Thermophysical properties and oxygen transport in the (U ,Pu1−)O2 lattice

Abstract Using molecular dynamics, the thermophysical properties of the (U x ,Pu 1− x )O 2 system have been investigated between 300 and 3200 K. The variation with temperature of lattice parameter, linear thermal expansion coefficient, enthalpy and specific heat at constant pressure, are explained in terms of defect formation and diffusivity on the oxygen sublattice. Vegard’s Law is approximately observed for the thermal expansion of the solid solutions below 2000 K. Deviation from Vegard’s Law above this temperature occurs due to the different superionic transition temperatures of the solid solutions (2200–2900 K). Similarly, a spike in the specific heat, associated with the superionic transition, occurs at lower temperatures in solid solutions that have a high Pu content. While oxygen diffusivity is higher in pure PuO 2 than in pure UO 2 , lower oxygen defect enthalpies in (U x ,Pu 1− x )O 2 solid solutions cause higher oxygen mobility than would be expected by interpolation between the diffusivities of the end members. In comparison to UO 2 and PuO 2 there is considerable variety of oxygen vacancy and oxygen interstitial sites in solid solutions generating a wide range of property values. Trends in the defect enthalpies are discussed in terms of composition and the lattice parameter of (U x ,Pu 1− x )O 2 . Comparison is made with previous work on (U x ,Th 1− x )O 2 .