Phase Field Simulation Study on Bubble Evolution Behavior of UO₂

A phase field model was established to study the evolution of gas bubbles in UO₂ under typical irradiation conditions. First, the phase field method was used to simulate the nucleation and growth of bubbles in UO₂ under different temperature conditions. The intragranular bubble evolution in UO₂ at 573, 673, 773, and 973 K was compared. At lower temperatures, bubbles nucleate rapidly, but their growth rate is slower, resulting in relatively uniform bubble sizes. As the temperature increases, the diffusion speed accelerates, and the bubble growth rate increases. The bubble size distribution becomes extremely uneven. Then, the bubble evolution under a horizontal temperature gradient of 2 K/μm ranging from 873 K to 1 073 K was investigated. It can be observed that there are significant differences in bubble nucleation and growth between the high-temperature and low-temperature regions. The nucleation rate is higher in the low-temperature region, but the growth rate is slower, whereas the nucleation rate is lower in the high-temperature region, but the growth rate is faster. The three-dimensional polycrystalline bubble evolution under a horizontal temperature gradient of 0.5 K/μm was also studied with a temperature distribution ranging from 873 K to 1 073 K. The bubbles at the higher-temperature end grow faster and quickly coalesce to form large intergranular channels, while the bubbles at the lower-temperature end grow more slowly, and no interconnected intergranular channels are formed. The rapid growth and interconnection of grain boundary bubbles also promote the diffusion of fission gas atoms within the grain, thereby suppressing the nucleation of fission gas in the vicinity of the grain boundaries. Temperature gradient is also a driving force for bubble migration. Bubbles exhibit a preferred migration direction that is related to the direction and magnitude of heat transfer. The migration of bubbles under temperature gradient conditions was investigated. The bubbles migrate toward the hot side of the simulation region, and the circular bubbles were stretched horizontally, with their morphology changing from circular to elliptical. To account for the effect of fission density on the diffusion and nucleation growth of Xe atoms, a source term representing the production of Xe was included in the diffusion equation. Higher fission rates increase the supersaturation of fission gas atoms in the matrix, promoting faster bubble growth. Consequently, both the porosity and the bubble average radius increase with higher fission rates. The resolution process of bubbles by introducing a random variable was also described. The simulation results show that bubble resolution slows down the growth rate of both porosity and bubble average size.