The thermodynamic modeling of the Uranium-Oxygen system

The thermodynamic modeling of the uranium-oxygen (U-O) system, which is of first importance in the development of a nuclear thermodynamic database, has been performed. The thermodynamic properties of the phases present in the U-O system are described using the compound energy model with ionic constituents for the solids and an ionic two-sublattice model for the liquid. For the uranium dioxide, the structure is described using three sublattices, one for the cations U3+, U4+ and U6+, one for the normal site of oxygen ions, and one for the interstitial oxygen ions. Vacancies are included in both oxygen sublattices. In this first approach, the homogeneity ranges of the U4O9-y and U3O8-y compounds are not represented. Phase diagram and thermodynamic properties, then, have been calculated from the optimized Gibbs energy parameters, assuming that the system is in thermodynamic equilibrium, i.e. by finding the minimum for the total free energy of the system. The thermodynamic calculation is conducted through CALPHAD (Calculation Phase Diagram) approach, with the help from the Thermo-Calc code. The results obtained show that the consistency between the calculated results and the experimental data is quite satisfactory