Environment and oxidation state of molybdenum in simulated high level nuclear waste glass compositions

Alkali borosilicate glasses containing between 20 and 35 wt% of a simulated high level nuclear waste stream with varying Li2O contents were melted under neutral (air) and reducing (nitrogen/hydrogen) conditions. XRD analysis of the as-cast glasses showed a tendency for the products to remain amorphous when melted under neutral conditions and for metallic silver to develop in the reduced melts. EXAFS analysis revealed (MoO4)2− tetrahedra in all glasses regardless of the sparge applied during melting. The glasses were heat treated to simulate an interruption to the cooling system used to prevent heat build-up in the vitrified product store. Powellite-type molybdate phases were found to develop in the heat treated samples and formed at lower waste loadings in glasses sparged with a reducing gas. A reduction in the quantity of Li2O lead to a reduction in the quantity of powellite-type molybdate phases. EDS showed the primary molybdate phase to be high in Sr and rare earth elements and TEM indicated that the presence of silver metal encouraged molybdate formation.