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.

Combination of uranium, peroxide, and mono- (Na, K) or divalent (Mg, Ca, Sr) cations under alkaline aqueous conditions results in the rapid formation of anionic uranyl triperoxide monomers (UTs), (UO(O)), exhibiting unique Raman signatures. Electronic structure calculations were decisive for the interpretation of the spectra and assignment of unexpected signals associated with vibrations of the uranyl and peroxide ions. Assignments were verified by O isotopic labeling of the uranyl ions supporting the computational-based interpretation of the experimentally observed peaks and the assignment of a novel asymmetric vibration of the peroxide ligands,(O).

Solid UO dissolution and uranium speciation in aqueous solutions that promote formation of uranyl peroxide macroanions was examined, with a focus on the role of alkali metals. UO powders were dissolved in solutions containing XOH (X = Li, Na, K) and 30% HO. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements of solutions revealed linear trends of uranium versus alkali concentration in solutions resulting from oxidative dissolution of UO, with X:U molar ratios of 1.0, showing that alkali availability determines the U concentrations in solution. The maximum U concentration in solution was 4.20 × 10 parts per million (ppm), which is comparable to concentrations attained by dissolving UO in boiling nitric acid, and was achieved by lithium hydroxide promoted dissolution. Raman spectroscopy and electrospray ionization mass spectrometry (ESI-MS) of solutions indicate that dissolution is accompanied by the formation of various uranyl peroxide cluster species, the identity of which is alkali concentration dependent, revealing remarkably complex speciation at high concentrations of base.

Uranium concentrations as high as 2.94 × 10 parts per million (1.82 mol of U/1 kg of HO) occur in water containing nanoscale uranyl cage clusters. The anionic cage clusters, with diameters of 1.5-2.5 nm, are charge-balanced by encapsulated cations, as well as cations within their electrical double layer in solution. The concentration of uranium in these systems is impacted by the countercations (K, Li, Na), and molecular dynamics simulations have predicted their distributions in selected cases. Formation of uranyl cages prevents hydrolysis reactions that would result in formation of insoluble uranyl solids under alkaline conditions, and these spherical clusters reach concentrations that require close packing in solution.

The first neutron diffraction study of a single crystal containing uranyl peroxide nanoclusters is reported for pyrophosphate-functionalized Na44K6[(UO2)24(O2)24(P2O7)12][IO3]2·140H2O (1). Relative to earlier X-ray studies, neutron diffraction provides superior information concerning the positions of H atoms and lighter counterions. Hydrogen positions have been assigned and reveal an extensive network of H-bonds; notably, most O atoms present in the anionic cluster accept H-bonds from surrounding H2O molecules, and none of the surface-bound O atoms are protonated. The D4h symmetry of the cage is consistent with the presence of six encapsulated K cations, which appear to stabilize the lower symmetry variant of this cluster. (31)P NMR measurements demonstrate retention of this symmetry in solution, while in situ (31)P NMR studies suggest an acid-catalyzed mechanism for the assembly of 1 across a wide range of pH values.

Herein, we report a new salt of a pyrophosphate-functionalized uranyl peroxide nanocluster {UPp} (1) exhibiting O molecular symmetry both in the solid and solution. Study of the system yielding 1 across a wide range of pH by single-crystal X-ray diffraction, small-angle X-ray scattering, and a combination of traditional P and diffusion-ordered spectroscopy (DOSY) NMR affords unprecedented insight into the amphoteric chemistry of this uranyl peroxide system. Key results include formation of a rare binary {U}·{UPp} (3) system observed under alkaline conditions, and evidence of acid-promoted decomposition of {UPp} (1) followed by spatial rearrangement and condensation of {U} building blocks into the {UPp} (2) cluster. Furthermore, P DOSY NMR measurements performed on saturated solutions containing crystalline {UPp} show only trace amounts (∼2% relative abundance) of the intact form of this cluster, suggesting a complex interconversion of {UPp}, {UPp}, and {UPp} ions.

Uranyl-peroxide capsules are the newestf amily of polyoxometalates. Although discovered 13 years previously with over 70 topologies reported, there is al ack in the fundamental understanding of assembly mechanisms, particularly the role of the alkali counterions.H erein, the reactionp athway anda ssembly of uranyl peroxide capsules is reported by tracking the conversion from K + uranyl triperoxide monomer to the K + uranyl-peroxide U 28 capsule by means of small-angle X-ray scattering and Raman spectroscopy.F or the first time, the K + uranyl-peroxide pentamer face is isolated and structurally characterized, giving credence to the long-held belief that these geometric faces serve as building blocks to the fully formed capsules. Once isolated and re-dissolved, the pentamer face undergoes rapid conversion to capsulef orms,u nderlining its high reactivity that challenges its isolation. Calorimetricm easurements of the studied speciesc onfirms the pentamer lies on the energy landscape between the monomer and capsule.Aqueousuranium speciation impactsall aspects of nuclear materials stewardship. These include safe storage, handling, and treatment of aqueous nuclear wastes;s torageo fs pent nuclear fuel;key steps of the cradle-to-grave nuclear fuel cycle;nuclear forensics;a nd environmental contamination. In the past 13 years, [1] uranyl-peroxide capsules have emerged as both a new molecule family akin to the transition-metalp olyoxometalates (POMs) [2] and as viable aqueous species in natural, [3] industrial, [4] and laboratory settings. Uranyl capsules have been borne out with now over 70 unique cluster topologies that have been structurally characterized, featuring variation in size (16 to 124 polyhedra), ligands (peroxide, hydroxide,o xalate, pyrophosphate) and heterometals in the capsule walls (Fe, V, W, Mo, Sm) and cavities (alkalis, alkaline earths, rare earths, Bi, Pb, Ta ,N b). [5] Peroxidei sf ormed by radiolysiso fw ater in the high radiation field of uranium ore deposits andf uel, and is stabilized by bondingt ou ranyl, forming the mineral studtite, [(UO 2 )(O 2 )(H 2 O) 2 ](H 2 O) 2 in both nature and as deposits on storednuclearf uel. [5a, 6] Peroxide is very effective for dissolving uranium in combination with am ild base or precipitating uranium in acidic to neutralc onditions.T he dissolution/precipitation behaviori so ne viable method to separateu raniumf rom other oxides (i.e. crude ore for yellowcake production).H owever,t he dichotomy of the stable uranyl-peroxideb ond deterring its controlled removal by heating,a nd the inherent tendencyo fp eroxide to disproportionatey ielding hazardouspressures of O 2 gas hasc reated dangerous scenarios with stored yellowcake [6] and other uranylp eroxide materials.With afocus on solid-state characterization and computation of uranyl peroxide capsules and related materials, we are gaining as tructural understanding of the uranyl-peroxide bond. [7] Less well-understood is its reactivity in conditions that promote the formation of soluble or precipitatedu rany...

More than 60 unique uranyl peroxide cage clusters have been reported that contain as many as 124 uranyl ions and that have overall diameters extending to 4 nm. They self-assemble in water under ambient conditions, are models for understanding structure-size-property relations as well as testing computational models for actinides, and have potential applications in nuclear fuel cycles. High-temperature drop solution calorimetry has been used to derive the enthalpies of formation of the salts of seven topologically diverse uranyl peroxide cage clusters containing from 22 to 28 uranyl ions that are bridged by various combinations of peroxide, pyrophosphate, and phosphite. The enthalpies of formation of these seven salts, as well as three salts of other uranyl peroxide clusters reported earlier, are dominated by the interactions of the alkali countercations with the clusters. There is an approximately linear relationship between the enthalpies of formation of the cluster salts and the charge density of the corresponding uranyl peroxide cluster, wherein salts containing clusters with higher charge densities have more negative enthalpies of formation.