Thermodynamic modelling of the plutonium–oxygen system

The published data for the thermodynamic functions and phase equilibria of the plutonium–oxygen system have been examined. Some inconsistencies have been found for oxygen chemical potential and vaporization data of [Pu ₂O ₃ + PuO _2−x] and PuO _2−x domains. As the original chemical potential data were not performed at the same temperature and O/Pu ratio, a chart with fixed temperature and composition ranges was built in order to compare all the experimental data. The discrepancies remain difficult to explain. Thermodynamic models of all the phases have been derived by the least-squares minimization procedure using the Thermo-Calc software. The compound energy formalism with the sublattice models (Pu ³⁺, Pu ⁴⁺) ₁(O ²⁻, Va) ₂ and (Pu ³⁺, Pu ⁴⁺) ₂(O ²⁻) ₃(O ²⁻, Va) ₁ have been chosen to account for the crystal structure, defect chemistry and thermodynamic properties of respectively PuO _2−x and PuO _1.61 phases. The liquid phase was described using the ionic two-sublattice model (Pu ³⁺) _P(O ²⁻, Va ^Q−, PuO ₂, O) _Q. The reliability of the refined parameters is demonstrated by calculation of the phase diagram, the thermodynamic properties of the phases and the equilibrium partial pressures in the Pu ₂O ₃–PuO ₂ region. Considering the large uncertainties on the experimental information, an overall good agreement was obtained. To improve the thermodynamic description of the system, some missing experimental data are listed.