Stability evaluation of fluorite structure phase in PuO2–UO2–ZrO2 system by thermodynamic modelling

A thermodynamic modelling was performed for PuO2–UO2–ZrO2 system based on the data available in the literature to evaluate stability of the fluorite structure phase in PuO2–UO2–ZrO2 system. The assessment of the heat capacity and phase diagram suggested that the deviation from the ideality in the PuO2–UO2 system is negligibly small. The calculated pseudo-ternary phase diagrams implied that the fluorite structure phase of PuO2–ZrO2 subsystem is more stable than that of UO2–ZrO2 subsystem in the lower temperature range and that the latter is more stable than the former in the higher temperature range. The effect of ZrO2 dissolution to PuO2 was larger both on the enthalpy and entropy of the fluorite structure phase than that to UO2 in the low ZrO2 composition range. On the other hand, UO2 had a larger effect on the stability of the fluorite structure phase in the high ZrO2 composition range.