Phase-field simulations of fission gas bubbles in high burnup UO2 during steady-state and LOCA transient conditions

Abstract

To improve the economics of commercial light water reactors, increased understanding of UO2 nuclear fuel with the high burnup structure (HBS) is required in both steady-state and transient conditions. Here, a phase-field model of the fission gas bubble microstructure in nuclear fuel is developed based on the Kim-Kim-Suzuki (KKS) formulation and implemented in Idaho National Laboratory's Marmot application for phase-field simulation of nuclear materials. The model includes the effects of gas pressure and the surface tension of the bubble-fuel matrix interface for arbitrary interfacial curvature. Simulations of bubble growth in the HBS region during steady-state conditions showed that initially overpressurized bubbles decreased in pressure during growth, but still remained above equilibrium pressure. During a loss-of-coolant accident (LOCA) transient, simulations of bubbles in the HBS region showed that bubble size did not change significantly. The bubble pressure in response to the LOCA transient was calculated for a variety of bubble sizes, initial pressures, and external restraint pressures.