Exploring Fission Enhanced Diffusion of Uranium in Uranium Dioxide Using Classical Molecular Dynamics Simulations

摘要

Fission fragment energy deposition in nuclear fuel has been experimentally observed to influence the diffusion of uranium. This deposition is initially dominated by inelastic interactions with the electronic structure. Subsequently, the energy is transported to the lattice through electron-phonon (e-p) coupling resulting in a thermal spike and an associated pressure gradient, which are presumed to contribute to diffusion enhancement. Molecular dynamics (MD) simulations were performed to investigate uranium diffusion enhancement in UO_2 while varying the e-p coupling. The model was composed of 10x60x60 unit cells and used a Buckingham potential. A two-temperature model captured energy deposition in the electronic subsystem and its transfer to a disordered region of the lattice. Experimentally, the fission enhanced diffusion coefficient of uranium in UO_2 is observed to be proportional to fission rate density with a proportionality constant between 10~(-31)-10~(-29)cm~5. The MD predicted proportionality constant showed reasonable agreement with experiment and decreased as the e-p coupling was weakened.