Phase Field Investigation on Grain Boundary Migration Affected by Intergranular Mobile Pores in UO₂ Fuels

The steep radial temperature gradients developed in UO₂ fuels during reactor operation can drive pore migration, making pore-grain boundary (GB) coupled migration an important mechanism governing microstructural evolution. Although pores are generally regarded as pinning features that hinder GB migration, the conditions under which mobile pores retard, co-migrate with, or promote GB migration remain poorly understood. In this study, we develop a phase field model coupling vapor-transport-driven pore migration and curvature-driven grain growth to investigate the coupled migration behavior between intergranular mobile pores and GBs. The simulations first focus on an idealized source-term-free system to isolate the effect of pore-GB migration coupling from irradiation-induced pore generation and growth. The results show that the effect of pores on GB migration depends on the relative migration rate of pores and GBs, which is determined by both the pore-to-GB mobility ratio and the corresponding driving-force ratio. When pores migrate more slowly than GBs, they retard GB migration and exhibit an effective pinning effect. In contrast, sufficiently mobile pores can co-migrate with GBs and promote apparent GB migration when the pore migration rate exceeds that of the GBs. Furthermore, to illustrate the regulating effect of pores on GB migration in UO₂ under a temperature gradient, we perform additional simulations under continuous irradiation, in which pores nucleate spontaneously, migrate along the temperature gradient, and interact with GBs, in agreement with experimental observations. Based on the simulated GB migration behavior, we construct a regime map that distinguishes pinning/retardation, weak interaction, and pore-assisted migration regimes. This work provides a mechanistic phase field interpretation of pore-GB coupled migration and offers insight into microstructural evolution in porous oxide fuel materials.