Microstructure Control of Uranium Dioxide Nuclear Fuel Pellets and Its Propertie

High energy ball milling (HEBM) and spark plasma sintering (SPS) was combined together to control the microstructure of UO2 nuclear fuel pellet. The main purpose of the former is to prepare a powder feedstock for the latter to densify into pellet form. As a result, UO2 nuclear fuel pellets with various microstructure features, i.e. grain size, porosity, and stoichiometry, have been prepared through various HEBM and SPS parameters. Microstructure characterization and properties measurement of sintered pellets established a correlation among the synthesizing parameters---microstructure control---properties. Those UO2 fuel pellets with controlled microstructure can serve as the model systems for fundamental understandings of fuel behaviors and obtain critical experimental data for Multiphysics MARMOT model validation.;UO2 fuels doped with oxide additives Cr2O 3 and TiO2 display larger grain size, improving fission product retention capability and thus accident tolerance. Spark plasma sintering (SPS) was applied to consolidate TiO2-doped UO2 powder with 0.5 wt % dopant concentration into dense pellet form. To induce the formation of the liquid phase and promote the sintering kinetics, the concentration of dopant was chosen to be above the solid solubility of TiO2 in UO2. The largest grain achieved is up to 80 microm by sintering at 1700 °C for 20 mins, and liquid U-Ti-O eutectic phase occurs at the triple junction of grain boundaries and significantly improves grain growth during sintering. The oxide additive also impedes the reduction of the initial hyperstoichiometric fuel powders to more stoichiometric pellets upon SPS. Thermal-mechanical properties of the sintered doped pellets including thermal conductivity and hardness are measured and compared with undoped pellets. The enlarged grain size (80 microm) and densification within short sintering duration highlight the immense possibility of SPS in fabricating large grain UO2 fuel pellets to improve fuel performance.;Nanocrystalline UO2+x powders are prepared by high-energy ball milling and subsequently consolidated into dense fuel pellets (> 95 % of theoretical density) under high-pressure (750 MPa) by spark plasma sintering at low sintering temperatures (600--700 ºC). The grain size achieved in the dense nano-ceramic pellets varies within 60~160 nm as controlled by sintering temperature and duration. The sintered fuel pellets are single-phase UO2+x with hyper-stoichiometric compositions as derived by x-ray diffraction, and micro-Raman measurements indicate that random oxygen interstitials and Willis clusters dominate the single-phase nano-sized oxide pellets of UO2.03 and UO2.11, respectively. The thermal conductivities of the densified nano-sized oxide fuel pellets are measured by laser flash, and the fuel stoichiometry displays a dominant effect in controlling thermal transport properties. A reduction of thermal conductivity is also observed for the dense nano-sized pellets as compared with the micron-sized counterpart reported in the literature.;Dense nano-sized UO2+x pellets subsequently isothermally annealed to study their effects on grain growth kinetics and microstructure stability. The grain growth kinetics is determined and analyzed focusing on the interaction between grain boundary migration, pore growth, and coalescence. Grains grow much bigger in nano-sized UO2.11 than UO2.03 upon thermal annealing, consistent with the fact that hyper-stoichiometric UO2+x is beneficial for sintering due to enhanced U ion diffusion from excessive O ion interstitials. The activation energies of the grain growth for UO2.03 and UO2.11 are determined as ~1.0 and 1.3~2.0 eV, respectively. As compared with the micron-sized UO2 in which volumetric diffusion dominates the grain coarsening with an activation energy of ~3.0 eV, the enhanced grain growth kinetics in nano-sized UO2+x suggests that grain boundary diffusion controls grain growth. The higher activation energy of more hyper-stoichiometric nano-sized UO2.11 may be attributed to the excessive O interstitials pinning grain boundary migration.;The grain growth kinetics of Dense nano-sized UO2+x pellets has also been studied by in-situ XRD technique. A synchrotron wide-angle X-ray scattering (WAXS) study was performed in situ at Advanced Photon Source (APS) at Argonne National Laboratory to reveal a real-time grain size evolution. The grain size is solely studied and estimated from the full width of half maximum (FWHM) of the obtained spectrum based on the modified Williamson-Hall analysis.