Fission gas bubbles and recrystallization-induced degradation of the effective thermal conductivity in U-7Mo fuels

Abstract

We have developed a mesoscale model to calculate the degradation of the effective thermal conductivity in irradiated U-Mo alloys caused by the fission-induced gas bubbles and recrystallization. The phase-field approach is employed to generate the grain microstructures of U-7Mo fuels with intra- and inter-granular gas bubbles. Based on the phase-field microstructures, the thermal conductivities of U-7Mo as a function of the fission density can be predicted by the developed mesoscale model. The predicted values of effective thermal conductivities are consistent with available experimental data although the grain structure and the distribution of gas bubbles were generated from the phase-field simulations that may not exactly correspond to experimental microstructures. Results show that the effective thermal conductivity decreases rapidly with recrystallization compared to the one prior to recrystallization, which can be attributed to the sudden increase of grain boundary densities and corresponding intergranular gas bubbles at high fission densities. Smaller grain size fuel structure has a lower thermal conductivity at the same fission density due to the increased grain boundary density. The current study can provide a better understanding of the fission-induced degradation mechanism of the thermal conductivity in U-Mo fuels.