High burnup structure (HBS), featuring nanoscalecrystals with microscale intergranular pores, was found inthe rim region of UO_2 fuel pellets as burnup exceedsapproximately 50 GWd/tU. Systematic investigation of thisunique radiation-induced microstructure has beenchallenging due to the associated severe radiation hazardsand limited volume available for sampling. In this study,nano-grained UO_2 fabricated by spark plasma sintering(SPS) was employed as a surrogate material for HBS tostudy the temperature dependence on the grain growthkinetics at elevated temperature typically found in the rimregion.UO_(2+x) samples with an initial grain size ofapproximately 104 nm were synthesized using the SPStechnique, followed by a hydrogen reduction procedure toreach the target stoichiometry. In this study, hyperstoichiometricUO_(2.03) was selected to simulate the hyperstoichiometryeffect caused by burnup. High-energysynchrotron radiation was applied to the nano-grainedUO_(2.11) samples with external heating at three differentnominal temperatures: 600°C, 700°C and 800°C. The wideangleX-ray scattering (WAXS) signals were collected by aHYDRA GE detector array, and interpreted by theWilliamson-Hall method to attain the grain size. Theactually sample annealing temperatures were found to be～615°C, ～730°C, and ～830°C according to the in-situmeasured lattice thermal expansion. The time-resolvedgrain growth behavior was observed in the nano-grainedUO_(2.03) annealing at ～730°C and ～830°C, whereasmarginal grain size change was found at ～615°C.This in-situ measured grain growth data will be usedto develop and validate advanced fuel performance codessuch as MARMOT. These codes are ultimately expected topredict the fuel performance of UO_2 with high fidelity.
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