Progress in the thermodynamic modelling of the O–U–Zr ternary system

The thermodynamic modelling of the O–U–Zr system is part of the basic knowledge of the corium, mixture formed at high temperature between fuel and other materials of the vessel (zircalloy, steel, control rods, …), which may interact with the concrete basemat during an hypothetical severe nuclear accident. Inconsistencies were previously detected in the available experimental information, phase diagram and thermodynamic properties, especially concerning the solubility of oxygen in uranium–zirconium liquid alloys and the extent of the oxygen–uranium liquid miscibility gap in the ternary system. The critical assessment was significantly improved in this work, both for the binary O–U, O–Zr, O2U–O2Zr, U–Zr, and the O–U–Zr ternary systems, taking into account the most recent experiments and evaluating ternary parameters for the main liquid and solid solutions. Calculated and experimental isothermal or isopleth phase diagrams are in satisfactory agreement, both qualitatively and quantitatively. However, experimental uncertainties still remain at high temperature.