Phase-field modeling of fission gas bubble growth on grain boundaries and triple junctions in UO ₂ nuclear fuel

A phase-field model of fission gas bubble evolution was developed and applied to gain an improved understanding of the microstructure-level processes leading to fission gas release from nuclear fuel, and to inform engineering-scale fission gas release models. The phase-field model accounts for multiple fuel grains and fission gas bubbles and tracks the local concentration of vacancies and gas atoms. The model was used to simulate the growth of grain boundary and triple junction bubbles in a hexagonal periodic 3D grain structure. The fractional coverage of the triple junctions and the number of fully saturated triple junctions (that is, triple junctions fully covered by the gas phase) was calculated, and correlations were developed between these quantities and the grain boundary coverage. The effects of initial triple junction bubble density, vacancy source strength, and bubble semi-dihedral angle on triple junction coverage and saturation were evaluated. High initial triple junction coverage and high bubble semi-dihedral angle can lead to triple junction saturation well before grain boundary percolation. The implications of these findings for engineering-scale fuel performance modeling are discussed.