Environmental behavior of ZrO2-MgO ceramics containing UO2

In this study a ZrO2-MgO inert matrix fuel containing UO2 (a PuO2 homolog) and Er2O3 (burnable poison) was evaluated under environmental conditions to assess its performance as a waste form should a direct disposal scheme be adopted. A Soxhlet degradation study was used to assess the durability of the ceramic in contact with water for 2000 h over a range of ZrO2 to MgO ratios. This quantified the amount of ZrO2 required to stabilize the material against degradation. A pulse flow dissolution study was used to investigate the stability of the material in aqueous environments relevant to a repository. Although the MgO phase was dissolved, the material was found to retain Zr, U, and Er oxides while immersed in brine, silicate-bicarbonate solution, or deionized water for 3700 h. Aqueous media with a high level of silicate formed a Mg2SiO4 phase at the surface, which retarded further degradation of the material. Brine was found to be the most aggressive solution in altering the material. The data provides an evaluation of the mixed inert matrix behavior under a range of potential repository conditions.