Evaluation of thermal conductivity of hyperstoichiometric UO _2+x by molecular dynamics simulation

The thermal conductivity of UO _2+x has been investigated by an equilibrium molecular dynamics (EMD) simulation up to 2000 K using the Born-Mayer-Huggins interatomic potential with the partially ionic model. In the present EMD system with the Green-Kubo method, the thermal conductivity was determined by the auto-correlation functions of energy and charge currents and the cross-coupling term. The thermal conductivity of UO _2+x decreased with an increase of x and temperature. Its temperature dependence was relatively small for large x values, which was attributed to phonon scattering by excess oxygens. In addition, the heat capacity was calculated using the phonon-level density deduced by the velocity auto-correlation function for constituent ions. The phonon velocity was also evaluated by the phonon- dispersion relationship. Using these thermal properties obtained by EMD calculations, the effect of excess oxygens on the phonon mean free path was discussed.