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 U.S.A. The drums contained an X-ray amorphous reactive form of uranium oxide that may have contributed to the pressurization. Heating hydrated uranyl peroxides produced during in situ mining can produce an amorphous compound, as shown by X-ray powder diffraction of material from impacted drums. Subsequently, studtite, [(UO2)(O2)(H2O)2](H2O)2, was heated in the laboratory. Its thermal decomposition produced a hygroscopic anhydrous uranyl peroxide that reacts 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 μ2-O atom, resulting in charge neutrality. A pair distribution function from neutron total scattering supports this structural model, as do (1)H- and (17)O-nuclear magnetic resonance spectra. The reactivity of U2O7 in water and with water in air is higher than that of other uranium oxides, and this can be both hazardous and potentially advantageous in the nuclear fuel cycle.

In Situ Generation of Organic Peroxide to Create a Nanotubular Uranyl Peroxide Phosphate

“…Uranium ore deposits emit α‐radiation, which can radiolytically split water molecules into hydroxyl and superoxide radicals and ultimately form solid uranyl peroxide phases . Additional findings demonstrate that uranyl peroxides are a component of yellow cake and form as an alteration phase on spent fuel rods and corium lavas at the Chernobyl site . Based upon the persistence of these phases, understanding the chemistry and speciation of uranyl peroxide compounds in complex systems is of high importance.…”

Thermodynamic studies of studtite thermal decomposition pathways via amorphous intermediates UO3, U2O7, and UO4

“…Hence, understanding the decomposition pathway of studtite may help to clarify aspects of UO 2 fuel degradation involving exposure to water. Studtite is often precipitated during production of uranium yellowcake from uranium ore processing, and decomposition of studtite or other uranium peroxides may release O 2 gas during transport and storage [14,15].…”

“…The decomposition of studtite upon heating under oxygen or argon atmospheres occurs in several steps [11,[15][16][17][18][19][20][21]. Irreversible dehydration to metastudtite begins around 60 °C.…”

“…The amorphous phase can persist to 550 -600 °C under heating rates of 10C per minute, beyond which -UO 2.9 crystallizes, followed by U 3 O 8 at higher temperature [11,17,18]. Among these intermediate decomposition products, the amorphous materials are interesting both because they appear to contain peroxide, and because their structures and reactivity with water are not fully understood [15,21]. Odoh et al observed the reaction of amorphous uranyl-bearing material, obtained by heating studtite with water [15]: such a reaction may contribute to pressurization of drums containing yellowcake due to the generation of oxygen from the reaction [14,15].…”

In Situ Generation of Organic Peroxide to Create a Nanotubular Uranyl Peroxide Phosphate

“…[4] Uranium ore deposits emit a-radiation, which can radiolytically split water molecules into hydroxyl and superoxide radicals and ultimately form solid uranyl peroxide phases. [5,6] Additional findings demonstrate that uranyl peroxides are ac omponent of yellow cake [7] and form as an alteration phase on spent fuel rods and corium lavas at the Chernobyl site. [8,9] Based upon the persistence of these phases,u nderstanding the chemistry and speciation of uranyl peroxide compounds in complex systems is of high importance.An expansive number of synthetic uranyl peroxide phases and nanocapsules have been characterized by Burns and coworkers, [6,10,11,13] with additional experimental and computational studies providing ab asis for the formation mechanism.…”