High-burn-up structure evolution in polycrystalline UO₂: Phase-field modeling investigation

Understanding the evolution of microstructures in nuclear fuels under high-burn-up conditions is critical for extending fuel refueling cycles and enhancing nuclear reactor safety. In this study, a phase-field model is 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 Kolmogorov–Johnson–Mehl–Avrami (KJMA) equation, and that recrystallization represents a process of free-energy reduction. Subsequently, the microstructural evolution in UO₂ was analyzed as the burn up increased. Gas bubbles acted as additional nucleation sites, thereby augmenting the recrystallization kinetics, whereas the presence of recrystallized grains accelerated bubble growth by increasing the number of grain boundaries. The observed variations in the recrystallization kinetics and porosity with burn-up closely align with experimental findings. Furthermore, the influence of grain size on microstructure evolution was investigated. Larger grain sizes were found to decrease porosity and the occurrence of high-burn-up structures.