Effect of grain size on irradiation-induced bubble evolution in high burn-up UO2: A phase-field study.

• A modified phase-field mode was developed to simulate the evolution of high burn-up structure in uranium dioxide. • The decrease in grain size led to an increase in grain boundary area, which accelerated the recrystallization. • The development of recrystallization and the decrease of grain size both enhance bubble evolution. • The enhancement of irradiation-induced bubbles becomes smaller as the grain size being much smaller. Due to economic reasons, uranium dioxide (UO 2) ceramic fuel needs to operate under high burn-up conditions. The high burn-up structure (HBS) and micrometric irradiation-induced bubbles within it cause safety issues for the fuel. In this study, a phase-field model was developed to simulate the evolution of fission bubbles under high burn-up conditions. This model simultaneously couples the evolution of recrystallized grains and bubbles through explicit nucleation methods. By simulating the development of HBS in polycrystalline UO 2 , it was indicated that the microstructure is consistent with experimental observation. The variation of porosity was divided into three stages and found to be in good agreement with experimental data. Additionally, the HBS fraction was compared with experimental measurements to validate this HBS model. Furthermore, the effect of recrystallization on bubble evolution was investigated using the validated model. It was found that recrystallization accelerated bubble evolution by increasing GB area. In addition, the influence of grain size on bubble evolution was studied by constructing initial structures with different grain sizes. Results showed that both the development of recrystallization and the decrease in grain size enhance bubble evolution, but the enhancement becomes smaller as the grain size decreases. The development of high burn-up structure in polycrystalline UO 2 for (a) 60, (b) 72, (c) 96 and (d) 120 GWd/tU [Display omitted] [ABSTRACT FROM AUTHOR]