Solubility of thorium in 14%Y-stabilized zirconia

Y-stabilized zirconia (YSZ) is one of the reference materials for immobilization of actinide elements for long-term and secure storage of radioactive waste. YSZ adapts different structural modifications, namely monoclinic, tetragonal (unstable) and cubic. Changes in relative Y content and synthesis temperature can be used to obtained specific structural form. However, for an intermediate Y content of ~5-17 at. % a mixture of cubic and unstable tetragonal modifications can form thus compromising corresponding long-term structural integrity. Nevertheless, for this Y concentration range a stabilized tetragonal (t′) phase can be obtained through a rapid quenching of the parent cubic modification. In this work we report studies of solubility of Th in 14%Y- stabilized (t′) zirconia phase.
Series of 14%Y-ZrO2 compounds with Th at. % content of 3, 6, 9 and 12 were synthesized by sol-gel method using salts of Zr(IV), Y(III) and Th(IV) as starting materials with a final sintering temperature of 1400 K. Subsequent powder X-ray diffraction measurements were performed in order to study underlying phase and structural evolutions. It was found that the cell parameters increase nearly linearly for the Th content from 3 to 9% and no cell increase is observed for the end-member with 12%Th. Additionally, ThO2 was found to precipitate in this sample. This may indicate that Th solubility limit in the 14%Y-ZrO2 system lies within the 9-12% range. Interestingly, Th content was reported to exceed 15% for the 25%Y-ZrO2 system, as concluded from electron microscopy data. This sample, however, was found to be 25%Y-ZrO2–ThO2 phase separated. Considering that in the current study traces of ThO2 have been found already in the 9%Th sample, conditions of thermal treatment with the resulting phase equilibria may impact the Th solubility. Further details on structural and phase properties in the Th-14%Y-ZrO2 system will be presented.