Three-dimensional phase-field simulations of intragranular gas bubble evolution in irradiated U-Mo fuel

The evolution of fission gas bubbles in irradiated materials plays a critical role in the microstructural processes that leads to dimensional changes of U-Mo alloy fuels, e.g., fuel swelling. Although the intergranular bubbles-induced fuel swelling has been long-discussed for U-Mo fuel, there are very few computational studies of the formation of intragranular gas bubbles and its impact on fuel swelling. To this end, we develop a three-dimensional phase-field model to investigate the evolution of intragranular gas bubbles in U-Mo fuel. Fission induced defect formation and annihilation processes, such as vacancy-interstitial recombination, fission gas atom resolution, and interactions with dislocations and grain boundaries are incorporated in the model. Simulations show that the intragranular gas bubbles can be stabilized to certain sizes due to the balance between the generation and annihilation of defects. The intragranular gas bubbles induced fuel swelling is predicted to be comparable to experimental measurements. The effects of the irradiation and fuel fabrication conditions (i.e., fission rate, fuel grain size, and mechanical work induced deformation) on the bubble evolution and the resultant swelling are investigated. The current simulations provide a better understanding of intragranular gas bubble-induced swelling and a solid foundation for the future study of the nucleation and growth of intergranular gas bubbles and recrystallization in U-Mo fuel.