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.