Our latest paper is now available from the Journal of Nuclear Materials:

• M.W.D.Cooper, S.T. Murphy, M.J.D. Rushton and R.W. Grimes, “Thermophysical properties and oxygen transport in the (U_x,Pu_1-x)O₂ lattice”, Journal of Nuclear Materials, 461 206–214 (2015) http://dx.doi.org/10.1016/j.jnucmat.2015.03.024

In this paper we build upon a recent body of work using a many-body potential approach (see potentials page) to investigate the thermophysical and diffusion properties of actinide oxides. Using an updated version of the PuO₂ parameter set we have studied the thermal expansion, specific heat capacity, oxygen diffusion and the oxygen point defect energies of (U_x,Pu_1−x)O₂. The results shows that the non-uniform cation lattice has a much smaller effect on the oxygen diffusion in (U_x,Pu_1-x)O₂ compared to (U_x,Th_1-x)O₂. This is expained in terms of the lattice parameter missmatch between the end member oxides and the role this plays in the defect energies.

Abstract

Using molecular dynamics, the thermophysical properties of the (U_x,Pu_1−x)O₂ 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 PuO₂ than in pure UO₂, lower oxygen defect enthalpies in (U_x,Pu_1−x)O₂ solid solutions cause higher oxygen mobility than would be expected by interpolation between the diffusivities of the end members. In comparison to UO₂ and PuO₂ 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)O₂. Comparison is made with previous work on (U_x,Th_1−x)O₂.