A molecular dynamics study of solid and liquid UO2

The authors present an extensive series of molecular dynamics simulations of UO 2 in the solid and liquid states, in which they calculate the ionic diffusion coefficients and some of the important thermodynamic quantities. The simulations are based on a rigid-ion model derived from the new shell model potentials of Jackson and co-workers (1986) and make use of recently developed constant-pressure and constant-temperature techniques. The simulations confirm that UO 2 is an oxygen superionic conductor, as suggested by recent neutron scattering experiments. The temperature of the diffuse transition to the superionic regime is in satisfactory agreement with experiment, as is the melting point of the model system. The thermal expansion coefficient, specific heat and bulk modulus for the solid agree well with experiment below about 2500 K but are less satisfactory near the melting point; they suggest that the differences may be due to the effect of electronic excitations. The volume increase on melting and thermodynamic quantities of the liquid are sensitive to details of the inter-ionic potentials and are in only fair agreement with experiment.