Chemical Transformations of Hydrated Uranyl Fluoride

The chemical behavior of uranyl fluoride (UO2F2), a byproduct of the nuclear fuel cycle, is of significant interest for nuclear security applications. Two phases of uranyl fluoride (UO2F2 and [(UO2F2)(H2O)]7·4H2O) have been previously identified; these structures and the phase transition between them are further characterized in this work. In addition, the stability of uranyl fluoride is assessed under varying environmental conditions. While previous studies have suggested that uranyl fluoride may degrade upon exposure to high humidity, the chemical pathway of degradation was not well understood. This work demonstrates that uranyl fluoride undergoes a chemical reaction with water vapor to form a novel uranyl hydroxide hydration product. This species, shown to be structurally similar to the uranyl hydroxide mineral schoepite, can be further hydrated to form a uranyl peroxide species. The unexpected and novel nonphotochemical formation of uranyl peroxide from multiple uranyl hydroxide species is explained by unusually high uranyl ion reactivity in these reactant species. While the uranyl ion is typically fairly inert, strong sigma-donating equatorial ligands and strong interactions between uranyl oxygens and interlayer water molecules weaken the uranyl ion in these species such that an increase in the water vapor pressure can induce a redox reaction that is normally dependent on the photoexcitation of the uranyl ion.