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

Abstract A thermodynamic modelling was performed for PuO 2 –UO 2 –ZrO 2 system based on the data available in the literature to evaluate stability of the fluorite structure phase in PuO 2 –UO 2 –ZrO 2 system. The assessment of the heat capacity and phase diagram suggested that the deviation from the ideality in the PuO 2 –UO 2 system is negligibly small. The calculated pseudo-ternary phase diagrams implied that the fluorite structure phase of PuO 2 –ZrO 2 subsystem is more stable than that of UO 2 –ZrO 2 subsystem in the lower temperature range and that the latter is more stable than the former in the higher temperature range. The effect of ZrO 2 dissolution to PuO 2 was larger both on the enthalpy and entropy of the fluorite structure phase than that to UO 2 in the low ZrO 2 composition range. On the other hand, UO 2 had a larger effect on the stability of the fluorite structure phase in the high ZrO 2 composition range.