Fully Coupled Simulation of Oxygen and Heat Diffusion for (U, Pu)O2 Fuel in Both FBR and LWR

Oxygen redistribution with the high temperature gradient is one of the important fuel performance concerns in fast reactor (FBR) and light water reactor (LWR) oxide (U, Pu)O2 fuel during irradiation, and will affect nuclear fuel materials properties, power distribution and overall performance of the fuel. This paper focuses on the oxygen and heat diffusion within (U, Pu)O2 fast reactor and light water reactor fuel. In this study, the correlations from the literature are used for density, thermal conductivity, specific heat, and oxygen to metal ratio redistribution. Three dimensional burnup dependent oxygen diffusion and heat diffusion models are fully-coupled in steady states and transients to account for the effects on each other. The models are implemented into COMSOL Multiphysics to perform this analysis. The simulation results show that the temperature profile change has relatively larger impact on oxygen/metal ratio distribution compared to oxygen/metal ratio distribution change’s impact on temperature distribution. With regard to the oxygen/metal ratio’s effect on temperature distribution, fast reactor and light water reactor show different trends. For fast reactor application, with the oxygen/metal ratio’s increase on the outer surface, the fuel temperature decreases. However, for light water reactor application, with the oxygen/metal ratio increase on the outer surface, the fuel temperature increases, which is opposite to fast reactor application. For different oxygen/metal ratio boundary conditions, with the oxygen/metal ratio increase on the outer surface, the oxygen/metal ratio increases over the whole fuel. For the fast reactor application, the inner surface has the lowest oxygen/metal ratio, and the outer surface has the highest oxygen/metal ratio. However, this trend in light water reactor application is exactly opposite to fast reactor application. For the start-up transient scenario, the rapid changes in time-dependent temperature and oxygen/metal ratio distributions are observed, which are due to a rapid change in heat generation. Comparing fast reactor and light water reactor simulation results, we can observe that the temperature change is relatively more obvious in light water reactor than fast reactor.