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.
