Abstract

Abstract Understanding the evolution of microstructure in nuclear fuels under high burn-up conditions is a critical concern aimed at extending fuel refueling cycles and enhancing nuclear reactor safety. In this study, a phase-field model has been proposed to examine the evolution of high burn-up structures in polycrystalline UO2. The formation and growth of recrystallized grains were initially investigated. It was demonstrated that recrystallization kinetics adhere to the KJMA equation, and recrystallization represents a process of free energy reduction. Subsequently, the microstructure evolution in UO2 was analyzed as burn-up increased. Gas bubbles acted as additional nucleation sites, thereby augmenting recrystallization kinetics, while the presence of recrystallized grains accelerated bubble growth by increasing the number of grain boundaries. The observed variations in recrystallization kinetics and porosity with burn-up closely align with experimental findings. Furthermore, the influence of grain size on microstructure evolution was deliberated upon. Larger grain sizes were found to decrease porosity and the occurrence of high burn-up structures.

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