Evaluation of SrMoO₄ in repository simulating tests

The SrMoO₄ compound, a host for strontium in tailored nuclear waste ceramics, is highly stable in deionized water, NaCl, KCl, and the acidic MgCl₂ under hydrothermal conditions of 100, 200 and 300°C as indicated by the fact that <0.5% strontium was released under these conditions. In Na₂SO₄ solution, SrMoO₄ is somewhat reactive, as indicated by the release of ~2 and 6% of the original strontium and molybdenum inventories, respectively, into solution at 200 and 300°C. More molybdenum than strontium was detected in solution because of the crystallization of celestite, SrSO₄. Thus, anions such as SO^2-₄ influence the reactivity of SrMoO4. The SrMoO₄ phase was found to be least stable in CaCl₂ solution as indicated by the release of 93.5, 96.1, and 100% of the strontium from the initial SrMoO₄ at 100, 200, and 300°C, respectively. However, only 0.02% of the original inventory of molybdenum in SrMoO₄ was detected in solution. The removal of molybdenum from solutions was due to the formation of CaMoO₄ powellite as revealed by x-ray diffraction. Thus, CaMoO₄ is more stable than SrMoO₄ under these hydrothermal conditions. These results show that SrMoO₄ is stable in Na⁺, K⁺, and Mg²⁺ solutions but not in Ca²⁺-containing solutions.