Nano-grained UO2 grain growth kinetics at 600-800°C: An in-situ synchrotron x-ray study

High burnup structure (HBS), featuring nanoscale crystals with microscale intergranular pores, was found in the rim region of UO2 fuel pellets as burnup exceeds approximately 50 GWd/tU. Systematic investigation of this unique radiation-induced microstructure has been challenging due to the associated severe radiation hazards and limited volume available for sampling. In this study, nano-grained UO2 fabricated by spark plasma sintering (SPS) was employed as a surrogate material for HBS to study the temperature dependence on the grain growth kinetics at elevated temperature typically found in the rim region. UO2+x samples with an initial grain size of approximately 104 nm were synthesized using the SPS technique, followed by a hydrogen reduction procedure to reach the target stoichiometry. In this study, hyper-stoichiometric UO2.03 was selected to simulate the hyper-stoichiometry effect caused by burnup. High-energy synchrotron radiation was applied to the nano-grained UO2.11 samples with external heating at three different nominal temperatures: 600ºC, 700ºC and 800ºC. The wide-angle X-ray scattering (WAXS) signals were collected by a HYDRA GE detector array, and interpreted by the Williamson-Hall method to attain the grain size. The actually sample annealing temperatures were found to be ~615ºC, ~730ºC, and ~830ºC according to the in-situ measured lattice thermal expansion. The time-resolved grain growth behavior was observed in the nano-grained UO2.03 annealing at ~730ºC and ~830ºC, whereas marginal grain size change was found at ~615ºC. This in-situ measured grain growth data will be used to develop and validate advanced fuel performance codes such as MARMOT. These codes are ultimately expected to predict the fuel performance of UO2 with high fidelity.