Isolation and Reactivity of Uranyl Superoxide

The high radiation field associated with spent nuclear fuel (U^IV O₂ ) pellets produces an array of reactive radical species that impact the corrosion and formation of secondary alteration phases. Dioxygen radicals are important as radiolysis products, but the interaction between these reactive oxygen species and U^VI O₂²⁺ and its effects on the resultant alteration phases is unclear. We report the first example of a U^VI superoxide compound and explore its reactivity in the environments relevant to the storage of spent nuclear fuel. We utilized X-ray diffraction and Raman scattering techniques to demonstrate that the uranyl superoxide reacts with CO₂ in air to afford a mixed uranyl peroxide/carbonate within 3 days, both in solution and under atmospheric conditions. An additional transformation occurs over the course of 3 months to form a potassium U^VI carbonate (grimselite), which also occurs as an alteration product on Chernobyl corium. Our results demonstrate the presence and significance of the superoxide anion in the alteration of spent nuclear fuel and indicate the impact of uranyl superoxide chemistry on high prevalence of carbonate in the secondary phases of spent nuclear fuel.