Solid-liquid equilibria in the ThO2 –ZrO2 system: An experimental study

In this work, solid-liquid equilibria of the pseudo-binary system ThO2-ZrO2 are investigated. A series of different molar ratios (80:20, 60:40, 40:60, and 20:80 ThO2:ZrO2) of ThO2 and ZrO2 were synthesised at JRC-Karlsruhe using an external sol-gel method. The XRD data of samples sintered at 1650 °C show two-phase behaviour throughout the range of compositions with small solubility (<5%) of ZrO2 in ThO2 and limited solubility of ThO2 in ZrO2. These results were supported with Raman spectroscopy and SEM measurements to have a better understanding of the distribution of the components within each other. Solid/liquid equilibria in the ThO2-ZrO2 system are investigated here for the first time by laser heating coupled with fast optical thermometry under a controlled atmosphere. A minimum solidus point exists in this system for intermediate compositions (close to Th0.4Zr0.6O2) at approximately 2750 K. These results contribute to assessing the thermal stability of the fuel-cladding system in a ThO2-fuel based reactor. Moreover, when compared with other similar mixed oxide systems, the present data confirm the essential role played by oxygen chemistry in the definition of the studied phase boundaries.