Examination of the impact of electron–phonon coupling on fission enhanced diffusion in uranium dioxide using classical molecular dynamics

Fission energy deposition in nuclear fuel has been experimentally observed to influence diffusion in uranium dioxide (UO2). This deposition is initially dominated by inelastic interactions with the electronic structure. Subsequently, energy is transferred to the lattice through electron–phonon (e–p) coupling resulting in a thermal spike and an associated pressure spike, which are presumed to contribute to diffusion enhancement. Molecular dynamics (MD) simulations were performed to investigate uranium diffusion enhancement in UO2while varying the e–p coupling. The model was composed of 10 × 60 × 60 unit cells and used a Buckingham potential. A two-temperature model captured energy deposition in the electronic subsystem and its transfer to the atomic lattice. Experimentally, the fission enhanced diffusion coefficient (D*) of uranium in UO2is observed to be athermal and proportional to fission rate density. For fission rate densities that are reported in experiment, the MD predictedD* was found to be on the order of 10−18cm2/s, in reasonable agreement with experimental trends, and to decrease as e–p coupling was weakened.