Multiphysics Modeling for Restructuring in the High Burnup UO2 Fuel

To improve the economics of commercial nuclear power production, utilities are seeking to increase the allowable burnup limit of UO₂ fuel. However, exposure to extended burnup leads to the formation of a refined grain structure that generates the risk of performance degradation and fragmentation in UO₂ fuels. Therefore, it is important to understand the HBS formation mechanisms and their impact on fuel performance. It is hypothesized that defect accumulation and dislocation interaction within the grains cause the realignment of dislocations into grain boundaries (GBs), leading to the new subgrain formation, which over time transforms into new grains. An integrated multiphysics modeling approach is developed coupling crystal plasticity (CP) and phase-field model (PFM) is developed to capture the grain subdivision behavior and concurrent fission product evolution at the mesoscale. Here, the CP captures the inhomogeneous local dislocation density, deformation, and orientation distribution of the polycrystalline material, while the PFM will demonstrate the microstructural evolution leading to grain subdivision. The coupled model elucidates the role of stress and inelastic deformation on the restructuring observed in the high burnup fuel.