering trial was completed, the same program was re-run to measure thethermal expansion during the 2nd run [93].In this study, the rst approach was preferred over the second one as re-running the sinteringtrial could potentially cause additional densication of the sintered samples.Hence, the tooling deformation study was used to draw the following conclusions:1. The error in the densication curves due to permanent tooling deformation values couldbe up to 1.4%.2. The displacement during empty die test was a ected by both temperature and pressurevariation. Hence, to isolate the e ect of pressure and temperature, the pressure was794.3. Densication of ThO2applied at the start of the SPS experiments.3. Thermal expansion also contributed to the displacement during SPS experiments. Thecontribution from thermal expansion could be estimated by measuring the displacementduring cooling, which could be subtracted from the displacement data obtained duringsintering part of the experiment.4.3. Densication of ThO2The densication of ThO2 was studied to examine the e ect of di erent processingconditions (e.g., peak sintering temperature, peak pressure, dwelling time, heating rate) onthe as-sintered density. The e ect of high thermal conductivity additives, such as SiC anddiamond, was also studied to identify their e ect on the densication of ThO2. The e ect of Udoping was also studied by SPS processing two solid solutions of Th0.95U0.05O2 and Th0.9U0.1O2.The purpose of U doping was intended to simulate the e ect of Pu doping on ThO2 for disposalof Pu from spent fuel and weapons. A consolidated list of all SPS experiments and materialsused in this study can be found in Appendix B.4.3.1. E ect of Sintering TemperatureThe e ect of the sintering temperature on the densication of ThO2 has been studied byexamining each of the temperature related process parameters: peak sintering temperature,heating rate and dwelling time at peak sintering temperature.4.3.1.1. E ect of Peak Sintering TemperatureThe e ect of peak sintering temperature on the as-sintered RD of ThO2 was studied byvarying the peak sintering temperature for di erent pressure and dwelling time conditions,804.3. Densication of ThO2as shown in Figure 4.7a. Within the temperature range studied (1500◦C-1800◦C), theas-sintered RD of ThO2 SPS processed at 50 MPa - 10 min increased with increasing peaksintering temperature; however, this e ect seemed to stagnate when the as-sintered RD roseabove 95%. For example, there was a statistically insignicant drop in the as-sintered RDfrom 97.2% to 96.5% with an increase in the peak sintering temperature from 1600◦C to1700◦C at 50 MPa - 5 min. A similar observation was made at 1600◦C to 1700◦C for 30 MPa -10 min and 50 MPa - 5 min test conditions.a bFigure 4.7: E ect of peak sintering temperature on the densication: a) As-sintered RDvs. peak sintering temperature (for various pressure and dwelling time conditions) and b)Densication rate vs. instantaneous RD for 1700◦C - 50 MPa - 10 min sample.The stagnation e ect of the temperature on the densication can be traced back to theplot between the instantaneous RD and the densication rate constructed from real timedisplacement data during SPS at ’1700◦C - 50 MPa - 10 min’, as seen in Figure 4.7b. Thedensication rate rose steadily as sintering progressed, until it reached a peak at ∼60% afterwhich it plateaued. Eventually, the densication rate started dropping at RD value of 75%and reached a negligible value at 85%. The decrease could be because of various mechanismsthat are active during various stages of sintering. First stage of sintering proceeds bytemperature and pressure driven mechanisms, such as particle rearrangement and plastic814.3. Densication of ThO2 ow of powder particles, that contribute to rapid densication (RD <80%) [142][143]. Thesecond stage of sintering is typically dominated by pore migration and grain growth, whichare controlled by time driven di usion kinetics (RD >80%)[144].The instantaneous RD and densication rate curves from the various peak sinteringtemperatures at 50 MPa - 10 min appeared to closely match, as seen in Figure 4.8a. All thederivative curves consistently had an "asymmetric bell" shape with the maximumdensication rate at around 1300◦C, followed by a subsequent drop to zero at ∼1450◦C. The"asymmetric bell" peak appeared to be a combination of roughly three peaks represented bythe ’P1’, ’P2’ and ’P3’ markers in Figure 4.8a. This observation was consistent withliterature, where the results on the SPS processing of nanocrystalline ThO2 by Tyrpekl et al.also revealed the presence of multiple densication peaks [108]. Tyrpekl et al. reported twopeaks that were interpreted as the formation of agglomerates by sintering of particles andsubsequent sintering of these agglomerates amongst each other, corresponding to the ’P2’and ’P3’ peaks in the present work. The rst peak was the result of the pressure rampingduring heating, unlike Tyrpekl et al.’s study, where the pressure was kept constant.a bFigure 4.8: Comparison of densication and rst derivative curves based on peak sinteringtemperature for 50 MPa pressure and 10 min: a) 1500◦C-1700◦C and b)1700◦C-1800◦C.824.3. Densication of ThO2When sintering at 1600◦C and 1700◦C, the instantaneous RD curves suggested theevolution of a fourth peak above ∼1450◦C. This was investigated further using the datafrom the ’1800◦C - 50 MPa - 10 min’ experiment. As shown in Figure 4.8b, there was in fact afourth peak ’P4’ at 1600◦C, which was not reported in past literature on SPS of ThO2[105][145][108]. It is important to note that all the studies in the literature were carried outat a peak sintering temperature lower than 1700◦C. In addition to ’P4’, there was also a shiftin the position of the primary ’asymmetric bell’ peak towards a lower temperature, whenThO2 was sintered at ’1800◦C - 50 MPa - 10 min’. This was contradictory to the three SPStrials at temperatures lower then ’1800◦C - 50 MPa - 10 min’, which appeared to closelyretrace each other. The shift was evident in the instantaneous RD curve also, causinginitial/green RD to appear higher than that of the 1700◦C. This suggests that there is ananomaly in the construction of instantaneous RD curve for 1800◦C, causing these e ects.There were two theories put forward to address this peak shift anomaly:1. In situ reduction hypothesis: The instantaneous RD curves in Figure 4.8 were backcalculated by assuming the nal as-sintered RD to correspond to the point of maximumram displacement. Hence, it is possible that the RD in-situ at this point was higherthan that measured post SPS using Archimedes immersion method. This would bepossible if there was a reaction between ThO2 and the graphite tooling leading to theformation of denser ThC (10.6 g/cm3) or Th metal (11.7 g/cm3) [146]. Th metal phasewould have short life as it is extremely sensitive to oxidation upon contact with water oratmosphere. Since, immersion densities were measured at University of Saskatchewan,there was a time delay between SPS and immersion density measurement, during whichthe Th metal could have possibly reoxidised. Hence, the measured density could belower than the instantaneous RD at the maximum displacement. The instance at whichthe denser ThC or Th metal phase formed would re ect as ’P4’, since in theory there834.3. Densication of ThO2was an increase in the density.2. Tooling creep hypothesis: Alternatively, it is also possible that the shift and ’P4’ maybe an artifact caused by the creep deformation of the graphite tooling. If the graphitetooling had undergone permanent deformation under these conditions, it would exhibitits own deformation behaviour at high temperatures.Hypothesis 1 was further explored and is expanded upon in Section 4.3.3.3. Hypothesis 2was investigated using an "empty die test" using the same SPS program. The displacementdata from a fully prepared tooling, but with no powder, was used to investigate thepossibility of creep deformation of the graphite tooling. The tooling deformation aftersubtraction of thermal expansion (calculated from displacement data during cooling) isplotted in Figure 4.9a. The resultant curve was tted to a 3rd degree polynomial equationwith an R2 value of 0.99741. This function, describing the tooling deformation, wassubtracted from sintering curve obtained at ’1800◦C - 50 MPa - 10 min’. As seen in Figure4.9b, the secondary densication domain was still present in the net curve obtained aftersubtraction of polynomial function, thus removing the possibility of a tooling deformationartifact.844.3. Densication of ThO2a bFigure 4.9: Tooling deformation from SPS run at 1800◦C - 50 MPa - 10 min: a) Polynomialcurve t of tooling deformation during empty die test and b) Displacement data before andafter subtraction of tooling deformation.4.3.1.2. E ect of Heating RateThe standard deviation of as-sintered RD appeared to be ∼3% for SPS trials at variousheating rates, as shown in Figure 4.10 for SPS samples from ’1800◦C - 40 MPa - 10 min’ at50◦C/min, 75◦C/min and 100◦C/min. The increasing heating rate resulted in a correspondingincrease in the densication rate (time based), which was also evident from the increase inthe Full Width Half Maximum (FWHM) of the densication rate peaks, as seen in Figure4.10a. However, when plotted against temperature, the densiciation curves had negligibledi erence in position and height, as seen in Figure 4.10b. Thus, within the conditions tested,the e ect of the heating rate on the RD was negligible.854.3. Densication of ThO2a bFigure 4.10: E ect of heating rate on the densication of ThO2 during SPS: a) As a functionof time and b) As a function of temperature SPS processed at ’1800◦C - 40 MPa - 10 min’with heating rates 50◦C/min, 75◦C/min and 100◦C/min.4.3.1.3. E ect of Dwelling TimeThe common methodology for identifying the e ect of dwelling time on the RD wasby conducting SPS trials of varying dwelling times and then measuring the density of theas-sintered material. However, as shown in Figure 4.11a, this methodology yieldedinconsistent results for two sintering temperatures (1600◦C - 50 MPa and 1700◦C - 50 MPa).At the 1600◦C sintering temperature, the as-sintered RD seemed to have increased withincreasing dwelling time, whereas at 1700◦C the nal RD seemed to have decreased withincreasing dwelling time. Thus, the dwelling time and densication were further studied byanalysing the instantaneous RD during dwelling. As shown in Figure 4.11b at 1500◦C, therewas a negligible change (+0.8%) in the density during the holding period. However, in thecase of the 1600◦C and 1700◦C SPS experiments, there was a signicant increase (+8.7% and+12.6%, respectively) in the instantaneous RD during the dwelling period. Anothernoticeable feature of these curves was their slope. At 1500◦C, the slope of the curve wasnearly zero, suggesting that there was no e ect of dwelling time at this temperature.864.3. Densication of ThO2However, at 1700◦C the densication curve had a constant positive slope. At 1700◦C, theslope was parabolic suggesting a stagnation of RD as it approached value S95%.a bFigure 4.11: E ect of dwelling time on the densication of ThO2 during SPS: a) As-sinteredRD vs. dwelling time for various dwelling times and b) Instantaneous RD curve duringdwelling time.4.3.2. E ect of PressureThe e ect of pressure on the relative density during SPS processing could be acombination of two factors:1. E ect of pressure at room temperatures.2. E ect of pressure at high processing temperature, when temperature and pressure ramprates were coupled.4.3.2.1. E ect of Pressure at Room TemperatureThe displacement measurements obtained during cold pressing of various fuelpowders used in this study are shown in Figure 4.12a. The response of these powders to874.3. Densication of ThO2pressure was consistent to the observation made in the literature on powder metallurgy ofcold pressing of powder materials [147][148][149][150][151]. Almangour et al. summarisedvarious mechanisms that are active during cold pressing of powders and their e ect on thegreen density, as seen in Figure 4.12b. The low pressure regime is dominated by powderparticle rearrangement, causing rapid increase in the green density (i.e. rapid ramdisplacement), whereas in the high pressure regime, the powder material starts behavinglike a solid, with reduced displacement associated with particle fracture and plasticdeformation [151].a bFigure 4.12: E ect of cold compaction pressure on fuel powders used in this study: a)Displacement during cold pressing and b) Stages of cold compaction (Source: PowderMetallurgy Science, 2nd Ed., German, Randall M., (1994) ISBN 1-878954-42-3 Metal PowderIndustries Federation, 105 College Road East, Princeton, New Jersey, USA.) [151].As seen in Figure 4.12a, the pure UO2 and ThO2 powders appeared to have compactedmore during cold pressing than the composite and mixed oxide powders. This could be dueto the poor owability caused by the ball milling process used for the preparation of theblends. The ball milling process could have decreased the particle size of precursor powders,decreasing the apparent density of material.884.3. Densication of ThO24.3.2.2. E ect of Pressure During High TemperatureThe e ect of pressure on the nal RD of SPS processed pellets at high temperatureseemed to be inconclusive, irrespective of temperature and dwelling time conditions, as seenin Figure 4.13a. It is important to note that in the case of ’1700◦C - 10 min’ and ’1600◦C - 10min’ the pressure ramping was coupled to the heating rate, whereas the ’1800◦C - 0.5 min’test was carried out at a constant pressure. However, irrespective of these two approachesthere was no statistically indeterminate e ect of peak sintering pressure on as-sintered RD.a aFigure 4.13: E ect of pressure on the densication of ThO2 during SPS: a) Pressure vs. as-sintered RD (the error bars in 1800◦C - 0.5 min represent sample set of varying heatingrates: 50◦C/min, 75◦C/min and 100◦C/min) and b) Densication curves for di erent peaksintering temperatures as a function of instantaneous pressure.The e ect of pressure ramping during SPS was analysed by plotting the instantaneousRD vs. pressure curves for SPS, when the temperature and pressure proles were coupled.The derivative curves showed peaks similar to those observed in instantaneous RD vs.temperature curves as shown in Subsection 4.3.1.1. However, the positions of the peaksappeared to be at di erent pressure values for di erent peak sintering temperatures. Thesevalues were 40.6 MPa, 45.8 MPa, 49.1 MPa for sintering experiments at 1500◦C, 1600◦C and894.3. Densication of ThO21700◦C, respectively. Upon close examination, it was evident that the temperature at whichthese pressure values were attained was approximately 1250◦C. This temperature coincidedwith the maxima point for instantaneous RD vs. temperature curves shown in Figure 4.7. Asa result, rather than the pressure in uencing the instantaneous RD, the results indicate thatthe temperature had a dominant e ect.4.3.3. Characterisation of SPS processed ThO2 PelletsThe SPS processed pellets were characterised by light and electron microscopy toidentify the densication mechanisms that might have been active during the SPS process.XRD was used to identify crystallographic changes to the material, as a result of SPSprocessing.4.3.3.1. Visual ObservationThe precursor ThO2 powder was white in colour, and appeared to darken during SPSprocessing, as seen in Figure 4.14. The change in the coloration of ThO2 during sintering hasbeen investigated by several researchers in the past and is attributed to O vacancyconcentration gradients [152][153][154][155][108]. Conventional sintering of ThO2 under areducing atmosphere of 5 vol% H2 in Ar was found to result in a greyish colorationcompared to no change under synthetic dry air, as reported by Baena et al., due to anincrease in oxygen vacancies [152]. Tyrpekl et al. also reported a change in the coloration ofThO2 from white (raw powder) to greyish yellow after SPS processing at 1700◦C [108]. Boththese studies claimed that the reduction of oxygen stoichiometry was too low to bemeasured by chemical analyses or lattice parameter measurement [108][152]. Hence, as inthese studies the present work could not establish evidence for oxygen vacancyconcentration gradients in SPS processed pellets.904.3. Densication of ThO2Figure 4.14: SPS processed ’1700◦C - 50 MPa - 10 min’ ThO2 showing greyish discolorationon the surface.4.3.3.2. MicroscopyAs shown in Figure 4.15a, the dark coloration was visible in the macrograph of ThO2SPS processed at ’1600◦C - 30 MPa - 10 min’. The pellet changed from dark grey to yellowishwhite after thermal etching in air, as seen in Figure 4.15b, hence proving that the discolorationwas due to reduced oxygen stoichiometry. A similar observation was made for SPS processedThO2 at ’1600◦C - 30 MPa - 10 min’ and ’1800◦C - 50 MPa - 10 min’, as shown in Figure C.1and C.2 in Appendix D.914.3. Densication of ThO2a cb dFigure 4.15: Optical microscopy results from ThO2 SPS processed at ’1600◦C - 30 MPa - 10min’ macrographs (stitched from 4x images) from: a) Unetched specimen and b) Etchedspecimen and micrographs from: c) Etched from a region away from edge and d) Unetchedmicrograph from edge of sample showing faceted boundaries.The region near the centre the pellet appeared to have a dense microstructure with grainsize∼ 50 µm, as seen in Figure 4.15c. Additionally, the grain boundaries observed were curvedrather than linear, suggesting pore breakaway phenomenon [156][157]. This observation alsoindicated that grain growth and densication were proceeding in unison and also the pore areshrinking by the migration of vacancies through grains rather than through grain boundariesduring this stage of sintering [156][157]. As seen in Figure 4.15d, the edge of the specimenappeared to show faceted particle like boundaries which could be Prior Particle Boundaries(PPB), since they resemble perpendicular boundaries of precursor ThO2 particles (as seen inFigure 4.1a).924.3. Densication of ThO2Secondary electron imaging of fractured and polished SPS processed ThO2 providedsuggestions for multiple densication and grain growth mechanisms that were active duringSPS. Fractography of ThO2 samples (Figure 4.16a) processed at ’1500◦C - 50 MPa - 10 min’showed equiaxed polyhedral grains. The orthogonal morphology of ThO2 particles appearedto have transformed into polyhedrons with broader grain boundary angles, that arecommonly observed in sintered solids. The transformation of particle morphology might bedue to surface di usion or vapour transport, as was illustrated in Figure 2.1. The drivingforce behind this change might be the minimisation of surface energy by rounding sharpcorners of the grains/particles. The fracture surface at a higher peak sintering temperatureof the 1600◦C sample showed clear evidence of particle coalescence, as seen in Figure 4.16.The grains of ThO2 processed at 1600◦C had a more pronounced evidence of neck formationand growth, thus showing the progression of densication with increasing peak sinteringtemperature.a bFigure 4.16: Secondary electron micrographs from fractured ThO2 SPS processed pellets at’50 MPa - 10 min’ and peak sintering temperature of: a) 1500 ◦C and b) 1600 ◦C.934.3. Densication of ThO2The ThO2 pellet processed at 1700◦C appeared less porous than the pellet processed at1600◦C, as seen in Figure 4.17a. The grains in the fractures surface were well joined incomparison to the specimens sintered at 1500◦C and 1600◦C. The polished surface alsoindicated a dense microstructure with no visible pores, as seen in Figure 4.17b. A triple grainboundary is shown in the inset with no signs of entrapped pores, suggesting a completedensication of the material. There was no visible change in the grain size on the fracturesurface as sintering temperature increased from 1500◦C to 1700◦C. Since a relatively largergrain size is often preferred for nuclear fuel pellets, the ’1700◦C - 50 MPa - 10 min’ sinteringcondition appears to be the optimum SPS conditions for ThO2, due to the minimal graingrowth and complete densication. This result is which was consistent with Muta et al. andTypekl et al.’s observations [105][106][108].a bFigure 4.17: SEM micrographs from ThO2 SPS processed at ’1800 ◦C - 50 MPa - 10 min’: a)Fractured surface and b) Polished surface (inset showing a triple grain boundary).SPS processed pellet SPS processed at 1800◦C, as seen in Figure 4.18, underwentextensive grain growth. Many grains became visible with a low power microscope and944.3. Densication of ThO2appeared to the extent that they were visible macroscopically as white crystals in etchedsample (Figure C.2). The sample was extremely brittle, as seen in Figure 4.18a, where twointergranular fractures extending across the pellet. The intergranular fractures andbrittleness might be due to reduced strength caused by grain coarsening [158]. Theweakening of grain boundaries could be also the result of intragranular pores forming at thecentre of the grains as seen in Figure 4.18b, which could have formed by the migration ofvacancies away from pores present at the grain boundaries. Typically, nuclear fuel pelletsare fabricated with 4% porosity, wherein intragranular porosity is preferred to trap ssiongas and reduce fuel swelling. [159][160][161][67][18][76]. Hence, the presence ofintraganular pores might be benecial for SPS processed ThO2 during service.a bFigure 4.18: Optical micrograph from ThO2 SPS processed at ’1800◦C 50 MPa - 10 min’: a)Stitched image from 97 micrographs showing a sample wide intergranular fracture and b)Intragranular pores in unetched specimen.Figure 4.19a shows that intergranular pores were also present in a pellet sintered at1800◦C, as observed in Figure 4.19a. One such pore that formed at a triple grain boundary isshown in Figure 4.19b. These intergranular pores might be impinging further grain growth,which is also commonly observed in irradiated UO2 [162]. The pores present at grain954.3. Densication of ThO2boundaries hinder grain coalescence by pinning grain boundaries [163].a bFigure 4.19: SEM micrographs from ThO2 SPS processed at ’1800◦C - 50 MPa - 10 min’: a)Polished surface showing grain growth and b) An entrapped pore impinging grain growth.The grain size for SPS processed ThO2 was measured using EBSD technique for eight SPSprocessed samples of varying sintering temperature, pressure and dwelling time (detailedresults can be found in Figure C.3). The grain size values obtained from EBSD were plottedagainst the as-sintered RD values, as shown in Figure 4.20a. The results suggest that duringthe nal stages of SPS processing, there was a clear evidence for rapid grain growth, possiblydue to enhanced di usion caused by either the mobility of oxygen vacancies or due to theelectric eld [152]. It was also evident that there was resemblance between this plot and thatfrom Ge et al.’s study on the e ect of SPS conditions on UO2, reproduced in Figure 4.20b [67].Although the magnitude of grain growth in Ge et al. was lower than that observed in thepresent study, the same trend of grain growth preceding densication was observed. Hence,the earlier hypothesis that intergranular pores impinged grain growth was supported by thisobservation. At a lower density, there would be more intergranular pores impinging grain964.3. Densication of ThO2growth.a bFigure 4.20: Grain size vs. RD: a) Results of this work and b) Ge et al.’s SPS study on UO2(Reprinted from Journal of the European Ceramic Society, vol. 34, L. Ge, G. Subhash, R. H.Baney, and J. S. Tulenko, "In uence of Processing Parameters on Thermal Conductivity ofUranium Dioxide Pellets Prepared by Spark Plasma Sintering," pp 1791-1801, © (2014), withpermission from Elsevier) [67].Although, the ability of SPS to retain nanocrystallinity has been often credited as one of itsunique features, grain growth at the nal stages of SPS was often observed but downplayed[164][72][73][65]. Several studies in the past have reported enhanced grain growth duringSPS, especially in ionic solids [67][76][78][68][83]. Tyrpekl et al. claimed that the electriceld during SPS would improve di usion of ionic species like U+4 and O2- in the presenceof vacancies, improving both densication and grain growth [68]. The discoloration of ThO2during SPS has already proved (similar to UO2) oxygen vacancies were created in ThO2 latticeduring SPS.Another similarity in grain size measurements between Ge et al.’s and this study was thehigh standard deviation for large grain size values (S50 µm). This hinted in-homogeneity ingrain size for the coarse grained ThO2 pellets was investigated by performing grain sizemeasurements along radial direction, as shown in Figure 4.21. These measurements were974.3. Densication of ThO2performed using Abram’s three circle intercept method on optical micrographs. SPSprocessed pellet at ’1600◦C - 30 MPa - 10 min’ had bigger grains towards the edge of thepellet, whereas in case of the pellet SPS processed at ’1800◦C - 50 MPa - 10 min’ the biggergrains were observed near the center of the pellet.Figure 4.21: Grain size measurements along radial direction for ThO2 SPS processed at’1800◦C - 50 MPa - 10 min’ and ’1600◦C - 30 MPa - 10 min’ (connecting lines included forconvenience).These opposite grain growth trends could be a result of several factors such as directionof heat conduction, heat retention during cooling, electric eld induced di usion of ionicspecies, etc. For example, the bigger grains observed near the edge of the pellet could bedue to the direction of heat transfer in SPS, which is typically radially inwards for electricallyinsulating materials [76]. This is because the joule heat is generated externally by the passageof electric current through the graphite die, unlike in electrically conductive materials wherethe electric current is owing through the powder itself, hence Joule heating occurs internally.This phenomenon was validated by both Tyrpekl et al. and Nili et al. as well [76][165].The observation of coarser grains away from the edge of the ’1800◦C - 50 MPa - 10 min’pellet could be due to two reasons:984.3. Densication of ThO21. Electric eld induced di usivity: Tyrpekl et al. had observed coarser grains at thepositive electrode of the SPS machine when processing UO2. A higher concentrationof O2- ions was observed near the positive electrode, which resulted in enhanceddi usivity and grain growth in this region [165]. The presence of oxygen vacancieswas validated indirectly in the present work by observing the color change during SPSof ThO2, which in turn proved that electric eld induced di usion of O2- ions wastheoretically possible. Consequently, the region closer to the center of the punchwould have experienced a higher voltage, causing more grain growth in this region[166].2. Heat retention: Another proposition is related to the poor thermal conductivity ofThO2 (8 W/mK). Hence, the region at the center of the pellet could have retained moreheat during cooling, causing an increased grain growth in this region.In order to investigate this phenomenon further a detailed coupled thermo-electric simulationwould be required, which was found to be out of the scope for this dissertation, and was hencesuggested as a future work.4.3.3.3. XRD AnalysisThe following results were identied from the XRD analysis of eight di erent SPSprocessed ThO2 pellets, as shown in Figure 4.22a.1. The peaks corresponding to uorite type cubic phase were detected in all eight SPSprocessed pellets.2. The (111) peak in ’1700◦C - 50 MPa - 15 min’ and ’1800◦C - 50 MPa - 10 min’ scansappeared to have reduced intensity compared to that of precursor ThO2 powder and994.3. Densication of ThO2other SPS processed pellets. The coarser grains present in these pellets could havecaused the intensity of peaks to deviate from the peak intensity ratio of ThO2 powder.The slit size (100 µm) of the XRD machine was comparable to the grain size of thesepellets (∼100 µm). Hence, the pattern could be collected from one or more grain crystalsmaking the SPS-ed material to appear textured.3. Th metal was detected in ’1800◦C - 50 MPa - 10 min’, possibly due to reduction of ThO2during SPS.4. The low intensity peaks of carbonaceous compounds, such as ThC and graphite, weredetected; however, due to the reduced condence in this analysis, they were not indexedin samples other than the ’1800◦C - 50 MPa - 10 min’. The occurrence of carbonaceouscompounds could have been a surface phenomenon, as seen in Figure 4.22b, where theywere seen in the crevices of grain boundaries.1004.3. Densication of ThO2abcFigure 4.22: XRD analyses of SPS processed ThO2: a) XRD patterns, b) PLP measurementsand c) Carbonaceous compounds detected in crevice between grains on the surface of ThO2SPS processed at ’1700◦C - 70 MPa - 10 min’.There was no detectable variation in PLP, with the exception of the ’1800◦C - 50 MPa- 10 min’ sample, as observed in Figure 4.22c. Thus, under the investigated SPS conditionsthe ThO2 remained stable. However, ThO2 was not stable during SPS at ’1800◦C - 50 MPa- 10 min’, which was proven by the detection of Th metal and lower PLP value of ThO2.The formation of Th metal and the reduction of PLP for ThO2 phase could be related viathe formation of Th metal, which would lower the concentration of Th in the cubic uoritephase. The cubic uorite phase was a solid solution of ThO2 (a=0.560 nm) and CeO2 (a=0.540nm), CeO2 was stated as an impurity (1050 ppm) by the manufacturer of the precursor ThO21014.3. Densication of ThO2powder. A reduction of Th in (Th,Ce)O2 would lower the lattice parameter of this phase, wasalso suggested by Muta et al. [105].The formation of Th metal at higher temperature could have also contributed to thesecondary stage of densication that was detected in the densication curves reported inSection 4.3.1.1. Th metal (/=11.7 g/cm3) has a higher density compared to ThO2 (/=10.0g/cm3), hence its formation could have appeared as an additional densication regime in theSPS densication curves. However, since none of the density values measured by immersiondensity exceeded 10.0 g/cm3, it is possible that the Th metal formed only in trace amountson the surface and readily reoxidised upon contact with air or water during densitymeasurements. The in situ formation of Th metal would make the nal density measured byimmersion density measurement to be di erent from the actual density at the end of the SPScycle. Hence, for calculating MSC of ThO2, a di erent approach was used to calculatedensication curves for MSCs.The detection of Th metal in the pellet processed at ’1800◦C - 50 MPa - 10 min’ wasinvestigated further by reexamining the sample after aging for nearly a year in ambientatmosphere and temperature. The sample was polished for EBSD using standardmetallography techniques and rescanned using XRD. Another, XRD scan was taken after thesample was thermally etched and oxidised in air (at 1300◦C for 30 min). The aged andreoxidised samples showed no peaks corresponding to Th metal, suggesting that thereduction was either a surface phenomenon or Th metal got reoxidised during aging inambient conditions. The PLP of ThO2 phase has also been restored to the same value as theprecursor ThO2 powder and other SPS pellets. The carbonaceous compounds were alsoabsent in both polished and thermally etched samples, proving that their presence was asurface phenomenon.1024.3. Densication of ThO2a bFigure 4.23: E ect of natural aging, polishing and thermal etching on ThO2 pellet SPSprocessed at ’1800◦C - 50 MPa - 10 min’: a) XRD patterns and b) PLP.4.3.4. E ect of Additives on Densication of ThO2The e ect of additives on the densication of ThO2 was studied for SiC, diamond andU doping. This analysis was performed using the experiments designed to construct andvalidate Master Sintering Curves (MSCs) for these materials (1800/1500◦C - 60 MPa - 0.5/5min and heating rate was varied from 50◦C/min to 150◦C/min). However, for comparing theas-sintered RD between these composites, only the data from 1800◦C - 60 MPa - 0.5 min wasused. For comparison, the results from ThO2 processing under the same conditions was alsoused.4.3.4.1. E ect of SiC on the Densication of ThO2The e ect of SiC addition on the average as-sintered RD was evident in 3 wt% additionof 1 µm and 15 nm SiC to ThO2, as seen in Figure 4.24a. The average as-sintered RD ofcomposites increased for both 1 µm and 15 nm SiC particle addition, with more densicationobserved for ner 15 nm addition. The observed increase in the average as-sintered RD may1034.3. Densication of ThO2be due to the enhancement of thermal conductivity caused by the SiC addition. Theenhancement of thermal conductivity was later proven in a parallel study published byco-authors [167]. Similar observation has been made in YSZ-CNT composites due to theaddition of CNTs, which is a higher thermal conductivity additive [102]. Thermalconductivity plays a critical role in improving the densication rate of materials during ahigh heating rate process like SPS. Since the composites are electrically non-conductive, thejoule heat is generated externally to the pellet in the tooling, hence the conduction of heatto the composite powder would be in uenced by the pellet’s thermal conductivity.a bFigure 4.24: E ect of 3 wt% SiC addition to ThO2 during SPS at ’1800◦C - 60 MPa - 0.5 min’:a) Average as-sintered RD and b) Instantaneous RD and derivative curves from 75◦C/minheating rate.The average as-sintered RD was also dependent on the particle size of the SiC additive.The RD decreased with increasing particle size, as shown in Figure 4.24a. The e ciency ofSiC particle in improving thermal conductivity would be dependent on its distribution withincomposite, which was expected to be better with ner Sic particles [89]. The enhancement ofdensication was also re ected in the instantaneous RD curves shown in Figure 4.24b, wherethe primary densication rate peaks shifted to a lower temperature for materials with theSiC addition. There was a second peak at ∼1670◦C for 15 nm SiC addition, which could be1044.3. Densication of ThO2sintering of ThO2-SiC or SiC-SiC particle interfaces. However, this peak was only evident in15 nm SiC sample, possibly due to a higher population of ThO2-SiC and SiC-SiC interfaces.The e ect of the SiC particle size was also evident, when 5wt% SiC was added to ThO2, asshown in Figure 4.25a. The average as-sintered RD of pellets with 40 µm SiC additive showed astatistically insignicant increase compared to pure ThO2. This validated the prior conclusionthat densication enhancement was dependent on the distribution of the SiC particles, andlikely deteriorated with increasing particle size. The primary densication peak shifted toa even lower temperature with increasing concentration from 3 wt% to 5 wt% 1 µm SiC, asshown in Figure 4.25b. The second densication peak related to densication of ThO2-SiC andSiC-SiC interfaces had also shifted to a lower temperature of 1417◦C compared to >1650◦C for5 wt% 1 µm addition. A similar analysis was impossible to perform on 5 wt% 40 µm additionsince the derivative peak was much broader possibly due to the convolution of individualsmaller peaks.a bFigure 4.25: E ect of 5 wt% SiC addition to ThO2 during SPS at ’1800◦C - 60 MPa - 0.5 min’:a) Average as-sintered RD and b) Instantaneous RD and derivative curves from 75◦C/minheating rate.The e ect of SiC concentration (wt%) on the as-sintered RD of ThO2-SiC is shown inFigure 4.8a. The average as-sintered RD for each composition was calculated from the SPS1054.3. Densication of ThO2trials at three di erent heating rates, since heating rate was found to have negligible e ecton densication as shown in Section 4.3.1.2. The ThO2 - 40 µm SiC composite showed nostatistically relevant change to the densication, while the ThO2 - 1 µm SiC composite showeda corresponding increase in as-sintered RD with increase in 1 µm SiC content. Similarly, the15 nm addition also showed an increase in as-sintered RD with increasing SiC content until3 wt% SiC addition, beyond which there was no statistically signicant e ect on as-sinteredRD. The increase in densication with increase in SiC content is possibly due to enhancementof thermal conductivity, due to SiC addition. However, beyond a certain SiC content (forexample; 3 wt% in case of 15 nm SiC), the SiC content seemed to create no e ect, possibly dueto segregation of nanometric SiC particles [102]. Figure 4.8 shows the e ect of SiC contenton the instantaneous RD curves of ThO2 - 15 nm SiC. The primary densication peak showedinsignicant change in position with increasing SiC content, whereas the second peak shiftedto a lower temperature (1647◦C from ∼ 1750◦C for pure ThO2). A further increase in SiCcontent shifted the peak to 1573◦C for 3 wt%, but the 15 wt% addition saw the peak shiftto a higher temperature (1719◦C). A possible explanation for this phenomenon, under theassumption that the second peak corresponds to inter-particle densication of SiC - SiC orThO2 - SiC interfaces, would be that the peak position is dependent on the distribution of SiCparticles.1064.3. Densication of ThO2a bFigure 4.26: E ect of SiC concentration on densication of ThO2 - SiC: a) As-sintered RDand b) Instantaneous RD from SPS at ’1800◦C - 60 MPa - 0.5 min’.At lower concentrations (i.e. below 3 wt%), a higher SiC content, could improvedensication by thermal conductivity enhancement. However, a further increase mightcounteract this e ect due to agglomeration, which causes more SiC - SiC contacts and poordistribution within the matrix. In case of nanometric particles like CNTs or SiC, the particlestend to agglomerate due to Van der Waal’s or electrostatic forces and at a higherconcentration the higher population of SiC particles would increase the likely-hood of suchagglomerates forming as seen in some past studies [102][168].Thus, the e ect of SiC addition on the RD was dependent on both the particle size andconcentration. The ner SiC particles were found to improve the as-sintered RD, due toimproved distribution of SiC particles. A judicious increase in the SiC wt% was observed toincrease the as-sintered RD of ThO2-SiC composite; however as the wt% of SiC increasedthere were more agglomerations especially in the case of ner SiC particles, causing noimprovement in the nal RD.Figure 4.27a shows the results from XRD analysis of ThO2-SiC composites. The only peakswhich could be detected with statistical signicance corresponded to ThO2 and SiC and no1074.3. Densication of ThO2additional peaks were detected in the composite samples, suggesting no reaction betweenSiC and the matrix occurred during SPS processing. The microstructure of SPS processedThO2- 5 wt% 1 µm SiC showed a uniform distribution of SiC particles in the ThO2 matrix, asseen in Figure 4.27b. The closeup of an individual SiC particle showed coherence with theThO2 matrix, with no interfacial cracks. Prior studies on UO2-SiC composites often reportedcracking and debonding between SiC particles and UO2 matrix, due to the mismatch in CTEbetween SiC ( df = 4O4 × 10−6◦C−)) and UO2 ( df = 9O9 × 10−6◦C−)) [89]. The CTE ofThO2 ( df = 8O5 × 10−6◦C−)) is lower than that of UO2, which could be the reason behindthe better interfacial contact between SiC and ThO2 compared to SiC-UO2 [138].a bFigure 4.27: a) XRD analysis and b) SEM microstructure of SPS processed ThO2- 5 wt% 1 µmSiC ’1800◦C - 60 MPa - 0.5 min’.4.3.4.2. E ect of Diamond on Densication of ThO2The e ect of diamond addition to ThO2 seemed contrary to the observation made forthe SiC-ThO2 composites. Increasing the diamond content was found to reduce the averageas-sintered RD of ThO2, as seen in Figure 4.28a. The adverse e ect of diamond on thedensication of ThO2 could be either due to CTE mismatch between the additive and thematrix or due to the agglomeration of diamond particles [169][98]. Chen et al. claimed that1084.3. Densication of ThO2CTE mismatch between diamond ( df = 1O1 × 10−6◦C−)) and UO2 created micro-cracks atthe interfaces during cooling [98]. The densication curves of ThO2-diamond compositesalso showed a shift in primary densication peak towards a higher temperature, as seen inFigure 4.27b, possibly due to the creation of more diamond-diamond interfaces byagglomeration of diamond particles. Agglomeration is more likely in diamond powder thanSiC powder due to the lower density of diamond (1.6 g/cm3) in comparison to SiC (3.2g/cm3).a bFigure 4.28: E ect of diamond addition to ThO2: a) As-sintered RD versus diamond wt% andb) Instantaneous RD curves of pellets SPS processed at ’1800◦C - 60 MPa - 0.5 min’.The SPS processed ThO2-diamond pellets appeared structurally and chemically stableaccording to the XRD results as seen in Figure 4.29a. With the exception of carbonaceouscompounds, such as ThC2 and graphite in two pellets, there were no additional phasesdetected using XRD analysis. The graphite phases could be either forming on the surface ofthe pellet in areas in contact with graphoil or at the interface between diamond and ThO2[98]. There was no indication of any relation between PLP and SPS conditions or diamondcontent. Figure 4.29b, shows the microstructure of ThO2-3 wt% diamond SPS processed at1500◦C - 60 MPa - 5 min, which appeared highly porous in its surface, possibly due to the1094.3. Densication of ThO2poor interfacial bonding between diamond and ThO2 substrate causing it to fall o , as seenin Figure 4.29c. The pores appeared to be a few magnitudes bigger than the intact diamondparticles by visual estimation. Hence, it is likely that the pores were formed from thediamond agglomerates that aked o from the surface during polishing.a bcFigure 4.29: Characterisation of SPS processed ThO2-diamond composites: a) XRD analysisand b) Optical micrograph of ThO2-3 wt% diamond SPS processed at ’1500◦C - 60 MPa - 5min’.1104.3. Densication of ThO24.3.4.3. E ect of U Doping on Densication of ThO2The e ect of U doping on the densication of ThO2 during SPS processing was evidentfrom the RD curves as seen in Figure 4.30a. Unlike, the analysis on SiC and diamond addition,the ball milling process was carried out for 12 hours in order to enable the synthesis of a solidsolution (Th,U)O2 of UO2 and ThO2. However, it was not possible to make a comparison of theas-sintered RD values, since the pellets were processed at di erent peak SPS temperatures.A shift in the densiciation rate peak was observed towards lower temperatures, as seen inFigure 4.30b. A shift in peak densication temperature was expected due to the lower meltingpoint of UO2. A non-linear variation in the peak densication rate temperature suggests apossible enhancement of densication due to U doping. Past research by Tyrpekl et al. hadshown that densication during SPS of UO2 is enhanced under electric eld, due to improveddi usivity of U+3/+4 [108]. Hence, an increase U doping should have a similar e ect on thedensity of (Th,U)O2. The peak densication rate temperature of Th0.9U0.1O2 and Th0.95U0.05O2showed a minor deviation (Th0.9U0.1O2: 985◦C v.s. Th0.95U0.05O2: 1005◦C), suggesting thatincreasing the U doping did not proportionally lower the densication peak temperature.The enhancing e ect of U doping was not reported in previous studies on SPS processing of(Th,U)O2 composites.1114.3. Densication of ThO2a bFigure 4.30: Analyses of densication results from SPS of (Th,U)O2: a) Instantaneous RDcurves and b) E ect of U doping on the temperature of peak densication rate (connectingline is for convenience).XRD analysis of the (Th,U)O2 showed that some residual peaks of the UO2, revealingthat some UO2 was undissolved in the (Th,U)O2 solid solution, as seen in Figure 4.31a. Butthese residual peaks were absent in the XRD pattern of SPS processed pellets, indicating thesolutionising of undissolved UO2 in (Th,U)O2 mixed oxide phase during SPS. There wereseveral similarities between the solutionising process and the densication of mixed oxidesduring SPS. The high temperature accompanied by the electric eld would have resulted inthe enhancement of solutionising process, due to enhanced di usion under electric andthermal eld [108][107]. However, the deconvolution of the (111) peak of (Th,U)O2 showedan additional smaller peak with higher d-spacing, as seen in Figure 4.31b. This indicated thepossibility of a secondary mixed oxide phase forming that had a higher Th contentcompared to the major phase.1124.3. Densication of ThO2a bFigure 4.31: Analyses of densication results from SPS of (Th,U)O2 samples and peaksintering temperature: a) Densication curves and b) E ect of U doping on the peakdensication rate temperature (connecting line is for convenience).The optical micrograph in Figure 4.32a showed a visibly di erent white secondary phasein a grey colored matrix in the Th0.95U0.05O2 pellet SPS processed at ’1700◦C (75◦C/min) - 60MPa - 0.5 min’. A lighter colour should correspond to higher ThO2 concentration. Hence, itis likely that this phase was the shoulder peak observed in Figure 4.31.Additionally, grain size was measured using optical micrographs taken from variouspositions along the radial direction (radial position = 0 is the center of the pellet), as shownin 4.32b. There was clear grain coarsening at the periphery of the pellet in comparison tothe center of the pellet. The grain coarsening at the periphery of the pellet could be eithercaused by the enhanced di usion of UM+ ions near the die surface or due to the highertemperature at the periphery in comparison to the center [108].1134.3. Densication of ThO2a bFigure 4.32: Optical micrograph of Th0.95U0.05O2 SPS processed at ’1700◦C - (75◦C/min) - 60MPa - 0.5 min’: a) Secondary oxide with higher Th content and b) Grain size prole alongradial direction of pellet.4.3.4.4. Summary on the E ect of AdditivesThe e ect of additives on the as-sintered RD of ThO2 is summarised in Figure 4.33.Although the error bars overlap, in general, the addition of SiC and U improved thedensication of ThO2, with the highest improvement observed for the 3 wt% (15 nm) SiCadditive. The enhanced densication was possibly related to improved thermal conductivityand enhanced di usivity due to SiC doping. It was also evident that the distribution of SiCin ThO2 matrix was critical in enhancement of densication. Hence, RD increased withdecreasing SiC particle size irrespective of SiC concentration. Although, an increase in theconcentration of SiC increased the RD until 5 wt% SiC addition, at higher concentrationlevels the SiC particles agglomerated, causing no change in RD. However, not all highthermal conductivity additives enhanced densication. For example, the addition ofdiamond was found to have a negative e ect, which was credited to the CTE mismatchbetween diamond and ThO2 or agglomeration of diamond particles.1144.3. Densication of ThO2Figure 4.33: E ect of additives on the as-sintered RD of ThO2Macroscopic examination of representative composites is provided in Table 4.2, 4.3 and 4.4.In general, the thermal etching lightened the colour of SPS processed pellets irrespective of thecomposition of the pellet, hence the as-sintered pellets might have oxygen vacancies similarto pure ThO2. There was a visible inhomogeneity in SPS processed ThO2-1 wt% 15 nm SiC(Figure 4.36) and Th0.95U0.05O2-1300◦C (Figure 4.40) pellets in their periphery, causing themto crack during thermal etching. The inhomogeneities could have caused a CTE mismatchbetween these regions and the rest of the pellet, resulting in cracking during thermal etching.The inhomogeneity in ThO2-1 wt% 15 nm SiC was not symmetrical and was observedonly in the right periphery. Hence, the inhomogenity in ThO2-1 wt% 15 nm SiC would bedue to uneven distribution of SiC particles in the powder blend, possibly due to settling of1154.3. Densication of ThO2ThO2 particles while drying after the powder blending. This could be another reason behindthe secondary densication rate peak in 1 wt% 15 nm SiC addition corresponding to SiC-SiCsintering shifted to higher temperature as the uneven distribution caused a higher populationof these interfaces.In the case of the Th0.95U0.05O2 pellet sintered at 1300◦C, the inhomogeneity was present inthe periphery of the whole quadrant that was examined. As revealed by the XRD scans, theprecursor mixed oxide powders had a secondary U-rich oxide phase, that was solutionisedduring SPS. However, the thermal gradient during SPS, may have caused the periphery tobecome solutionised faster than the interior region resulting in the inhomogeneity identiedhere. It likely that a lower concentration of U (Th0.95U0.05O2 pellet in Figure 4.42 and 4.43),there was less segregation, thereby was visually homogeneous and survived thermal etchingwith no cracks.1164.3. Densication of ThO2As SPS processed Thermally EtchedThO2-5 wt% 1 µm SiC ’1800◦C (100◦C/min) - 60 MPa - 0.5 min’.Figure 4.34: Macrograph of unetched ThO2-5wt% 1 µm SiC ’1800◦C (100◦C/min) - 60 MPa -0.5 min’.Figure 4.35: Macrograph of etched ThO2-5wt% 1 µm SiC ’1800◦C (100◦C/min) - 60 MPa -0.5 min’.ThO2-1 wt% 15 nm SiC ’1800◦C (50◦C/min) - 60 MPa - 0.5 min’.Figure 4.36: Macrograph of unetched ThO2-1wt% 15 nm SiC ’1800◦C (50◦C/min) - 60 MPa -0.5 min’.Figure 4.37: Macrograph of etched ThO2-1wt% 15 nm SiC ’1800◦C (50◦C/min) - 60 MPa -0.5 min’.Table 4.2: Stereoscopic examination of SPS processed and thermally etched ThO2-SiCcomposites1174.3. Densication of ThO2As SPS processed Thermally EtchedThO2-3 wt% diamond ’1500◦C (100◦C/min) - 60 MPa - 5 min’.Figure 4.38: Macrograph of unetched ThO2-3wt% diamond ’1500◦C (100◦C/min) - 60 MPa -5 min’.Figure 4.39: Macrograph of etched ThO2-3wt% diamond ’1500◦C (100◦C/min) - 60 MPa -5 min’.Th0.95U0.05O2 ’1300◦C (50◦C/min) - 60 MPa - 0.5 min’.Figure 4.40: Macrograph of unetchedTh0.95U0.05O2 ’1300◦C (50◦C/min) - 60 MPa- 0.5 min’.Figure 4.41: Macrograph of etchedTh0.95U0.05O2 ’1300◦C (50◦C/min) - 60 MPa- 0.5 min’.Table 4.3: Stereoscopic examination of SPS processed and thermally etched ThO2-3 wt%diamond composite and Th0.95U0.05O21184.3. Densication of ThO2As SPS processed Thermally EtchedTh0.95U0.05O2 ’1700◦C (75◦C/min) - 60 MPa - 0.5 min’.Figure 4.42: Macrograph of unetchedTh0.95U0.05O2 ’1700◦C (75◦C/min) - 60 MPa- 0.5 min’.Figure 4.43: Macrograph of etchedTh0.95U0.05O2 ’1700◦C (75◦C/min) - 60 MPa- 0.5 min’.Table 4.4: Stereoscopic examination of SPS processed and thermally etched Th0.95U0.05O2119Chapter 5: Part II: Comparison of SPSand CS for IXAFThis chapter focuses on the comparison of SPS and Conventional Sintering (CS) techniques forthe fabrication of novel Internally and Externally cooled Annular Fuel (IXAF). IXAF was chosenas it was geometrically more complex than solid fuel pellet and highlighted the contrastbetween SPS and CS. In IXAF, the increased contact area between the fuel powder and thetooling could in uence the tooling-powder friction, Electrical Contact Resistance (ECR),Thermal Contact Resistance (TCR), etc. The near net-shaping capability of SPS was alsoinvestigated using dimensional assessment, optical microscopy and stereomicroscopy. Some ofthe observations were also explored by basic Finite Element Modelling (FEM) tools.5.1. Preliminary Results from SPS of Surrogate IXAFIXAF has never been fabricated using SPS before, hence the primary objective ofthis study was to design and fabricate a custom SPS tooling setup. The tooling design wasoptimised through several iterations; however, to eliminate radiotoxic hazards, the radioactivefuel powder was substituted with surrogate powder of 8 mol% Yttria Stabilised Zirconia (8-YSZ), which had similar Coe cient of Thermal Expansion (CTE) as UO2 (CTEUO2/ZrO2 at 23◦C∼1 × 10−5/◦C). The product of these iterations was the nal design used for the SPS of fuelmaterial, as shown in Figure 5.1a. The di erence in thermal shrinkage between graphitePlease note that aside from this section all the pellets used in this chapter were fabricated from natural UO21205.1. Preliminary Results from SPS of Surrogate IXAF(CTEgraphite at 23◦C=5.41×10−6/◦C [170]) and 8-YSZ during post SPS cooling caused the pelletto shrink more than the core rod creating an interference t as seen in Figure 5.1b. The resultsfrom SPS of YSZ played a key role in guiding the die design for IXAF.a bFigure 5.1: SPS of IXAF: a) Cross-section view of nal tooling and its various componentsand b) Surrogate pellet attached to core rod.The dimensions of the IXAF tooling were selected based on the rationale provided by Niliet al. to reduce thermal heterogeneity and punch deformation during SPS [74]. The ratio ofthe height of the die to that of pellet ( hdiehpellet) plays a critical role in the temperature distributionwithin the pellet during SPS, since a longer die would isolate the pellet from the temperaturehotspots at the top and bottom punch-spacer contact areas [74]. The hdiehpelletwas 10 for thetooling used for pellets sintered in Chapter 4, whereas it was increased to 14 for the SPS ofIXAF. The punch lengths were determined, such that after full densication, the die surfacewas only 0.5 mm away from the spacer surface, thus ensuring minimal length of punchesprotruding outside the die. The reduction of punch length protruding outside the die reducedthe risk of buckling/bulging of the punches [74].In contrast to the SPS experiments from previous chapter, in this chapter SPS experimentswere carried out using green pellets as a precursor material. The rationale behind using greenpellets was:1215.1. Preliminary Results from SPS of Surrogate IXAF• To achieve a direct comparison between IXAF processed through SPS and commercialfuel fabrication by CS (green pellets are typically used as a precursor material for CS).• To accommodate more fuel material inside the SPS tooling prior to sintering, sinceadditional sintering cavity volume was consumed by the core rod.• For the ease of preparation, since lling the tooling cavity with powder would be morechallenging with the core rod in place. Due to the dimensions of the die cavity, crevicesin grafoil and tight spaces that could be susceptible to powder ingress, which duringsintering would form undesired lips/overhangs or even the powder could react withexposed graphite.• To reduce the length of punches protruding outside die of the at the start of the SPSexperiment, thus reducing the hotspot size, and to reduce the possibility of buckling orbending [74].Representative samples from the surrogate material (8-YSZ) experiments can be seen inFigure 5.2. As seen in Figure 5.2a, the excessive hoop stress caused by di erential thermalshrinkage between the graphite tooling and the 8-YSZ material resulted in radial fracturesupon removal of the core rod. The pellets also showed two hairline fractures culminatingfrom the inner edge, proving that the residual stress was building up in the inner diameterof the pellet. The e ect of the changes made to the SPS tooling design can be seen in Figure5.2b, c, d. The modications made to the tooling were targeted at providing more room for theas-sintered pellet to shrink during cooling. The rst modication was to add another layer ofgrafoil between the core rod and the pellet, in order to reduce die wall friction. It was foundthat using two wraps of grafoil around the core rod, instead of one wrap, provided additionalcushioning yielding a less fragmented 8-YSZ pellet, as seen in Figure 5.2b.1225.1. Preliminary Results from SPS of Surrogate IXAFa bc dFigure 5.2: Selected samples from tooling design study using 8-YSZ: a) Solid core-rod andone wrap of grafoil, b) Solid core-rod and two wraps of grafoil, c) Annular core-rod and onewrap of grafoil, d) Annular core-rod and two wraps of grafoil.The second modication was to use a core rod with a central hole of 2.5 mm insteadof a solid core rod. The 8-YSZ pellet sintered with such annular core rod and one wrap ofgrafoil exhibited only two radial cracks, as seen in Figure 5.2c. The residual stress in theannular pellet-core rod combination was modelled using a coupled thermomechanical FEM,to evaluate the e ect of the central hole, as seen in Figure 5.3. The model proved that thethe hoop stress that had build-up in the inner edge of annular pellet decreased in the caseof an experiment with an annular core rod. The model showed that some of the stress wastransferred from the pellet to inner edge of the core rod during cooling. The two modications(extra grafoil wrap and annular core rod) were combined to fabricate the nal tooling for SPS1235.2. Density Resultsof IXAF, of di erent die ID and core rod OD dimensions, as was discussed in Section 3.5.Figure 5.3: E ect of a centre hole in core rod on residual stress distribution within SPS-edIXAF.5.2. Density ResultsThis sections details the density results from the comparison of IXAF pellets (fabricatedusing natural UO2) from CS and SPS techniques. The CS and SPS IXAF samples had a highas-sintered RD of ∼98%, as shown in Table 5.1. Although, the SPS-ed IXAF had a higheraverage as-sintered RD than the CS-ed IXAF, the standard deviation for both the sample setsoverlapped. Hence, there was no reliable correlation between the sintering method and the as-sintered RD. However, CS was carried out at a higher temperature and a longer process timethan SPS, hence these experiments proved that comparable as-sintered RD can be achievedusing SPS at a lower processing temperature and time. On a industrial scale, however CS istypically carried out as a batch process to compensate for the longer processing time. On theother hand, there have been studies that showed that SPS could be scaled up to sinter multiplepellets in one trial [20][74]. Hence, despite the varying process conditions, the production1245.2. Density Resultsrates for both CS and SPS could theoretically be matched.Table 5.1: As-sintered density results from comparison of CS and SPS for IXAF.CS SPSAverage greendensity (g/cm3) 5.66 ± 0.01 5.62 ± 0.03As-sintered density(g/cm3) 10.72 ± 0.10 10.75 ± 0.06As-sintered RD (%) 97.9 ± 0.9 98.2 ± 0.5The CS-ed samples were prepared from the same batch of green pellets in the samesintering trial, hence the variation in density between individual samples was expected to beminimal. Despite being processed in the same sintering run, the CS-ed IXAF pellets had ahigher standard deviation than SPS-ed IXAF pellets sintered in four di erent runs withvariation in tooling. A possible explanation for this could be the presence of microscopiccracks that are in uencing the density measurement in the as-sintered state. Depending onthe size of these cracks water could be seeping in or not, causing the density values to varybetween pellets. Nonetheless, the observation that despite the change in tooling conditionsthe SPS-ed IXAF had a narrower standard deviation than the CS-ed IXAF could also be atestament to the high degree of repeatability of the SPS process.5.2.1. E ect of Tooling Clearance on The Densication During SPS ofIXAFUnlike the CS-ed IXAFs, the SPS-ed IXAFs were unique which could be traced back tothe exact SPS tooling conditions. The as-sintered RDs of SPS-ed IXAFs varied from 97.1% to1255.2. Density Results98.5%, as shown in Figure 5.4a. Assuming that the variation in the as-sintered RD was relatedto the di erence between sintering trials, a direct correlation between the as-sintered RDsand (tooling-green pellet) clearance was evident. The e ect was possibly due to the in uenceon the ECR and TCR, which resulted in a di erent average temperature of IXAF during SPS.Similar observation were made in the past while studying the e ect of die wall friction anddie-punch tolerance by Manière et al. and Grasso et al., respectively [171][172].a bcFigure 5.4: a) Correlation between (tooling-green pellet) clearance and as-sintered RD andb) Grasso et al.’s study on the e ect of die-punch clearance on pellet temperature (Reprintedfrom Materials Transactions, vol. 49, no. 12, S. Grasso, Y. Sakka, and G. Maizz, "E ects ofInitial Punch-Die Clearance in Spark Plasma Sintering Process," pp 2899-2906, (2008), © withpermission from Elsevier) [172] c) Manière et al.’s study on e ect of die wall friction duringSPS (Reprinted from Scripta Materialia, vol. 116, C. Manière, L. Durand, and C. Estournè,"Powder/die friction in the Spark Plasma Sintering process: Modelling and experimentalidentication," pp 139-142 (2016), © with permission from Elsevier) [171].Grasso et al. studied the e ect of a die-punch clearance by varying the punch OD for a1265.2. Density Resultsxed die ID of 20 mm. The study was carried out by xing the electric current applied throughthe tooling during SPS, wherein the sample temperature increased with increasing die-punchclearance, as shown in Figure 5.4b. A higher die-punch clearance increased the ECR, resultingin more Joule heat generation at the punch interface, causing a higher sample temperature.An increase in the die-punch clearance from 0.02 mm to 0.2 mm caused sample temperatureto increase by 200◦C. Although, this study was carried out to investigate the e ect of punch-die clearance, the inferences from Grasso et al.’s study should be valid for green pellet-toolingclearance during SPS of IXAF. Since UO2 is a poor electrical conductor compared to graphite,this overheating phenomenon would be localised to the four circumferential edges of thesample (top and bottom inner and outer diameter edges), where the electric current wouldcut through the pellet. Hence, the higher clearance in the "Loose loose" (See Figure 5.4a) SPStrial could be causing the pellet to experience higher temperature than in other congurations,resulting in the highest as-sintered RD.Additionally, the clearance could also a ect the friction between the green pellet and thedie or the core rod. Manière et al. modelled and validated the e ect of friction coe cientat the die-powder interface during SPS of alumina on the densication during SPS. A higherfriction coe cient resulted in a lower RD and inhomogeneous density distribution, as shownin Figure 5.4b. Hence, the total friction force during SPS in the present study should decreasein the order "Tight tight", "Loose tight", "Tight loose" and "Loose loose" and the as-sintered RDin the opposite order. The "Tight loose" and "Loose tight" IXAF congurations may experiencesimilar net frictional forces, however "Tight loose" might be higher than "Loose tight", dueto the higher contact area in the latter conguration. Both Manière et al.’s and Grasso etal.’s studies were carried out on solid pellets, hence ideally the e ects of tooling friction andclearance should be further enhanced in the case of an annular pellet due to the added innerdiameter interface.1275.2. Density ResultsThe in situ displacement data obtained from SPS trials was used to plot theinstantaneous RD curves (back calculated from the as-sintered RD after correcting forthermal expansion), as shown in Figure 5.5a. The unique feature in the instantaneous RDcurves were perturbations, which were most signicantly noticeable in the "Tight tight"conguration. The origin of these perturbations was studied in detail in the later part of thisseection. Aside from the perturbations, the instantaneous RD curves were similar for allfour SPS trials. Therefore, the "Tight tight" instantaneous RD curve was selected for furtheranalysis as a function of temperature, as illustrated in Figure 5.5b. The densication ratecurve appeared to be symmetric bell shaped curve. The sintering onset temperature was∼600◦C, the peak densication rate was observed at ∼850◦C. Beyond ∼1100◦C, thedensication rate reduced to zero, and any increase in the RD was likely due to graingrowth. The observations made here were in agreement with the sintering curve reportedby Chen et al. on SPS of a cylindrical pellet of UO2. A maximum SPS temperature ∼1100◦Cwould be recommended for pellets where grain size control is critical, since only 96% RD isrecommended for fuel pellets.1285.2. Density Resultsa bFigure 5.5: Instantaneous RD curves from SPS of IXAF: a) Comparison of time basedinstantaneous RD curves and b) Instantaneous RD and densication rate curves of "Loosetight" specimen plotted against temperature.The in situ voltage (k ) and current (I) data were analysed in the form of power(e = k I) and total circuit resistance (g = hI) to study the evolution of the perturbationsobserved in instantaneous RD curves as seenn in Figure 5.6. The total circuit resistancedecreased as sintering progressed, due to the decreasing length (l) of current path (g = lS; /is resistivity, V is cross-sectional area) as the sample densied, eventually reaching a steadyvalue at ∼13 min, beyond which displacement was negligible. The power plots also hadperturbations analogous to those in the densication curve (refer to Figure 5.5a), hence the uctuations of power might be causing the sample temperature to rise and drop causing athermal expansion and shrinkage response, respectively, in the displacement curve. Theorigin of these perturbations could be traced back to the peaks (marked by green arrows) intotal circuit resistance during the sintering process. A peak in the resistance could have1295.2. Density Resultscaused the PID controller to temporarily increase the power input to the tooling causingoverheating, which would be followed by a rapid drop in power, causing the temperature todrop below a desired setpoint. This cycle would continue until the controller would havere-calibrated the PID values, after which the perturbations faded away.Figure 5.6: In situ power and total circuit resistance of data from SPS of IXAF.A possible explanation for these peaks in resistance during SPS, was the pellet-toolingclearance in uencing the electric current path during SPS, as illustrated in Figure 5.7. Inthe case of non-conductive materials, the electric current cuts through the materials at thesample edges rather than going around the sample through the graphite [173]. In case thereis a gap between the tooling and the sample, the current is forced to ow through the die/corerod rather than across the edge of material, hence increasing the length of current path andresistance, as shown in the illustration. Such a phenomenon could occur during SPS of IXAFdue to three reasons:1305.2. Density ResultsFigure 5.7: E ect of pellet tooling clearance on electric current path during SPS.1. Thermal expansion at core rod-IXAF interface: During SPS, the thermal expansionof the pellet could cause the inner diameter surface to loose contact with the core rod,causing a spike in resistance; however, as sintering progressed the fuel material wouldbe pressed against the core rod, causing the resistance to drop. This process could repeatitself based on the kinetics of thermal expansion and densication, which might be thereason behind di ering peak positions and number of peak occurrences for each of thefour SPS trials. Since fuel expands more than graphite during heating, this phenomenoncan take place only at the core rod-fuel interface and not the die-IXAF interface.2. Thermal inhomogenity at the die-IXAF interface: The above explanation is valid,provided the die, core rod and fuel experience negligible temperature di erenceduring SPS. However, thermal inhomogeneity during SPS is a commonly observedphenomenon [76][74][68]. Hence, if the die is at a higher temperature than the fuel, itcould expand more, causing the die to loose contact with the fuel during SPS. Once1315.3. Visual Observation of As-sintered Pelletsagain, this contact would be reestablished as sintering progressed causing a drop inresistance.3. Initial pellet tooling clearance at the die-IXAF/core rod-IXAF interfaces:Additionally, initial clearance could also cause a drop in resistance, when the fuelmaterial comes in contact with tooling during the initial stages of SPS.All three of these phenomena are likely to occur, during SPS of IXAF. Hence, acomprehensive explanation of the resistance peak positions would require a coupledelectro-thermo-mechanical simulation which also accounts for densication. Such ananalysis was recommended for future work. However, these changes to currentcharacteristics proved that a change in tooling dimensions by wear and tear could a ect thedensity of SPS-ed IXAF.5.3. Visual Observation of As-sintered PelletsAll eight of CS-ed IXAF pellets were intact in their as-sintered condition, as seen inFigure 5.8a. Only one edge on one of the samples was chipped, as seen in Figure 5.8b, probablydue to distortion during sintering. Die-wall friction during cold pressing could result in alower green density on the surface than in the interior, resulting in more shrinkage on thesurface than in the interior, leading to the chipping.1325.3. Visual Observation of As-sintered Pelletsa bFigure 5.8: Conventional sintering of UO2 IXAF: a) As-sintered samples and b) Surfacedefect on the corner of a pellet.Out of the four SPS trials, only two pellets were extracted intact from the tooling in as-sintered state, as seen in Figure 5.9. All the pellets were easily removed from the die, howeverthey remained tightly attached to the core rod despite the modications to the die design(center hole on the core rod and thicker grafoil lining) to ease their removal from core rod.After removal of the outer grafoil layer, the "Tight tight" pellet fractured and separated intothree sectors by radial fractures, as seen in Figure 5.9a, possibly due to excessive residualhoop stress. All the other three pellets remained intact attached to the core rod after removalof outer grafoil layer. The "Loose tight" and "Tight loose" pellets were successfully separatedfrom the core rod using a hydraulic press, as seen in Figure 5.9b and c. However, this was avery delicate procedure and required an evenly distributed applied load. When the load wasmisaligned, the sample fractured, as in case of the "Loose loose" sample seen in Figure 5.9d.1335.3. Visual Observation of As-sintered Pelletsa bc dFigure 5.9: As sintered SPS-ed IXAF pellets of varying clearances: a) "Tight Tight", b) "Loosetight", c) "Tight loose" and d) "Loose loose".5.3.1. Modelling of Residual Stress Distribution During SPS of IXAFThe cracking behaviour observed during separation of the core rod from the pellet,would imply that having a tight tooling-green pellet clearance may deteriorate the integrity1345.3. Visual Observation of As-sintered Pelletsof the sintered pellet. However, the thermo-mechanical FEM analysis proved that it ismerely the geometric di erences between four trials that was in uencing the residual stressbuildup during cooling. The same static thermomechanical model used for modellingdi erence in stress buildup by adding a central hole to the core rod was used to model thedi erence in residual stress for the four die pellet congurations, as shown in Figure 5.10a.The material properties were changed to that of UO2 from 8-YSZ for this model. The modelwas built to simulate the hoop stress build-up during cooling from 1300◦C to 23◦C under theassumption that the pellets sintered to the exact dimensions of sintering cavity (ID=OD ofcore rod and OD=ID of die). The numerical values of the residual stress obtained from thismodel were likely not fully accurate, since the model relied only on elastic behavior of thematerial, however in theory the plastic deformation, recrystallisation, creep, etc. wouldreduce the stress close to yield strength of the material. Despite this inaccuracy, the thermalstress values obtained from each of the model simulations should allow qualitativecomparison of the di erent tooling congurations. Due to this, the thermal stress valueswere plotted in arbitrary unit (a.u), as seen in Figure 5.10b. The thermal stress in thesamples seem to a ect the integrity of the samples, which in turn is a ected by the size ofthe core rod.1355.3. Visual Observation of As-sintered Pelletsa bFigure 5.10: Residual stress distribution within IXAF samples modelled using ABAQUS: a)Stress build-up around inner edge of SPS-ed IXAF and b) Max. residual stress vs. C.S.A.The model results indicate that the "Tight tight" pellet had the highest residual hoop stressamongst all the pellets, thus having the highest tendency for fracture. The e ect of the as-sintered pellet geometry on the residual hoop stress can be explained by the di erence inthe Cross Sectional Area (C.S.A) for each of these pellets, which was plotted alongside theresidual hoop stress in Figure 5.10b. The residual stress would be distributed over a largercross-section with increasing C.S.A., hence larger C.S.A implied lower residual stress values.The inferences from visual observation of the integrity of SPS-ed IXAF and FEM alsoprovided insight to identify the e ect of the tooling wear on the integrity of the as-sinteredpellet. The observations suggest that wear and tear of tooling has only positive e ect onthe integrity and residual stress in the as-sintered IXAF. The clearance between the greenpellet and core rod and die increases with tool wear and increasing this clearance reduces theresidual stress in the samples, leading to an improved as-sintered integrity.1365.3. Visual Observation of As-sintered Pellets5.3.2. Dimensional Comparison of CS-ed and SPS-ed IXAFTable 5.2 summarizes the dimensions of the CS-ed and SPS-ed samples. The OD, IDand height of the CS-ed pellets shrunk uniformly in comparison to the green pellet by∼120%, resulting in an ∼180% shrinkage in total volume. The Loose-tight SPS-ed IXAFpellet showed only axial shrinkage, highlighting the fundamental di erence between theseSPS and CS technique. Another observation from these results is the high standarddeviation for the dimensions in SPS-ed IXAF, possibly due to the grafoil that remainsattached to certain parts of the pellets after sintering. The presence of this grafoil layerresidue may not be an issue for fuel applications since most CANDU fuel sheaths are coatedwith a graphite based CANLUB® spray to reduce TCR between sheath and fuel pellet andthus improve the fuel performance [174].Table 5.2: Change in dimensions of IXAF during conventional sintering.Condition ofsample OD (mm) ID (mm)Height(mm)Geometricvolume(×10+mm3)Immersionvolume(= msssvwnsity)(×10+mm3)Green 15.74±0.01 6.07±0.03 11.10±0.12 1.84±0.05 1.84±0.02CS-ed IXAF 12.76±0.01 4.95±0.02 9.23±0.07 1.00±0.02 0.98±0.01SPS-ed "TightLoose" IXAF 16.26±0.11 5.89±0.09 5.6 1.01±0.09 0.95( Grwwneintwrwv)Ue % 123.3±0.2 122.6±1.1 120.3±2.2 183.4±9.8 187.8±4.0( Grwwneintwrwv)ebe % 96.8±0.7 103.0±2.1 198.2 182.2±21.2 193.7A side by side comparison of the green pellet, CS-ed and SPS-ed IXAF is shown in Figure5.11. The CS-ed IXAF underwent volumetric shrinkage during densication, while the1375.4. Stereomicroscopy ResultsSPS-ed pellets experienced axial shrinkage due to the uniaxial pressure during sinteringprocess. Additionally, the conventionally sintered pellets seem to have maintained thechamfer from green pellet form, however, since a at faced punch was used for SPS, thechamfer was attened during SPS processing. Hence, for future research, the cross-sectionof the SPS tooling should be modied such that it matched the nal pellet cross-sectiondimensions rather than matching the green pellet dimensions. The cross-section of greenpellet could be modied to match the nal IXAF cross-section in the case of SPS.Figure 5.11: Side by side (left to right) comparison of green pellet, CS-ed and SPS-ed IXAF.5.4. Stereomicroscopy ResultsStereomicroscopy was performed on CS-ed and SPS-ed UO2 IXAF samples to identifymacroscopic defects in the as-sintered materials. Two separate specimens were prepared forexamining the surface and cross-section microstructures for both CS-ed and SPS-ed IXAF.1385.4. Stereomicroscopy Results5.4.1. Stereomicroscopy of CS-ed IXAFThe stereo micrograph of the surface specimen of a representative CS-ed IXAF pelletrevealed several radial cracks, as seen in Figure 5.12a. As proposed earlier, these radial crackswould have likely in uenced the immersion density measurement, causing a higher standarddeviation in the as-sintered RD. Although the pellet remained consolidated in the as-sinteredstate, two of the radial cracks nearly fully extended through the entirety of the pellet. Oneof these cracks can be seen at a higher magnication in Figure 5.12b, which also showed thepresence of a circumferential crack acting as a bridge between two radially aligned cracks.These radial cracks indicate development of stress within the pellet probably due to di erentshrinking rates of the skin of the pellet and it’s interior. This shrinkage gradient could be theresult of green density gradient imposed during cold compaction.a bFigure 5.12: Stereomicrograph of surface specimen of CS-ed IXAF: a) Stereo micrographstitched from 66 images taken at 4X magnication and b) One of the images used forstitching showing a radial crack at 4X magnication.As seen in Figure 5.13a, there were several radial cracks present across the cross-sectionspecimen too, suggesting that the cracks were not just a surface phenomenon. The average1395.4. Stereomicroscopy Resultscrack length of all macroscopic radial cracks was measured to be ∼0.8 mm, indicating thatthe crack-free width of the pellet at certain regions could be only ∼66% of the total width,assuming the cracks were present on both inner and outer edges. As seen in Figure 5.13b, ahigh magnication image also revealed axial bridges between these radial cracks, which werevisible as circumferential cracks in the surface specimen. Two segments in the corner of thespecimen appeared to be entirely surrounded by cracks, as shown in Figure 5.13c, indicatingthe possibility of chipping, which had happened in one of the pellets shown earlier in Figure5.8ba bFigure 5.13: Stereomicrograph of cross-section specimen of CS-ed IXAF: a) Stereomicrograph stitched from 153 4X images and b) Higher magnication image of regionswhere axial cracks connect radial cracks.The presence of these cracks might adversely a ect the in-reactor performance. Thee ect of cracks on the in-reactor fuel performance is well documented, since fuel typicallycracks during service. MacDonald et al. showed that pellet cracking results in lowering the1405.4. Stereomicroscopy Resultse ective thermal conductivity of the fuel during service [175]. The formation of these crackswas not reported by previous researchers who examined the microstructure of the CS-edIXAF [134][176]. However, Chen et al. observed cracking in 3 mol% YSZ that was sintered inannular form, which has a similar CTE (∼1.2 times) as UO2 [177]. The crackingphenomenon was attributed to a green density gradient, due to die wall friction during coldpressing, as illustrated in Figure 5.14. Die wall friction during cold pressing could haveresulted in a lower green density in the skin of the green pellet. As a result, during sintering,the skin would have shrunk more than the inside, resulting in the formation of a radialcrack. When these radial cracks happen in close proximity of each other, circumferentialcracks might enable bridging the gap between such radial cracks.Figure 5.14: An exaggerated schematic showing the e ect of green density gradient onradial cracks observed in CS-ed IXAF.5.4.2. Stereomicroscopy of SPS-ed IXAF"Tight tight" SPS-ed IXAF was chosen as the representative sample, as it had alreadyfractured. As seen in Figure 5.15a, the stitched stereo micrograph of the surface specimen ofSPS-ed IXAF revealed several crevice type defects that appeared to be interconnected.1415.4. Stereomicroscopy ResultsAdditionally, the region closer to the core rod had more porosity, whereas the periphery ofthe pellet appeared dense. Both these defects, porosity and crevice-defects, could be due tothe dish prole in the green pellets, as illustrated by the schematic in Figure 5.15c. Thegrafoil disk might be folding into itself in the dish region when pressure ramps up duringSPS processing, since the punch had a at face. Similarly, the di erence in the densitybetween the OD and annular ID region might be due to the more material being present inthe periphery region than in the annular region. However, as densication progressed, thematerial should become more homogeneous due to particle rearrangement and graingrowth.1425.4. Stereomicroscopy Resultsab bFigure 5.15: Stereomicrograph of SPS-ed IXAF: a) Stereo micrograph stitched from 1294X magnication images (the yellow squares represent locations of micrographs later inFigure 5.18), b) Higher magnication image of the crevice type defect and c) Exaggeratedschematic of evolution surface defects during SPS of IXAF.The stereo micrograph from the cross-section specimen, as seen in Figure 5.16a showedthat the crevice defects were limited to the surface. However, there was high concentrationof porosity at the bottom corner of the sample, as marked in Figure 5.16b. This high porosityregion likely corresponded to a "neutral zone", that is common occurrence in sintering and1435.5. Optical Microscopy Resultspowder metallurgy of complex part geometries [178]. A "neutral zone" of low densicationoccurs in regions where the tooling remains stationary relative to the part, hence the powdermaterial close to this regions fails to densify due to die-wall friction [178]. Such a "neutralzone" could be occurring between the bottom punch and the core rod due to the lack of relativemotion between these components. Hence, for future work on SPS of IXAF, it is recommendedto re-design the tooling to eliminate "neutral zone", by using a oating core rod [179].a bFigure 5.16: Stereomicrograph of SPS-ed IXAF: a) Stitched image of polished surface(the yellow squares represent locations of micrographs in Figure 5.18) and b) Highermagnication image of "neutral zone".5.5. Optical Microscopy ResultsOptical microscopy was carried out for SPS-ed and CS-ed IXAF to identify the e ect ofIXAF processing on the material’s microstructure. The same specimens prepared forstereomicroscopy was used for optical microscopy also.1445.5. Optical Microscopy Results5.5.1. Optical Microscopy of CS-ed IXAFFigure 5.17a and b show two representative images taken from the surface and cross-section specimens of CS-ed IXAF. There was no noticeable di erence in the grain structureor densication at these two sections of the sample. The microstructure appeared dense, withthe only visible defects being the cracks identied in Figure 5.17c. The intersection regionbetween a radial and axial crack is shown in Figure 5.17c. Formation of both intergranularand transgranular fractures was observed, as seen in Figure 5.17d.a bc dFigure 5.17: Etched microstructure of CS-ed IXAF: a) Surface, b) Cross-section, c) Image of acrack showing crack healing and d) Intergranular and transgranular mode of fracture.1455.5. Optical Microscopy Results5.5.2. Optical Microscopy of SPS-ed IXAFFigure 5.18 shows the general microstructure of the SPS-ed IXAF before etching. Inagreement with the stereo micrographs, the microstructure at the periphery (refer to Figure5.18a) appeared denser compared to the rest of the pellet, as seen in Figure 5.18b and c. Thecrevices described earlier were visible at higher magnication (Figure 5.18b) with porositysurrounding the crevices. A ring shaped region (marked by box with letter ’c’ in Figure5.15a) around the core rod also appeared to be highly porous, with interconnected poresseen in Figure 5.18c. The region surrounding the core rod could be more dense, since it wasexperiencing higher temperature due to the electric current passing through the core rod.This was investigated later using coupled electro-thermal FEA model will be described inSection 5.6.2.1.1465.5. Optical Microscopy Resultsa b cd e fFigure 5.18: Unetched microstructure of SPS-ed "Tight tight" IXAF: a) Dense regions at theperiphery of the surface of pellet, b) Crevices in the mid section, c) Porosity surroundingthe core rod, d) Representative high density region in the cross-section sample, e) Interfacebetween dense and porous region in the cross section and f) High magnication image ofporous region (the location of micrographs are identied in Figures 5.15 and 5.16).Figure 5.18d shows a representative image of a dense region observed in the cross-sectionspecimen of the SPS-ed IXAF pellet. The interface between the "neutral zone" and denseregion in the cross-section of SPS-ed IXAF was clearly demarcated as seen in Figure 5.15e. Thepores in the neutral zone were bigger than in the bulk of the material, with an average porediameter of∼ 10 µm and the total porosity was∼23% (Figure 5.18f). Hence, as-sintered RD inthis region is below the acceptable 96% for commercial fuel pellets, which would be concernfor the application of SPS-ed IXAF in a reactor. The lack of relative motion between the bottompunch and the core rod might be creating a triangular region in the cross section, above whichthe powder particles were allowed to slide against each other, while below this region the1475.5. Optical Microscopy Resultsparticles may be sintering with a reduced applied load. The distinct demarcation between the"neutral zone" and una ected region supported this hypothesis. Hence, application of SPS forfabrication of IXAF, would require the elimination of the "neutral zone".Figure 5.19a shows a region that was color tinted by the etching process duringmetallographic sample preparation. The color of the grains here possibly corresponds to theorientation of grains, which appeared to be randomly oriented. Figure 5.19b shows theetched cross-section of SPS-ed "Tight tight" IXAF specimen at a region that was in contactwith the core rod. The bulk of the microstructure had mostly equiaxed grains, but a fewcolumnar grains were present on the inner diameter region that was in contact with thecore rod. The columnar grains observed here were similar to those present in irradiated fuelaround a central hole of a fuel pellet [180][181]. The driving force for columnar graingrowth in irradiated fuel is the high thermal gradient (S1000◦C/cm) between the centerlineof the fuel and its periphery [180][181]. Initially, the columnar grains were believed to haveformed as a result of the solidication of fuel after service; later, MacEwan et al. proved thatpore migration propelled by a radial thermal gradient resulted in the formation of columnargrains even at the temperatures below the fuel’s melting point [180][181]. MacEwan et al.validated this theory using a simulation experiment, where electric current was passedthrough a tungsten rod inserted in the centre of an annular fuel pellet with a water jacketcooling its periphery, as shown in Figure 5.19b [181]. The experimental setup is very similarto the SPS of IXAF, hence the formation of columnar grains in SPS of IXAF also might be aresult of a similar phenomenon.1485.5. Optical Microscopy ResultsabcFigure 5.19: Microstructure of SPS-ed IXAF: a) Cross-sectional microstructure at a regionthat was in contact with core-rod, b) Equiaxed grains at a region in the interior of the pelletand c) MacEwan et al.’s in-service fuel simulation experiment (Republished with permissionof John Wiley & Sons, Inc., from "Grain Growth in Sintered Uranium Dioxide: II, ColumnarGrain Growth," Journal of American Ceramic Society, J. R. MacEwan and V. B. Lawson, vol.45, no. 1 and pp 42-46 (1962); © permission conveyed through Copyright Clearance Center,Inc.) [181].During SPS of IXAF, the precise current distribution (or density) through the core rodand the die would in uence the radial thermal gradient, which in turn may have aided in theformation of the columnar grains. The core rod could be experiencing elevatedtemperatures relative to the die, due to its lower cross-sectional area compared to the die.Such a thermal gradient could propel void migration towards the core rod as proven by1495.6. Grain Size Analysis of CS-ed and SPS-ed IXAFMacEwan et al. [181]. Another driving force for pore migration could be the electric eldapplied during SPS. Tyrpekl et al. demonstrated that the electric eld caused high oxygenstoichiometry near the positive punch, due to the migration of O2- ions under the incidentelectric eld [68][182][78]. The mobility of O2- ions would be accompanied by vacancydi usion in the opposite direction. Hence, in theory an electric eld assisted di usion ofvacancies is also a possible driving force for columnar grain growth. Nonetheless, theformation of columnar grains during IXAF SPS could lead to potential application of SPS forsimulation of in-service fuel microstructure. It is important to note that these columnargrains were observed only in the region that was approximately half way height across thecross-section of the SPS-ed "Tight tight" IXAF. Hence, the thermal gradient causing theformation of columnar grains could be localised.5.6. Grain Size Analysis of CS-ed and SPS-ed IXAFThe same surface and cross-section specimens of CS-ed and SPS-ed IXAF, that wereused for stereomicroscopy and optical microscopy were also used for grain size analysis.5.6.1. Grain Size Analysis of CS-ed IXAFThe primary objective of performing grain size analysis was to identify anyinhomogenity in grain size distribution. Hence micrographs were captured at variouslocations on lines along radial and axial directions as seen in Figure 5.20a for both surfaceand cross-section specimens. Grain size measurements were carried out at various pointsalong two radial direction lines on the surface specimen (’Surf-r1’ and ’Surf-r2’). On thecross-section specimen, two axial direction lines (’CS-a1’ and ’CS-a2’) and a radial directionline (’CS-r’) were used for measuring, as seen in Figure 5.20a.1505.6. Grain Size Analysis of CS-ed and SPS-ed IXAFa bFigure 5.20: Grain size analyses of CS-ed IXAF: a) Line proles used for grain sizemeasurement, b) Grain size along various lines indicated in rst gure.As seen in Figure 5.20b, there was no apparent variation in the grain size across the axialand radial directions for all of the linear grain size proles. The average grain size values werewithin the standard deviation of each other, with the maximum standard deviation being 1.0µm.5.6.2. Grain Size Analysis of SPS-ed IXAFA similar analysis was carried out for SPS-ed IXAF too with the three radial directionschosen as seen in Figure 5.21a. The grain size values from these three radial line proles wereaggregated into a grain size vs. radial distance graph as shown in Figure 5.21b. The resultsshowed that the grains at the inner and outer edge were∼ 2µm (∼33% of average grain size)smaller to the grains in the middle region as seen in Figure 5.21b. The parabolic curve t alsoindicated possible grain growth in the middle region of the pellet compared to its edges.1515.6. Grain Size Analysis of CS-ed and SPS-ed IXAFa cFigure 5.21: Grain size analysis of surface specimen of "Tight tight" SPS-ed IXAF: a) Radialline prole used for selecting grain size measurement locations, b) Aggregated grain size vs.radial distance.Due to the smaller cross-sectional area of SPS-ed IXAF, grain size was measured along aseries of locations, as shown in Figure 5.22a, to generate a grain size distribution map. Thegrain size distribution map revealed two hotspots of coarse grains (∼14 µm) and a generaltrend of grain coarsening in the central region of the cross-section, as seen in Figure 5.22b.One of these hotspots was observed close to the core rod, whereas the second one was closeto the top/positive punch. Despite the di erent locations of the two hotspots, they had anidentical visible separation (∼1-2 mm) from the nearest tooling surface. These hotspots werefurther investigated using coupled electrothermal FEA.1525.6. Grain Size Analysis of CS-ed and SPS-ed IXAFa bFigure 5.22: Grain size analysis of cross-section specimen of SPS-ed "Tight tight" IXAF: a)Locations used for generating grain size map and b) Grain size map of sample cross-section.The SPS-ed IXAF had a higher grain size than CS IXAF, as summarised in Table 5.3. Thegrain size distribution in SPS-ed IXAF was inhomogenous in comparison to CS-ed IXAF, asshown by the higher standard deviations in Table 5.3. Irrespective of the implications of aninhomogenous grain size distribution, a larger grain size is generally deemed benecial forssion gas release. Hence, if SPS could be used to judiciously enhance grain growth in freshfuel, the benets of using SPS-ed IXAF might out weigh those of CS-ed IXAF. In order toachieve this, the mechanism of grain growth during SPS of IXAF has to be investigated indetail.Table 5.3: Comparison of grain size values of CS-ed and SPS-ed IXAF.CS-ed IXAF SPS-ed IXAFCross-section Surface Cross-section Surface6.9±0.8 µm 5.5±0.9 µm 8.2±2.0 µm 6.5±1.3 µm6.5±1.2 µm 7.6±2.0 µm1535.6. Grain Size Analysis of CS-ed and SPS-ed IXAF5.6.2.1. Modelling of Thermal Field During SPS of IXAFDespite the shorter sintering time and the lower sintering temperature, the SPS processresulted in a material with a bigger grain size than in the case of CS. An electrothermal FEMsimulation was carried out by using the voltage data collected during SPS as the input tostudy the thermal prole in the IXAF pellet during processing and its possible relation tograin size. A detailed explanation of the coupled electrothermal modelling process can befound in Section 3.6.2.As shown in Figure 5.23, the experimental temperature recorded by the pyrometer in the1/8" hole on the die exterior appeared to closely match the FEA simulated data.Figure 5.23: Temperature of pyrometer point modelled using coupled electro-thermal FEMplotted with experimentally measured temperature data for "Loose tight" SPS trial.Figure 5.24 shows the progression of electric potential and temperature distribution acrossthe axisymmetric section of the SPS tooling and pellet during SPS process. An evident changein the electric potential across the punch spacer interface was possibly due to the contactresistance and lower cross-sectional area in this section of the tooling, as seen in Figure 5.24a[183]. Hence, the higher resistance in this region would increase the Joule heat generation atthe punch-spacer interface, making this interface a hotspot, as seen in Figure 5.24b. However,1545.6. Grain Size Analysis of CS-ed and SPS-ed IXAFthe pellet itself was su ciently far away from these hotspots and had a visibly homogeneouselectric and thermal eld (± 0.25 V and ± 50◦C) within the pellet, as seen in Figure 5.24c.a b cFigure 5.24: FEA model of SPS of "Loose tight" IXAF: c) Electric potential distribution att=11.00 min, a) Temperature distribution at t=3.33 min and b) Temperature distribution att=11.00 min (t=3.33 min corresponds to the ramping stage and t=11 min corresponds todwelling stage of SPS program).With the support of the coupled electro-thermal FEM, the factors that in uenced grainsize distribution in SPS-ed IXAF were explored. Two factors possibly contributed to graingrowth during SPS:1. E ect of thermal eld: The creation of thermal "hotspots" during SPS could lead tograin growth in the vicinity of these locations. The thermal eld within thecross-section of the IXAF was simulated using the model, where the temperature atthe center and four corners of the pellet cross-section was modelled (Figure 5.25a).There was no observable di erence in calculated temperatures between these1555.6. Grain Size Analysis of CS-ed and SPS-ed IXAFlocations within the frame of reference in Figure 5.25a. The temperature gradient(∆itottomoutwr = itottomoutwr − iuwntwr) was calculated with reference to thetemperature of the centre of pellet as shown in Figure 5.25b. The gradient was whichshowed Q 10◦C di erence in temperature between these points. Past literature fromFEM of UO2 during SPS also seemed to have yielded similar temperature distributionwithin the pellet [74][72]. Nili et al. observed a maximum temperature di erence of4.8◦C for a solid pellet of diameter 5mm and height 6 mm, whereas Tyrpekl et al.obtained temperature di erences of ∼4◦C for a 6 mm by 3 mm pellet [74][72].a bFigure 5.25: Temperature di erence between four edges, center and pyrometer point of"Loose tight" IXAF during SPS: a) Thermal prole for various locations and b) Temperatureof corners with reference to the temperature of the centre of the pellet.The quantitative data from Figure 5.25b was also visualised as temperaturedistribution maps, as shown in Figure 5.26. All the edges, with the exception of the topouter edge were hotter than the centre of the pellet during the heating phase ofsintering (tQ12 min), with bottom inner edge being the hottest followed by the topinner edge and the bottom outer edge. It was also evident that the face that was incontact with the core rod was hotter than the face in contact with the die (as seen in1565.6. Grain Size Analysis of CS-ed and SPS-ed IXAFFigure 5.26). Additionally, the data also proved that the top/positive punch was hotterin comparison to bottom/negative punch.1575.6. Grain Size Analysis of CS-ed and SPS-ed IXAFa bc de fFigure 5.26: Evolution of temperature distribution within as-sintered pellet: a) t=11.00 min,b) t=19.85 min, c) t=21.06 min, d) t=21.13 min, e) t=21.46 min and f) t= 22.10 min.1585.6. Grain Size Analysis of CS-ed and SPS-ed IXAFDuring the dwelling phase (12 minQ t Q21 min), the temperature gradientbecame more uniform (-1◦C to +2◦C; refer Figure 5.25b) compared to heating phase(-3◦C to +8◦C). During dwelling the OD edge was cooler than the ID edge, due to theradial heat loss by radiation and convection at the outer surface of the die. Although,most of these thermal gradients were insignicant in comparison to the peak SPStemperature of 1300◦C, it is important to note that previous studies by Nili andTyrpekl also credited similar thermal gradients to thermal gradient related grain sizeinhomogeneityDuring cooling (t S21 min) a thermal hotspot evolved near the interface of thesample and the top/positive punch, as seen in Figure 5.26c. The low thermalconductivity of the fuel might be responsible for the evolution of this thermal hotspot,as heat is conducted away from the fuel through the tooling. This hotspot appeared tobe in the same region as the second grain size hotspot that was identied near thepositive/top punch, hence it may have contributed to the non-uniform grain growth inthis region. As cooling proceeded, this hotspot moved towards the top outer edge ofthe pellet cross-section as shown in Figure 5.26. The top outer edge was ∼1◦C hotterthan the top inner edge probably because the top punch was longer than the bottompunch, causing more conduction heat loss through to the rams. The samephenomenon might have caused the bottom edges of the sample to cool down fasterthe bottom edges during the cooling phase (t S21 min) of SPS. The top outer edgeseemed to be cooling slower than the the top inner edge because during cooling, heatconduction might be dominating over radiation, hence the higher thermal mass of thedie caused the outer edges to cool faster than the light core rod.Hence, the FEA results showed two hotspots in temperature distribution mapsthat corresponded to the location of hotspots in the grain size distribution maps. The1595.6. Grain Size Analysis of CS-ed and SPS-ed IXAFthermal eld during heating and dwelling phase justied the possibility of graingrowth in the region that was in contact with the core rod. Although most of thesethermal gradients were insignicant in comparison to the peak SPS temperature of1300◦C, previous studies justied that these gradients could in uence grain growth[76][72]. It is important to note that this hotspot was only ∼1◦C and shortlived (Q0.5min). Hence, it is highly unlikely that the temperature distribution was the sole driverfor the grain size hotspot near the positive punch. However, it is possible that thethermal eld may have aided in its formation.2. E ect of electric eld: Electric eld enhances the di usivity of ionic species duringSPS, as has been demonstrated previously by Typekl et al. [68]. A higherconcentration of O2- ions was observed in the vicinity of the positive electrode duringSPS of UO2, which caused higher self di usion of U3+/4+ in this region causingenhanced densication and grain growth. It was also observed that the region withmost grain growth was not directly in contact with the positive electrode, butseparated from the surface. The surface that was in immediate contact with thepositive electrode had a lower concentration of O2- ions since UO2+x underwentreduction by reacting with graphite. The modelled electric potential distribution inthe cross-section of IXAF can be seen in Figure 5.27 during dwelling and heatingphase of SPS. There was a higher electric potential at the middle of the positive/toppunch. The higher electric potential here would increase O/M ratio in this region thatwould have resulted in the increased self di usion of U+3/+4 ions. It highly likely thatthe the electric eld contributed to the grain growth in this region.1605.7. Summary on CS and SPS of IXAFa bFigure 5.27: Evolution of electric eld during SPS within IXAF cross-section at: a) t=11 minand b) t=20.81 min.These two factors have been proven to contribute to the grain size inhomogenity in SPS-ed IXAF, however, resolving the contributions from each of these mechanism would requireadvanced characterisation (spatial O/M measurement) which was deemed beyond the scopeof this dissertation.5.7. Summary on CS and SPS of IXAFTable 5.4 summarises the ndings from the comparison of SPS and CS for IXAFfabrication. In summary, the results proved that SPS could be used to achieve similaras-sintered RD values at lower processing temperature and holding time. Both CS-ed andSPS-ed IXAF had several macroscopic defects that need to be rectied by modication of thetooling design. SPS-ed IXAF had a highly inhomogenous microstructure in comparison toCS-ed IXAF.1615.7. Summary on CS and SPS of IXAFTable 5.4: Comparison of conventional and spark plasma sintering for IXAF.Conventional Sintering Spark Plasma SinteringSinteringconditionsTemperature 1650◦C 1300◦CPressure 48.6 MPa 50 MPaHolding time 2 hours 0.167 hoursHeating rate 8.33◦C/min 100◦C/minTotal processtime ∼9 hours ∼0.5 hoursSintered density (g/cc) 10.719 ± 0.10 10.75 ± 0.06As-sintered RD (%) 97.9 ± 0.96 98.2 ± 0.5As-sintered pellet condition All pellets were intact Two out of four pelletsfailed due to radial cracksSintering shrinkage Volumetric AxialMacrostructure Several radial cracks. Several crevice typedefects on the surfaceMicrostructure Intergranular cracks Inhomogenous prositydistribution.Grain size 6.5±1.2 µm 7.6±2.0 µm162Chapter 6: Conclusions6.1. Densication Studies on ThO2 Based FuelThO2 based fuels studied in Part 1 of this dissertation, due to their higher thermalconductivity and melting point, which makes them attractive for accident tolerant fuelapplications. The addition of silicon carbide and diamond to ThO2 create to a composite fuelmaterial was explored. The sintering conditions for pure ThO2 were optimised by varyingsintering temperature, heating rate, pressure and dwelling time.During SPS of ThO2, an increase in peak sintering temperature resulted in acorresponding increase in the as-sintered RD. Above 95% as-sintered RD, the e ect of peaksintering temperature was negligible as this regime was dominated by grain growth andpore migration in ThO2. In the case of the heating rate, it’s e ect was negligible within theSPS conditions studied (1800◦C - 20/40/60 MPa - 0.5 min) for ThO2. The e ect of dwellingtime on densication was dependent on the dwelling temperature. At 1500◦C, there wasnegligible e ect of dwelling time, whereas at 1600◦C and 1700◦C the instantaneous RDincreased with increasing dwelling time. The addition of SiC was found to enhancedensication of ThO2 during SPS, by increasing thermal conductivity of the compositematerial [184]. Addition of diamond to ThO2 resulted in deterioration of densication,possibly due to CTE mismatch or agglomeration of diamond particles. U doped ThO2showed grain coarsening towards the periphery of the pellet, either due to a thermalgradient or enhanced ionic di usion near the surface of pellet.The as-sintered pellets were characterised to study the e ect of processing conditions onthe physical and chemical characteristics of fuel material. The as-sintered ThO2 appeared1636.2. Comparison of SPS and CS for IXAFdarker in comparison to ThO2 powder due to reduction during SPS process. The darkgreyish coloration of SPS-ed ThO2 in comparison to white precursor powder, proved thatthe ThO2 lattice developed oxygen vacancies during SPS. The change in color was reversedupon thermal etching.Phase analysis using XRD proved that ThO2 remained stable for peak SPS temperaturebelow 1800◦C, above which it formed trace Th metal. The reduction in PLP of cubic uoritephase also validated this conclusion. Microstructural investigation of as-sintered ThO2showed intragranular pores, that could have formed as a result of migration from grainboundaries or oxygen vacancy mobility during SPS. Radial grain size gradients of 11.1µm/mm and 16.2 µm/mm were identied for ThO2 SPS-ed at 1600◦C - 30 MPa - 10 min and1800◦C - 30 MPa - 10 min, respectively. The grain coarsening is believed to be an e ect ofthermal and electric gradients within ThO2 during SPS.6.2. Comparison of SPS and CS for IXAFA case study was carried out to compare SPS to CS, the commercial fuel manufacturingtechnique using IXAF. The tooling design for SPS was optimised using a surrogate material.The hoop stress on SPS-ed IXAF during cooling could be reduced by adding more grafoillayers between core rod and pellet or by drilling a central hole in the core rod. Despite themodication to SPS IXAF tooling, there was still excessive hoop stress in SPS-ed IXAF. Thehoop stress caused "Tight tight" IXAF to fracture without any load, whereas the "Loose loose"pellet fractured due to uneven load application.CS at 1650◦C - 2 hours and SPS at 1300◦C - 50 MPa - 10 min yielded comparable pellets of98% as-sintered RD. The clearance between green IXAF and tooling in uenced its as-sinteredRD during SPS. A higher clearance was found to yield higher as-sintered RD probably due1646.3. Future Workto reduced die-powder friction or higher ECR. The clearance also played a crucial role in thecreation of perturbations in instantaneous RD curves for IXAF during SPS. The perturbationswere traced to peaks in tooling resistance during SPS, probably due to di erent kinetics ofdensication and thermal expansion.The CS-ed IXAF underwent uniform shrinkage along all the measured dimensions,whereas the SPS-ed IXAF underwent axial shrinkage only. Microstructural investigationrevealed several radial cracks in CS-ed IXAF, covering nearly half its width. These crackswere related to the green density gradient caused by die-wall friction during cold pressing.The CS-ed IXAF had a homogeneous grain size distribution across the pellet with anaverage grain size of ∼ 7 µm. The SPS-ed IXAF had several surface defects caused by thedish prole of green pellet that was in contact with the at edge of the punches.A neutral zone was observed in the cross-section of SPS-ed IXAF between the bottompunch and the core rod, since they remained stationary relative to each other during SPS.The SPS-ed IXAF had a heterogeneous grain size distribution with visible grain coarsening attwo "hotspots" of coarse grains in its cross-section.6.3. Future WorkThe following suggestions are recommended for future work related to this dissertation.1. Investigation of grain coarsening in ThO2 SPS-ed at 1800◦C - 50 MPa - 10 min.using FEM:Coarser grains were observed away from the center of pellet resulting in an"abnormal" grain growth behavior. This observation was perceived as an e ect of eitherelectric eld or thermal eld distribution during SPS, hence a detailed investigationrequires a coupled electro-thermal model.2. Improving accuracy of MSCs: The accuracy of MSCs generated in this study could1656.3. Future Workbe improved by using data from more SPS trials.3. Low temperature IXAF SPS: The temperature used for SPS-ing IXAF was more thanthe ideal temperature. SPS of IXAF at ∼1100◦C is recommended for future studies.4. Redesigned SPS IXAF tooling: The SPS tooling for IXAF need to be redesigned tomatch the dimensions of nal fuel pellet. The tooling design also needs to eliminate theevolution of the neutral zone between the bottom punch and the core rod.5. Investigation of grain growth in SPS-ed IXAF: In order to validate the conclusionfrom FEM model of SPS of IXAF and the relation between thermal and electric eld onthe grain coarsening of SPS-ed IXAF, an advanced spatial characterisation of O/U ratiois required.166Bibliography[1] “BP: World Reserves of Fossil Fuels - knoema.com.”[2] K. E. Kunkel, T. R. Karl, H. Brooks, J. Kossin, J. H. Lawrimore, D. Arndt, L. Bosart,D. Changnon, S. L. Cutter, N. 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Brochu, “Modeling197residual porosity in thick components consolidated by spark plasma sintering,” ScriptaMaterialia, vol. 76, pp. 53–56, 2014.198Appendix AA.1. History of Sintering TheoryMost of research that led to the development of the modern sintering theory took placehappened between 1945 and 1955 [185]. A large portion of the literature from this era is yet tobe digitized and therefore it remains inaccessible. AS a result, the following text summarizesseveral review articles available in the open literature [186][187][188][189][190][191][192].Early publications on sintering theory suggest that during sintering, particles join byvarious mechanisms such as di usion or plastic ow [193]. Two soviet physicists, Ya.Frenkeland B.Ya.Pines developed two di erent theories for sintering [194][195]. Frenkel was the rstscientist to point out that Gibbs energy minimization was the driving force behind sintering[196]. As shown in Figure A.1, the total surface at the end of sintering would be lower thaninitial surface area.Figure A.1: Surface area reduction by particle coalescence.As for the mechanism of sintering, Frenkel’s solution relied on the assumption claim199A.1. History of Sintering Theorythat solids start owing like liquids at temperatures close to their melting point. He justiedthis by using the Arrhenius relation of viscosity to temperature (Equation 1) [195].(i ) = (exp(fki) (1)where, (i ) is viscosity at any given temperature, ( is a constant, Q is the activation energy,k is the Boltzman’s constant and T is the temperature. Frenkel was also able to calculate therelation between the neck radius x and time t and viscosity  during the coalescence of twospherical particles (Equation 2):x+r=3t2(2)where r is the radius of the particle and is the surface energy. However, severalscientists didn’t agree to Frenkel’s assumption that solids could be treated as liquids at hightemperatures B.Ya. Pines was one among them.Figure A.2: Two spherical particles necking during sintering.Pines was in agreement with Frenkel’s theory that surface energy minimizationwas the driving force. However, he claimed that the pores in between particles are lled by200A.1. History of Sintering Theorythe di usion of vacancies rather than by viscous ow and deformation. According to Pines,there would be a high vacancy concentration gradient between the pores and the materialsurrounding it. This concentration gradient caused di usion of vacancies towards particlesurface, thus eventually closing all the pores. The works done by Frenkel and Pines were keyto the understanding of the onset of sintering. However, both these models had their ownshortcomings, since neither of these theories could explain sintering of ceramics. Ceramicsare extremely hard in nature, hence the major sintering mechanism in ceramics could notinvolve plastic ow.Robert Coble, a General Electric scientist working on the development of pore freeceramics, came up with an alternative mechanism. Coble’s sintering theory was the rstattempt for an integrated sintering theory, which was not only applicable to polymers andmetals, but also to ceramics. Coble suggested that in addition to all the mechanisms thatwere proposed so far an additional grain boundary di usion process also contributed tosintering [197][198]. Coble’s four stage sintering theory is summarized in Figure A.3. In rststage of sintering, cohesive forces cause particles to come in contact with each other. Duringthe second stage of sintering, the particles join together by neck formation. The particlemorphology changes to tetrakaidecahedron, a 14 sided polyhedron, during the third andintermediate stage of sintering. Coble proved the presence of tetrakaidecahedron by usingmicroscopic evidence. Coble also found that tetrahedral pores exist between thetetrkaidechedral particles.201A.1. History of Sintering TheoryFigure A.3: Coble’s sintering stages [197][198] (Reprinted from R. L. Coble, “SinteringCrystalline Solids. I. Intermediate and Final State Di usion Models,” Journal of AppliedPhysics, vol. 32, pp 787–792 (1961), © with the permission of AIP Publishing).Coble’s papers were critical in describing a sintering process in a chronologicalorder. However, they lacked clarity in terms of sintering mechanisms, which was addressedby M.F. Ashby. Ashby also developed the Ashby diagrams, which could be used to ndwhich di usion mechanism was active at a certain neck radius, x, temperature ,i , and time,t [38]. Figure A.4 shows such a diagram for UO2 particles of diameter 10µm. As shown inthe diagram, the x-axis is temperature and y-axis is the logarithm of the ratio between neckradius and the particle radius (xs). The diagram contains contour lines which show therelation between these two factors at various sintering times. The diagram also has variousregions which identify by the largest contributor to the neck radius growth at a giventemperature. In addition to UO2, Ashby also calculated sintering diagrams for copper andsilver at various particle sizes.202A.1. History of Sintering TheoryFigure A.4: Sintering diagram of spherical UO2 particles of diameter 10µm (Reprinted fromActa Metallurgica, vol. 116, M. F. Ashby, "A First Report on Sintering Diagrams", pp 275-289(1974), © with permission from Elsevier) [38].Ashby’s diagrams provided a "holistic" approach to sintering theory. However, theyare applicable only under ideal conditions, especially since they neglect pressure. Hence,David S. Wilkinson, a student under Ashby modied the Ashby diagrams so that they takeinto account pressure too. Wilkinson achieved this by adding an additional mechanism, powerlaw creep, which gets activated in pressure assisted sintering [199]. Also, rather than using(xs), Wilkinson plotted the sintering diagrams with the density values on the y-axis, thusmaking them more relevant. Wilkinson used the data obtained from two sites in Antarticaand Greenland, to validate his model on densication of snow into ice. The densicationrates were close to the rates calculated from his model, thus providing experimental proof ofAshby’s sintering diagrams.203A.1. History of Sintering TheoryAll the sintering mechanisms proposed so far were based on the assumption thatsintering proceeds by the interaction between various particles, hence called Discrete ParticleTheory. Although, Ashby’s Sintering Maps were close to predicting density, the chart relieson the calculating density based on a loose relationship between the density and the neckgrowth rate. A continuum mechanics based approach was attempted to predict sinteringkinetics with the basic assumption that sintering body is a two phase mixture of solid and thepores [190].G.L Scherer made a breakthrough in the development of the continuum approach tosintering in 1977 [200]. The theory was rheological in nature and was inspired by Frenkel’stheory of sintering. Instantaneous density of sintering body was calculated based on the bulkviscosity. Scherer considered the body being sintered consisted of cells like shown in FigureA.5a). Here, the incomplete corners and edges of the cube represent a pore in the sinteringbody. Scherer was able to validate his model by experimental data obtained from sinteringof SiO2 soot [201]. As shown in Figure A.5b), the experimentally obtained relative densityplotted against reduced time (a variable based on time) shows a very good correlation withdata calculated from Scherer’s model.204A.2. Nuclear Fuela bFigure A.5: Scherer’s continuum based sintering model: a) Model of a cell element(Republished with permission of John Wiley and Sons, Inc., from "Sintering of Low-DensityGlasses: I, Theory,", Journal of American Ceramic Society, G. W. Scherer, vol. 60, no. 5-6 andpp 236-239 (1977); © permission conveyed through Copyright Clearance Center, Inc.) [200]and b) Experimental data obtained from sintering of SiO2 plotted against Scherer’s relation(Republished with permission of John Wiley and Sons, Inc., from "Sintering of Low-DensityGlasses: II, Experimental Study,", Journal of American Ceramic Society, G. W. Scherer and D.Bachman, vol. 60, no. 5-6 and pp 239-243 (1977); © permission conveyed through CopyrightClearance Center, Inc.) [201].There were several other continuum-based models developed after Scherer’smodel succeeded. With the improvement of computing power, the models became morecomplicated and accurate. Increased interest in PM and sintering also led to the availabilityof more sintering data for testing these models. A review of all these models would bebeyond the scope of this study.A.2. Nuclear FuelIn theory, there are two types of nuclear reactors: ssion- and fusion- based. Fusionreactors are still in their infancy stage and not yet commercially viable. In this study, thefocus was on nuclear fuel for ssion reactors, hence the literature review was limited to this205A.2. Nuclear Fueltopic. A nuclear ssion reactor functions by the controlled ssion of an atom’s nucleus, whichrelease heat, that is used to boil water and run steam through turbines attached to a generator.A.2.1. Nuclear Fuel IsotopesA brief understanding of the type of isotopes involved in nuclear fuel is necessary inorder to understand the wide variety of nuclear fuel material that are available [202].There are three major types isotopes that are relevant in nuclear ssion reactions:1. Fissionable isotopes: These isotopes undergo nuclear ssion, after absorbing eithera thermal (1eV) or fast (1MeV) nuetron. These isotopes include: 233U, 235U, 238U, 239Pu,240Pu, 241Pu, etc.2. Fissile isotopes: Fissile isotopes are a sub-group within ssionable isotopes whichcould ssion by absorbing just a thermal neutron. These isotopes include: 233U, 235U,239Pu, 240Pu, etc.3. Fertile isotopes: Fertile isotopes are capable of producing ssionable isotopes uponabsorption of a neutron. These isotopes include: 238U, 232Th, etc.Currently, nearly all nuclear reactors worldwide use an uranium-based nuclear fuel.Based on the reactor conditions, a certain composition of 235U is required for the nuclear fuel.Natural Uranium consists of 99.27% 238U and 0.72% 235U. The activity of Natural Uranium is lowto sustain chain reaction for certain reactor designs. Hence, the concentration of 235U wouldbe increased by enrichment [203]. For example GEN III Pressurized Water Reactor (PWR) andBoiling Water Reactor (BWR) need an enrichment of 3.5% and 2.5%, respectively. Accordingto the world nuclear association, the CANDU and Magnox reactors could use natural uraniumas fuel without any enrichment [202].206A.2. Nuclear FuelThorium based fuels on the other hand is mostly used in research reactors andrarely used commercially. Thorium has signicant advantages over uranium, such asnon-proliferation, longer life, increased safety, etc., despite which thorium was rarely usedand researched for nuclear energy [204][205][206][207]. India has made signicant progressin usage of Thorium through its fast breeder reactor program, that is used to convert fertile232Th into ssile 233Th by using neutrons [208][209][210]. It was found from bothexperiments and calculations that the CANDU reactors can support natural Thoriumdioxide as fuel as well [211][212][213][214].There are two reasons why ThO2 has higher accident tolerance than UO2:1. Melting point: A fuel meltdown is a highly undesirable outcome in a nuclear reactor.Hence, having a fuel material with a higher melting point is directly correlated to safety.For example, ThO2 has a higher melting point than UO2, hence ThO2 is considered saferin case of meltdown events.2. Thermal conductivity: UO2 has a low thermal conductivity at high temperaturescompared to ThO2. Hence, the core of the pellet would be at a higher temperaturethan the periphery of the fuel pellet. In the case of a LoCA, this would cause the coreof the fuel pellet to meltdown before the periphery of the fuel pellet. Improving thethermal conductivity of the fuel is indirectly increasing the accident tolerance ofnuclear fuel.A.2.2. Fabrication of Nuclear Fuel PelletsApart from a very few research reactors, most reactors use UO2 as a fuel, hence thissection focuses on its processing [203]. Uranium is extracted from uranium ore in the formof U3O8 or yellow cake. This is chemically converted to UF6, which is then enriched in a207A.3. Additional Literature on ECAS of Nuclear Fuel Materialscentrifuge to the requirement. Once enriched, the UF6 is converted back to U3O8. The U3O8 isreduced to UO2 by using Hydrogen gas. The UO2 powder is then pressed into a green pellet,by using a binder such as polyvinyl alcohol. This green pellet is then placed in an oven andbaked at high temperature (1650◦C) for a long duration (24h). The nal RD of pellet is around94%.A.3. Additional Literature on ECAS of Nuclear FuelMaterialsThe literature review on ECAS of nuclear fuel 2.2 was limited to UO2 and ThO2 to limitthe scope of literature review chapter. Additional literature on alternative fuel materials suchas Uranium Carbide (UC), Uranium Silicides (USix), Uranium Nitride (UN) and CERMETs isincluded in the following sections.A.3.1. SPS of Uranium NitrideThe rst ever published study on SPS processing of nuclear fuel materials was onUranium nitride (UN) in 2009 by Muta et al. [215]. UN was studied for its viability as a fuelmaterial for Fast Breeder Reactors (FBR) and Accelerator Driven Sub-critical Reactors(ADSR). UN powder is pyrophoric and tends to oxidise in normal atmospheric composition,hence conventional sintering (CS) of UN is challenging and requires pre-processing ofpowders in form of ball milling or addition of sintering additives. Additionally, americiumnitride (AmN) which is often added to enhance fuel properties, could decompose duringconventional sintering. Muta et al. used three di erent fuel materials (UN, UN-60 mol%TiNand Ti0.4U0.6N) prepared by ball milling followed by SPS. The SPS processed pellets were208A.3. Additional Literature on ECAS of Nuclear Fuel Materialssubjected to mechanical and thermophysical property evaluation. Upon comparison toexisting literature on conventionally sintered materials, the SPS pellets had a higher density,reduced grain growth, higher Vickers hardness and higher thermal conductivity (refer toFigure A.6). The higher thermal conductivity was attributed to the absence of impurities atthe grain boundaries, which could increase phonon scattering. The authors credited thesurface cleaning abilities of the SPS process to be the reason behind the absence ofimpurities at the grain boundaries.Figure A.6: Thermal conductivity of nuclear fuel materials prepared by Muta et al. (UN,UN-60 mol%TiN and Zr0.6Ti0.4N) using SPS compared to previous literature [215] (Reprintedfrom Journal of Nuclear Materials, vol. 389, H. Muta, K. Kurosaki, M. Uno and S. Yamanaka,“Thermophysical Properties of Several Nitrides Prepared by Spark Plasma Sintering,” , pp186-190, © (2009), with permission from Elsevier).Malkki et al. utilised SPS to sinter UN that was produced by hydriding uranium metaland then reacting the UH3 with nitrogen gas [216]. The authors claim that the carbothermicnitriding created aky powder with poor sinterablility compared to the proposed process209A.3. Additional Literature on ECAS of Nuclear Fuel Materialswhich produced a ne powder. The authors reported RD value around 99%. However, around9.2% of the microstructure consisted of secondary inclusions. Most of these inclusions wereof uranium sequestrinitride phase (U2N3), which was an intermediate product of the nitridingprocess. A more detailed version of this work was published as a licentiate thesis by Malkki[217]. In addition to the results from the UN study, the thesis also showcased results fromSPS of (U,Zr)N and ZrN. The solid solutions (U0.7Zr0.3N and U0.85Zr0.15N) were prepared bytwo routes: ball milling followed by SPS and post sintering heat treatment or sintering of apowder prepared by nitriding uranium-zirconium alloys.A Swedish research group under the supervision of Dr. Janne Wallenius and Dr. MikaelJolkkonen studied the processing and performance of uranium nitride and silicide fuels atKTH Stockholm. A patent application was led earlier in 2011 by Wallenius et al. on nitridefuels prepared by SPS [218]. The patent covered processing of (U,Pu,Am)N, (U,Pu,Am,Cm)N,(U,Pu,Am,Zr)N and (U,Pu,Am,Cm,Zr)N by SPS. It had also proposed an additionalpost-sintering heat treatment for elemental homogenisation in case of mixed nitrides.Johnson et al., from the Swedish research group, studied the nature of porosity developedduring SPS and its relation to sintering parameters while sintering UN by SPS [219]. Thepellet sintered at 1600◦C and 134 MPa had a density of 99.8%. A comparison of open porosityagainst total porosity showed that below 3% porosity all pores become closed. According tothe EDS chemical analysis results, trace impurities of Ce, La, Fe, Si and F were found at thegrain boundaries. The source of these impurities was the raw material used in this study. Theauthors also found that some of the Si impurity dissolved into the UN phase to form smallamounts (2.5 vol%) of U(N,Si) phase.In a follow up article, Johnson et al. studied grain growth in SPS processed UN usingElectron Back Scatter Di raction (EBSD) [164]. The primary objective of this study was tocorrelate the e ect of SPS parameters to the nal grain size of the sintered pellets. In general,210A.3. Additional Literature on ECAS of Nuclear Fuel Materialsgrain growth was observed to happen after densication. It was found that grain growthcould be kept to a minimum without a ecting RD by using higher pressure and lower sinteringtemperature. Additionally, it was also found that the grain growth proceeds by the dissolutionof low angle grain boundaries [164].Johnson et al. studied composites of UN and 5 vol% to 25 vol% Uranium silicide (U3Si2)sintered using SPS, with a goal of improving the sinterability of UN by incorporating U3Si2as a liquid phase sintering aid. U3Si2 was prepared by arc melting U and Si followed bymanual crushing [118]. Two di erent methods of blending UN-U3Si2 were studied: manualmixing and ball milling. Manual mixing resulted in a porous, heterogeneous as-sinteredmicrostructure along with the formation of U3Si5 around U3Si2 particles. In comparison, theball milled powder had a dense and homogeneous structure with U3Si2 phase only connedto the particle interfaces as intended in the primary objective of this research. A detailedversion of this study was documented by Raftery in a thesis [220]. In addition to the resultsdiscussed above, a study was carried out to examine the impact of sintering temperature onthe microstructure of UN-10 wt% U3Si2. Figure A.7a shows a typical result, with amicrostructure showing UN grains encapsulated by a U3Si2 phase. A ternary Si-rich phasewas also reported here, the volume fraction of which increased with increasing sinteringtemperature.211A.3. Additional Literature on ECAS of Nuclear Fuel Materialsa bFigure A.7: a) SE image of UN-10 wt% U3Si2 showing UN in encapsulated in U3Si2 phase(Reprinted from A. Raftery, (2015) "Fabrication and Characterization of UN-USix NuclearFuel". Master thesis, KTH Stockholm, pp 37, © by permission from author) [220] and b)Oxidation mechanism of UN (inter-granular) and UN-10 wt% U3Si2 [62] (Reprinted from K.Johnson, (2015) "High Performance Fuels for Water-Cooled Reactor Systems". Ph.D thesis,KTH Stockholm, pp 81 and 84, © by permission from author).A third article by Johnson et al. focused on the oxidation resistance of SPS-ed UN andUN-10 wt% U3Si2 [221]. The primary motivation of the authors was to study the behaviorof these materials in a high temperature steam environment; however, the preliminary studywas carried out in air. The materials were subjected to air at a constant heating rate (5◦C/mintill 800◦C) in a thermogravimetric analyser (TGA). In addition to these SPS-ed pellets, samplesof commercial UO2 pellet and arc melted U3Si2 and U3Si5 were also subjected to air oxidation.The mass increase during oxidation decreased in the order: UO2, UN, UN-10 wt% U3Si2, U3Si2and U3Si5. An analysis of mass increase curves as a function of temperature, showed an onsettemperature for oxidation. The oxidation onset temperature of UN was increased from 440◦Cto 450◦C due to the 10wt% U3Si2 addition. Hence, the study revealed a slight increase inoxidation resistance for UN-10 wt% U3Si2 compared to UN.From the same research group, Lopes et al. followed up this work with a degradation studyof the same SPS-ed material (UN and UN-10 wt% U3Si2) materials in extreme steam (5 MPa and300◦C) environment [222]. The results (refer Figure A.7b) showed that UN oxidised along thegrain boundaries, causing inter-granular cracks, which further accelerated oxidation. The net212A.3. Additional Literature on ECAS of Nuclear Fuel Materialsmass increase increased with decreasing RD and increasing grain size, showing that a highdensity pellet with ne grains might be better suited for this application. Microstructuralexamination showed UN-10 wt%U3Si2 cracks intra-granular manner unlike UN, which wascredited to the presence of U3Si2 phase at the grain boundaries. Johnson published a doctoralthesis including the results from the above ve research articles [219]. Two patents were ledby Jolkonen et al. on high RD 99.8% UN pellets, which are resistant to oxidation in steamatmosphere. [218][223].Jolkkonen et al. studied the behavior of SPS-ed UN in superheated steam [224]. Thisstudy proposed a hydrolysis reaction between UN and super-heated steam to form UO2,ammonia and hydrogen gas. The SPS-ed pellets of varying RD (78%, 87%, 96% and 98%) wereexposed to superheated steam/Argon in the temperature range of 400◦C-425◦C. Thehydrogen and nitrogen evolution from the hydrolysis experiment were monitored using anonline mass spectrometer. The results showed that porosity of the pellets increased theirreactivity, making them fragment due to gas buildup in the interior of the pellet. The XRDscan from post-hydrolysis pellet showed just a UO2 phase with slight nitrogen content (0.1wt%). The authors claim that the high reactivity of UN to steam under these conditionscould be exploited to extract 15N from spent UN fuel (in form of ammonia) for reuse.A.3.2. SPS of Uranium Carbide (UC)UC has been considered as a potential fuel material in fast breeder reactors, due to itsfavourable thermophysical properties. Salvato et al. used SPS for the synthesis of UC. Thegoal of their study was to use SPS to heat-treat a powder mixture of UO2 and C for acarbothermic reduction (CTR) process to synthesise UC. A modied SPS die setup was used,as seen in Figure A.8b, to allow degassing and avoid pressing of the powder during CTR.The authors claim that using SPS rather than a conventional furnace would allow for the213A.3. Additional Literature on ECAS of Nuclear Fuel Materialsapplication of higher heating rates (200◦C in this case), hence avoiding grain growth andsintering of fresh UC. The retention of nanocrystallinity could be benecial for thesinterability of these powders as pointed out in the previous studies.Figure A.8: Salvato et al.’s die setup for CTR synthesis of UC (Reprinted from CeramicsInternational, vol. 45, no. 6, D. Salvato, J. F. Vigier, O. Dieste Blanco, L. Martel, L. Luzzi, J.Somers and V. Tyrpekl, “Innovative Preparation Route for Uranium Carbide Using CitricAcid As A Carbon Source,” pp 1661-1675, © (2006), with permission from Elsevier) [225].The sintering was carried out at 1700◦C, 100 MPa and 10 minutes holding time to obtaina RD of 95%. The starting powder used for sintering had Q5% UO2 content which increasedslightly during sintering (Q7%). A higher carbide phase of chemical formula UC2 had alsoformed during sintering. It is suspected that this phase formed due to a reaction withunreacted carbon or graphite tooling.A.3.3. SPS of Uranium SilicidesLopes et al. studied the sintering of uranium silicide (U3Si2) and U3Si2-Mo compositesusing SPS for use as an ATF material. U3Si2 was found to have lower ssion product solubility,thus raising concerns for its application in LWRs. In order to study this behavior, the authors214A.3. Additional Literature on ECAS of Nuclear Fuel Materialssintered a blend of U3Si2 and 1.5 at% Molybdenum (Mo), which is a common ssion product.The as-sintered U3Si2 pellet had nanoscale porosity (refer to Figure A.9a), which resembledirradiation induced features in spent fuel. The authors believe these pores were the result ofthe multiple step processing route followed here (arc melting, crushing and SPS). As shown inFigure A.9, in the case of U3Si2-1.5 at%Mo, there was evidence of Mo segregation in the formof U2Mo3Si4 and U4Mo(MoxSi1-x)Si2 phases. Hence, as predicted by the authors, Mo showedpoor solubility in U3Si2, questioning its application as an ATF fuel material.a bFigure A.9: Lopes et al.’s observations from SPS of U3Si2 and U3Si2- 1.5 at% Mo: a) Nanoscaleporosities in U3Si2 resembling HBS and b) U-Mo-Si intermetallics in U3Si2-1.5 at% Mo(Reprinted from Journal of Nuclear Materials, vol. 496, no. 6, D. A. Lopes, A. Benarosch, S.Middleburgh and K. D. Johnson, “Spark Plasma Sintering and Microstructural Analysis ofPure and Mo Doped U3Si2 Pellets,” pp 234-271, © (2017), with permission from Elsevier)[226].Mohamad et al. synthesised U3Si2 by arc melting followed by crushing and SPS processing[119]. The resultant pellet was subjected to dilatometry, high temperature-XRD, DSC andthermal di usivity measurement. The thermal conductivity of U3Si2 was found to increasewith increasing temperature making it a better fuel at high temperature (S250◦C) than UO2.The mechanical property evaluation found U3Si2 to have a better fracture toughness than UO2and UN, thus the authors concluded U3Si2 fuel might be more resistant to cracking. These two215A.3. Additional Literature on ECAS of Nuclear Fuel Materialsfeatures might make U3Si2 an inherently better ATF material than UO2 and UN.A.3.4. SPS of CERMET FuelsCERMET (CERamics in a METal matrix) nuclear fuel materials were studied widely forspace power generation applications such as Radioisotope Thermoelectric Generators (RTGs),Advanced Sterling Radioisotope Generators (ASRG), and General Purpose Heating Systems(GPHS) [227][60][228][229]. The metallic addition to the ceramic matrix results in a higherthermal conductivity that is required for this application. The concept of utilising SPS forthis application was introduced by O’Brien et al. in 2009 using a surrogate CERMET material(CeO2-tungsten (W)) [57]. Later on in 2013, O’Brien et al. fabricated UO2-50 vol% W-25Re(tungsten with 25 at%Re) CERMET pellet by SPS at 1500◦C with a RD of ∼98%. A surfacelayer of WC was found to have formed as a result of reaction between graphite and tungsten.As seen in Figure A.10, EDS mapping of pellets showed good dispersion of UO2 particles in aW-25Re matrix. An increase in Vickers hardness was observed for the metallic phase in caseof the SPS pellets compared to previous studies. The authors claim that this might be due tothe presence of ceramic phases or due to the dissolution of Re during regular processing ofthis material. UO2-Mo was sintered.216A.3. Additional Literature on ECAS of Nuclear Fuel MaterialsFigure A.10: Elemental mapping of UO2-50 vo% W-25Re material for space nuclear fuelapplications (Reprinted from Journal of Nuclear Materials, vol. 433, R. C. O’Brien and N. D.Jerred, “Spark Plasma Sintering of W–UO2 Cermets,” , pp 50-54, © (2013), with permissionfrom Elsevier) [230].In a similar study, Tucker et al. demonstrated that SPS could be used for sintering UO2-40vol% W CERMET material [231][232]. In this study by O’Brien, the SPS-ed UO2-40 vol% Wfuel material was considered for Nuclear Thermal Propulsion (NTP) applications. Unlike theprevious study, the addition of 0.5 wt% polyethylene powder was not required prior to mixingin order to obtain a uniform distribution of UO2. Also, in this study the metallic addition waspure W metal without any Re. Pellets sintered at 1600◦C, 1700◦C, 1750◦C, 1800◦C and 1850◦Chad high RDs increasing progressively (S0.97), with the highest level being 99.5%. The as-sintered microstructure revealed a good distribution of UO2 particles in the W matrix.Tucker et al. published a follow-up study with an advanced characterisation (TransmissionElectron Microscopy (TEM) and Local Electrode Atom Probe (LEAP)) carried out on the UO2-W CERMETs [233]. A new phase (U0.1WO3) was detected at the boundary between UO2 and Wfrom TEM of sample SPS-ed at 1600◦C. The LEAP analysis showed that there was an uneven217A.3. Additional Literature on ECAS of Nuclear Fuel Materialsdistribution of oxygen vacancies causing U and O to exist in the form of UO, UO2 and UO3.A.3.5. EDC and FS of Nuclear Fuel MaterialsFlash Sintering (FS) is a relatively modern technique with the rst publication in 2010by Cologna et al. on ultra rapid sintering of Yttria Stabilised Zirconia (YSZ) [234][235]. YSZwhich typically sinters at around 1300◦C using SPS, was sintered at 850◦C in under 5 secondsbyCologna et al. using FS. FS is typically carried out as a two stage process, where the rst stepinvolves heating the powder to a certain temperature until it is electrically conductive andthen an electric current is passed through it, which triggers a thermal runaway event called" ash phenomenon" [55][234][236]. One of the di erences between SPS and ash sinteringis that FS uses higher electric elds ∼100 V/cm compared to SPS <3 V/cm [166][237]. Aconference proceeding by Yurlova et al. was the rst non-SPS based ECAS technique fornuclear fuel material. UN was sintered by Electric Discharge Sintering (EDS) with 2 to 3 kVunder 200 MPa. The most dense pellet had a RD of 0.96 [238].McCellan et al. sintered UO2 by ash sintering for the rst time, at 600◦C to obtain apellet of 94% RD. Figure A.8 shows displacement curves reported by Valdez et al., showingthe di erence in displacement rates for ash sintering, SPS and conventional sintering. Itwas also evident that ash sintering could be accomplished at a lower temperature (600◦C)than SPS (1000◦C).218A.3. Additional Literature on ECAS of Nuclear Fuel Materialsa bFigure A.11: Impact of electric eld on densication of: a) UO2.16 and b) UO2.00 (Reprintedfrom Journal of Nuclear Materials, vol. 505, J. A. Valdez, D. D. Byler, E. Kardoulaki, J. S.C. Francis, K. J. McClellan, “Flash Sintering of Stoichiometric and Hyper-stoichiometricUrania,” pp 85-93, © (2018), with permission from Elsevier) [239] .Flash sintering of UO2+x (x=2.00, 2.08 and 2.16) was modeled and experimented by Rafteryet al. [240]. The experiments were conducted inside a dilatometer with two external platinumleads that applied the electric eld. The experimental and modelling works concluded thatan increase in O/M ratio lowered temperature and electric eld required for ash event. Thiswas explained by the increase in electrical conductivity due to hyperstoichiometry of UO2+x,which caused an early onset of thermal runaway event.The same experimental setup was used by Valdez et al. for ash sintering of UO2+x (x=0.0and 0.16) [239]. Cold pressed pellets were heated to 600◦C, when an electric eld of 188 V/cmwas applied for 2 to 3 minutes. XRD scans from UO2.16 powder showed peaks of U3O8 inaddition to UO2 uorite; however, after sintering, only the UO2 phase was detected. Theas-sintered pellets had an inhomogenous structure with microcracks and varying regions of219A.3. Additional Literature on ECAS of Nuclear Fuel Materialsdensity, which was suspected to be caused by varying di erential heat transfer gradients.A.3.6. Application of SPS for Fuel Waste ManagementPopa et al. studied the SPS of cheralite (CaTh(PO4)2) as for its potential application innuclear waste storage [112]. CaTh(PO4)2 is a temperature sensitive material that undergoesdecomposition at high temperature (S1270◦C) to form ThO2, hence SPS was used to decreaseprocessing time. The sintering was carried at a high heating rate of 200◦C/min to 1310◦C with1 minute dwelling time. The as-sintered material had phase fraction of 99% CaTh(PO4)2 andremainder ThO2. SPS at 1255◦C and zero holding time resulted in no ThO2 formation witha RD of 0.96. These results suggest that SPS could be used for sintering CaTh(PO4)2 withoutany phase transformation for nuclear waste management applications.A.3.7. Post Irradiation Examination of SPS-ed FuelCappia et al. carried out Post Irradiation Examination (PIE) of irradiated SPS-ed UO2,UO2-10 vol% SiC and UO2-10 vol% diamond [91]. Each fuel type was assembled into separaterodlets for irradiation study. The burnup values for the UO2 and UO2-diamond was the same at7.2 GWd/tHM, whereas it was 4.9 GWd/tHM for UO2-SiC. The ssion gas release was lowerfor UO2-10 vol% SiC and UO2-10 vol% diamond at 0.63% and 1.08% respectively, comparedto 2.24% for UO2. Gamma spectrometry along the rods revealed irregular spikes for UO2-diamond, which was related to Cs137 migration. Neutron radiography and ceramography(refer to Figure A.12) revealed disruptions in fuel structure, that was claimed to be a resultof thermal stress imposed by CTE mismatch between the additive (SiC and diamond) and theUO2 matrix. Based on these results, the authors suggested that these fuel concepts may notbe optimal for further studies.220A.3. Additional Literature on ECAS of Nuclear Fuel Materialsa b cFigure A.12: Post irradiation ceramography of SPS-ed: a) UO2 b) UO2-10 vol% SiC c) UO2-10vol% diamond (Reprinted from Journal of Nuclear Materials, vol. 517, F. Cappia, J. M. Harp,and K. McCoy, “Post-irradiation Examinations of UO2 Composites As Part of The AccidentTolerant Fuels Campaign,” pp 97-105, © (2019), with permission from Elsevier) [91].A.3.8. Summary of ECAS of Nuclear Fuel MaterialsThe literature review of ECAS of nuclear fuel materials has been summarised in FigureA.1.221A.3.AdditionalLiteratureonECASofNuclearFuelMaterialsTable A.1: Summary of materials and processing conditions for ECAS processing of nuclear materials.Material Reference Method Composition Temperature(◦C)Pressure(MPa)Dwelltime(mins)RD(%)UNpureMuta et al. J. Nuc. Mat.,389 2009 [215]SPSpure UN 1500 100 10 90Malkki et al. J. Nuc. Mat.,452 2014 [216] pure UN 1650 50 3 99Malkki, Pertti Licentiatethesis., KTH Stockholm,2015 [217]pure UN 1450, 1550& 1650 50 3 & 1065 -90Johnson et al. J. of Nuc.Mat. 473 2016 [219] pure UN 1450 & 1600 11-134 -68 -99.8Johnson et al. J. of Nuc.Mat. 503 2018 [164] pure UN 1450 & 165045, 90 &135 2-1591-99.9Yurlowa et al. Phy.Procedia 72 2016 [238] EDC pure UN - 160-210 -82 -95TiN Muta et al. J. Nuc. Mat.,389 2009 [215]SPSUN-60 mol% TiN 1600 100 19 96ZrNZr0.6U0.4N 1600 100 19 96Malkki, Pertti Licentiatethesis., KTH Stockholm,2015 [217]Zr0.3U0.7N 1600 50 2-1590 -94Zr0.15U0.85N 1600 50 2-1588-91222A.3.AdditionalLiteratureonECASofNuclearFuelMaterialsContinuation of Table A.1Material Reference Method Composition Temperature(◦C)Pressure(MPa)Dwelltime(mins)RD(%)U3Si2Johnson et al. J. of Nuc.Mat. 477 2016 [118] UN- 5 wt% U3Si2 1450 135 -13.92g/cm3UN U3Si2Johnson et al. J. of Nuc.Mat. 477 2016 [118]SPSUN- 10 wt% U3Si2 1450 135 -13.66-13.72g/cm3UN- 20 wt% U3Si2 1450 135 -13.72g/cm3UN- 25 wt% U3Si2 1450 135 -13.82g/cm3UO2 pureGe et al. J. Nuc. Mat., 4352013 [66] pure urania 850-1525 40 0.5-2078 -98Chen et al. J. Nuc. Mat.,454 2014 [241] pure urania 850 36 2 90Ge et al., J. of Eur. CerSoc., 34, 2014 pure UO2.16750, 850,1050, 1150,1350 & 145040 0.5, 5, 10& 2078 -97Tyrpekl et al. J. Nuc.Mat., 466 2015 [77]pure UO2.11 1000 70 5 98UO2.03 1000 70 5 93Cologna et al. Cer. Int., 422016 [65] UO2.2 1000 & 1600 70 5 97223A.3.AdditionalLiteratureonECASofNuclearFuelMaterialsContinuation of Table A.1Material Reference Method Composition Temperature(◦C)Pressure(MPa)Dwelltime(mins)RD(%)UO2 pureYao et al. J. Nuc. Mat.,469 2016 [242]SPSpure urania 1500 40 5 98Yao et al. Sci. Rep., 8(2987) 2016 [243] pure UO2+x 1150 40 5-20 98Ironman et al. Nuc. Tec.,200 2017 [20] pure urania 1050 40 5 ∼97Li et al. J. Scr. Mat., 1422018 [28] pure urania 1200 25 5 96Li et al. Cer. Int., 44 2018[94] pure urania 620-1500 25 1076 -98Papynov et al.Radiochemistry, 60 2018[69][70]pure urania 1250, 1450& 1650 24.5 5 ∼98pure urania 1100 141 5 ∼98Yao et al. J. Amer.Cer. Soc., 101 2018[73][78][64][82]pure UO2.03 104 nm, 700 750 5 96pure UO2.03 144 nm, 700 750 20 99pure UO2.11 66 nm, 600 750 20 95pure UO2.13 166 nm, 700 750 20 98224A.3.AdditionalLiteratureonECASofNuclearFuelMaterialsContinuation of Table A.1Material Reference Method Composition Temperature(◦C)Pressure(MPa)Dwelltime(mins)RD(%)UO2 pureTyrpekl et al. J. Amer.Cer. Soc., 100 2017 [72]SPSpure UO2 11 nm, 800-900 750 20 ∼90Tyrpekl et al. Sci. Rep., 7(46625) 2017 [68] pure UO2.16 100 nm,600, 800 &1000 70 0-60Gong et al. J. of Nuc. Mat.,2019 [80]pure UO2.16 7.2 µm 1600 40 5 96pure UO2.16 1.8 µm 1300 40 30 96pure UO2.16 125 nm 700 500 5 97McCellan K. EFASintering Proceedings,2016 [244]FSpure UO2.16 600 - - 94Raftery et al. J. Nuc. Mat.,493 2017 [240]pure UO2.0, UO2.08& UO2.16600 - - -Sprouster et al.Materialia., 2 2018[237]pure UO2.0, UO2.12& UO2.23250-700 - - -Valdez et al. J. of Nuc.Mat., 505 2018 [239]pure UO2.00 600 - ∼3 81pure UO2.16 600 - ∼2 91U3O8Gong et al. MRS Comm.,8 2018 [71]SPSUO2.11- ∼ 6 mol%U3O81600 30 5 ∼95225A.3.AdditionalLiteratureonECASofNuclearFuelMaterialsContinuation of Table A.1Material Reference Method Composition Temperature(◦C)Pressure(MPa)Dwelltime(mins)RD(%)UO2 SiCYeo et al. Mater. Res. Soc.Sym. Proc., 1444 2012[86]SPSUO2-10 vol% SiC 1200-1600 40 582 -97Yeo et al. J. Nuc. Mat., 4422013 [89]UO2-5 vol% 0.6 µmSiC 1350 36 5 95UO2-5 vol% 1 µmSiC 1350 36 5 95UO2-5 vol% 1 µmSiC 1450 36 5 97UO2-10 vol% 1 µmSiC 1450 36 5 97UO2-15 vol% 1 µmSiC 1450 36 5 95UO2-20 vol% 1 µmSiC 1450 36 5 94UO2-5 vol% 9 µmSiC 1450 36 5 95UO2-5 vol% 16.9µm SiC 1450 36 5 95226A.3.AdditionalLiteratureonECASofNuclearFuelMaterialsContinuation of Table A.1Material Reference Method Composition Temperature(◦C)Pressure(MPa)Dwelltime(mins)RD(%)Yeo et al. J. Nuc. Mat.,442 2013 [89]UO2-5 vol% 55 µmSiC 1450 36 5 95UO2SiCYeo et al. J. Nuc. Mat., 4332013 [92]SPSUO2-10 vol% SiCwhisker 1200-1600 40 582-97UO2-10 vol% SiCpowder 1200-1600 40 582-96Chen et al. J. Nuc. Mat.,454 2014 [241]UO2-5 vol% SiCpowder 850 36 2 70Li et al. Cer. Int., 44 2018[94] UO2-10 vol% SiC 640-1500 25 1045-98DiamondYeo, Sungwan, PhDthesis, Univ. of Florida,2013 [90]UO2-10 vol%diamond 1400-1500 36 591-96Chen et al., Nuc. Eng. andDes., 294, 2015 [97]UO2-5 vol% 0.25µm diamond 1300-1600 36 591-95UO2-5 vol% 3 µmdiamond 1300-1600 36 594-97227A.3.AdditionalLiteratureonECASofNuclearFuelMaterialsContinuation of Table A.1Material Reference Method Composition Temperature(◦C)Pressure(MPa)Dwelltime(mins)RD(%)UO2Diamond Chen et al., Nuc. Eng. andDes., 294, 2015 [97]SPSUO2-5 vol% 12 µmdiamond 1300-1600 36 595-98UO2-5 vol% 25 µmdiamond 1300-1600 36 595-98W-25ReO’Brien et al., J. of Nuc.Mat., 433 2013 [230]UO2-50 vol%W-25Re 1500 37.5 20 98WTucker et al., J. ofNuc. Mat., 486 2017[232][233][231]UO2-40 vol% W 1600-1850 50 2097-99SWCNT Cartas et al., Trans.Amer. Nuc. Soc., 1092013[95][114]UO2-5,10vol%SWCNT 1300 40 5 95MWCNT UO2-5,10vol%MWCNT 1300 40 5 95GNP Yao et al. Sci. Rep., 8(2987) 2016 [243]UO2+x- 1-5 wt%GNP 1500-1600 40 5-2092-95ZrB2Ge, Lihao, Ph.D Thesis,Univ. of Florida, 2014[160]UO2-1.5 vol% ZrB2 1600 36 0.5 98Cr2O3UO2-0.1 wt% Cr2O3 1450 & 1600 36 0.5 & 20 98UO2-0.15 wt%Cr2O31450 & 1600 36 0.5 & 20 97228A.3.AdditionalLiteratureonECASofNuclearFuelMaterialsContinuation of Table A.1Material Reference Method Composition Temperature(◦C)Pressure(MPa)Dwelltime(mins)RD(%)UO2Cr2O3Ge, Lihao, Ph.D Thesis,Univ. of Florida,2014[160]SPSUO2-0.2 wt% Cr2O3 1450 & 1600 36 0.5 & 2096-97TiO2Yao et al. J. Nuc. Mat.,4692016 [242] UO2-0.5 wt% TiO21500, 1600& 1700 40 5 & 2097-98CsITyrpekl et al. J. Nuc. Mat.,466 2015 [77]UO2.11-1 mol% CsI 1000 70 5 95UO2.11-2.5 mol%CsI 1000 70 5 99UO2.03-1 mol% CsI 1000 70 5 91UO2.03-2.5 mol%CsI 1000 70 5 96BeO Li et al. J. Scr. Mat., 1422018 [28] UO2- 10 vol% 1400 25 10 95Ti3SiC2Li et al. Cer. Int., 44 2018[103]UO2-10 vol%Ti3SiC21400 50 1 98U3Si2pureMohamed et al. J. Nuc.Sci. Tech., 55 2018 [119] pure U3Si2 850 75 10 96Lopes et al. J. Nuc. Mat.,496 2017 [226] pure U3Si2 1000-1300 50 3-1592-99229A.3.AdditionalLiteratureonECASofNuclearFuelMaterialsContinuation of Table A.1Material Reference Method Composition Temperature(◦C)Pressure(MPa)Dwelltime(mins)RD(%)UO2 MoLopes et al. J. Nuc. Mat.,496 2017 [226]SPSU3Si2-1.5 at% Mo 1200 50 6 99UC UO2Salvato et al. Cer. Int., 432017 [245]UC-3 wt% UO2 1700 100 1088-95UC-5 wt% UO2 1700 100 10 68ThO2 pureMuta et al., Jour. Nuc.Mat., 434 2012[105] pure ThO2 1600 50 - 94Scott et al., J. of Nuc. Mat.,485 2017 [145] pure ThO2 1500-1700 40 587-94Tyrpekl et al., J. of Eur.Cer. Soc., 36 2016 [108]pure ThO2 (13 nm) 1600 70 1095-1.00pure ThO2 (19 nm) 1600 70 1093-99pure ThO2 (34 nm) 1600 70 1094-96ThO2 (12h and 18hball milled) 1300-1700 40 588-98La2O3Scott et al., J. of Nuc. Mat.,485 2017 [145] Th0.91La0.09O1.955 1300-1700 40 589-94230A.3.AdditionalLiteratureonECASofNuclearFuelMaterialsContinuation of Table A.1Material Reference Method Composition Temperature(◦C)Pressure(MPa)Dwelltime(mins)RD(%)ThO2La2O3Scott et al., J. of Nuc. Mat.,485 2017 [145]SPSTh0.77La0.27O1.885 1500-1700 40 593-96Muta et al., Jour. Nuc.Mat., 434 2012[105]Th0.95La0.05O1.975 1600 50 - 93Th0.90La0.1O1.95 1600 50 - 90Y2O3Th0.95Y0.05O1.975 1600 50 - 98Th0.90Y0.1O1.95 1600 50 - 98Nd2O3Th0.95Nd0.05O1.975 1600 50 - 94Th0.90Nd0.1O1.95 1600 50 - 100CeO2Th0.95Ce0.05O2 1600 50 - 96Th0.90Ce0.1O2 1600 50 - 95Muta et al., Jour. Nuc.Mat., 483 2017[109]Th0.8Ce0.2O2.00 1600 100 10 94Th0.8Ce0.2O1.97 1600 100 10 96Th0.8Ce0.2O1.91 1600 100 10 97GdO2 Muta et al., Jour. Nuc.Mat., 434 2012[105]Th0.95Gd0.05O2 1600 50 - 95Th0.90Gd0.1O2 1600 50 - 95231A.3.AdditionalLiteratureonECASofNuclearFuelMaterialsContinuation of Table A.1Material Reference Method Composition Temperature(◦C)Pressure(MPa)Dwelltime(mins)RD(%)ThO2 UO2Muta et al., Jour. Nuc.Mat., 434 2012[105]SPSTh0.95U0.05O2 1600 50 - 94Th0.90U0.1O2 1600 50 - 92Th0.90U0.1O2 1600 50 - 92Muta et al., Jour. ofNuc. Sci. and Tech., 502013[106]Th1-xUxO2 x=0, 0.1,0.2,.....,1.0 1600 50 1090-95Balice et al., Jour. of Nuc.Mat., 498 2018[111]Th0.50.5UO2 1300 & 1700 68 2 96Th0.850.15UO21300, 1540& 1700 68 296-98Saoudi et al., Jour. of Nuc.Mat., 500 2013[107]UO2- 79 wt% ThO2 1300 35 5 96UO2- 93 wt% ThO2 1300 35 5 95CaTh(PO4)2 Popa et al., J. of Eur. Cer.Soc., 36 2016 [112] pure 1255-1310 70-106 0-195-99232Appendix BB.1. SPS Tooling Preparation ProcedureThe procedure listed below was followed sequentially for all the SPS experimentscarried out during the course of this research:1. Step 1: A graphite die was prepared by placing a 0.005" thick grafoil sheet in the innerdiameter of the die, as shown in Figure B.1. The grafoil was added to reduce the frictionbetween the die and powder during the SPS process. Additionally, it also preventedreactions between the powder and the graphite tooling during SPS.a bc dFigure B.1: Tooling components used for SPS process: a) Graphite die, b) Grafoil lining ofthe die, c) Grafoil disks and d) Graphite punches (20 mm and 12.5 mm) (Reprinted fromA. Prasad, (2017) "Spark Plasma Sintering of Cerium Dioxide and Its Composites". MAScthesis, University of British Columbia, pp 26, © by permission from author) [136].233B.1. SPS Tooling Preparation Procedure2. Step 2: Once the grafoil was placed on the inner surface of the die, the bottom punchwas inserted. This was followed by the addition of a grafoil disk in order to protect thebottom punch surface. The disc acted as a barrier between the powder and the punch.Figure B.1c,d show the grafoil disks and graphite punches that were used.3. Step 3: The die was placed inside an analytical balance (Fisher Scientic®) then lledwith powder, as shown in Figure B.2a . This enabled accurate die ll. A completelylled die set-up is shown in Figure B.2b. This step was carried out in a static glove-boxto avoid the spread of radiotoxic contamination.234B.1. SPS Tooling Preparation Procedurea bcFigure B.2: Stages of SPS die preparation: a) Die ready for lling powder inside theweighing balance, b) Fully prepared tooling for SPS and c) Static glovebox used forpreparing tooling for SPS (Reprinted from A. Prasad, (2017) "Spark Plasma Sintering ofCerium Dioxide and Its Composites". MASc thesis, University of British Columbia, pp 27, ©by permission from author) [136].4. Step 4: Once the die was lled with the powder, the top punch was inserted into the235B.1. SPS Tooling Preparation Proceduredie with a grafoil disk in between the punch and the powder. The completely lleddie was then placed inside the SPS chamber with a carbon sleeve around it, which wasused to reduce the radiation heat loss. Graphite spacers were used to act as an interfacebetween the stainless steel rams and the punches, as shown in Figure 3.3a.5. Step 5: The completely lled die was placed inside the sintering chamber, and then thepyrometer was aligned at a hole on the die surface, as seen in Figure B.3.a bFigure B.3: Focusing of pyrometer on the die surface: a) Not focused and b) Focused(Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of Cerium Dioxide and ItsComposites". MASc thesis, University of British Columbia, pp 27, © by permission fromauthor) [136].6. Step 6: Once the die was set up inside the chamber, the chamber was evacuated andlled with Ar gas, and the SPS process was started.B.1.0.1. SPS ProgramThe iTools® software was used to create a sintering recipe/program based on the desiredSPS parameters. Once the program was started, the sintering process ran uninterruptedlyuntil completion. Figure B.4 shows one of the programs from the iTools® software. Theprogram was completed in six steps:236B.1. SPS Tooling Preparation ProcedureFigure B.4: A screenshot of the iTools® software (the red box indicates the rst step of theprogram) (Reprinted from A. Prasad, (2017) "Spark Plasma Sintering of Cerium Dioxide andIts Composites". MASc thesis, University of British Columbia, pp 28, © by permission fromauthor) [136].1. Step 1: Ramp the temperature from room temperature to 600 ◦C at 100 ◦C/min heatingrate. Dwell the pressure at 5 MPa.2. Step 2: Dwell at 600 ◦C for 15 seconds. Dwell at 5 MPa for the same duration. This stagewas intended to give the pyrometer enough time to catch up with the actual temperatureincrease.3. Step 3: Ramp the temperature to 1200 ◦C at 100 ◦C/min. Ramp the pressure to 50 MPaat 8 MPa/min.4. Step 4: Hold the temperature and pressure under these peak conditions for 5 minutes.5. Step 5: Ramp down temperature to 20 ◦C at 200 ◦C/min and ramp down pressure to5MPa at 20 MPa/min.6. Step 6: End the program and reset the controller.A detailed list of all the SPS conditions and materials studied in this dissertation can befound in Table B.2 in Appendix B. However, the conditions used for MSC experiments andthe rationale behind their choice are discussed in the subsection below.237B.2. Repeatability Test SPS ProgramB.2. Repeatability Test SPS ProgramThe program shown in Figure B.5, was used to test the repeatability and error in SPSdata between repetitions.Figure B.5: SPS program used for SPS-ing Al powder for repeat-ability test.The repeatability of data acquired during SPS was studied by performing two SPS trialsusing pure Aluminium powder (10.0 g each trial) using the same SPS program and tooling.The details of the SPS program can be found in Figure B.5 in Appendix B. The temperature,pressure, electric potential, current, power (e = k × I) and displacement obtained fromthese two experiments were plotted against each other, as shown in Figure B.6. In theory,if the experiments were mutually repeatable, the slope of these plots should be unity, anydeviation could be considered as experimental error.238B.2. Repeatability Test SPS Programa bc de fFigure B.6: Comparison of SPS data obtained from repeatability test: a) Temperature, b)Pressure, c) Electric potential, d) Current, e) Input power and f) Ram displacement.239B.3. Theoretical Density ChartThe standard deviation of the process values was calculated using in Equation B.1: =1NNXi5)p(∆ei)2 (B.1)Where, ∆ei is the di erence in time step i,N is the total number of time steps. The maximumdeviation (i.e., maximum value of ) from slope = 1 was observed for input power followed bycurrent, electric potential, displacement, temperature and pressure respectively. The variationin displacement of 0.4 mm could be due to variation in powder weight between sintering trialsthat were only made accurate to 0.1 g. The variation in temperature and pressure could be dueto least counts of thermocouple and load gauge used for recording temperature and pressure.The variation in input power, voltage and current could be due to perturbations in PID processcontrol. In summary, these experiments validated the high accuracy and repeatablility of theSPS system.B.3. Theoretical Density ChartThe theoretical densities of pure materials were obtained taken from literature, unlessspecied by the manufacturers. The theoretical density of composites was calculated using"rule of mixtures" relation, as seen in Equation B.2:/uompositw =/S/TlS/T +lT/S(B.2)Where /uompositw is the density of a composite containing A and B, /S is the density of of A,/T is the density of of B, lS is the weight of of A and lT is the weight of of B. Thetheoretical density of Th1-xUxO2 was also calculated using "rule of mixtures" relation asmentioned in Equation B.3:/fh1−xgxa2 = 10O00× (1− x) + 10O96× x (B.3)240B.3. Theoretical Density ChartTable B.1: Theoretical density of materials used in this dissertation [246]Material Theoretical density (g/cm3)ThO2 10.00SiC 3.20ThO2-1 wt% SiC 9.79ThO2-3 wt% SiC 9.40ThO2-5 wt% SiC 9.04ThO2-15 wt% SiC 7.58Diamond 3.51ThO2-3 wt% Diamond 10.96ThO2-5 wt% Diamond 9.15UO2 9.47Th0.95U0.05O2 10.05Th0.95U0.05O2 10.10241B.4. Consolidated List of SPS TrialsB.4. Consolidated List of SPS TrialsAll the SPS trials performed for this dissertation are summarized in Table B.2.Table B.2: Consolidated list of SPS experiments performed in this study.No. MaterialSintering conditions Density(g/cm3) RD (%)Temp.(◦C)Pressure(MPa)Dwelltime(min.)Heatingrate(◦C/min.)Coolingrate(◦C/min.)1 UO2 1300 60 0.5 50 50 10.82 98.72 UO2 1300 60 0.5 75 75 10.87 99.13 UO2 1300 60 0.5 100 100 10.83 98.84 UO2 1000 60 5 100 100 10.32 94.15 UO2 (IXAF) 1300 50 10 100 200 10.80 98.56 UO2 (IXAF) 1300 50 10 100 200 10.78 98.17 UO2 (IXAF) 1300 50 10 100 200 10.78 98.18 UO2 (IXAF) 1300 50 10 100 200 10.65 97.29 ThO2 1500 50 10 100 200 9.05 90.510 ThO2 1600 30 10 100 200 9.76 97.611 ThO2 1600 50 5 100 200 9.73 97.312 ThO2 1600 50 10 100 200 9.41 94.113 ThO2 1600 50 15 100 200 9.58 95.814 ThO2 1700 30 10 100 200 9.75 97.515 ThO2 1700 30 10 100 200 9.73 97.316 ThO2 1700 50 5 100 200 9.64 96.017 ThO2 1700 50 10 100 200 9.81 98.118 ThO2 1700 50 15 100 200 9.85 98.5242B.4. Consolidated List of SPS TrialsNo. MaterialSintering conditions Density(g/cm3) RD (%)Temp.(◦C)Pressure(MPa)Dwelltime(min.)Heatingrate(◦C/min.)Coolingrate(◦C/min.)19 ThO2 1700 70 10 100 200 9.97 99.720 ThO2 1800 50 10 100 200 9.78 97.821 ThO2 1500 60 5 100 200 9.45 94.522 ThO2 1500 60 5 150 200 9.23 92.323 ThO2 1500 60 5 150 200 9.34 93.424 ThO2 1500 60 0.5 50 200 9.57 95.725 ThO2 1800 60 0.5 50 50 9.44 94.426 ThO2 1800 60 0.5 75 75 9.55 95.527 ThO2 1800 60 0.5 100 100 9.36 93.628 ThO2 1800 60 0.5 125 125 9.70 97.029 ThO2 1800 60 5 75 75 9.60 96.030 ThO2 1800 20 0.5 50 50 9.52 95.231 ThO2 1800 20 0.5 75 75 9.79 97.932 ThO2 1800 20 0.5 100 100 9.35 93.533 ThO2 1800 40 0.5 50 50 9.27 92.734 ThO2 1800 40 0.5 75 75 9.33 93.335 ThO2 1800 40 0.5 50 50 9.69 96.936 ThO2 - 5 wt%40µm SiC 1800 60 0.5 50 50 8.64 95.637 ThO2 - 5 wt%40µm SiC 1800 60 0.5 75 75 8.56 94.738 ThO2 - 5 wt%40µm SiC 1800 60 0.5 100 100 8.78 97.2243B.4. Consolidated List of SPS TrialsNo. MaterialSintering conditions Density(g/cm3) RD (%)Temp.(◦C)Pressure(MPa)Dwelltime(min.)Heatingrate(◦C/min.)Coolingrate(◦C/min.)39 ThO2 - 5 wt%40µm SiC 1500 60 5 100 100 7.70 85.240 ThO2 - 5 wt% 1µm SiC 1800 60 0.5 50 50 8.92 98.741 ThO2 - 5 wt% 1µm SiC 1800 60 0.5 75 75 8.84 97.842 ThO2 - 5 wt% 1µm SiC 1800 60 0.5 100 100 8.94 99.043 ThO2 - 5 wt% 1µm SiC 1500 60 5 100 100 8.21 90.844 ThO2 - 3 wt% 1µm SiC 1800 60 0.5 50 50 9.03 96.645 ThO2 - 3 wt% 1µm SiC 1800 60 0.5 75 75 9.23 98.746 ThO2 - 3 wt% 1µm SiC 1800 60 0.5 100 100 9.06 96.947 ThO2 - 3 wt% 1µm SiC 1300 60 5 100 100 7.56 80.848 ThO2 - 3 wt%15 nm SiC 1800 60 0.5 50 50 8.73 92.949 ThO2 - 3 wt%15 nm SiC 1800 60 0.5 75 75 9.53 101.550 ThO2 - 3 wt%15 nm SiC 1800 60 0.5 100 100 9.05 96.351 ThO2 - 3 wt%15 nm SiC 1450 60 5 100 100 8.73 92.952 ThO2 - 1 wt%15 nm SiC 1800 60 0.5 50 50 9.49 97.0244B.4. Consolidated List of SPS TrialsNo. MaterialSintering conditions Density(g/cm3) RD (%)Temp.(◦C)Pressure(MPa)Dwelltime(min.)Heatingrate(◦C/min.)Coolingrate(◦C/min.)53 ThO2 - 1 wt%15 nm SiC 1800 60 0.5 75 75 9.52 97.254 ThO2 - 1 wt%15 nm SiC 1800 60 0.5 100 100 9.62 98.255 ThO2 - 1 wt%15 nm SiC 1500 60 5 100 100 9.05 92.556 ThO2 - 15 wt%15 nm SiC 1800 60 0.5 50 50 7.46 98.357 ThO2 - 15 wt%15 nm SiC 1800 60 0.5 75 75 7.30 96.258 ThO2 - 5 wt%Diamond 1800 60 0.5 50 50 8.16 89.259 ThO2 - 5 wt%Diamond 1800 60 0.5 75 75 8.12 88.760 ThO2 - 5 wt%Diamond 1800 60 0.5 100 100 8.21 89.761 ThO2 - 5 wt%Diamond 1500 60 5 100 100 8.39 91.762 ThO2 - 3 wt%Diamond 1800 60 0.5 100 100 8.57 89.763 ThO2 - 3 wt%Diamond 1800 60 0.5 100 100 8.78 88.764 ThO2 - 3 wt%Diamond 1800 60 0.5 50 50 8.79 89.265 ThO2 - 3 wt%Diamond 1500 60 5 100 100 8.10 85.566 Th0.90U0.10O2 1700 60 0.5 50 50 9.78 97.3245B.4. Consolidated List of SPS TrialsNo. MaterialSintering conditions Density(g/cm3) RD (%)Temp.(◦C)Pressure(MPa)Dwelltime(min.)Heatingrate(◦C/min.)Coolingrate(◦C/min.)67 Th0.90U0.10O2 1700 60 0.5 75 75 9.95 99.068 Th0.90U0.10O2 1300 60 0.5 100 100 9.93 98.869 Th0.90U0.10O2 1000 60 5 100 100 9.49 94.470 Th0.95U0.05O2 1700 60 0.5 50 50 9.86 97.771 Th0.95U0.05O2 1700 60 0.5 75 75 9.74 96.572 Th0.95U0.05O2 1300 60 0.5 100 100 10.03 99.473 Th0.95U0.05O2 1000 60 5 100 100 9.47 93.8246Appendix CC.1. Stereo Microscopy of SPS-ed ThO2a aFigure C.1: Stereomicrographs of SPS-ed (’1600◦C - 30 MPa - 10 min’) ThO2 pellet: a)Unetched, b) Etched.247C.1. Stereo Microscopy of SPS-ed ThO2a aFigure C.2: Stereomicrographs of SPS-ed (’1800◦C - 50 MPa - 10 min’) ThO2 pellet: a)Unetched, b) Etched.248C.2. Electron Back Scatter Di raction of SPS-ed ThO2C.2. Electron Back Scatter Di raction of SPS-ed ThO2a b c de f g hi j kFigure C.3: EBSD maps from pellets sintered at varying temperatures: a) ’1500◦C 50 MPa10 min’, b) ’1600◦C 50 MPa 10 min’, c) ’1700◦C - 50 MPa - 10 min’, d) ’1800◦C - 50 MPa - 10min’, e) ’1700◦C - 50 MPa - 5 min’, f) ’1700◦C - 50 MPa - 15 min’, g) ’1700◦C - 30 MPa - 10min’, h) ’1700◦C - 70 MPa - 10 min’, i) Grain size vs. peak sintering temperature, j) Grainsize vs. peak sintering pressure, i) Grain size vs. sintering holding time.249Appendix DD.1. Master Sintering Curve TheoryThe Master Sintering Curve (MSC) theory was developed by H.Su and L. Johnson. Thetheory enables the prediction of sintering behaviour based on experiments and the knowledgeof a minimal number of powder characteristics [247]. Powder characterisation is expensiveand requires sophisticated instruments, whereas sintering trials are faster and cheaper. Thisinspired Su et al. to develop a model capable of predicting the density of sintered materialsfrom data obtained during sintering trials. In a prior work, Johnson et al. developed thefollowing equation for linear shrinkage rate of a material during sintering [248][247].− dLLdt=Ωki(ΓvDvG++ΓtDttG4) (D.1)Where, L is the height of the sintered pellet, hence− vLLvtis the linear shrinkage rate, is thesurface energy, Ω is the atomic volume, i is the temperature, G is the grain diameter, Dv isthe coe cient of volume di usion, Dt is the coe cient of bulk di usion,  is the width ofgrain boundary, Γv and Γt are scaling parameters for volume and grain boundary di usionrespectively, and k is the Boltzmann’s constant. Despite Johnson’s model being simple,determination of the input constants required sophisticated experimentation [248][247].Su et al. devised a way to avoid using these constants to predict the as-sintered density.Their model relied on the data obtained while sintering, to create a MSC, that could be used topredict the density of sintered pellet for future sintering trials. The derivation of MSC modelwas as follows:250D.1. Master Sintering Curve Theory1. The linear shrinkage rate can written in terms of the density:− dLLdt=d/3/dt(D.2)where / is the instantaneous density of a material undergoing sintering.2. Equation D.1 and D.2 was combined to give Equation D.3. An assumption thatdensication kinetics are dominated by either volume di usion or grain boundarydi usion was made. It was also assumed that Γ, i.e di usion coe cient, G i.e grainsize, are dependent on only density during sintering. This assumption was justied byexperimental studies, but only for materials with density below 90% [249][250].d/3/dt=Ω(Γ(/))Dki (G(/))n(D.3)where f is the activation energy. When grain boundary di usion is dominant, D =Dt and n = 4. When volume di usion is dominant, D = Dv and n = 3.3. Equation D.3 can be modied to account for the Arrhenius relationship betweendi usion coe cient D and temperature i .d/3/dt=Ω(Γ(/))D(ki (G(/))nexp(−fgi) (D.4)4. Equation D.4 can be rearranged in the following manner to bring all the temperaturedependent terms to one side.k(G(/))nd/3D(Ω(Γ(/))/=1iexp(−fgi)dt (D.5)251D.1. Master Sintering Curve Theory5. Integrating Equation D.5 on both sides with the desnity and time limits /( to / and 0 tot, respectively:k3ΩD(Z 0(G(/))n(Γ(/))/d/ =Z t(1iexp(−fgi)dt (D.6)6. The Left Hand Side (LHS) and Right Hand Side (RHS) can be dened separately and theequation can be simplied as follows: (/) =k3ΩD(Z 0(G(/))n(Γ(/))/d/ (D.7)(tP i (t)) =Z t(1iexp(−fgi)dt (D.8) (/) = (tP i (t)) (D.9)Equation D.9 denes the thermal history of sintering and instantaneous Relative Density(RD) as shown in Figure D.1a. The / andi (t) data is collected during sintering, by tracking theposition of pistons during pressure sintering, whereas in case of pressure-less sintering, thedata can be collected using dilatometry. The only variable in the MSC construction process isf, which can be determined from / vs. (i (t)) from multiple sintering trials. Typically, this252D.1. Master Sintering Curve Theoryis done by carrying out sintering trials at varying heating rates, such as 8◦C/min, 15◦C/min,30◦C/min and 45◦C/min for alumina, as seen in Figure D.1a. The "best tting" f is found byplotting the average Mean Residual Squares (MRS) (Equation D.10) between the MSCs frommultiple sintering trials for varying f values, as shown in Figure D.1b [251].Mgh =vuut 1/x − /oZ foPNi5)(iavg− 1)2Nd/ (D.10)Where, /x and /o is initial and nal RD; i and svy are initial and average (i (t)); N is thenumber of sintering trials. The f value of the material would be the one for which the MRSvalue was at a minimum. As seen in Figure D.1a, the MSCs plotted at varying heat ratesappear to retrace each other for the "best t" activation energy f.a bFigure D.1: Example of MSC: a) MSC of alumina with activation energy value of 488 kJ/moland b) Mean residual square method used for calculation of best t activation energy Qfrom a study by Cho et al. (Republished with permission of John Wiley and Sons, Inc., from"Master Sintering Curve: A Practical Approach to Sintering", Journal of American CeramicSociety, D. L. Johnson and H. Su, vol. 79 no. 12 (2005); © permission conveyed throughCopyright Clearance Center, Inc.) [252][253].In order to predict RD from MSC, the average MSC is calculated from multiple MSCs for253D.1. Master Sintering Curve Theorymultiple sintering trials. Subsequently, the (i (t)) is calculated by using the f value and thethermal history of sintering trial for which the RD has to be predicted. Once the (i (t)) iscalculated, it is compared to the MSC to nd the corresponding RD. According to Su et al.,deviations from predicted density are expected at RD>95%, since grain growth is neglectedin the MSC model [252]. Since, grain growth is a contributor to consolidation at higher RD,thus the accuracy of MSC predictions at high RD deteriorates [252]. An alternate approachto constructing MSCs is by the Minimum Perpendicular Curve Distance (MPCD) approachpresented by Pouchly et al., which claimed to yield improved prediction results at higherRDs [251]. Therefore, the MPCD technique as dened by Equation D.11, was used in thisdissertation:MeCD =Z foPNi5)(eDi(/))/x − /i d/ (D.11)Where PD is the perpendicular distance between curves.D.1.0.1. Limitations of Master Sintering Curve TheoryMSC is mostly an empirical model based on some underlying equations deningsintering mechanics. This section outlines some of the limitation of MSC and assumptionsmade in this thesis [254]. Frueh et al. compiled activation energies calculated using MSCfrom alumina powder and showed that the activation energies varied 290 to 1064 kJ/mol[254]. It was found that powder characteristics such as purity and morphology of particlesin uenced the activation energy values. Additionally, processing conditions and processingtechniques also in uenced the activation energy values according to Frueh et al. [254]. Thefundamental reasoning behind this was that MSC assumed sintering to proceed by a singlemechanism and denes a activation energy for it. However, in reality a wide variety ofmechanisms could contributing to sintering as shown in Section 2.1.1.1 [254]. Powder254D.2. SPS Conditions for MSC Experimentscharacteristics and processing conditions could in uence the contributing mechanismscausing the activation energy values to vary, despite keeping the material same. Frueh et al.also acknowledges the signicance of MSC as a useful tool for creating time-temperaturedensifciation maps that are usefulThe accuracy and capability of sintering models could be improved by using moremechanism specic models such as the constitutive model by Olevsky et al. [255]. Olvesky’smodel was e ective in creating a densication map for Al powder, which could be extendedto other conductive materials provided material properties are available [256][257]. Theintial model was developed with consideration of three factors: electromigration, externalload and surface tension, however Hendrickx et al. extended the model to account forparticle rearrangement mechanism as well [256]. The model was also used to predict theresidual porosity in the pellet after SPS by Milligan et al. [257].Although the constitutive model developed by Olvesky et al. was more comprehensive,however applies only to conductive materials and requires material properties to develop amodel. Hence, a more empircal model such as MSC was used for this study since it requiresonly data from prior SPS trials.D.2. SPS Conditions for MSC ExperimentsThe MSCs were constructed and validated by two sets of SPS experiments inspired fromthe experimental design used by Ge et al. [26]. The peak temperatures were varied accordingto literature or approximately set as 70-80% of the melting point of the material. Two typesof SPS conditions were used for MSC experiments:• MSC construction experiments: The objective of this sintering condition was toobtain a pellet with the highest as-sintered RD as possible, to relate instantaneous RD to255D.2. SPS Conditions for MSC Experimentsthermal history (log()). However, to determine MSC using the MPCD or MRS method,a minimum of three experiments were required. Hence, the peak sintering temperaturewas xed and heating rates were varied from 50◦C/min to 200◦C/min. The dwellingtime at the peak sintering temperature was maintained at 0.5 min.• MSC validation experiments: One validation experiment was carried out at atemperature lower than the MSC construction experiment. The peak SPS temperaturefor MSC validation experiment was 200◦C-300◦C lower than the MSC constructionexperiment. The dwelling time was increased to 5 min to simultaneously examine thee ect of both temperature and dwelling time using the same SPS program. Theinstantaneous RD data obtained during this experiment were compared against theinstantaneous RD data predicted from MSC to validate their accuracy.Table D.1 shows the constant heating rate and isothermal conditions used for the variousmaterials studied. The pressure was xed at 60 MPa for all MSC experiments throughoutsintering cycle, to remove the e ect of load variation during SPS on the instantaneous RD.The cooling rate was kept same as the heating rate, since the displacement data during coolingwas used to eliminate thermal expansion contribution to displacement data. The heating andcooling rate were kept the same to eliminate any time dependence on thermal expansion.256D.2. SPS Conditions for MSC ExperimentsTable D.1: MSC experimental conditions used in this study.Material MSC construction conditions MSC validationconditionUO21300◦C (100◦C/min.) - 60 MPa - 0.5 min.1300◦C (75◦C/min.) - 60 MPa - 0.5 min.1300◦C (50◦C/min.) - 60 MPa - 0.5 min.1000◦C, 60 MPa - 5 min.ThO21800◦C (125◦C/min.) - 60 MPa - 0.5 min.1800◦C (100◦C/min.) - 60 MPa - 0.5 min.1800◦C (75◦C/min.) - 60 MPa - 0.5 min.1800◦C (50◦C/min.) - 60 MPa - 0.5 min.1500◦C, 60 MPa - 5 min.ThO2-5 wt%40 µm SiC1800◦C (100◦C/min.) - 60 MPa - 0.5 min.1800◦C (75◦C/min.) - 60 MPa - 0.5 min.1800◦C (50◦C/min.) - 60 MPa - 0.5 min.1500◦C, 60 MPa - 5 min.ThO2-5 wt% 1µm SiC1800◦C (100◦C/min.) - 60 MPa - 0.5 min.1800◦C (75◦C/min.) - 60 MPa - 0.5 min.1800◦C (50◦C/min.) - 60 MPa - 0.5 min.1500◦C, 60 MPa - 5 min.ThO2-1 wt%15 nm SiC1800◦C (100◦C/min.) - 60 MPa - 0.5 min.1800◦C (75◦C/min.) - 60 MPa - 0.5 min.1800◦C (50◦C/min.) - 60 MPa - 0.5 min.1500◦C, 60 MPa - 5 min.ThO2-5 wt%diamond1800◦C (100◦C/min.) - 60 MPa - 0.5 min.1800◦C (75◦C/min.) - 60 MPa - 0.5 min.1800◦C (50◦C/min.) - 60 MPa - 0.5 min.1500◦C, 60 MPa - 5 min.Th0.9U0.1O21700◦C (100◦C/min.) - 60 MPa - 0.5 min.1700◦C (75◦C/min.) - 60 MPa - 0.5 min.1300◦C (50◦C/min.) - 60 MPa - 0.5 min.1000◦C, 60 MPa - 5 min.Th0.95U0.05O21300◦C (100◦C/min.) - 60 MPa - 0.5 min.1300◦C (75◦C/min.) - 60 MPa - 0.5 min.1300◦C (50◦C/min.) - 60 MPa - 0.5 min.1000◦C, 60 MPa - 5 min.257D.3. MSC of ThO2 and Its CompositesD.3. MSC of ThO2 and Its CompositesThis section presents the results from the construction of MSC for UO2, ThO2 and itscomposites. A unique approach developed and was subsequently adopted for the constructionof the MSC for UO2 to evaluate its validity. The entire methodology has been described herefor future researchers to follow.D.3.1. MSC of UO2Experiments with UO2 were chosen for the early proof of concept studies, and to allowthe comparison of the MSC obtained in this study to MSC results published by Ge et al. on SPSof UO2 [66]. The primary requirement for the construction of MSC is the determination of theinstantaneous RD as a function of temperature and time during a sintering process. However,the displacement values (used to calculate instantaneous RD) obtained from SPS may havecontributions from the pressure change (preloading and unloading), thermal expansion ofthe dies, data loss during pyrometer "warm-up", etc. These contributions were isolated in thepresent study. Figure D.2, shows a set of representative displacement results from the SPSdata acquisition system. This data was later used for MSC construction.258D.3. MSC of ThO2 and Its CompositesFigure D.2: Unaltered displacement data obtained from SPS of UO2 at 1300◦C (75◦C/min) -60 MPa - 0.5 min plotted alongside temperature and pressure.A basic understanding of the raw displacement data collected during an SPS process isrequired to understand the construction of MSCs in this dissertation. The displacement datacan be divided into seven segments as summarized:• Segment 1 - Preloading: This segment involved the application of maximum pressureused during an SPS experiment. The pressure was applied before the start of heating.As seen in Figure D.2, the most displacement in the entire SPS trial occurred duringthis segment. Hence, it was critical to remove the pressure related displacement bypreloading rather than ramping the pressure during heating.• Segment 2 and 3 - Pyrometer "warm-up": The pyrometer used in this study had a259D.3. MSC of ThO2 and Its Compositesminimum cut-o temperature of 464◦C. Between 464 - 600◦C, the accuracy of thepyrometer was not optimised. During third stage, the materials used in this studyshowed negligible displacement during this stage. As a result, displacement data fromsegments 2 and 3 was neglected.• Segment 4 - Densication: This segment represented the actual materialdensication as a function of the temperature during an SPS experiment. In this stage,the displacement had contributions from thermal expansion of graphite tooling andthe densication of sintered material.• Segment 5 - Thermal expansion: Once the material has densied, the rate of thermalexpansion exceeds the rate of densication, which causes negative ram displacementas seen in Segment 4.• Segment 6 - Thermal shrinkage: During cooling, the tooling and the sinteredmaterial underwent shrinkage, causing a positive ram displacement measurement.• Segment 7 - Unloading and cool down to room temperature: Once thetemperature cooled to 600◦C, the pressure was removed and the tooling was allowedto cool down to RT.D.3.1.1. Step by Step Construction of MSC of UO2The following steps were followed for the construction of MSC of UO2.• Step 1: The thermal contraction identied in Segment 7 of Figure D.2 was separatelyplotted in Figure D.3a. Although thermal expansion is typically a polynomial functionof displacement, for this study it was assumed to be a linear function of temperaturewith an R2 value of 0.99959. Hence, this linear function could be used to calculate260D.3. MSC of ThO2 and Its Compositesthe thermal expansion contribution of tooling assembly under 60 MPa pressure (or adi erent pressure value of choosing).a bFigure D.3: Removal of thermal expansion contribution from sintering data: a) Thermalexpansion data obtained from the sintering of UO2 at 1300◦C (75◦C/min) - 60 MPa - 0.5 minand b) Corrected ram displacement plotted with actual displacement data.• Step 2: The next step was to obtain the new corrected displacement (duorr) after addingthermal expansion (∆L) to the actual recorded displacement (d) by following EquationD.12:duorr = d+ ∆L (D.12)As shown in Figure D.3b, this operation resulted in the removal of thermal expansioncontributions that were evident in unaltered displacement data.• Step 3: The displacement data was converted to the instantaneous height (hinst inEquation D.13) of the pellet during SPS by using the nal height (hxinsl) of the pelletthat was measured after SPS process using digital calipers.261D.3. MSC of ThO2 and Its Compositeshinst = duorrL + hxinsl − d6(( (D.13)d6(( was the corrected displacement during cooling at 600◦C just before the load wasreleased.• Step 4: The instantaneous height was used to calculate the instantaneous density (/instLand instantaneous RD (gODOinstL) using Equations D.14 and D.15:/instL =lpowvwrhinstL.0O6252(D.14)gODOinstL =/instL/thworL(D.15)Where, lpowvwr was the weight of precursor powder, and /thworL was the theoreticaldensity of the material.• Step 5: The densication part of the curve was extracted and saved separately. Toobtain a good t for MSC, the most common practice in the literature involves usingthree sintering runs at di erent heating rates [251][26]. As a result, UO2 was sintered at50◦C/min, 75◦C/min and 100◦C/min The data extracted from these three trials is shownin Figure D.4.262D.3. MSC of ThO2 and Its CompositesFigure D.4: Densication curves extracted from sintering runs at 50◦C/min, 75◦C/min and100◦C/min for UO2.• Step 6: The next step was to identify the activation energy ,f, using the softwaredeveloped by Pouchly et al. [251]. The criteria used to determine the activation energyf was by minimising the Mean Perpendicular Curve Distance (MPCD) for varying fvalues. As shown in Figure D.5a, the software calculated the MPCD for ten equidistantf values within a specied range (100 kJ/mol to 1000 kJ/mol). Once a local minimumwas obtained, a narrower range (660 kJ/mol to 760kJ/mol) was selected. The secondlocal minimum was selected as the nal f value (640 kJ/mol) and the MSC wereaveraged from the three sintering curves at this f value (refer to Figure D.5b).263D.3. MSC of ThO2 and Its Compositesa bFigure D.5: MSC calculation by MPCD method: a) MPCD minimisation for Q and b)Averaged MSC curve obtained at Q value 640 kJ/mol plotted against the sintering curvesfrom various heating rates.• Step 7: The MSC obtained in Step 6 was used to predict the instantaneous RD curvefor the validation experiment. This was done by calculating the log((iP t)) by usingthe temperature i vs. time t data obtained from the the validation experiment.As shown in Figure D.13 the predicted instantaneous RD curve closely matchedthe real data. The as-sintered RD for the pellet by measured by immersion method was94.1% whereas the predicted value was 94.7%, thus validating the accuracy of MSCcalculation procedure.264D.3. MSC of ThO2 and Its Compositesa bFigure D.6: Validation of MSC by comparison of calculated and actual RD: a) As a functionof time and b) As a function of temperature.D.3.2. MSC of ThO2The MSC of ThO2 had a higher MPCD compared to UO2 despite using data from 4trials compared to 3 used for UO2, as seen in Figure D.7a. The higher MPCD indicates ahigher deviation between the four trials, probably due to the secondary densication regimeduring SPS of ThO2 due to Th metal formation. The higher MPCD value also in uenced thestandard deviation () between predicted curve and experimental data (See Figure D.7b). Inorder to improve the accuracy of MSC of ThO2, either more sintering trials would have to becarried out or the MSC should be limited to a temperature below which there is no Th metalformation.265D.3. MSC of ThO2 and Its Compositesa bFigure D.7: a) MSC of ThO2 and b) Instantaneous RD predicted by MSC for SPS of ThO2 at’1500◦C - 60 MPa - 10 min’.D.3.3. MSC of ThO2-5 wt% 40 µm SiCThe MSC of ThO2-5 wt% 40 µm SiC had improved accuracy as seen in Figure D.8b,where the standard deviation in the instantaneous RD between the predicted curve and theexperimental data was only 3.5%. This was probably due to the lower MPCD value which waslower than the MPCD of ThO2 as shown in Figure D.8b. However, at the moment there is noreasonable explanation for the lower MPCD value than pure ThO2.266D.3. MSC of ThO2 and Its Compositesa bFigure D.8: a) MSC of ThO2-5 wt% (40 µm) SiC and b) Instantaneous RD predicted by MSCfor SPS of ThO2-5 wt% (40 µm) SiC at ’1500◦C - 60 MPa - 0.5 min’.D.3.4. MSC of ThO2-5 wt% 1 µm SiCThe MSC of ThO2-5 wt% 1 µm SiC had a better accuracy in comparison to the MSC ofpure ThO2, as shown in Figure D.9a, but reduced accuracy than ThO2-5 wt% 40 µm SiC. Thestandard deviation in MSC prediction was 6.0%, also possibly due to higher MPCD value, asseen in Figure D.9b. A secondary densication regime was evident (similar to pure ThO2),which could be a ecting the accuracy of MSC. The secondary densication regime would bea result of Th formation or reaction between ThO2 and SiC.267D.3. MSC of ThO2 and Its Compositesa bFigure D.9: a) MSC of ThO2-5 wt% (1 µm) SiC and b) Instantaneous RD predicted by MSCfor SPS of ThO2-5 wt% (1 µm) SiC at ’1500◦C - 60 MPa - 5 min’.D.3.5. MSC of ThO2-1 wt% 15 nm SiCThe MSC of ThO2-1 wt% 15 nm SiC had the lowest standard deviation among all the SiCcomposites as seen in Figure D.10b. The standard deviation in the prediction was only 1.8%.Most of the error was limited to the nal stage of SPS, probably due to the Th formation.a bFigure D.10: a) MSC of ThO2-1 wt% (15 nm) SiC and b) Instantaneous RD predicted by MSCfor SPS of ThO2-1 wt% (15 nm) SiC at ’1500◦C - 60 MPa - 5 min’.268D.3. MSC of ThO2 and Its CompositesD.3.6. MSC of ThO2-5 wt% DiamondThe MSC of ThO2-5 wt% Diamond (refer Figure D.11) was accurate in predictinginstantaneous RD below 70%, however error above 70% caused the predicted curve to havean average error of 4.1%.a bFigure D.11: a) MSC of ThO2-5 wt% Diamond and b) Instantaneous RD predicted by MSCfor SPS of ThO2-5 wt% Diamond at ’1500◦C - 60 MPa - 5 min’.D.3.7. MSC of Th0.9U0.1O2MSC of Th0.9U0.1O2 had the least standard deviation (0.9%) amongst all the MSCsconstructed in this dissertation, as shown in Figure D.12. The was also lower than that ofpure ThO2 or its composites.269D.3. MSC of ThO2 and Its Compositesa bFigure D.12: a) MSC of Th0.9U0.1O2 and b) Instantaneous RD predicted by MSC for SPS ofTh0.9U0.1O2 at ’1000◦C - 60 MPa - 5 min’.D.3.8. MSC of Th0.95U0.05O2MSC of Th0.95U0.05O2 had the least MPCD amongst all the MSCs constructed inthis dissertation. The standard deviation of prediction was also similar to Th0.9U0.1O2. TheSPS of both (Th,U)O2 mixed oxides had a better accuracy probably since they were carriedout at lower temperature (1500◦C) compared to others (1800◦C). This could be the reasonbehind pure UO2 also having better accuracy. A high temperature SPS might be causingphase transformation, hence in uencing the accuracy of MSCs.270D.3. MSC of ThO2 and Its Compositesa bFigure D.13: a) MSC of Th0.9U0.1O2 and b) Instantaneous RD predicted by MSC for SPS ofTh0.9U0.1O2 at ’1000◦C - 60 MPa - 5 min’.D.3.9. Summary on MSC of ThO2 and Its CompositesFigure D.14 summarises the results from the construction of MSCs for ThO2 and itscomposites. The activation energy of pure and mixed oxide fuel materials were observedto be lower than the SiC and diamond composites as shown in Figure D.14a. An analysisFrueh et al. revealed that "activation energy" is merely a tting factor and is not a measure ofenergy required for the activation of a sintering mechanism [254]. The plot between MPCDand standard deviation showed a some correlation as seen in Figure D.14d. Hence, by addingmore data from SPS trials, MPCD could be further minimised, thereby reducing standarddeviation of MSC prediction.271D.3. MSC of ThO2 and Its Compositesa bc dFigure D.14: Summary of MSC results: a) Activation energy, b) MPCD, c) Variance and d)MPCD vs. variance.D.3.10. MSC DatabaseA MSC database has been created to enable access to the MSC data generated from thisstudy. The MSC data could be used for estimation of density for future sintering trials orcould be further improved in accuracy by using data from more SPS trials. The database hasbeen created in two formats.1. Tabulated data: The number of data points in the averaged MSC curve ranged from272D.3. MSC of ThO2 and Its Composites500 - 1000 points. This data has been added to Appendix D.2. Simplied formula: The tabulated data was extensive to be presented here, hence asimplied binomial dose response formula (refer to Equation D.16) was tted on theMSC curve to generate the data shown in Table D.2. The tting was carried out untilconvergence using Levenberg Marquardt Algorithm. The Coe cient Of Determination(COD) was also provided as metric for accuracy of tting./ = V) + (V2 − V))p1 + 10(L1−)h1+1− p1 + 10(L2−)h2(D.16)Table D.2: Simplied MSC database generated using binomial dose response equation.Material V) V2 L) L2 h) h2 pCOD(g2)UO2-0.83 ±1.130.98 ±0.00-38.24± 0.01-62.24± 9.490.32 ±0.000.05 ±0.000.26 ±0.16 0.999ThO20.38 ±0.001.32 ±0.03-18.51± 0.17-7.94 ±0.100.61 ±0.100.28 ±0.000.06 ±0.01 0.999ThO2-5 wt%40 µm SiC0.50 ±0.001.09 ±0.01-23.50± 0.20-20.93± 0.130.12 ±0.000.64 ±0.050.77 ±0.03 0.998ThO2-5 wt%1 µm SiC0.51 ±0.001.23 ±0.05-45.08± 1.28-44.79± 0.020.06 ±0.000.78 ±0.050.79 ±0.02 0.998ThO2-1 wt%15 nm SiC0.52 ±0.000.94 ±0.00-30.32± 0.02-36.11± 0.070.77 ±0.020.25 ±0.010.44 ±0.01 0.999ThO2-5 wt%Diamond0.48 ±0.001.64 ±0.17-27.55± 1.49-30.78± 0.460.09 ±0.001.59 ±0.690.91 ±0.09 0.996Th0.9U0.1O20.45 ±0.000.96 ±0.00-10.66± 0.01-10.76± 0.012.96 ±0.260.72 ±0.020.24 ±0.03 0.999Th0.95U0.05O20.19±0.141.21 ±0.15-18.18± 0.18-17.2 ±0.070.52 ±0.01-0.89 ±0.160.74 ±0.06 0.999273D.4. Master Sintering Curve DataD.4. Master Sintering Curve DataD.4.1. Master Sintering Curve Data of UO2Table D.3: Master Sintering Curve Data of UO2. /  /  /  /-46.62242 0.30631 -34.31741 0.86204 -29.17227 0.93067 -28.70553 0.94998-46.44034 0.30631 -34.22394 0.86402 -29.16345 0.93122 -28.7025 0.94998-46.25979 0.30631 -34.13102 0.86989 -29.1548 0.93163 -28.69949 0.94998-46.08074 0.30631 -34.03866 0.87316 -29.14633 0.93163 -28.6965 0.94998-45.90319 0.30952 -33.94684 0.87555 -29.13801 0.93163 -28.69353 0.94998-45.7271 0.31327 -33.85558 0.87924 -29.12985 0.93163 -28.69058 0.94998-45.55246 0.32146 -33.76485 0.88258 -29.12184 0.93163 -28.68765 0.94998-45.37926 0.32501 -33.67466 0.88538 -29.11398 0.93163 -28.68474 0.94998-45.20747 0.32548 -33.585 0.88804 -29.10625 0.93163 -28.68185 0.95005-45.03709 0.3278 -33.49587 0.89193 -29.09866 0.9317 -28.67897 0.95005-44.86808 0.32965 -33.40725 0.89268 -29.0912 0.93218 -28.67612 0.95005-44.70044 0.32965 -33.31916 0.89465 -29.08387 0.93238 -28.67329 0.951-44.53414 0.3319 -33.23158 0.89684 -29.07666 0.93238 -28.67047 0.951-44.36918 0.33353 -33.1445 0.89772 -29.06957 0.93238 -28.66767 0.951-44.20553 0.33845 -33.05793 0.89909 -29.06259 0.93238 -28.66489 0.951-44.04319 0.34486 -32.97185 0.90059 -29.05572 0.93238 -28.66213 0.93743-43.88213 0.34595 -32.88628 0.90148 -29.04896 0.93238 -28.65939 0.93743-43.72233 0.3512 -32.80119 0.90148 -29.0423 0.93238 -28.65666 0.93743-43.5638 0.35523 -32.71659 0.90229 -29.03574 0.93238 -28.65395 0.9377-43.4065 0.35782 -32.63247 0.90332 -29.02928 0.93238 -28.65125 0.9377274D.4. Master Sintering Curve Data /  /  /  /-43.25042 0.36342 -32.54883 0.90427 -29.02291 0.93238 -28.64858 0.9377-43.09556 0.36621 -32.46566 0.90475 -29.01664 0.93238 -28.64592 0.95114-42.94189 0.37044 -32.38296 0.90475 -29.01046 0.93238 -28.64327 0.95114-42.78941 0.37617 -32.30073 0.90475 -29.00436 0.93238 -28.64064 0.95114-42.6381 0.38006 -32.21896 0.90475 -28.99834 0.93286 -28.63803 0.95114-42.48794 0.38177 -32.13765 0.90475 -28.99241 0.93313 -28.63543 0.95114-42.33892 0.3862 -32.05679 0.90543 -28.98656 0.9334 -28.63285 0.95114-42.19104 0.38818 -31.97638 0.90557 -28.98079 0.9334 -28.63028 0.95114-42.04427 0.39139 -31.89642 0.907 -28.97509 0.9334 -28.62773 0.95114-41.8986 0.39493 -31.8169 0.90748 -28.96947 0.9334 -28.6252 0.95114-41.75403 0.39589 -31.73782 0.90871 -28.96392 0.9334 -28.62268 0.95114-41.61053 0.39834 -31.65918 0.90966 -28.95843 0.9334 -28.62017 0.95114-41.4681 0.39923 -31.58097 0.91055 -28.95302 0.9334 -28.61768 0.95114-41.32673 0.40169 -31.50318 0.9111 -28.94767 0.9334 -28.6152 0.95114-41.1864 0.40476 -31.42582 0.91226 -28.94239 0.9334 -28.61273 0.95114-41.0471 0.40715 -31.34889 0.91226 -28.93717 0.9334 -28.61028 0.93784-40.90883 0.41015 -31.27237 0.91226 -28.93202 0.9334 -28.60785 0.93784-40.77156 0.41438 -31.19626 0.91226 -28.92692 0.9334 -28.60542 0.95121-40.63529 0.41949 -31.12057 0.91301 -28.92188 0.9334 -28.60301 0.95121-40.50001 0.4212 -31.04528 0.91437 -28.91691 0.9334 -28.60062 0.95162-40.3657 0.42509 -30.9704 0.91451 -28.91198 0.9334 -28.59823 0.95162-40.23236 0.42945 -30.89592 0.91471 -28.90712 0.9334 -28.59586 0.95162-40.09998 0.43362 -30.82184 0.91512 -28.9023 0.9334 -28.59351 0.95162-39.96854 0.43914 -30.74815 0.91512 -28.89754 0.93368 -28.59116 0.95203-39.83804 0.44446 -30.67485 0.91512 -28.89283 0.93368 -28.58883 0.95203275D.4. Master Sintering Curve Data /  /  /  /-39.70846 0.45244 -30.60194 0.93941 -28.88817 0.93422 -28.58651 0.95203-39.5798 0.45763 -30.52942 0.93941 -28.88356 0.93422 -28.5842 0.95203-39.45204 0.46588 -30.45728 0.93941 -28.879 0.93429 -28.58191 0.95203-39.32518 0.47168 -30.38552 0.94029 -28.87449 0.93429 -28.57962 0.95203-39.19921 0.48028 -30.31413 0.9422 -28.87002 0.93429 -28.57735 0.95203-39.07411 0.48683 -30.24312 0.9422 -28.8656 0.93429 -28.57509 0.95203-38.94988 0.49372 -30.17248 0.94261 -28.86123 0.93429 -28.57285 0.95271-38.82651 0.50218 -30.10221 0.94336 -28.85689 0.93429 -28.57061 0.95271-38.70399 0.51098 -29.9721 0.94534 -28.8526 0.93429 -28.56838 0.95271-38.58231 0.51746 -29.91923 0.92037 -28.84835 0.93429 -28.56617 0.95271-38.46147 0.52421 -29.87211 0.94657 -28.84415 0.93429 -28.56397 0.93791-38.34145 0.53745 -29.82961 0.92256 -28.83998 0.93436 -28.56178 0.93791-38.22225 0.54461 -29.79089 0.94684 -28.83585 0.93477 -28.5596 0.93818-38.10385 0.553 -29.75535 0.92385 -28.83176 0.93477 -28.55743 0.93818-37.98625 0.56405 -29.7225 0.94807 -28.82771 0.93477 -28.55527 0.93818-37.86944 0.57436 -29.69196 0.94828 -28.8237 0.93477 -28.55312 0.93818-37.75342 0.58275 -29.66342 0.92563 -28.81972 0.93477 -28.55099 0.93859-37.63817 0.59694 -29.63665 0.92563 -28.81578 0.93477 -28.54886 0.93859-37.52369 0.60601 -29.61143 0.92631 -28.81188 0.93477 -28.54674 0.93859-37.40997 0.61577 -29.5876 0.94889 -28.80801 0.93477 -28.54464 0.93866-37.297 0.62682 -29.565 0.94923 -28.80417 0.93477 -28.54254 0.93866-37.18478 0.63787 -29.54352 0.92672 -28.80037 0.93477 -28.54046 0.93866-37.07329 0.64626 -29.52306 0.94944 -28.7966 0.93477 -28.53838 0.93866-36.96253 0.65718 -29.50352 0.92685 -28.79287 0.93477 -28.53631 0.939-36.8525 0.67205 -29.48481 0.92767 -28.78916 0.93484 -28.53426 0.939276D.4. Master Sintering Curve Data /  /  /  /-36.74318 0.68078 -29.46688 0.92774 -28.78549 0.93538 -28.53221 0.939-36.63458 0.69074 -29.44967 0.94957 -28.78185 0.93538 -28.53018 0.939-36.52667 0.69934 -29.4331 0.92863 -28.77823 0.93538 -28.52815 0.95291-36.41946 0.71421 -29.41715 0.92863 -28.77465 0.93559 -28.52613 0.95291-36.31293 0.72472 -29.40176 0.94971 -28.7711 0.93559 -28.52412 0.95291-36.20709 0.73304 -29.3869 0.94978 -28.76757 0.93559 -28.52212 0.95291-36.10193 0.74157 -29.37253 0.92917 -28.76408 0.93559 -28.52013 0.95291-35.99743 0.75248 -29.35862 0.92917 -28.76061 0.93559 -28.51815 0.95291-35.8936 0.76176 -29.34514 0.9302 -28.75717 0.93559 -28.51618 0.95291-35.79042 0.77213 -29.33207 0.9302 -28.75375 0.93559 -28.51422 0.95291-35.68789 0.78366 -29.31938 0.9302 -28.75037 0.93559 -28.51226 0.95291-35.586 0.78768 -29.30705 0.9304 -28.74701 0.93559 -28.51032 0.95291-35.48476 0.79894 -29.29506 0.9304 -28.74367 0.93559 -28.50838 0.95291-35.38414 0.8014 -29.2834 0.9304 -28.74036 0.93559 -28.50645 0.95291-35.28415 0.80481 -29.27203 0.9304 -28.73708 0.93559 -28.50453 0.95291-35.18478 0.80801 -29.26096 0.9304 -28.73382 0.93565 -28.50262 0.95291-35.08602 0.81293 -29.25016 0.9304 -28.73058 0.93565 -28.50072 0.95305-34.98788 0.82207 -29.23963 0.9304 -28.72737 0.93606 -28.49882 0.95305-34.89033 0.82732 -29.22934 0.9304 -28.72418 0.93606 -28.49694 0.95305-34.79338 0.82998 -29.2193 0.9304 -28.72102 0.93606 -28.49506 0.95305-34.69703 0.84294 -29.20948 0.93067 -28.71788 0.93654 -28.49319 0.95305-34.60126 0.84629 -29.19987 0.93067 -28.71476 0.93654-34.50607 0.84929 -29.19048 0.93067 -28.71166 0.93702-34.41146 0.8572 -29.18128 0.93067 -28.70858 0.93702277D.4. Master Sintering Curve DataD.4.2. Master Sintering Curve Data of ThO2Table D.4: Master Sintering Curve Data of ThO2. /  /  /  /-31.93597 0.43418 -20.12899 0.52468 -15.94479 0.91208 -15.4545 0.92192-31.77691 0.43418 -20.07329 0.52883 -15.93758 0.91208 -15.45215 0.92192-31.61961 0.43418 -20.01795 0.53336 -15.93048 0.91208 -15.44982 0.92192-31.46403 0.43418 -19.96297 0.53751 -15.92351 0.91235 -15.4475 0.92192-31.31014 0.43418 -19.90834 0.54099 -15.91664 0.9124 -15.4452 0.92192-31.15791 0.43418 -19.85405 0.54583 -15.90988 0.9124 -15.4429 0.92192-31.00733 0.43418 -19.80012 0.55077 -15.90322 0.9124 -15.44062 0.92192-30.85835 0.43418 -19.74653 0.55788 -15.89667 0.9124 -15.43835 0.92198-30.71097 0.43418 -19.69328 0.56245 -15.89021 0.91277 -15.43609 0.92203-30.56514 0.43418 -19.64036 0.56819 -15.88384 0.91314 -15.43384 0.92203-30.42085 0.43418 -19.58778 0.57313 -15.87757 0.91361 -15.4316 0.92203-30.27807 0.43602 -19.53553 0.58029 -15.87139 0.91424 -15.42938 0.92203-30.13677 0.43912 -19.4836 0.58676 -15.86529 0.91445 -15.42717 0.92203-29.99694 0.44717 -19.432 0.59276 -15.85928 0.91445 -15.42497 0.92203-29.85856 0.44744 -19.38072 0.60002 -15.85335 0.91445 -15.42277 0.92203-29.72159 0.44812 -19.32976 0.60665 -15.8475 0.91445 -15.42059 0.92203-29.58602 0.44849 -19.27911 0.61344 -15.84173 0.91445 -15.41843 0.92203-29.45183 0.44944 -19.22878 0.62222 -15.83603 0.91445 -15.41627 0.92203-29.31899 0.45044 -19.17875 0.62875 -15.83041 0.91445 -15.41412 0.92203-29.18749 0.45144 -19.12903 0.63754 -15.82486 0.91445 -15.41198 0.92203-29.0573 0.45175 -19.07962 0.64527 -15.81938 0.91445 -15.40986 0.92203-28.9284 0.45201 -19.0305 0.65358 -15.81397 0.91445 -15.40774 0.92203278D.4. Master Sintering Curve Data /  /  /  /-28.80079 0.45243 -18.98168 0.66179 -15.80862 0.91445 -15.40564 0.92203-28.67442 0.45243 -18.93316 0.66879 -15.80334 0.91445 -15.40354 0.92203-28.5493 0.45243 -18.88493 0.67658 -15.79812 0.91445 -15.40145 0.92203-28.4254 0.45243 -18.83699 0.68484 -15.79297 0.91445 -15.39938 0.92203-28.30269 0.45243 -18.78934 0.69247 -15.78787 0.91487 -15.39731 0.92203-28.18118 0.45243 -18.74197 0.70231 -15.78284 0.91487 -15.39526 0.92203-28.06083 0.45364 -18.69488 0.70799 -15.77786 0.91487 -15.39321 0.92203-27.94162 0.4558 -18.64807 0.71651 -15.77294 0.91613 -15.39117 0.92203-27.82356 0.45743 -18.60154 0.72667 -15.76807 0.91619 -15.38915 0.92203-27.70661 0.45885 -18.55529 0.73693 -15.76326 0.91619 -15.38713 0.92224-27.59076 0.45912 -18.50931 0.74398 -15.7585 0.91629 -15.38512 0.92224-27.476 0.45912 -18.46359 0.75055 -15.75379 0.91629 -15.38312 0.92224-27.36231 0.45912 -18.41815 0.75897 -15.74913 0.91629 -15.38113 0.92224-27.24968 0.45912 -18.37296 0.76865 -15.74452 0.91629 -15.37915 0.92224-27.13809 0.45912 -18.32805 0.7786 -15.73996 0.91629 -15.37718 0.92224-27.02752 0.45912 -18.28339 0.78549 -15.73545 0.91629 -15.37522 0.92224-26.91797 0.45912 -18.23899 0.79591 -15.73098 0.91687 -15.37326 0.92224-26.80941 0.45912 -18.19484 0.80349 -15.72656 0.91724 -15.37132 0.92224-26.70185 0.46043 -18.15095 0.81017 -15.72219 0.91745 -15.36938 0.92224-26.59525 0.46291 -18.10731 0.81785 -15.71785 0.91745 -15.36745 0.92224-26.48961 0.46564 -18.06392 0.82737 -15.71356 0.91745 -15.36553 0.92224-26.38491 0.46575 -18.02077 0.83137 -15.70932 0.91745 -15.36362 0.92224-26.28115 0.46601 -17.97788 0.84053 -15.70511 0.91745 -15.36172 0.92224-26.17831 0.46601 -17.93522 0.84773 -15.70094 0.91745 -15.35982 0.92224-26.07638 0.46601 -17.8928 0.85258 -15.69682 0.91745 -15.35794 0.92224279D.4. Master Sintering Curve Data /  /  /  /-25.97534 0.46601 -17.85063 0.85763 -15.69273 0.91745 -15.35606 0.92224-25.87519 0.46601 -17.80869 0.86462 -15.68868 0.91745 -15.35419 0.92224-25.77591 0.46601 -17.76698 0.86768 -15.68467 0.9175 -15.35233 0.92224-25.67749 0.46601 -17.72551 0.87162 -15.68069 0.9175 -15.35047 0.92224-25.57992 0.46601 -17.68427 0.87578 -15.67675 0.91792 -15.34863 0.92224-25.48319 0.46659 -17.64325 0.8773 -15.67285 0.91792 -15.34679 0.92224-25.38728 0.4678 -17.60247 0.88009 -15.66898 0.91792 -15.34496 0.92224-25.2922 0.46822 -17.5619 0.88162 -15.66514 0.91792 -15.34314 0.92224-25.19792 0.46838 -17.52157 0.88351 -15.66134 0.91866 -15.34132 0.92224-25.10444 0.46838 -17.48145 0.8853 -15.65757 0.91866 -15.33951 0.92224-25.01174 0.46838 -17.44155 0.88699 -15.65384 0.91866 -15.33771 0.92224-24.91982 0.46838 -17.40187 0.88835 -15.65013 0.91866 -15.33592 0.92224-24.82867 0.46838 -17.36241 0.8892 -15.64646 0.91866 -15.33413 0.92224-24.73827 0.46838 -17.32316 0.89004 -15.64282 0.91866 -15.33236 0.92224-24.64862 0.46838 -17.28412 0.89041 -15.63921 0.91913 -15.33058 0.92224-24.55971 0.46885 -17.24529 0.89156 -15.63562 0.91913 -15.32882 0.92213-24.47153 0.4698 -17.20667 0.89283 -15.63207 0.91919 -15.32706 0.92287-24.38407 0.47174 -17.16826 0.89377 -15.62855 0.91919 -15.32531 0.92287-24.29732 0.4719 -17.13005 0.89483 -15.62505 0.91919 -15.32357 0.92287-24.21128 0.4719 -17.09205 0.89498 -15.62158 0.91919 -15.32184 0.92287-24.12592 0.4719 -17.05425 0.89609 -15.61814 0.91919 -15.32011 0.9234-24.04126 0.4719 -17.01665 0.89704 -15.61473 0.91929 -15.31839 0.9234-23.95727 0.4719 -16.97925 0.89746 -15.61134 0.91929 -15.31667 0.9234-23.87395 0.4719 -16.94204 0.89809 -15.60798 0.9195 -15.31496 0.9234-23.79129 0.4719 -16.90504 0.89856 -15.60465 0.9195 -15.31326 0.9234280D.4. Master Sintering Curve Data /  /  /  /-23.70928 0.4719 -16.86822 0.89977 -15.60134 0.91955 -15.31157 0.9234-23.62792 0.4719 -16.8316 0.90019 -15.59806 0.91955 -15.30988 0.9234-23.5472 0.4719 -16.79517 0.90082 -15.5948 0.91971 -15.3082 0.9234-23.46711 0.4719 -16.75893 0.90124 -15.59156 0.91971 -15.30652 0.9234-23.38764 0.4719 -16.72288 0.90193 -15.58835 0.91971 -15.30485 0.9234-23.30879 0.4719 -16.68701 0.90219 -15.58516 0.91971 -15.30319 0.9234-23.23054 0.4719 -16.65133 0.90251 -15.582 0.91971 -15.30153 0.9234-23.1529 0.47217 -16.58302 0.90382 -15.57886 0.91992 -15.29988 0.9234-23.07585 0.47248 -16.55252 0.90419 -15.57574 0.92034 -15.29824 0.9234-22.99939 0.47332 -16.52401 0.90445 -15.57264 0.92034 -15.2966 0.9234-22.92351 0.47427 -16.49726 0.9053 -15.56957 0.92108 -15.29497 0.9234-22.8482 0.47464 -16.47207 0.90545 -15.56651 0.92108 -15.29335 0.9234-22.77347 0.47501 -16.44826 0.90545 -15.56348 0.92108 -15.29173 0.9234-22.69929 0.47532 -16.42568 0.90545 -15.56047 0.92108 -15.29012 0.9234-22.62567 0.47538 -16.40422 0.90545 -15.55748 0.92108 -15.28851 0.9234-22.5526 0.47627 -16.38377 0.90598 -15.55451 0.92108 -15.28691 0.9234-22.48006 0.47669 -16.36424 0.90635 -15.55156 0.92108 -15.28531 0.9234-22.40807 0.47748 -16.34555 0.90666 -15.54863 0.92108 -15.28372 0.9234-22.33661 0.47748 -16.32763 0.90666 -15.54572 0.92108 -15.28214 0.9234-22.26567 0.47748 -16.31043 0.90724 -15.54283 0.92108 -15.28056 0.9234-22.19525 0.47748 -16.29387 0.9073 -15.53996 0.92108 -15.27899 0.9234-22.12534 0.47748 -16.27793 0.90735 -15.53711 0.92108 -15.27742 0.9234-22.05594 0.47748 -16.26255 0.90735 -15.53427 0.92108 -15.27586 0.9234-21.98704 0.47748 -16.2477 0.90793 -15.53146 0.92108 -15.27431 0.9234-21.91864 0.47748 -16.23334 0.90808 -15.52866 0.92108 -15.27276 0.9234281D.4. Master Sintering Curve Data /  /  /  /-21.85073 0.47748 -16.21943 0.90824 -15.52588 0.92108 -15.27121 0.9234-21.78331 0.47748 -16.20596 0.90824 -15.52312 0.92108 -15.26967 0.9234-21.71636 0.47748 -16.1929 0.90882 -15.52037 0.92108 -15.26814 0.9234-21.6499 0.47748 -16.18021 0.90903 -15.51765 0.92108 -15.26661 0.9234-21.5839 0.47748 -16.16789 0.90903 -15.51493 0.92108 -15.26509 0.9234-21.51837 0.47748 -16.1559 0.90903 -15.51224 0.92108 -15.26357 0.92361-21.45329 0.47827 -16.14424 0.90908 -15.50956 0.92108 -15.26206 0.92361-21.38868 0.47916 -16.13289 0.90935 -15.5069 0.92108 -15.26055 0.92361-21.32451 0.48058 -16.12182 0.90935 -15.50426 0.92108 -15.25905 0.92361-21.26079 0.48137 -16.11102 0.90998 -15.50163 0.92108 -15.25756 0.92361-21.19751 0.48222 -16.10049 0.90998 -15.49902 0.92108 -15.25606 0.92361-21.13467 0.48337 -16.09021 0.90998 -15.49642 0.92108 -15.25458 0.92361-21.07226 0.48511 -16.08017 0.91019 -15.49384 0.92108 -15.2531 0.92361-21.01028 0.48637 -16.07035 0.91019 -15.49127 0.92108 -15.25162 0.92361-20.94872 0.48748 -16.06075 0.91072 -15.48872 0.92108 -15.25015 0.92361-20.88758 0.48916 -16.05136 0.91087 -15.48619 0.92108 -15.24868 0.92361-20.82685 0.49105 -16.04217 0.91103 -15.48367 0.92108 -15.24722 0.92361-20.76653 0.49269 -16.03316 0.91103 -15.48116 0.92108 -15.24577 0.92361-20.70663 0.495 -16.02434 0.91161 -15.47867 0.92108 -15.24432 0.92361-20.64712 0.497 -16.0157 0.91161 -15.47619 0.92108 -15.24287 0.92361-20.58801 0.49921 -16.00722 0.91161 -15.47372 0.92108 -15.24143 0.92361-20.52929 0.50079 -15.99891 0.91177 -15.47127 0.92108 -15.23999 0.92361-20.47097 0.50416 -15.99075 0.91177 -15.46884 0.9215 -15.23856 0.92392-20.41303 0.50721 -15.98275 0.91177 -15.46641 0.9215 -15.23713 0.92392-20.35547 0.51026 -15.97488 0.91177 -15.464 0.92161 -15.23571 0.92392282D.4. Master Sintering Curve Data /  /  /  /-20.2983 0.51294 -15.96716 0.91177 -15.46161 0.92192 -15.23429 0.92392-20.24149 0.51599 -15.95957 0.91177 -15.45923 0.92192 -15.23288 0.92392-20.18506 0.52026 -15.95212 0.91177 -15.45686 0.92192283D.4. Master Sintering Curve DataD.4.3. Master Sintering Curve Data of ThO2-5 wt% 40 µm SiCTable D.5: Master Sintering Curve Data of ThO2-5 wt% 40 µm SiC /  /  /  /-44.50503 0.49656 -33.91398 0.52606 -27.62192 0.60855 -23.4475 0.73066-44.42953 0.49656 -33.87206 0.52629 -27.59526 0.6093 -23.42904 0.73137-44.35432 0.49656 -33.83025 0.52657 -27.56865 0.61005 -23.41061 0.73197-44.27937 0.49656 -33.78856 0.52685 -27.54211 0.6108 -23.39222 0.73272-44.2047 0.49656 -33.74697 0.52724 -27.51562 0.61163 -23.37385 0.73339-44.1303 0.49656 -33.7055 0.5274 -27.48918 0.61195 -23.35553 0.73426-44.05616 0.49656 -33.66414 0.52779 -27.4628 0.61325 -23.33723 0.73465-43.98229 0.49656 -33.62289 0.52799 -27.43648 0.61369 -23.31897 0.73501-43.90869 0.49683 -33.58175 0.52815 -27.41022 0.61456 -23.30074 0.73568-43.83535 0.49738 -33.54073 0.52835 -27.38401 0.61503 -23.28254 0.73639-43.76228 0.49794 -33.49981 0.5287 -27.35785 0.61618 -23.26437 0.73777-43.68946 0.4981 -33.459 0.52914 -27.33176 0.61677 -23.24623 0.73856-43.61691 0.4981 -33.4183 0.52941 -27.30571 0.6172 -23.22813 0.73903-43.54461 0.49829 -33.3777 0.52981 -27.27973 0.61752 -23.21006 0.73959-43.47258 0.49857 -33.33722 0.53012 -27.2538 0.61835 -23.19202 0.73994-43.40079 0.49881 -33.29684 0.53048 -27.22792 0.6191 -23.17402 0.74077-43.32926 0.49881 -33.25657 0.53083 -27.2021 0.61965 -23.15604 0.7418-43.25799 0.49881 -33.21641 0.53107 -27.17633 0.62016 -23.1381 0.74247-43.18696 0.49881 -33.17635 0.53131 -27.15061 0.62111 -23.12018 0.7431-43.11619 0.49881 -33.1364 0.53158 -27.12496 0.6219 -23.1023 0.74334-43.04566 0.49881 -33.09655 0.53198 -27.09935 0.62242 -23.08445 0.74405-42.97539 0.49881 -33.05681 0.53237 -27.0738 0.62285 -23.06663 0.74476284D.4. Master Sintering Curve Data /  /  /  /-42.90536 0.49881 -33.01717 0.53253 -27.0483 0.62364 -23.04885 0.74551-42.83557 0.49881 -32.97764 0.53281 -27.02286 0.62439 -23.03109 0.74693-42.76603 0.49881 -32.93821 0.53305 -26.99747 0.62494 -23.01337 0.7476-42.69673 0.49881 -32.89888 0.5334 -26.97213 0.62565 -22.99567 0.7482-42.62767 0.49881 -32.85966 0.53372 -26.94684 0.62636 -22.97801 0.74879-42.55885 0.49881 -32.82054 0.53407 -26.92161 0.62719 -22.96038 0.74958-42.49027 0.49892 -32.78152 0.53447 -26.89643 0.62802 -22.94277 0.75001-42.42193 0.4994 -32.7426 0.53478 -26.87131 0.62866 -22.9252 0.75072-42.35382 0.49952 -32.70378 0.53506 -26.84623 0.62905 -22.90766 0.75128-42.28595 0.49983 -32.66507 0.53545 -26.82121 0.63004 -22.89015 0.75199-42.21832 0.49983 -32.62645 0.53581 -26.79624 0.63035 -22.87267 0.75325-42.15091 0.49983 -32.58794 0.53609 -26.77132 0.63083 -22.85522 0.75444-42.08374 0.50023 -32.54952 0.53617 -26.74645 0.63197 -22.8378 0.75515-42.01679 0.5005 -32.51121 0.53628 -26.72164 0.63245 -22.82041 0.75566-41.95008 0.5007 -32.47299 0.53656 -26.69688 0.633 -22.80305 0.75606-41.88359 0.5007 -32.43487 0.53684 -26.67216 0.63383 -22.78572 0.75677-41.81733 0.5007 -32.39685 0.53727 -26.6475 0.63466 -22.76842 0.75752-41.7513 0.5007 -32.35893 0.53763 -26.62289 0.63501 -22.75115 0.75823-41.68549 0.50094 -32.3211 0.53778 -26.59833 0.63576 -22.73391 0.75929-41.6199 0.5011 -32.28337 0.5381 -26.57382 0.63663 -22.7167 0.75969-41.55454 0.50133 -32.24574 0.53838 -26.54936 0.6373 -22.69952 0.76032-41.48939 0.50133 -32.2082 0.53853 -26.52496 0.63766 -22.68237 0.76119-41.42447 0.50133 -32.17076 0.53881 -26.5006 0.63888 -22.66525 0.76309-41.35976 0.50141 -32.13342 0.53917 -26.47629 0.63944 -22.64816 0.76356-41.29528 0.50157 -32.09617 0.53956 -26.45203 0.63991 -22.63109 0.76411285D.4. Master Sintering Curve Data /  /  /  /-41.231 0.50157 -32.05902 0.53984 -26.42782 0.64062 -22.61406 0.76474-41.16695 0.50165 -32.02196 0.54031 -26.40366 0.64149 -22.59705 0.76502-41.10311 0.50169 -31.98499 0.54063 -26.37955 0.64196 -22.58008 0.76585-41.03948 0.50185 -31.94812 0.54086 -26.35549 0.64256 -22.56313 0.76735-40.97606 0.50185 -31.91134 0.54122 -26.33148 0.64307 -22.54621 0.7679-40.91286 0.50204 -31.87465 0.54154 -26.30752 0.64421 -22.52932 0.76838-40.84986 0.50224 -31.83806 0.54185 -26.2836 0.64489 -22.51246 0.76917-40.78707 0.5024 -31.80156 0.54201 -26.25974 0.64512 -22.49563 0.77118-40.72449 0.50256 -31.76515 0.54221 -26.23592 0.64643 -22.47883 0.77166-40.66212 0.50256 -31.72883 0.54241 -26.21215 0.64706 -22.46205 0.77237-40.59996 0.50256 -31.6926 0.54264 -26.18843 0.64765 -22.44531 0.77276-40.538 0.50279 -31.65647 0.54292 -26.16476 0.64805 -22.42859 0.7741-40.47624 0.50299 -31.62042 0.54312 -26.14113 0.64852 -22.4119 0.77497-40.41469 0.50331 -31.58447 0.54347 -26.11755 0.64887 -22.39524 0.77545-40.35333 0.50355 -31.5486 0.54367 -26.09402 0.65018 -22.37861 0.77616-40.29218 0.50362 -31.51282 0.54398 -26.07054 0.65093 -22.362 0.77663-40.23123 0.50374 -31.47714 0.54466 -26.04711 0.65132 -22.34542 0.77726-40.17048 0.5039 -31.44154 0.54497 -26.02372 0.6516 -22.32888 0.77967-40.10992 0.50426 -31.40603 0.54521 -26.00038 0.65267 -22.31235 0.78007-40.04956 0.50426 -31.37061 0.54541 -25.97708 0.65357 -22.29586 0.78098-39.9894 0.50449 -31.33528 0.54576 -25.95384 0.65417 -22.2794 0.78157-39.92943 0.50461 -31.30003 0.54608 -25.93064 0.6546 -22.26296 0.78287-39.86966 0.50461 -31.26487 0.54631 -25.90748 0.65539 -22.24655 0.78366-39.81008 0.50461 -31.2298 0.54655 -25.88437 0.65594 -22.23016 0.78504-39.75069 0.50461 -31.19481 0.54667 -25.86131 0.65646 -22.21381 0.78587286D.4. Master Sintering Curve Data /  /  /  /-39.6915 0.50461 -31.15992 0.54703 -25.8383 0.65721 -22.19748 0.78654-39.63249 0.50461 -31.1251 0.54726 -25.81533 0.65816 -22.18118 0.78725-39.57367 0.50461 -31.09037 0.54758 -25.7924 0.65887 -22.16491 0.78903-39.51504 0.50461 -31.05573 0.54778 -25.76952 0.65942 -22.14866 0.78982-39.4566 0.50461 -31.02117 0.54805 -25.74669 0.65974 -22.13244 0.79101-39.39835 0.50461 -30.9867 0.54829 -25.7239 0.66029 -22.11625 0.79219-39.34028 0.50461 -30.95231 0.54849 -25.70116 0.66139 -22.10008 0.7929-39.2824 0.50461 -30.91801 0.54876 -25.67846 0.6621 -22.08395 0.79334-39.2247 0.50461 -30.88379 0.54904 -25.65581 0.66262 -22.06783 0.79582-39.16718 0.50461 -30.84965 0.54928 -25.6332 0.66325 -22.05175 0.7965-39.10985 0.50461 -30.8156 0.54955 -25.61064 0.66372 -22.03569 0.79721-39.05269 0.50461 -30.78162 0.54979 -25.58812 0.66443 -22.01966 0.79748-38.99572 0.50461 -30.74774 0.55011 -25.56564 0.66515 -22.00366 0.80005-38.93893 0.50461 -30.71393 0.55042 -25.54321 0.6659 -21.98768 0.80068-38.88231 0.50461 -30.6802 0.55066 -25.52083 0.66684 -21.97173 0.80135-38.82588 0.50461 -30.64656 0.55113 -25.49848 0.66767 -21.9558 0.80242-38.76962 0.50461 -30.613 0.55137 -25.47619 0.66811 -21.9399 0.80412-38.71354 0.50461 -30.57952 0.55192 -25.45393 0.66882 -21.92403 0.80475-38.65763 0.50461 -30.54612 0.55228 -25.43172 0.66898 -21.90818 0.8057-38.6019 0.50461 -30.5128 0.55255 -25.40955 0.66953 -21.89236 0.80657-38.54634 0.50461 -30.47956 0.55287 -25.38743 0.66992 -21.87657 0.80779-38.49095 0.50461 -30.4464 0.55307 -25.36535 0.67111 -21.8608 0.80901-38.43574 0.50461 -30.41332 0.55338 -25.34331 0.67166 -21.84506 0.80992-38.3807 0.50461 -30.38031 0.55413 -25.32131 0.67233 -21.82934 0.81059-38.32582 0.50461 -30.34739 0.55445 -25.29936 0.67324 -21.81365 0.81202287D.4. Master Sintering Curve Data /  /  /  /-38.27112 0.50461 -30.31455 0.55473 -25.27745 0.67383 -21.79798 0.81281-38.21659 0.50461 -30.28178 0.55492 -25.25558 0.67458 -21.78234 0.81395-38.16222 0.50461 -30.2491 0.55524 -25.23376 0.67514 -21.76673 0.81529-38.10803 0.50461 -30.21649 0.5556 -25.21198 0.67561 -21.75114 0.81632-38.054 0.50461 -30.18395 0.55599 -25.19024 0.67612 -21.73558 0.81672-38.00013 0.50461 -30.1515 0.55635 -25.16854 0.67672 -21.72004 0.8177-37.94643 0.50461 -30.11912 0.55654 -25.14688 0.67715 -21.70453 0.81889-37.8929 0.50461 -30.08682 0.55686 -25.12527 0.67755 -21.68904 0.81991-37.83952 0.50461 -30.0546 0.55729 -25.1037 0.67909 -21.67358 0.82078-37.78632 0.50461 -30.02245 0.55793 -25.08217 0.67972 -21.65814 0.82161-37.73327 0.50461 -29.99038 0.55824 -25.06068 0.68015 -21.64273 0.82264-37.68038 0.50461 -29.95838 0.55848 -25.03923 0.68059 -21.62734 0.8241-37.62766 0.50473 -29.92646 0.55875 -25.01782 0.68086 -21.61198 0.82501-37.57509 0.50477 -29.89461 0.55931 -24.99646 0.68134 -21.59665 0.8258-37.52269 0.50512 -29.86284 0.55954 -24.97513 0.68272 -21.58134 0.82611-37.47044 0.50532 -29.83114 0.55978 -24.95385 0.68311 -21.56605 0.82769-37.41835 0.50544 -29.79952 0.56006 -24.93261 0.68343 -21.55079 0.82872-37.36642 0.50572 -29.76797 0.56065 -24.9114 0.68398 -21.53555 0.82951-37.31464 0.50588 -29.7365 0.56097 -24.89024 0.68426 -21.52034 0.83026-37.26302 0.50619 -29.70509 0.56148 -24.86912 0.68517 -21.50515 0.83085-37.21155 0.50619 -29.67377 0.56187 -24.84804 0.6858 -21.48998 0.8318-37.16024 0.50659 -29.64251 0.56235 -24.827 0.68695 -21.47484 0.83322-37.10908 0.5067 -29.61133 0.56286 -24.806 0.6873 -21.45973 0.83433-37.05808 0.50702 -29.58022 0.56326 -24.78504 0.68793 -21.44464 0.83496-37.00722 0.50726 -29.54918 0.56365 -24.76411 0.68852 -21.42957 0.83551288D.4. Master Sintering Curve Data /  /  /  /-36.95652 0.50753 -29.51822 0.5644 -24.74323 0.68924 -21.41453 0.8363-36.90597 0.50773 -29.48732 0.56476 -24.72239 0.68951 -21.39951 0.8382-36.85557 0.50773 -29.4565 0.56507 -24.70159 0.69014 -21.38452 0.83879-36.80532 0.50797 -29.42575 0.56527 -24.68083 0.69046 -21.36955 0.83946-36.75521 0.50809 -29.39507 0.56594 -24.6601 0.69074 -21.3546 0.84009-36.70526 0.5084 -29.36446 0.56689 -24.63942 0.69247 -21.33968 0.84065-36.65545 0.50852 -29.33392 0.56725 -24.61877 0.69291 -21.32478 0.84215-36.60579 0.50876 -29.30345 0.5676 -24.59817 0.69315 -21.30991 0.84337-36.55627 0.50919 -29.27305 0.56815 -24.5776 0.69354 -21.29505 0.84432-36.50691 0.50951 -29.24272 0.56879 -24.55707 0.69401 -21.28023 0.84491-36.45768 0.5099 -29.21246 0.5695 -24.53658 0.69449 -21.26542 0.8476-36.4086 0.51014 -29.18227 0.57017 -24.51613 0.69579 -21.25064 0.84831-36.35966 0.5103 -29.15215 0.57044 -24.49571 0.69658 -21.23589 0.8491-36.31087 0.51065 -29.1221 0.57179 -24.47534 0.69702 -21.20691 0.85044-36.26222 0.51077 -29.09212 0.57194 -24.455 0.69753 -21.19311 0.85115-36.21371 0.51101 -29.0622 0.57273 -24.4347 0.69804 -21.17974 0.85202-36.16534 0.51129 -29.03235 0.57325 -24.41444 0.69887 -21.16676 0.85328-36.11711 0.51144 -29.00257 0.57455 -24.39422 0.69958 -21.15417 0.85431-36.06902 0.5118 -28.97286 0.57506 -24.37403 0.69982 -21.14193 0.85463-36.02107 0.51208 -28.94322 0.5755 -24.35388 0.70021 -21.13002 0.85534-35.97326 0.51231 -28.91364 0.57574 -24.33377 0.70096 -21.11843 0.85629-35.92559 0.51259 -28.88413 0.57629 -24.3137 0.70148 -21.10715 0.85719-35.87805 0.51279 -28.85469 0.57657 -24.29366 0.70203 -21.09614 0.85787-35.83065 0.51346 -28.82531 0.57684 -24.27366 0.7031 -21.08542 0.85877-35.78339 0.51389 -28.796 0.57803 -24.2537 0.70361 -21.07495 0.85937289D.4. Master Sintering Curve Data /  /  /  /-35.73626 0.51405 -28.76675 0.57842 -24.23378 0.70416 -21.06472 0.85976-35.68927 0.51441 -28.73757 0.57882 -24.21389 0.7048 -21.05473 0.86059-35.64241 0.51468 -28.70846 0.58016 -24.19404 0.70531 -21.04497 0.86181-35.59569 0.51484 -28.67941 0.58063 -24.17423 0.70582 -21.03542 0.86253-35.5491 0.51508 -28.65043 0.58154 -24.15445 0.70645 -21.02608 0.86304-35.50264 0.51523 -28.62151 0.58221 -24.13471 0.70689 -21.01693 0.86336-35.45631 0.51551 -28.59266 0.58292 -24.115 0.70768 -21.00797 0.86418-35.41012 0.51571 -28.56387 0.58356 -24.09533 0.70807 -20.99919 0.86486-35.36406 0.51602 -28.53514 0.58415 -24.0757 0.70863 -20.99059 0.86545-35.31813 0.51642 -28.50648 0.58462 -24.0561 0.70926 -20.98215 0.8664-35.27232 0.51666 -28.47789 0.58482 -24.03654 0.71032 -20.97388 0.86821-35.22665 0.51681 -28.44935 0.58549 -24.01702 0.71139 -20.96576 0.86884-35.1811 0.51709 -28.42088 0.58671 -23.99753 0.7119 -20.95778 0.8692-35.13569 0.51768 -28.39248 0.58826 -23.97808 0.7123 -20.94996 0.86959-35.0904 0.51808 -28.36413 0.58889 -23.95866 0.71261 -20.94227 0.87125-35.04524 0.51828 -28.33585 0.58936 -23.93928 0.71309 -20.93471 0.87181-35.0002 0.51863 -28.30764 0.58991 -23.91993 0.71364 -20.92728 0.87271-34.95529 0.51903 -28.27948 0.59039 -23.90062 0.71415 -20.91998 0.87327-34.91051 0.51918 -28.25139 0.59094 -23.88135 0.71463 -20.9128 0.87366-34.86585 0.5195 -28.22335 0.59205 -23.86211 0.71613 -20.90574 0.87493-34.82132 0.51966 -28.19538 0.59288 -23.8429 0.71696 -20.89879 0.87568-34.77691 0.51993 -28.16748 0.59343 -23.82373 0.71755 -20.89194 0.87576-34.73262 0.52029 -28.13963 0.59505 -23.80459 0.71806 -20.88521 0.87745-34.68846 0.52061 -28.11184 0.59548 -23.78549 0.71826 -20.87858 0.87809-34.64442 0.5208 -28.08412 0.59619 -23.76642 0.71893 -20.87204 0.87974290D.4. Master Sintering Curve Data /  /  /  /-34.6005 0.52108 -28.05645 0.59679 -23.74739 0.7196 -20.86561 0.88282-34.55671 0.52147 -28.02885 0.59758 -23.72839 0.72083 -20.85927 0.88361-34.51303 0.52183 -28.00131 0.59821 -23.70943 0.7213 -20.85302 0.8844-34.46947 0.52222 -27.97382 0.59876 -23.6905 0.72186 -20.84685 0.8844-34.42604 0.52254 -27.9464 0.59998 -23.67161 0.72245 -20.84078 0.88452-34.38272 0.52286 -27.91904 0.60113 -23.65275 0.72304 -20.83479 0.88614-34.33952 0.52325 -27.89173 0.6022 -23.63392 0.72379 -20.82888 0.88614-34.29644 0.52349 -27.86449 0.60267 -23.61513 0.72442 -20.82304 0.88614-34.25348 0.52369 -27.8373 0.60299 -23.59637 0.72513 -20.81729 0.88618-34.21064 0.524 -27.81017 0.60334 -23.57764 0.72604 -20.81161 0.88618-34.16791 0.52432 -27.78311 0.60476 -23.55895 0.72663 -20.80601 0.88709-34.1253 0.52455 -27.7561 0.60504 -23.54029 0.72711 -20.80047 0.88863-34.08281 0.52487 -27.72915 0.60567 -23.52166 0.7275 -20.79501 0.88891-34.04043 0.52507 -27.70225 0.60622 -23.50307 0.72814 -20.78961 0.89124-33.99816 0.52531 -27.67542 0.60725 -23.48451 0.72928 -20.78428 0.89128-33.95601 0.5257 -27.64864 0.6078 -23.46599 0.73015291D.4. Master Sintering Curve DataD.4.4. Master Sintering Curve Data of ThO2-5 wt% 1 µm SiCTable D.6: Master Sintering Curve Data of ThO2-5 wt% 1 µm SiC. /  /  /  /-115.35957 0.46303 -72.75359 0.53031 -53.6628 0.6536 -45.15835 0.84282-114.84001 0.46303 -72.55448 0.53031 -53.55824 0.65583 -45.15041 0.84287-114.32527 0.48603 -72.35651 0.53031 -53.45411 0.65827 -45.14262 0.84318-113.81528 0.48738 -72.15968 0.53031 -53.35041 0.65853 -45.13497 0.84329-113.30998 0.48909 -71.96397 0.53031 -53.24715 0.65962 -45.12745 0.84557-112.8093 0.48909 -71.76938 0.53031 -53.14431 0.66118 -45.12006 0.84557-112.31318 0.48909 -71.57589 0.53031 -53.04189 0.66258 -45.11279 0.84588-111.82155 0.48909 -71.38349 0.53031 -52.9399 0.66497 -45.10564 0.84588-111.33436 0.48909 -71.19218 0.53031 -52.83832 0.66648 -45.09861 0.84609-110.85154 0.48909 -71.00194 0.53031 -52.73716 0.6671 -45.09168 0.8464-110.37304 0.48909 -70.81278 0.53031 -52.63641 0.66819 -45.08487 0.84697-109.89879 0.48909 -70.62467 0.53031 -52.53607 0.66995 -45.07816 0.84874-109.42875 0.49055 -70.43761 0.53031 -52.43614 0.67084 -45.07156 0.84895-108.96286 0.492 -70.25159 0.53031 -52.33662 0.67245 -45.06505 0.84941-108.50106 0.49304 -70.0666 0.53031 -52.2375 0.67431 -45.05864 0.85165-108.04329 0.49309 -69.88263 0.53047 -52.13878 0.67644 -45.05232 0.85217-107.58951 0.49345 -69.69968 0.53057 -52.04045 0.67753 -45.0461 0.85232-107.13967 0.49345 -69.51774 0.53083 -51.94253 0.67847 -45.03996 0.853-106.6937 0.49345 -69.33679 0.53109 -51.845 0.68008 -45.0339 0.853-106.25157 0.49345 -69.15683 0.53156 -51.74786 0.68195 -45.02793 0.853-105.81322 0.49345 -68.97786 0.53176 -51.6511 0.68392 -45.02205 0.85346-105.3786 0.49345 -68.79986 0.53223 -51.55474 0.68501 -45.01624 0.85346292D.4. Master Sintering Curve Data /  /  /  /-104.94767 0.49345 -68.62282 0.53239 -51.45876 0.68594 -45.0105 0.85476-104.52038 0.49345 -68.44675 0.53259 -51.36316 0.68683 -45.00484 0.85476-104.09668 0.49345 -68.27162 0.53317 -51.26794 0.68802 -44.99926 0.85492-103.67654 0.49345 -68.09743 0.53342 -51.1731 0.68989 -44.99374 0.85518-103.25989 0.49345 -67.92418 0.53405 -51.07863 0.69238 -44.9883 0.85528-102.84671 0.49413 -67.75186 0.53415 -50.98453 0.69331 -44.98292 0.85606-102.43694 0.49459 -67.58046 0.53441 -50.89081 0.69394 -44.97761 0.85653-102.03055 0.49548 -67.40997 0.53477 -50.79745 0.69461 -44.97236 0.85653-101.62749 0.49626 -67.24038 0.53529 -50.70446 0.69461 -44.96717 0.85814-101.22773 0.49698 -67.07169 0.5355 -50.61184 0.69461 -44.96205 0.85814-100.83121 0.49771 -66.9039 0.53576 -50.51958 0.69591 -44.95698 0.8586-100.43791 0.4987 -66.73698 0.53592 -50.42767 0.69695 -44.95197 0.85917-100.04778 0.49989 -66.57095 0.53654 -50.33613 0.69819 -44.94703 0.86037-99.66079 0.50062 -66.40578 0.53701 -50.24494 0.69902 -44.94213 0.86104-99.2769 0.5014 -66.24148 0.53721 -50.1541 0.69902 -44.93729 0.86104-98.89606 0.50217 -66.07803 0.53758 -50.06362 0.69902 -44.93251 0.86104-98.51825 0.50326 -65.91544 0.53784 -49.97348 0.70048 -44.92777 0.86135-98.14343 0.50498 -65.75368 0.53815 -49.88369 0.70209 -44.92309 0.86135-97.77156 0.50742 -65.59276 0.53893 -49.79425 0.7037 -44.91846 0.86167-97.40261 0.51022 -65.43268 0.53924 -49.70515 0.70448 -44.91387 0.86167-97.03654 0.51344 -65.27341 0.53981 -49.61639 0.70562 -44.90934 0.86167-96.67332 0.5152 -65.11497 0.53991 -49.52797 0.70702 -44.90485 0.86172-96.31292 0.51552 -64.95733 0.54038 -49.43989 0.70769 -44.90041 0.86172-95.9553 0.51552 -64.8005 0.54121 -49.35215 0.70961 -44.89601 0.86177-95.60043 0.51552 -64.64447 0.54178 -49.26474 0.7107 -44.89165 0.86203293D.4. Master Sintering Curve Data /  /  /  /-95.24828 0.51552 -64.48923 0.54199 -49.17766 0.71278 -44.88734 0.86203-94.89882 0.51552 -64.33477 0.54277 -49.09091 0.71382 -44.88308 0.86203-94.55202 0.51552 -64.1811 0.54308 -49.00449 0.71507 -44.87885 0.86234-94.20785 0.51552 -64.0282 0.54401 -48.91839 0.71735 -44.87466 0.8626-93.86628 0.51552 -63.87607 0.54433 -48.83262 0.71854 -44.87052 0.8626-93.52727 0.51552 -63.7247 0.54505 -48.74717 0.72067 -44.86641 0.86296-93.1908 0.51552 -63.57409 0.54547 -48.66204 0.72192 -44.86234 0.86296-92.85685 0.51552 -63.42423 0.54651 -48.57723 0.72332 -44.85831 0.86416-92.52537 0.51552 -63.27512 0.54723 -48.49274 0.72477 -44.85432 0.86416-92.19636 0.51552 -63.12674 0.54749 -48.40856 0.72633 -44.85036 0.86478-91.86977 0.51552 -62.97911 0.54869 -48.3247 0.7283 -44.84644 0.86478-91.54558 0.51552 -62.8322 0.54936 -48.24115 0.7296 -44.84255 0.86509-91.22377 0.51609 -62.68601 0.55056 -48.1579 0.73142 -44.8387 0.86509-90.9043 0.51629 -62.54054 0.55128 -48.07497 0.73323 -44.83488 0.86525-90.58716 0.51645 -62.39579 0.55201 -47.99234 0.73443 -44.8311 0.86525-90.27232 0.51645 -62.25174 0.55326 -47.91002 0.73583 -44.82734 0.86561-89.95975 0.51645 -62.1084 0.55409 -47.828 0.73723 -44.82362 0.86561-89.64943 0.51645 -61.96576 0.55486 -47.74628 0.73894 -44.81994 0.86592-89.34133 0.51645 -61.8238 0.55606 -47.66487 0.74035 -44.81628 0.86592-89.03543 0.51645 -61.68254 0.55684 -47.58375 0.74201 -44.81265 0.86831-88.73171 0.51645 -61.54196 0.55782 -47.50292 0.74491 -44.80906 0.86867-88.43014 0.51645 -61.40205 0.5586 -47.4224 0.74668 -44.80549 0.86867-88.13071 0.51645 -61.26282 0.56 -47.34216 0.74813 -44.80195 0.86976-87.83338 0.51645 -61.12425 0.56078 -47.26222 0.74979 -44.79844 0.8707-87.53814 0.51645 -60.98635 0.5626 -47.18257 0.75332 -44.79496 0.8707294D.4. Master Sintering Curve Data /  /  /  /-87.24496 0.51645 -60.84911 0.56307 -47.1032 0.75478 -44.79151 0.87111-86.95382 0.51645 -60.71252 0.56431 -47.02412 0.75644 -44.78808 0.87111-86.66471 0.51645 -60.57658 0.56535 -46.94533 0.76013 -44.78468 0.87111-86.37759 0.51645 -60.44128 0.56701 -46.86682 0.76101 -44.78131 0.87179-86.09246 0.51645 -60.30662 0.56769 -46.7886 0.76475 -44.77796 0.87179-85.80928 0.51645 -60.1726 0.5695 -46.71065 0.76604 -44.77464 0.87184-85.52804 0.51645 -60.03921 0.57065 -46.63298 0.76978 -44.77134 0.87184-85.24872 0.51645 -59.90644 0.5721 -46.5556 0.77316 -44.76807 0.87184-84.9713 0.51645 -59.7743 0.5736 -46.47848 0.77674 -44.76482 0.87246-84.69576 0.51645 -59.64277 0.57475 -46.40165 0.78037 -44.7616 0.87246-84.42208 0.51645 -59.51186 0.5763 -46.32508 0.78265 -44.7584 0.87246-84.15024 0.51645 -59.38155 0.5776 -46.24879 0.78515 -44.75522 0.87267-83.88023 0.51686 -59.25185 0.579 -46.17277 0.78743 -44.75207 0.87267-83.61202 0.51733 -59.12275 0.5802 -46.03254 0.79366 -44.74894 0.87267-83.3456 0.5178 -58.99425 0.58217 -45.9764 0.79631 -44.74583 0.87267-83.08094 0.51832 -58.86634 0.58336 -45.9267 0.79875 -44.74275 0.87309-82.81804 0.51889 -58.73902 0.5844 -45.88211 0.79942 -44.73968 0.87309-82.55687 0.51962 -58.61228 0.58612 -45.84167 0.80171 -44.73664 0.87335-82.29742 0.52055 -58.48612 0.58788 -45.80468 0.80249 -44.73362 0.87335-82.03967 0.52117 -58.36053 0.58913 -45.77059 0.80409 -44.73062 0.87345-81.78361 0.52195 -58.23552 0.59053 -45.73899 0.80487 -44.72764 0.87345-81.52921 0.52273 -58.11108 0.59131 -45.70953 0.80674 -44.72468 0.87366-81.27646 0.52309 -57.9872 0.59235 -45.68194 0.80742 -44.72174 0.87366-81.02534 0.52309 -57.86388 0.59395 -45.656 0.81095 -44.71882 0.87366-80.77585 0.52309 -57.74111 0.59499 -45.63152 0.81375 -44.71592 0.87366295D.4. Master Sintering Curve Data /  /  /  /-80.52795 0.52309 -57.6189 0.59593 -45.60835 0.81443 -44.71304 0.87412-80.28165 0.52309 -57.49724 0.59728 -45.58636 0.81474 -44.71018 0.87412-80.03692 0.52309 -57.37612 0.59821 -45.56542 0.81577 -44.70733 0.87412-79.79374 0.52309 -57.25555 0.59992 -45.54545 0.8164 -44.70697 0.87412-79.55211 0.52309 -57.13551 0.60122 -45.52635 0.81681 -44.63631 0.87599-79.31201 0.52309 -57.01601 0.60231 -45.50806 0.81707 -44.56589 0.88456-79.07341 0.52309 -56.89704 0.60314 -45.49051 0.82278 -44.49571 0.88674-78.83632 0.52309 -56.77859 0.60434 -45.47364 0.82418 -44.42577 0.9277-78.60072 0.52309 -56.66067 0.60584 -45.4574 0.82543 -44.35607 0.93195-78.36658 0.52309 -56.54327 0.60787 -45.44175 0.82642 -44.2866 0.93408-78.1339 0.52309 -56.42639 0.60901 -45.42664 0.82673 -44.21736 0.93746-77.90267 0.52309 -56.31002 0.61067 -45.41204 0.82694 -44.14836 0.93927-77.67286 0.52309 -56.19416 0.61249 -45.39791 0.82761 -44.0796 0.94317-77.44448 0.52309 -56.07881 0.61415 -45.38423 0.82797 -44.01106 0.94493-77.21749 0.52382 -55.96396 0.61534 -45.37097 0.82797 -43.94276 0.94685-76.9919 0.52455 -55.84961 0.617 -45.3581 0.82912 -43.87468 0.94945-76.76769 0.52491 -55.73576 0.61913 -45.3456 0.82969 -43.80683 0.95101-76.54484 0.52538 -55.6224 0.62142 -45.33345 0.83047 -43.73921 0.95168-76.32334 0.52636 -55.50953 0.62297 -45.32163 0.83057 -43.67181 0.95339-76.10319 0.52673 -55.39715 0.62458 -45.31012 0.83124 -43.60464 0.95495-75.88436 0.52714 -55.28525 0.62666 -45.29892 0.83166 -43.53769 0.95537-75.66686 0.52813 -55.17383 0.62879 -45.28799 0.83363 -43.47096 0.95682-75.45065 0.5287 -55.06289 0.63029 -45.27733 0.83405 -43.40445 0.95801-75.23574 0.52912 -54.95243 0.63325 -45.26693 0.83503 -43.33817 0.95817-75.02211 0.52964 -54.84244 0.6345 -45.25677 0.83566 -43.2721 0.95978296D.4. Master Sintering Curve Data /  /  /  /-74.80975 0.52989 -54.73291 0.6374 -45.24684 0.83685 -43.20625 0.96009-74.59865 0.53031 -54.62385 0.63922 -45.23713 0.83737 -43.14062 0.96108-74.3888 0.53031 -54.51526 0.6414 -45.22764 0.83976 -43.0752 0.9618-74.18018 0.53031 -54.40712 0.64291 -45.21835 0.84023 -43.01 0.9725-73.97279 0.53031 -54.29944 0.64478 -45.20925 0.84111 -42.94501 0.9725-73.76662 0.53031 -54.19222 0.64587 -45.20034 0.84111 -42.88023 0.97463-73.56164 0.53031 -54.08544 0.64737 -45.19161 0.84147 -42.81567 0.97701-73.35786 0.53031 -53.97912 0.64888 -45.18305 0.84147 -42.75131 0.97826-73.15527 0.53031 -53.87324 0.65007 -45.17466 0.84194 -42.68717 0.97951-72.95385 0.53031 -53.7678 0.65272 -45.16643 0.84282 -42.62323 0.97966297D.4. Master Sintering Curve DataD.4.5. Master Sintering Curve Data of ThO2-1 wt% 15 nm SiCTable D.7: Master Sintering Curve Data of ThO2-1 wt% 15 nm SiC. /  /  /  /-74.9337 0.47744 -44.31164 0.52595 -32.03744 0.7474 -28.87839 0.9243-74.59396 0.47744 -44.2004 0.52595 -31.98272 0.7474 -28.875 0.9243-74.25744 0.47744 -44.08976 0.52595 -31.92821 0.7474 -28.87164 0.9243-73.9241 0.48838 -43.97972 0.52595 -31.8739 0.74865 -28.8683 0.9243-73.59388 0.48993 -43.87027 0.52595 -31.8198 0.75035 -28.86499 0.9243-73.26676 0.49143 -43.76141 0.52595 -31.7659 0.75224 -28.8617 0.9243-72.94267 0.49438 -43.65313 0.52595 -31.71221 0.75442 -28.85844 0.9243-72.62159 0.51142 -43.54543 0.52595 -31.65871 0.75635 -28.8552 0.9243-72.30346 0.51147 -43.43831 0.52595 -31.60542 0.75805 -28.85199 0.9243-71.98826 0.51147 -43.33175 0.52595 -31.55232 0.75931 -28.8488 0.9243-71.67593 0.51147 -43.22576 0.52595 -31.49943 0.76284 -28.84564 0.9243-71.36644 0.51147 -43.12033 0.52595 -31.44673 0.76405 -28.84249 0.9243-71.05975 0.51147 -43.01545 0.52595 -31.39422 0.76715 -28.83937 0.9243-70.75582 0.51147 -42.91112 0.52595 -31.34191 0.77025 -28.83627 0.9243-70.45462 0.51147 -42.80734 0.52595 -31.28979 0.77189 -28.8332 0.9243-70.15611 0.51147 -42.70411 0.52595 -31.23787 0.7747 -28.83014 0.9243-69.86025 0.51147 -42.60141 0.52595 -31.18614 0.7763 -28.82711 0.9243-69.567 0.51147 -42.49925 0.52595 -31.13459 0.7793 -28.8241 0.9243-69.27634 0.51147 -42.39761 0.52595 -31.08324 0.7824 -28.8211 0.9243-68.98823 0.51147 -42.29651 0.52648 -31.03208 0.78376 -28.81813 0.9243-68.70263 0.51147 -42.19592 0.52682 -30.9811 0.78714 -28.81518 0.9243-68.41951 0.51147 -42.09585 0.5273 -30.93031 0.78995 -28.81225 0.9243298D.4. Master Sintering Curve Data /  /  /  /-68.13885 0.51147 -41.9963 0.52803 -30.8797 0.79203 -28.80934 0.9243-67.8606 0.51147 -41.89725 0.52851 -30.82928 0.79504 -28.80645 0.9243-67.58474 0.51147 -41.79871 0.529 -30.77904 0.8008 -28.80357 0.9243-67.31123 0.51147 -41.70067 0.52958 -30.72898 0.80467 -28.80072 0.9243-67.04005 0.51147 -41.60314 0.53016 -30.6791 0.8083 -28.79788 0.92459-66.77117 0.51147 -41.50609 0.53074 -30.62941 0.8144 -28.79507 0.92459-66.50456 0.51147 -41.40954 0.53113 -30.57989 0.81653 -28.79227 0.92503-66.24018 0.51147 -41.31347 0.53224 -30.53055 0.82413 -28.78949 0.92503-65.97801 0.51147 -41.21788 0.53287 -30.48139 0.82709 -28.78672 0.92503-65.71803 0.51147 -41.12278 0.5335 -30.43241 0.83072 -28.78398 0.92503-65.4602 0.51147 -41.02815 0.53418 -30.3836 0.83691 -28.78125 0.92503-65.2045 0.51147 -40.93399 0.5349 -30.33496 0.8404 -28.77854 0.92503-64.95091 0.51147 -40.8403 0.53544 -30.2865 0.84602 -28.77584 0.93244-64.69939 0.51147 -40.74708 0.53636 -30.23821 0.8496 -28.77316 0.93244-64.44992 0.51147 -40.65432 0.53732 -30.19009 0.85187 -28.7705 0.93244-64.20248 0.51147 -40.56202 0.53791 -30.14214 0.85706 -28.76786 0.93244-63.95704 0.51147 -40.47017 0.53849 -30.09437 0.86219 -28.76523 0.93244-63.71358 0.51147 -40.37877 0.53955 -30.04676 0.86514 -28.76261 0.93244-63.47207 0.51147 -40.28783 0.54037 -29.95655 0.87424 -28.76002 0.93244-63.23249 0.51307 -40.19732 0.54139 -29.91762 0.87913 -28.75743 0.93244-62.99481 0.51501 -40.10726 0.5425 -29.8819 0.88083 -28.75487 0.93244-62.75902 0.51767 -40.01764 0.54323 -29.84889 0.88296 -28.75231 0.93244-62.5251 0.52038 -39.92845 0.54405 -29.81821 0.88702 -28.74978 0.93244-62.29301 0.52057 -39.83969 0.54551 -29.78956 0.8879 -28.74725 0.93244-62.06274 0.52086 -39.75137 0.54614 -29.76269 0.89065 -28.74475 0.93244299D.4. Master Sintering Curve Data /  /  /  /-61.83426 0.52086 -39.66346 0.5472 -29.73738 0.89167 -28.74225 0.93244-61.60757 0.52086 -39.57599 0.54836 -29.71346 0.89298 -28.73977 0.93244-61.38262 0.52086 -39.48893 0.54962 -29.69079 0.89438 -28.73731 0.93316-61.15942 0.52086 -39.40229 0.55025 -29.66925 0.89729 -28.73486 0.93316-60.93792 0.52086 -39.31606 0.5519 -29.64872 0.89864 -28.73242 0.93316-60.71812 0.52086 -39.23024 0.55258 -29.62913 0.89927 -28.72999 0.93316-60.5 0.52086 -39.14483 0.55427 -29.61037 0.9 -28.72758 0.93316-60.28353 0.52086 -39.05982 0.55538 -29.5924 0.90077 -28.72519 0.93447-60.0687 0.52086 -38.97522 0.55674 -29.57514 0.90397 -28.7228 0.93447-59.85549 0.52086 -38.89101 0.55843 -29.55854 0.90494 -28.72043 0.93447-59.64388 0.52086 -38.80721 0.55911 -29.54255 0.90542 -28.71807 0.93447-59.43386 0.52086 -38.72379 0.56056 -29.52712 0.90595 -28.71573 0.93447-59.22539 0.52086 -38.64077 0.56211 -29.51223 0.90721 -28.71339 0.93447-59.01848 0.52086 -38.55813 0.56366 -29.49783 0.90804 -28.71107 0.93447-58.81309 0.52086 -38.47588 0.56555 -29.48389 0.90842 -28.70877 0.93447-58.60922 0.52086 -38.39401 0.56749 -29.47039 0.90973 -28.70647 0.93447-58.40685 0.52086 -38.31252 0.56879 -29.45729 0.90992 -28.70419 0.93447-58.20595 0.52086 -38.23141 0.57054 -29.44458 0.9107 -28.70191 0.93447-58.00652 0.52086 -38.15067 0.57238 -29.43223 0.91123 -28.69965 0.93447-57.80853 0.52086 -38.07031 0.57528 -29.42022 0.91133 -28.69741 0.93447-57.61198 0.52086 -37.99031 0.57741 -29.40853 0.91133 -28.69517 0.93447-57.41685 0.52086 -37.91068 0.57862 -29.39715 0.91176 -28.69294 0.93447-57.22311 0.52086 -37.83142 0.5809 -29.38606 0.91239 -28.69073 0.93447-57.03077 0.52086 -37.75252 0.58337 -29.37525 0.91288 -28.68853 0.93447-56.83979 0.52086 -37.67397 0.58608 -29.3647 0.91365 -28.68634 0.93447300D.4. Master Sintering Curve Data /  /  /  /-56.65017 0.52086 -37.59579 0.58743 -29.3544 0.91462 -28.68416 0.92566-56.46189 0.52086 -37.51795 0.59184 -29.34434 0.91496 -28.68199 0.92566-56.27494 0.52086 -37.44047 0.59281 -29.3345 0.91525 -28.67983 0.92566-56.08931 0.52086 -37.36334 0.59503 -29.32488 0.91588 -28.67768 0.93505-55.90497 0.52086 -37.28656 0.59794 -29.31548 0.91617 -28.67554 0.92585-55.72192 0.52086 -37.21012 0.60007 -29.30627 0.91636 -28.67341 0.92585-55.54014 0.52086 -37.13403 0.60138 -29.29725 0.91675 -28.6713 0.92585-55.35963 0.52086 -37.05827 0.60569 -29.28842 0.91714 -28.66919 0.92585-55.18036 0.52086 -36.98285 0.60665 -29.27976 0.91738 -28.66709 0.92585-55.00232 0.52086 -36.90777 0.61077 -29.27127 0.91815 -28.66501 0.92585-54.8255 0.52086 -36.83302 0.61285 -29.26294 0.91825 -28.66293 0.92585-54.6499 0.52086 -36.7586 0.61726 -29.25477 0.9184 -28.66086 0.92585-54.47548 0.52086 -36.68452 0.61842 -29.24676 0.9184 -28.65881 0.92585-54.30226 0.52086 -36.61075 0.6204 -29.23888 0.9184 -28.65676 0.92585-54.1302 0.52086 -36.53732 0.62491 -29.23115 0.91878 -28.65472 0.92585-53.95931 0.52086 -36.4642 0.62621 -29.22355 0.91907 -28.6527 0.92609-53.78956 0.52086 -36.39141 0.62849 -29.21609 0.91936 -28.65068 0.92609-53.62095 0.52086 -36.31893 0.63246 -29.20875 0.91941 -28.64867 0.92609-53.45346 0.52154 -36.24677 0.63498 -29.20153 0.91941 -28.64667 0.93549-53.28709 0.52198 -36.17493 0.6373 -29.19443 0.91941 -28.64468 0.92643-53.12182 0.5227 -36.1034 0.6403 -29.18744 0.92004 -28.6427 0.92643-52.95764 0.52357 -36.03217 0.64408 -29.18056 0.92038 -28.64072 0.92706-52.79455 0.52445 -35.96126 0.64611 -29.17379 0.92038 -28.63876 0.92706-52.63252 0.52508 -35.89065 0.64742 -29.16713 0.92038 -28.63681 0.93616-52.47155 0.5257 -35.82034 0.65115 -29.16057 0.92038 -28.63486 0.92716301D.4. Master Sintering Curve Data /  /  /  /-52.31164 0.52595 -35.75034 0.65362 -29.1541 0.92087 -28.63292 0.92716-52.15276 0.52595 -35.68064 0.65536 -29.14773 0.92087 -28.63099 0.92716-51.99491 0.52595 -35.61123 0.65768 -29.14145 0.92087 -28.62907 0.93699-51.83808 0.52595 -35.54212 0.65909 -29.13526 0.92087 -28.62716 0.93699-51.68226 0.52595 -35.47331 0.66238 -29.12916 0.92135 -28.62526 0.93699-51.52744 0.52595 -35.40479 0.66519 -29.12314 0.92159 -28.62336 0.92779-51.37361 0.52595 -35.33656 0.66625 -29.1172 0.92188 -28.62148 0.92779-51.22076 0.52595 -35.26861 0.66809 -29.11135 0.92193 -28.6196 0.93728-51.06888 0.52595 -35.20096 0.67003 -29.10557 0.92193 -28.61773 0.93728-50.91796 0.52595 -35.13359 0.67274 -29.09987 0.92193 -28.61586 0.93728-50.76799 0.52595 -35.0665 0.67511 -29.09424 0.92193 -28.61401 0.92914-50.61897 0.52595 -34.99969 0.67647 -29.08868 0.92193 -28.61216 0.93767-50.47088 0.52595 -34.93317 0.6786 -29.08319 0.92193 -28.61032 0.93767-50.32372 0.52595 -34.86692 0.67962 -29.07778 0.92193 -28.60849 0.93767-50.17748 0.52595 -34.80095 0.68213 -29.07243 0.92193 -28.60667 0.93767-50.03214 0.52595 -34.73525 0.68334 -29.06714 0.92193 -28.60485 0.92973-49.88771 0.52595 -34.66983 0.68567 -29.06192 0.92193 -28.60304 0.92973-49.74416 0.52595 -34.60467 0.68693 -29.05676 0.92193 -28.60124 0.92973-49.6015 0.52595 -34.53979 0.68886 -29.05166 0.92193 -28.59945 0.92973-49.45972 0.52595 -34.47517 0.68983 -29.04662 0.92193 -28.59766 0.93771-49.3188 0.52595 -34.41082 0.6924 -29.04164 0.92266 -28.59589 0.93786-49.17874 0.52595 -34.34673 0.69341 -29.03671 0.9229 -28.59411 0.93786-49.03953 0.52595 -34.28291 0.69496 -29.03184 0.9229 -28.59235 0.93786-48.90117 0.52595 -34.21935 0.69642 -29.02702 0.9229 -28.59059 0.93786-48.76364 0.52595 -34.15605 0.69806 -29.02226 0.9229 -28.58884 0.93035302D.4. Master Sintering Curve Data /  /  /  /-48.62695 0.52595 -34.093 0.69942 -29.01755 0.9229 -28.5871 0.938-48.49107 0.52595 -34.03021 0.70072 -29.01288 0.9229 -28.58536 0.938-48.356 0.52595 -33.96768 0.70198 -29.00827 0.9229 -28.58363 0.938-48.22175 0.52595 -33.9054 0.70295 -29.00371 0.9229 -28.58191 0.93137-48.08829 0.52595 -33.84337 0.70397 -28.99919 0.92309 -28.5802 0.932-47.95562 0.52595 -33.78159 0.70528 -28.99472 0.92309 -28.57849 0.932-47.82374 0.52595 -33.72006 0.70634 -28.9903 0.92309 -28.57679 0.932-47.69263 0.52595 -33.65878 0.70731 -28.98592 0.92309 -28.57509 0.932-47.5623 0.52595 -33.59774 0.70837 -28.98158 0.92309 -28.5734 0.93815-47.43273 0.52595 -33.53695 0.70944 -28.97729 0.92309 -28.57172 0.93815-47.30392 0.52595 -33.4764 0.71099 -28.97304 0.92363 -28.57004 0.93815-47.17586 0.52595 -33.41609 0.71292 -28.96883 0.92363 -28.56837 0.93859-47.04854 0.52595 -33.35602 0.71409 -28.96466 0.92387 -28.56671 0.93859-46.92196 0.52595 -33.29619 0.71505 -28.96053 0.9243 -28.56505 0.93859-46.79612 0.52595 -33.23659 0.71602 -28.95644 0.9243 -28.5634 0.93859-46.67099 0.52595 -33.17723 0.71723 -28.95239 0.9243 -28.56176 0.93859-46.54659 0.52595 -33.11811 0.71854 -28.94837 0.9243 -28.56012 0.95282-46.4229 0.52595 -33.05922 0.71951 -28.94439 0.9243 -28.55849 0.95282-46.29991 0.52595 -33.00056 0.72062 -28.94045 0.9243 -28.55687 0.95282-46.17763 0.52595 -32.94213 0.72183 -28.93654 0.9243 -28.55525 0.95282-46.05604 0.52595 -32.88393 0.72304 -28.93267 0.9243 -28.55363 0.95282-45.93514 0.52595 -32.82595 0.72382 -28.92883 0.9243 -28.55202 0.95282-45.81492 0.52595 -32.7682 0.72556 -28.92503 0.9243 -28.55042 0.93878-45.69538 0.52595 -32.71068 0.72764 -28.92126 0.9243 -28.54883 0.93878-45.57651 0.52595 -32.65338 0.72895 -28.91752 0.9243 -28.54724 0.93878303D.4. Master Sintering Curve Data /  /  /  /-45.4583 0.52595 -32.5963 0.73045 -28.91381 0.9243 -28.54565 0.93878-45.34075 0.52595 -32.53944 0.73219 -28.91014 0.9243 -28.54408 0.93878-45.22386 0.52595 -32.4828 0.73457 -28.90649 0.9243 -28.5425 0.94933-45.10762 0.52595 -32.42638 0.73636 -28.90288 0.9243 -28.54094 0.94933-44.99202 0.52595 -32.37018 0.73752 -28.89929 0.9243 -28.53937 0.94933-44.87706 0.52595 -32.31419 0.7395 -28.89574 0.9243 -28.53782 0.94933-44.76273 0.52595 -32.25842 0.74275 -28.89221 0.9243 -28.53627 0.94933-44.64903 0.52595 -32.20286 0.74459 -28.88871 0.9243 -28.53472 0.94933-44.53595 0.52595 -32.14751 0.74517 -28.88525 0.9243-44.42349 0.52595 -32.09237 0.74643 -28.8818 0.9243304D.4. Master Sintering Curve DataD.4.6. Master Sintering Curve Data of ThO2-5 wt% DiamondTable D.8: Master Sintering Curve Data of ThO2-5 wt% Diamond . /  /  /  /-76.5069 0.4456 -46.31447 0.50376 -33.76479 0.71644 -31.19582 0.86885-76.16297 0.44918 -46.19588 0.50417 -33.70522 0.71778 -31.1926 0.86885-75.82228 0.45903 -46.07794 0.50466 -33.64589 0.72038 -31.18941 0.83379-75.48477 0.46159 -45.96065 0.50529 -33.58678 0.72244 -31.18624 0.83384-75.1504 0.46351 -45.844 0.50564 -33.52791 0.7241 -31.18309 0.83384-74.81913 0.46718 -45.72799 0.50623 -33.46926 0.72638 -31.17996 0.8697-74.49091 0.46772 -45.6126 0.50667 -33.41084 0.72884 -31.17686 0.8706-74.1657 0.46772 -45.49785 0.50699 -33.35265 0.73157 -31.17378 0.8706-73.84346 0.46772 -45.38372 0.5073 -33.29468 0.73292 -31.17072 0.8706-73.52415 0.46772 -45.2702 0.50824 -33.23693 0.73556 -31.16768 0.8706-73.20773 0.46772 -45.15729 0.50873 -33.1794 0.7386 -31.16467 0.83437-72.89416 0.46772 -45.04499 0.50936 -33.12209 0.74039 -31.16167 0.87091-72.5834 0.46772 -44.9333 0.5099 -33.06501 0.7416 -31.1587 0.83442-72.27542 0.46772 -44.8222 0.51043 -33.00813 0.74406 -31.15574 0.87149-71.97017 0.46844 -44.71169 0.51138 -32.95148 0.74693 -31.15281 0.83446-71.66762 0.46965 -44.60176 0.51182 -32.89504 0.74926 -31.14989 0.8718-71.36774 0.47144 -44.49242 0.51232 -32.83881 0.75127 -31.147 0.83469-71.07049 0.4735 -44.38366 0.51352 -32.7828 0.75391 -31.14412 0.87239-70.77583 0.47412 -44.27548 0.51406 -32.72699 0.7566 -31.14126 0.87243-70.48374 0.47412 -44.16786 0.51469 -32.6714 0.7579 -31.13842 0.87243-70.19417 0.47412 -44.0608 0.51563 -32.61602 0.76068 -31.1356 0.83643-69.90709 0.47412 -43.9543 0.51648 -32.56084 0.76412 -31.1328 0.83643305D.4. Master Sintering Curve Data /  /  /  /-69.62248 0.47412 -43.84836 0.51715 -32.50587 0.76793 -31.13001 0.83684-69.3403 0.47412 -43.74298 0.51827 -32.45111 0.76963 -31.12725 0.83684-69.06053 0.47412 -43.63813 0.51917 -32.34808 0.77411 -31.1245 0.87279-68.78312 0.47412 -43.53383 0.51997 -32.30449 0.77666 -31.12177 0.83697-68.50805 0.47412 -43.43007 0.52114 -32.26487 0.77796 -31.11905 0.83697-68.23529 0.47412 -43.32685 0.52167 -32.22857 0.78033 -31.11635 0.83697-67.96481 0.47412 -43.22415 0.52275 -32.19507 0.7815 -31.11367 0.87283-67.69658 0.47412 -43.12198 0.52387 -32.16397 0.78355 -31.11101 0.87315-67.43058 0.47412 -43.02033 0.52485 -32.13495 0.78432 -31.10836 0.87315-67.16677 0.47412 -42.9192 0.52579 -32.10774 0.78535 -31.10573 0.83778-66.90514 0.47506 -42.81858 0.52691 -32.08214 0.78776 -31.10311 0.87337-66.64564 0.47609 -42.71847 0.52781 -32.05797 0.7887 -31.10051 0.87337-66.38827 0.4773 -42.61887 0.52866 -32.03507 0.78955 -31.09792 0.83804-66.13298 0.47869 -42.51977 0.52996 -32.01332 0.79049 -31.09535 0.87386-65.87976 0.48044 -42.42117 0.53063 -31.99261 0.7913 -31.0928 0.83822-65.62858 0.48223 -42.32306 0.53179 -31.97284 0.79278 -31.09026 0.83854-65.37941 0.48411 -42.22545 0.53282 -31.95393 0.79327 -31.08773 0.83854-65.13224 0.48478 -42.12832 0.53376 -31.93581 0.79421 -31.08522 0.83854-64.88703 0.48478 -42.03167 0.53502 -31.91841 0.79555 -31.08272 0.87418-64.64377 0.48478 -41.93551 0.53591 -31.90169 0.79618 -31.08024 0.83876-64.40243 0.48478 -41.83982 0.53681 -31.88558 0.79699 -31.07777 0.8744-64.16298 0.48478 -41.7446 0.5377 -31.87005 0.79761 -31.07532 0.83903-63.92542 0.48478 -41.64986 0.53873 -31.85506 0.79846 -31.07288 0.87462-63.6897 0.48478 -41.55558 0.53985 -31.84057 0.80066 -31.07045 0.87462-63.45582 0.48478 -41.46176 0.54057 -31.82654 0.80129 -31.06804 0.87516306D.4. Master Sintering Curve Data /  /  /  /-63.22375 0.48478 -41.3684 0.54187 -31.81296 0.80169 -31.06564 0.87516-62.99347 0.48478 -41.2755 0.54263 -31.79979 0.80214 -31.06325 0.83952-62.76496 0.48554 -41.18305 0.54379 -31.787 0.80303 -31.06088 0.83952-62.53819 0.48679 -41.09105 0.54478 -31.77458 0.80496 -31.05852 0.87548-62.31316 0.48805 -40.99949 0.54616 -31.76251 0.80536 -31.05617 0.83993-62.08983 0.48894 -40.90838 0.54693 -31.75076 0.8054 -31.05384 0.87592-61.8682 0.48921 -40.81771 0.54818 -31.73932 0.8059 -31.05151 0.84078-61.64823 0.48921 -40.72747 0.54966 -31.72818 0.80634 -31.0492 0.87628-61.42992 0.48921 -40.63766 0.55064 -31.71731 0.80652 -31.0469 0.87628-61.21324 0.48921 -40.54829 0.55158 -31.70671 0.80702 -31.04462 0.84109-60.99817 0.48921 -40.45934 0.55319 -31.69636 0.80733 -31.04234 0.87651-60.7847 0.48921 -40.37081 0.55418 -31.68625 0.80782 -31.04008 0.84131-60.57281 0.48921 -40.28271 0.5557 -31.67638 0.80818 -31.03783 0.84154-60.36248 0.48921 -40.19502 0.55669 -31.66672 0.80881 -31.03559 0.84154-60.15369 0.48921 -40.10775 0.55807 -31.65727 0.80881 -31.03337 0.84154-59.94643 0.48921 -40.02089 0.55955 -31.64802 0.81024 -31.03115 0.84154-59.74069 0.48921 -39.93444 0.56107 -31.63897 0.81055 -31.02894 0.84176-59.53643 0.48921 -39.8484 0.5626 -31.6301 0.81105 -31.02675 0.84176-59.33365 0.48962 -39.76275 0.56389 -31.62141 0.8114 -31.02457 0.877-59.13234 0.49011 -39.67751 0.56573 -31.61289 0.81145 -31.0224 0.84194-58.93247 0.491 -39.59267 0.56721 -31.60453 0.81163 -31.02023 0.84198-58.73403 0.49163 -39.50822 0.56783 -31.59633 0.8119 -31.01808 0.84198-58.53701 0.4919 -39.42416 0.56945 -31.58829 0.8123 -31.01594 0.84239-58.34138 0.4923 -39.34049 0.57092 -31.58039 0.8123 -31.01382 0.84239-58.14714 0.49261 -39.25721 0.57258 -31.57263 0.81436 -31.0117 0.87713307D.4. Master Sintering Curve Data /  /  /  /-57.95427 0.49387 -39.17431 0.57401 -31.565 0.81467 -31.00959 0.84257-57.76276 0.49427 -39.09179 0.57572 -31.55751 0.81485 -31.00749 0.84257-57.57259 0.49436 -39.00965 0.57773 -31.55015 0.81508 -31.0054 0.87736-57.38374 0.49436 -38.92789 0.57948 -31.5429 0.81557 -31.00332 0.8427-57.19621 0.49436 -38.8465 0.58068 -31.53578 0.81557 -31.00126 0.8778-57.00998 0.49436 -38.76548 0.58207 -31.52877 0.81584 -30.9992 0.84297-56.82503 0.49436 -38.68483 0.5836 -31.52188 0.81584 -30.99715 0.8431-56.64136 0.49436 -38.60454 0.58507 -31.51509 0.81588 -30.99511 0.8431-56.45895 0.49436 -38.52462 0.587 -31.5084 0.81655 -30.99308 0.87789-56.27778 0.49436 -38.44506 0.58915 -31.50182 0.81709 -30.99106 0.87789-56.09785 0.49436 -38.36586 0.58995 -31.49533 0.8174 -30.98905 0.87789-55.91914 0.49436 -38.28701 0.59165 -31.48894 0.81776 -30.98705 0.84387-55.74163 0.49436 -38.20851 0.59318 -31.48265 0.81781 -30.98505 0.84387-55.56533 0.49436 -38.13037 0.59564 -31.47644 0.81834 -30.98307 0.87825-55.39021 0.49436 -38.05258 0.59658 -31.47032 0.81857 -30.9811 0.87852-55.21626 0.49436 -37.97513 0.59815 -31.46429 0.81861 -30.97913 0.87852-55.04347 0.49436 -37.89802 0.59971 -31.45833 0.81884 -30.97717 0.87852-54.87184 0.49436 -37.82126 0.60191 -31.45246 0.81884 -30.97523 0.844-54.70134 0.49436 -37.74484 0.60406 -31.44667 0.81897 -30.97329 0.87883-54.53196 0.49436 -37.66875 0.60504 -31.44095 0.82108 -30.97136 0.87883-54.36371 0.49476 -37.593 0.60688 -31.43531 0.82161 -30.96943 0.87883-54.19655 0.49508 -37.51758 0.6097 -31.42974 0.82161 -30.96752 0.87883-54.03049 0.49553 -37.4425 0.61095 -31.42424 0.8217 -30.96562 0.87883-53.86552 0.49611 -37.36774 0.61225 -31.41881 0.82197 -30.96372 0.87915-53.70161 0.49615 -37.29331 0.61579 -31.41345 0.82224 -30.96183 0.87915308D.4. Master Sintering Curve Data /  /  /  /-53.53877 0.49624 -37.2192 0.61655 -31.40815 0.82224 -30.95995 0.87915-53.37698 0.49624 -37.14541 0.61852 -31.40291 0.8226 -30.95808 0.87915-53.21623 0.49624 -37.07194 0.62098 -31.39774 0.82305 -30.95621 0.87919-53.05651 0.49624 -36.99879 0.62259 -31.39263 0.82314 -30.95436 0.87919-52.89781 0.49624 -36.92596 0.62412 -31.38758 0.82336 -30.95251 0.87919-52.74012 0.49624 -36.85344 0.62595 -31.38259 0.82372 -30.95067 0.87919-52.58344 0.49624 -36.78123 0.62904 -31.37765 0.82457 -30.94884 0.87919-52.42775 0.49624 -36.70933 0.62985 -31.37277 0.82484 -30.94701 0.87919-52.27304 0.49624 -36.63774 0.63173 -31.36794 0.82484 -30.94519 0.87942-52.1193 0.49624 -36.56646 0.63307 -31.36317 0.82542 -30.94338 0.87942-51.96653 0.49624 -36.49548 0.63553 -31.35844 0.82555 -30.94158 0.87942-51.81472 0.49624 -36.4248 0.63808 -31.35377 0.82587 -30.93979 0.87968-51.66385 0.49624 -36.35442 0.63929 -31.34915 0.826 -30.938 0.87968-51.51391 0.49624 -36.28433 0.64095 -31.34458 0.82609 -30.93622 0.87968-51.36491 0.49624 -36.21455 0.64408 -31.34005 0.82663 -30.93445 0.87968-51.21683 0.49624 -36.14506 0.64565 -31.33557 0.82685 -30.93268 0.88009-51.06965 0.49624 -36.07586 0.64704 -31.33114 0.82743 -30.93092 0.88009-50.92338 0.49624 -36.00695 0.6487 -31.32675 0.82784 -30.92917 0.88009-50.77801 0.49624 -35.93833 0.65067 -31.32241 0.82824 -30.92743 0.88009-50.63352 0.49624 -35.86999 0.65443 -31.31811 0.82824 -30.92569 0.88009-50.48991 0.49624 -35.80194 0.6555 -31.31385 0.82851 -30.92396 0.88009-50.34718 0.49624 -35.73418 0.65702 -31.30963 0.82851 -30.92224 0.84476-50.2053 0.49624 -35.66669 0.65899 -31.30545 0.82882 -30.92052 0.88031-50.06428 0.49624 -35.59949 0.6607 -31.30132 0.82891 -30.91881 0.88031-49.92411 0.49624 -35.53256 0.66284 -31.29722 0.82931 -30.91711 0.88031309D.4. Master Sintering Curve Data /  /  /  /-49.78477 0.49624 -35.46591 0.66499 -31.29316 0.82931 -30.91541 0.84521-49.64627 0.49624 -35.39953 0.66714 -31.28913 0.82954 -30.91372 0.84521-49.50859 0.49624 -35.33343 0.66925 -31.28515 0.82954 -30.91204 0.88089-49.37173 0.49624 -35.2676 0.67028 -31.2812 0.82967 -30.91036 0.88121-49.23568 0.49624 -35.20203 0.67238 -31.27729 0.82967 -30.90869 0.88121-49.10043 0.49669 -35.13674 0.67471 -31.27341 0.82967 -30.90702 0.88121-48.96598 0.49691 -35.07171 0.67596 -31.26956 0.82972 -30.90537 0.88121-48.83231 0.49709 -35.00694 0.67807 -31.26575 0.8299 -30.90371 0.88139-48.69943 0.4975 -34.94244 0.67981 -31.26198 0.82999 -30.90207 0.88139-48.56732 0.49754 -34.8782 0.68129 -31.25823 0.8303 -30.90043 0.88139-48.43598 0.49799 -34.81422 0.68384 -31.25452 0.8303 -30.8988 0.88206-48.3054 0.49861 -34.7505 0.68608 -31.25084 0.83052 -30.89717 0.88206-48.17557 0.4987 -34.68703 0.68738 -31.24719 0.83075 -30.89555 0.88206-48.0465 0.49888 -34.62382 0.68935 -31.24357 0.83075 -30.89394 0.88206-47.91816 0.49933 -34.56086 0.69078 -31.23998 0.83079 -30.89233 0.88219-47.79056 0.49951 -34.49815 0.69311 -31.23642 0.83097 -30.89072 0.88219-47.66369 0.50005 -34.4357 0.69562 -31.23288 0.83097 -30.88913 0.88219-47.53754 0.50036 -34.37349 0.6966 -31.22938 0.86764 -30.88754 0.88219-47.41211 0.5005 -34.31153 0.69884 -31.2259 0.86764 -30.88595 0.88264-47.28739 0.50108 -34.24982 0.70045 -31.22246 0.86791 -30.88437 0.88264-47.16337 0.50157 -34.18835 0.7026 -31.21904 0.83227 -30.8828 0.88264-47.04005 0.50166 -34.12712 0.70386 -31.21564 0.83276 -30.88123 0.84584-46.91742 0.50215 -34.06614 0.70654 -31.21228 0.83276 -30.87967 0.88268-46.79548 0.5026 -34.00539 0.70825 -31.20893 0.86804 -30.87811 0.88268-46.67422 0.50278 -33.94489 0.70977 -31.20562 0.83303310D.4. Master Sintering Curve Data /  /  /  /-46.55364 0.50309 -33.88462 0.71178 -31.20233 0.86849-46.43372 0.50327 -33.82459 0.71425 -31.19906 0.83339311D.4. Master Sintering Curve DataD.4.7. Master Sintering Curve Data of Th0.9U0.1O2Table D.9: Master Sintering Curve Data of Th0.9U0.1O2. /  /  /  /-15.69701 0.45088 -12.51201 0.46699 -10.58281 0.74542 -9.2843 0.93374-15.62691 0.45088 -12.47215 0.46874 -10.55704 0.75367 -9.26622 0.93644-15.55756 0.45088 -12.43262 0.47009 -10.53143 0.76267 -9.24825 0.94059-15.48897 0.45088 -12.39341 0.47129 -10.50599 0.77182 -9.23036 0.94424-15.42111 0.45088 -12.35452 0.47349 -10.48072 0.77927 -9.21257 0.94544-15.35397 0.45088 -12.31593 0.47464 -10.4556 0.78852 -9.19488 0.94544-15.28755 0.45088 -12.27766 0.47599 -10.43065 0.79497 -9.17728 0.94544-15.22183 0.45088 -12.23969 0.47809 -10.40585 0.80222 -9.15977 0.94544-15.15679 0.45088 -12.20201 0.48019 -10.38122 0.80587 -9.14236 0.94544-15.09244 0.45088 -12.16464 0.48214 -10.35673 0.81753 -9.12504 0.95069-15.02875 0.45088 -12.12756 0.48349 -10.33241 0.82518 -10.09728 0.86603-14.96572 0.45088 -12.09076 0.48629 -10.30823 0.82993 -10.07456 0.86783-14.90334 0.45088 -12.05426 0.48769 -10.28421 0.83753 -10.05197 0.87138-14.84159 0.45088 -12.01804 0.49119 -10.26033 0.84168 -10.02952 0.87258-14.78048 0.45088 -11.98209 0.49199 -10.23661 0.84623 -10.0072 0.87548-14.71998 0.45088 -11.94643 0.49549 -10.21303 0.85418 -9.98502 0.87693-14.66008 0.45088 -11.91103 0.49644 -10.18959 0.85583 -9.96296 0.87953-14.60079 0.45088 -11.87591 0.49964 -10.1663 0.85913 -9.94104 0.88148-14.54209 0.45088 -11.84105 0.50159 -10.14316 0.86188 -9.91926 0.88368-14.48397 0.45088 -11.80646 0.50454 -10.12015 0.86393 -9.89759 0.88503-14.42642 0.45088 -11.77212 0.50624 -10.09728 0.86603 -9.87606 0.88688-14.36943 0.45088 -11.73805 0.50939 -10.07456 0.86783 -9.85466 0.88833312D.4. Master Sintering Curve Data /  /  /  /-14.313 0.45088 -11.70423 0.51349 -10.05197 0.87138 -9.83338 0.89013-14.25712 0.45088 -11.67066 0.51509 -10.02952 0.87258 -9.81222 0.89223-14.20178 0.45088 -11.63734 0.51829 -10.0072 0.87548 -9.79119 0.89243-14.14696 0.45088 -11.60426 0.52234 -9.98502 0.87693 -9.77028 0.89423-14.09268 0.45088 -11.57143 0.52519 -9.96296 0.87953 -9.74949 0.89473-14.03891 0.45088 -11.53884 0.52844 -9.94104 0.88148 -9.72882 0.89618-13.98564 0.45088 -11.50649 0.53194 -9.91926 0.88368 -9.70827 0.89718-13.93288 0.45088 -11.47438 0.53634 -9.89759 0.88503 -9.68784 0.89783-13.88062 0.45088 -11.44249 0.54079 -9.87606 0.88688 -9.66752 0.90013-13.82884 0.45088 -11.41084 0.54534 -9.85466 0.88833 -9.64732 0.90058-13.77754 0.45088 -11.37942 0.5471 -9.83338 0.89013 -9.62724 0.90188-13.72671 0.45088 -11.34822 0.55175 -9.81222 0.89223 -9.60727 0.90409-13.67636 0.45088 -11.31724 0.5574 -9.79119 0.89243 -9.58741 0.90419-13.62646 0.45088 -11.28648 0.56145 -9.77028 0.89423 -9.56767 0.90654-13.57702 0.45108 -11.25595 0.56615 -9.74949 0.89473 -9.54803 0.90679-13.52802 0.45158 -11.22562 0.5746 -9.72882 0.89618 -9.52851 0.90824-13.47947 0.45173 -11.19551 0.5787 -9.70827 0.89718 -9.50909 0.90879-13.43135 0.45218 -11.16562 0.5838 -9.68784 0.89783 -9.48979 0.91054-13.38367 0.45218 -11.13593 0.5892 -9.66752 0.90013 -9.47059 0.91269-13.33641 0.45218 -11.10644 0.5946 -9.64732 0.90058 -9.4515 0.91479-13.28956 0.45218 -11.07717 0.60025 -9.62724 0.90188 -9.43251 0.91644-13.24313 0.45218 -11.04809 0.60705 -9.60727 0.90409 -9.41363 0.91644-13.19711 0.45218 -11.01921 0.6129 -9.58741 0.90419 -9.39485 0.91849-13.15149 0.45218 -10.99054 0.62105 -9.56767 0.90654 -9.37617 0.91934-13.10626 0.45218 -10.96206 0.6274 -9.54803 0.90679 -9.3576 0.92004313D.4. Master Sintering Curve Data /  /  /  /-13.06143 0.45343 -10.93377 0.6344 -9.52851 0.90824 -9.33912 0.92169-13.01698 0.45448 -10.90567 0.64066 -9.50909 0.90879 -9.32075 0.92384-12.97292 0.45518 -10.87777 0.64731 -9.48979 0.91054 -9.30248 0.92499-12.92923 0.45638 -10.85005 0.66016 -9.47059 0.91269 -9.2843 0.93374-12.88591 0.45693 -10.82252 0.66621 -9.4515 0.91479 -9.26622 0.93644-12.84296 0.45819 -10.79517 0.67306 -9.43251 0.91644 -9.24825 0.94059-12.80037 0.45884 -10.76801 0.68091 -9.41363 0.91644 -9.23036 0.94424-12.75814 0.46004 -10.74102 0.68831 -9.39485 0.91849 -9.21257 0.94544-12.71626 0.46134 -10.71422 0.69611 -9.37617 0.91934 -9.19488 0.94544-12.67473 0.46254 -10.68759 0.70421 -9.3576 0.92004 -9.17728 0.94544-12.63354 0.46369 -10.66113 0.71396 -9.33912 0.92169 -9.15977 0.94544-12.5927 0.46429 -10.63485 0.72226 -9.32075 0.92384 -9.14236 0.94544-12.55218 0.46624 -10.60874 0.73272 -9.30248 0.92499 -9.12504 0.95069314D.4. Master Sintering Curve DataD.4.8. Master Sintering Curve Data of Th0.95U0.05O2Table D.10: Master Sintering Curve Data of Th0.95U0.05O2. /  /  /  /-31.93597 0.43418 -20.12899 0.52468 -15.94479 0.91208 -15.4545 0.92192-31.77691 0.43418 -20.07329 0.52883 -15.93758 0.91208 -15.45215 0.92192-31.61961 0.43418 -20.01795 0.53336 -15.93048 0.91208 -15.44982 0.92192-31.46403 0.43418 -19.96297 0.53751 -15.92351 0.91235 -15.4475 0.92192-31.31014 0.43418 -19.90834 0.54099 -15.91664 0.9124 -15.4452 0.92192-31.15791 0.43418 -19.85405 0.54583 -15.90988 0.9124 -15.4429 0.92192-31.00733 0.43418 -19.80012 0.55077 -15.90322 0.9124 -15.44062 0.92192-30.85835 0.43418 -19.74653 0.55788 -15.89667 0.9124 -15.43835 0.92198-30.71097 0.43418 -19.69328 0.56245 -15.89021 0.91277 -15.43609 0.92203-30.56514 0.43418 -19.64036 0.56819 -15.88384 0.91314 -15.43384 0.92203-30.42085 0.43418 -19.58778 0.57313 -15.87757 0.91361 -15.4316 0.92203-30.27807 0.43602 -19.53553 0.58029 -15.87139 0.91424 -15.42938 0.92203-30.13677 0.43912 -19.4836 0.58676 -15.86529 0.91445 -15.42717 0.92203-29.99694 0.44717 -19.432 0.59276 -15.85928 0.91445 -15.42497 0.92203-29.85856 0.44744 -19.38072 0.60002 -15.85335 0.91445 -15.42277 0.92203-29.72159 0.44812 -19.32976 0.60665 -15.8475 0.91445 -15.42059 0.92203-29.58602 0.44849 -19.27911 0.61344 -15.84173 0.91445 -15.41843 0.92203-29.45183 0.44944 -19.22878 0.62222 -15.83603 0.91445 -15.41627 0.92203-29.31899 0.45044 -19.17875 0.62875 -15.83041 0.91445 -15.41412 0.92203-29.18749 0.45144 -19.12903 0.63754 -15.82486 0.91445 -15.41198 0.92203-29.0573 0.45175 -19.07962 0.64527 -15.81938 0.91445 -15.40986 0.92203-28.9284 0.45201 -19.0305 0.65358 -15.81397 0.91445 -15.40774 0.92203315D.4. Master Sintering Curve Data /  /  /  /-28.80079 0.45243 -18.98168 0.66179 -15.80862 0.91445 -15.40564 0.92203-28.67442 0.45243 -18.93316 0.66879 -15.80334 0.91445 -15.40354 0.92203-28.5493 0.45243 -18.88493 0.67658 -15.79812 0.91445 -15.40145 0.92203-28.4254 0.45243 -18.83699 0.68484 -15.79297 0.91445 -15.39938 0.92203-28.30269 0.45243 -18.78934 0.69247 -15.78787 0.91487 -15.39731 0.92203-28.18118 0.45243 -18.74197 0.70231 -15.78284 0.91487 -15.39526 0.92203-28.06083 0.45364 -18.69488 0.70799 -15.77786 0.91487 -15.39321 0.92203-27.94162 0.4558 -18.64807 0.71651 -15.77294 0.91613 -15.39117 0.92203-27.82356 0.45743 -18.60154 0.72667 -15.76807 0.91619 -15.38915 0.92203-27.70661 0.45885 -18.55529 0.73693 -15.76326 0.91619 -15.38713 0.92224-27.59076 0.45912 -18.50931 0.74398 -15.7585 0.91629 -15.38512 0.92224-27.476 0.45912 -18.46359 0.75055 -15.75379 0.91629 -15.38312 0.92224-27.36231 0.45912 -18.41815 0.75897 -15.74913 0.91629 -15.38113 0.92224-27.24968 0.45912 -18.37296 0.76865 -15.74452 0.91629 -15.37915 0.92224-27.13809 0.45912 -18.32805 0.7786 -15.73996 0.91629 -15.37718 0.92224-27.02752 0.45912 -18.28339 0.78549 -15.73545 0.91629 -15.37522 0.92224-26.91797 0.45912 -18.23899 0.79591 -15.73098 0.91687 -15.37326 0.92224-26.80941 0.45912 -18.19484 0.80349 -15.72656 0.91724 -15.37132 0.92224-26.70185 0.46043 -18.15095 0.81017 -15.72219 0.91745 -15.36938 0.92224-26.59525 0.46291 -18.10731 0.81785 -15.71785 0.91745 -15.36745 0.92224-26.48961 0.46564 -18.06392 0.82737 -15.71356 0.91745 -15.36553 0.92224-26.38491 0.46575 -18.02077 0.83137 -15.70932 0.91745 -15.36362 0.92224-26.28115 0.46601 -17.97788 0.84053 -15.70511 0.91745 -15.36172 0.92224-26.17831 0.46601 -17.93522 0.84773 -15.70094 0.91745 -15.35982 0.92224-26.07638 0.46601 -17.8928 0.85258 -15.69682 0.91745 -15.35794 0.92224316D.4. Master Sintering Curve Data /  /  /  /-25.97534 0.46601 -17.85063 0.85763 -15.69273 0.91745 -15.35606 0.92224-25.87519 0.46601 -17.80869 0.86462 -15.68868 0.91745 -15.35419 0.92224-25.77591 0.46601 -17.76698 0.86768 -15.68467 0.9175 -15.35233 0.92224-25.67749 0.46601 -17.72551 0.87162 -15.68069 0.9175 -15.35047 0.92224-25.57992 0.46601 -17.68427 0.87578 -15.67675 0.91792 -15.34863 0.92224-25.48319 0.46659 -17.64325 0.8773 -15.67285 0.91792 -15.34679 0.92224-25.38728 0.4678 -17.60247 0.88009 -15.66898 0.91792 -15.34496 0.92224-25.2922 0.46822 -17.5619 0.88162 -15.66514 0.91792 -15.34314 0.92224-25.19792 0.46838 -17.52157 0.88351 -15.66134 0.91866 -15.34132 0.92224-25.10444 0.46838 -17.48145 0.8853 -15.65757 0.91866 -15.33951 0.92224-25.01174 0.46838 -17.44155 0.88699 -15.65384 0.91866 -15.33771 0.92224-24.91982 0.46838 -17.40187 0.88835 -15.65013 0.91866 -15.33592 0.92224-24.82867 0.46838 -17.36241 0.8892 -15.64646 0.91866 -15.33413 0.92224-24.73827 0.46838 -17.32316 0.89004 -15.64282 0.91866 -15.33236 0.92224-24.64862 0.46838 -17.28412 0.89041 -15.63921 0.91913 -15.33058 0.92224-24.55971 0.46885 -17.24529 0.89156 -15.63562 0.91913 -15.32882 0.92213-24.47153 0.4698 -17.20667 0.89283 -15.63207 0.91919 -15.32706 0.92287-24.38407 0.47174 -17.16826 0.89377 -15.62855 0.91919 -15.32531 0.92287-24.29732 0.4719 -17.13005 0.89483 -15.62505 0.91919 -15.32357 0.92287-24.21128 0.4719 -17.09205 0.89498 -15.62158 0.91919 -15.32184 0.92287-24.12592 0.4719 -17.05425 0.89609 -15.61814 0.91919 -15.32011 0.9234-24.04126 0.4719 -17.01665 0.89704 -15.61473 0.91929 -15.31839 0.9234-23.95727 0.4719 -16.97925 0.89746 -15.61134 0.91929 -15.31667 0.9234-23.87395 0.4719 -16.94204 0.89809 -15.60798 0.9195 -15.31496 0.9234-23.79129 0.4719 -16.90504 0.89856 -15.60465 0.9195 -15.31326 0.9234317D.4. Master Sintering Curve Data /  /  /  /-23.70928 0.4719 -16.86822 0.89977 -15.60134 0.91955 -15.31157 0.9234-23.62792 0.4719 -16.8316 0.90019 -15.59806 0.91955 -15.30988 0.9234-23.5472 0.4719 -16.79517 0.90082 -15.5948 0.91971 -15.3082 0.9234-23.46711 0.4719 -16.75893 0.90124 -15.59156 0.91971 -15.30652 0.9234-23.38764 0.4719 -16.72288 0.90193 -15.58835 0.91971 -15.30485 0.9234-23.30879 0.4719 -16.68701 0.90219 -15.58516 0.91971 -15.30319 0.9234-23.23054 0.4719 -16.65133 0.90251 -15.582 0.91971 -15.30153 0.9234-23.1529 0.47217 -16.58302 0.90382 -15.57886 0.91992 -15.29988 0.9234-23.07585 0.47248 -16.55252 0.90419 -15.57574 0.92034 -15.29824 0.9234-22.99939 0.47332 -16.52401 0.90445 -15.57264 0.92034 -15.2966 0.9234-22.92351 0.47427 -16.49726 0.9053 -15.56957 0.92108 -15.29497 0.9234-22.8482 0.47464 -16.47207 0.90545 -15.56651 0.92108 -15.29335 0.9234-22.77347 0.47501 -16.44826 0.90545 -15.56348 0.92108 -15.29173 0.9234-22.69929 0.47532 -16.42568 0.90545 -15.56047 0.92108 -15.29012 0.9234-22.62567 0.47538 -16.40422 0.90545 -15.55748 0.92108 -15.28851 0.9234-22.5526 0.47627 -16.38377 0.90598 -15.55451 0.92108 -15.28691 0.9234-22.48006 0.47669 -16.36424 0.90635 -15.55156 0.92108 -15.28531 0.9234-22.40807 0.47748 -16.34555 0.90666 -15.54863 0.92108 -15.28372 0.9234-22.33661 0.47748 -16.32763 0.90666 -15.54572 0.92108 -15.28214 0.9234-22.26567 0.47748 -16.31043 0.90724 -15.54283 0.92108 -15.28056 0.9234-22.19525 0.47748 -16.29387 0.9073 -15.53996 0.92108 -15.27899 0.9234-22.12534 0.47748 -16.27793 0.90735 -15.53711 0.92108 -15.27742 0.9234-22.05594 0.47748 -16.26255 0.90735 -15.53427 0.92108 -15.27586 0.9234-21.98704 0.47748 -16.2477 0.90793 -15.53146 0.92108 -15.27431 0.9234-21.91864 0.47748 -16.23334 0.90808 -15.52866 0.92108 -15.27276 0.9234318D.4. Master Sintering Curve Data /  /  /  /-21.85073 0.47748 -16.21943 0.90824 -15.52588 0.92108 -15.27121 0.9234-21.78331 0.47748 -16.20596 0.90824 -15.52312 0.92108 -15.26967 0.9234-21.71636 0.47748 -16.1929 0.90882 -15.52037 0.92108 -15.26814 0.9234-21.6499 0.47748 -16.18021 0.90903 -15.51765 0.92108 -15.26661 0.9234-21.5839 0.47748 -16.16789 0.90903 -15.51493 0.92108 -15.26509 0.9234-21.51837 0.47748 -16.1559 0.90903 -15.51224 0.92108 -15.26357 0.92361-21.45329 0.47827 -16.14424 0.90908 -15.50956 0.92108 -15.26206 0.92361-21.38868 0.47916 -16.13289 0.90935 -15.5069 0.92108 -15.26055 0.92361-21.32451 0.48058 -16.12182 0.90935 -15.50426 0.92108 -15.25905 0.92361-21.26079 0.48137 -16.11102 0.90998 -15.50163 0.92108 -15.25756 0.92361-21.19751 0.48222 -16.10049 0.90998 -15.49902 0.92108 -15.25606 0.92361-21.13467 0.48337 -16.09021 0.90998 -15.49642 0.92108 -15.25458 0.92361-21.07226 0.48511 -16.08017 0.91019 -15.49384 0.92108 -15.2531 0.92361-21.01028 0.48637 -16.07035 0.91019 -15.49127 0.92108 -15.25162 0.92361-20.94872 0.48748 -16.06075 0.91072 -15.48872 0.92108 -15.25015 0.92361-20.88758 0.48916 -16.05136 0.91087 -15.48619 0.92108 -15.24868 0.92361-20.82685 0.49105 -16.04217 0.91103 -15.48367 0.92108 -15.24722 0.92361-20.76653 0.49269 -16.03316 0.91103 -15.48116 0.92108 -15.24577 0.92361-20.70663 0.495 -16.02434 0.91161 -15.47867 0.92108 -15.24432 0.92361-20.64712 0.497 -16.0157 0.91161 -15.47619 0.92108 -15.24287 0.92361-20.58801 0.49921 -16.00722 0.91161 -15.47372 0.92108 -15.24143 0.92361-20.52929 0.50079 -15.99891 0.91177 -15.47127 0.92108 -15.23999 0.92361-20.47097 0.50416 -15.99075 0.91177 -15.46884 0.9215 -15.23856 0.92392-20.41303 0.50721 -15.98275 0.91177 -15.46641 0.9215 -15.23713 0.92392-20.35547 0.51026 -15.97488 0.91177 -15.464 0.92161 -15.23571 0.92392319D.4. Master Sintering Curve Data /  /  /  /-20.2983 0.51294 -15.96716 0.91177 -15.46161 0.92192 -15.23429 0.92392-20.24149 0.51599 -15.95957 0.91177 -15.45923 0.92192 -15.23288 0.92392-20.18506 0.52026 -15.95212 0.91177 -15.45686 0.92192320321E.1. Engineering Drawings of "Tight tight" Tooling ComponentsAppendix EE.1. Engineering Drawings of "Tight tight" ToolingComponentsFigure E.1: Engineering drawing of the bottom punch for "Tight tight" IXAF SPS setup(Dimensions in mm).322E.1. Engineering Drawings of "Tight tight" Tooling ComponentsFigure E.2: Engineering drawing of the die for "Tight tight" IXAF SPS setup (Dimensions inmm).323E.1. Engineering Drawings of "Tight tight" Tooling ComponentsFigure E.3: Engineering drawing of the core rod for "Tight tight" IXAF SPS setup(Dimensions in mm).324E.1. Engineering Drawings of "Tight tight" Tooling ComponentsFigure E.4: Engineering drawing of the top punch for "Tight tight" IXAF SPS setup(Dimensions in mm).325E.2. Engineering Drawings of "Loose tight" Tooling ComponentsE.2. Engineering Drawings of "Loose tight" ToolingComponentsFigure E.5: Engineering drawing of the bottom punch for "Loose tight" IXAF SPS setup(Dimensions in mm). 326E.2. Engineering Drawings of "Loose tight" Tooling ComponentsFigure E.6: Engineering drawing of the die for "Loose tight" IXAF SPS setup (Dimensions inmm).327E.2. Engineering Drawings of "Loose tight" Tooling ComponentsFigure E.7: Engineering drawing of the core rod for "Loose tight" IXAF SPS setup(Dimensions in mm).328E.2. Engineering Drawings of "Loose tight" Tooling ComponentsFigure E.8: Engineering drawing of the top punch for "Loose tight" IXAF SPS setup(Dimensions in mm).329E.3. Engineering Drawings of "Tight loose" Tooling ComponentsE.3. Engineering Drawings of "Tight loose" ToolingComponentsFigure E.9: Engineering drawing of the bottom punch for "Tight loose" IXAF SPS setup(Dimensions in mm). 330E.3. Engineering Drawings of "Tight loose" Tooling ComponentsFigure E.10: Engineering drawing of the die for "Tight loose" IXAF SPS setup (Dimensionsin mm).331E.3. Engineering Drawings of "Tight loose" Tooling ComponentsFigure E.11: Engineering drawing of the core rod for "Tight loose" IXAF SPS setup.332E.3. Engineering Drawings of "Tight loose" Tooling ComponentsFigure E.12: Engineering drawing of the top punch for "Tight loose" IXAF SPS setup.333E.4. Engineering Drawings of "Loose loose" Tooling ComponentsE.4. Engineering Drawings of "Loose loose" ToolingComponentsFigure E.13: Engineering drawing of the bottom punch for "Loose loose" IXAF SPS setup.334E.4. Engineering Drawings of "Loose loose" Tooling ComponentsFigure E.14: Engineering drawing of the die for "Loose loose" IXAF SPS setup.335E.4. Engineering Drawings of "Loose loose" Tooling ComponentsFigure E.15: Engineering drawing of the core rod for "Loose loose" IXAF SPS setup.336E.4. Engineering Drawings of "Loose loose" Tooling ComponentsFigure E.16: Engineering drawing of the top punch for "Loose loose" IXAF SPS setup.337
Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Characterization of spark plasma sintered nuclear fuel materials
Text