Thermodynamic modelling of the (U, Pu, Np)O2±x mixed oxide

To assess the phase stability of actinide oxides with a fluorite structure which has a variable composition (MO2±x), thermodynamic modelling has been performed for O-Pu-U-Np quaternary systems on the basis of thermodynamic data available in the literature. Using the thermodynamic data for O-Pu, O-U and O-Np binary systems obtainable in the literature, the thermodynamic modelling for O-Pu-U-Np quaternary system was carried out by a means of CALculation of PHAse Diagram (CALPHAD) technique. Using this model, the oxygen potentials for (Pu0.3U0.7)O2, (Pu0.3U0.65Np0.05)O2 and (Pu0.3U0.58Np0.12)O2 were calculated between 1123 K and 1973 K and compared with the experimental data. Interaction parameters required for calculation of the Gibbs free energy of the solid solution phase were estimated for O-Pu-U-Np system.