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 (MO 2±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 CAL-culation of PHAse Diagram (CALPHAD) technique. Using this model, the oxygen potentials for (Pu 0.3 U 0.7 )O 2 , (Pu 0.3 U 0.65 Np 0.05 )O 2 and (Pu 0.3 U 0.58 Np 0.12 )O 3 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. 2005 Elsevier B.V. All rights reserved.