Structure and reactivity of X-ray amorphous uranyl peroxide, U2O7

Abstract

Recent accidents resulting in worker injury and radioactive contamination occurred due to pressurization of uranium yellowcake drums produced in the western USA. The drums contained an unexpected X-ray amorphous reactive form of uranium oxide, U2O7. Heating hydrated uranyl peroxides produced during in situ mining unintentionally produced U2O7. It is a hygroscopic anhydrous uranyl peroxide that reacts rapidly with water to release O2 gas and form metaschoepite, a uranyl-oxide hydrate. Quantum chemical calculations indicate that the most stable U2O7 conformer consists of two bent (UO2)2+ uranyl ions bridged by a peroxide group bidentate and parallel to each uranyl ion, and a 2-O atom, resulting in charge neutrality. A pair distribution function from neutron total scattering supports this structural model. The reactivity of U2O7 in water and with water in air is much higher than other uranium oxides, and this can be both hazardous and potentially advantageous in the nuclear fuel cycle.