The high radiation field associated with spent nuclear fuel (UIVO2) pellets produces an array of reactive radical species that impact the corrosion and formation of secondary alteration phases. Dioxygen radicals are important as radiolysis products, but the interaction between these reactive oxygen species and UVIO22+ and its effects on the resultant alteration phases is unclear. We report the first example of a UVI superoxide compound and explore its reactivity in the environments relevant to the storage of spent nuclear fuel. We utilized X‐ray diffraction and Raman scattering techniques to demonstrate that the uranyl superoxide reacts with CO2 in air to afford a mixed uranyl peroxide/carbonate within 3 days, both in solution and under atmospheric conditions. An additional transformation occurs over the course of 3 months to form a potassium UVI carbonate (grimselite), which also occurs as an alteration product on Chernobyl corium. Our results demonstrate the presence and significance of the superoxide anion in the alteration of spent nuclear fuel and indicate the impact of uranyl superoxide chemistry on high prevalence of carbonate in the secondary phases of spent nuclear fuel.

Current synthetic pathways for uranyl peroxide materials introduce high initial concentrations of aqueous H2O2 that decline over time. Alternatively, in situ generation of organic peroxide would maintain constant concentrations of peroxide over prolonged periods of time and open new pathways to novel uranyl peroxide compounds. Herein, we demonstrate this concept through the synthesis of a nanotube‐like uranyl peroxide phosphate (NUPP), Na12[(UO2)(μ‐O2)(HPO4)]6(H2O)40, making use of the inhibited autoxidation of benzaldehyde in benzyl alcohol solutions in the presence of phosphonate ligands. The unique feature of NUPP is the bent dihedral angle U‐(μ‐O2)‐U (123.9°±0.4° to 124.6°±0.5°), which allows hexameric uranyl peroxide macrocycles to adopt the nanotubular topology and prevents the formation of nanocapsules. Raman spectroscopy of the solution phase confirms our mechanistic understanding of the reaction pathway and confirms that consistent levels of peroxide are generated in situ over an extended period of time.

Naturally occurring uranium is a widespread contaminant present in the water resources around the abandoned uranium mines in the southwest United States. A novel method for rapid uranium detection has been recently developed that relies on the sequestering of uranium by amidoximated polyacrylonitrile (AO-PAN) polymer mats and uses the Raman-active (ν1) symmetric stretch as the signal. The Raman signals obtained from uranium bearing AO-PAN were challenging to interpret due to an unknown uranyl speciation on the surface of the mats. Herein, we provide the synthesis and structural characterization of six model coordination compounds that contain acetamidoxime/benzamidoxime (AAO/BAO) coordinated to the uranyl cation: [UO2(η1-AAO)­(NO3)2(H2O)] (1), [UO2(η1-AAO)2(NO3)2] (2), [UO2(η2-BAO)2(CH3OH)2] (3), [(UO2)3(η2-BAO)3(μ2-NO3)3] (4), [(UO2)4(μ3-O)2(μ2-BAO)4(η1-BAO)4(H2O)2]­(NO3)4 (5), and [(UO2)4(μ3-O)2(μ2-BAO)4(η1-BAO)6Na­(NO3)2]­(NO3)3 (6). Solid-state Raman spectra of 1–6 showed dramatic differences in the uranyl ν1 symmetric stretch depending on the coordination of the amidoxime functional group. The assignments made from the solid-state Raman spectra were used to deconvolute the solution-state Raman spectra of uranyl–acetamidoxime/benzamidoxime methanol solutions at different metal to ligand molar ratios. At low molar ratios (1 U:1 AAO/BAO and 1 U:2 AAO/BAO) the dominant species is the uranyl coordinated via the η1-oxygen atom of the oxime group, while at high molar ratios (1 U:3 AAO/BAO and 1 U:4 AAO/BAO) the dominant species are a tetrameric uranyl−μ3-O-η1-amidoxime complex similar to compounds 5 and 6 and a uranyl−η2-amidoxime complex similar to compounds 3 and 4. Solid-state Raman spectra showed good agreement with Raman signals obtained from the uranyl–AO-PAN mats, demonstrating that binding motifs between uranyl and amidoxime in compounds 5 and 6 are the most representative of the uranyl species on the surface of the AO-PAN mats.

Actinyl-actinyl interactions are particularly prevalent for the pentavalent neptunyl cation (Np(V)O 2 ) + where these interactions appear either as a T-or D-shape (diamond-shape). Tshaped interactions have been previously identified in high concentration Np(V) solutions containing simple anions (NO 3 À , ClO 4À , Cl À ) whereas D-shaped have only been isolated in the solid-state in the presence of carboxylate ligands. In this study, Density Functional Theory (DFT) calculations were paired with Raman spectroscopy to evaluate the formation of D-shaped interactions in the presence of aliphatic (R = H (formate), CH 3 (acetate), CH 2 CH 3 (propionate)) and aromatic (R = C 6 H 5 (benzoate), C 6 H 4 OH (4-hydroxybenzoate), C 5 H 4 N (isonicotinate)) carboxylate ligands. DFT studies indicate that the ΔG to form hydrated T-and D-shaped forms are not spontaneous but become so with the addition of the carboxylate ligands. Raman spectra of the Np(V) carboxylate solutions contained vibrational modes associated with the D-shaped interactions, but spectral changes observed over time indicate a dynamic system. Crystallization experiments from the Np(V) carboxylate systems confirmed the presence of D-shaped dimers for the aromatic carboxylates, suggesting that the choice of the anion in solution favors actinyl-actinyl interactions even at low concentrations (� 20 mM) of Np(V).

Mechanochemical reaction of UO3 with metal peroxides (M2O2) yields U(vi) triperoxide materials without producing radioactive solvent wastes.