Molecular dynamics simulation of Xe bubble nucleation in nanocrystalline UO2 nuclear fuel

We have performed molecular dynamics (MD) simulations to investigate the dynamical interactions between vacancy defects, fission gas atoms (Xe), and grain boundaries in a model of polycrystalline UO2 nuclear fuel with average grain diameter of about 20 nm. We followed the mobility and aggregation of Xe atoms in the vacancy-saturated model compound for up to 2 ns. During this time we observed the aggregation of Xe atoms into nuclei, which are possible precursors to Xe bubbles. The nucleation was driven by the migration of Xe atoms via vacancy-assisted diffusion. The Xe clusters aggregate faster than grain boundary diffusion rates and are smaller than experimentally observed bubbles. As the system evolves towards equilibrium, the Xe atom cluster growth slows down significantly, and the lattice relaxes around the cluster. These simulations provide insights into fundamental physical processes that are inaccessible to experiment.