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• 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]

Evolution of bubbles in polycrystalline composite fuel over burnup [Display omitted] • The fission bubbles in the UN-UO 2 composite nuclear fuel was simulated using a new phase-field model. • The nucleation rate of bubbles in UN phase was slower than that in UO 2 phase. • The growth rate of bubbles in UN phase was faster than that in UO 2 phase. A phase-field model was developed to simulate the evolution of fission bubbles in UN-UO 2 composite ceramic nuclear fuel. The model was firstly used to simulate the formation of UO 2 intracrystalline bubbles, and the change of porosity obtained from the simulation was compared with the experimental data. The bubble evolution of UN-UO 2 composite fuels in the UN and UO 2 phase regions was then investigated for different burnups and doping concentrations. It was found that the nucleation rate of bubbles in the UN phase was slower than that in the UO 2 phase, but the growth rate of bubbles in the UN phase was faster, so that the bubbles in the UO 2 phase had a larger number density at low fuel consumption. It is specifically analyzed how the surface energy, diffusion coefficient and uranium density affect the difference in bubble evolution between the two phases. [ABSTRACT FROM AUTHOR]

Tungsten (W) is widely recognized as a promising material for plasma-facing components in nuclear fusion reactors, and the evolution of bubbles in polycrystalline tungsten significantly impacts its properties. This study developed a phase-field model considering elastic inhomogeneity to investigate the evolution of radiation-induced bubbles. The Gibbs free energy of helium bubbles was derived using the Van der Waals equation. An explicit nucleation algorithm accounting for the position of grain boundaries (GBs) was coupled to the model. Furthermore, the model also considered the inequality between the internal pressure of the bubble and the surface tension. The growth kinetics of a single bubble with elastic heterogeneity were examined, and based on these findings, the evolution of bubbles under irradiation was simulated, yielding results consistent with experimental observations. Furthermore, the influence of GBs on radiation-induced bubbles was investigated using the developed model. The simulation results demonstrated that both GBs, as preferential nucleation regions and as sinks for vacancy absorption, had an impact on bubble nucleation and growth. The superior radiation resistance observed in nanocrystalline tungsten was dominantly attributed to the absorption of vacancies by GBs. [ABSTRACT FROM AUTHOR]