Oxidation States and Local Environment of Molybdenum in a Simplified Nuclear Waste Glass Under Reducing Conditions

ABSTRACT

Although borosilicate glass remains the predominant matrix for nuclear waste immobilization, it exhibits a limited tolerance to molybdenum (Mo), a major fission product in numerous high‐level radioactive waste streams. When oversaturated, Mo precipitates as a “yellow phase”, which compromises the durability of the waste glass. The multivalent nature of Mo offers a potential strategy to improve its incorporation into the glass matrix through redox control during vitrification. Nonetheless, this reduction process and the mechanisms underlying enhanced Mo solubility have not yet been fully understood. In this work, Mo‐containing borosilicate glasses were synthesized by adding varying amounts of tannic acid as a reducing agent. The solubility of Mo was observed to rise as the amount of reducing agent was increased. The oxidation states and local coordination of Mo were examined by X‐ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and molecular dynamics simulations. The analyses showed that Mo ⁶⁺ in the glass could be partly converted to lower oxidation states, such as Mo ^{3 +} , Mo ^{4 +} , Mo ^{5 +} , and metallic Mo. The reduced field strength of Mo–O units at lower valence states under reducing conditions facilitates their incorporation into the glass network, and their easier dispersion within the glass contributes to the increased solubility of Mo.