Oxidation State Of Uranium Research Articles

Chernobyl fuel microparticles: uranium oxidation state and isotope ratio by HERFD-XANES and SIMS

Chernobyl fuel microparticles: uranium oxidation state and isotope ratio by HERFD-XANES and SIMS

On the origin of low-valent uranium oxidation state

The significant interest in actinide bonding has recently focused on novel compounds with exotic oxidation states. However, the difficulty in obtaining relevant high-quality experimental data, particularly for low-valent actinide compounds, prevents a deeper understanding of 5f systems. Here we show X-ray absorption near-edge structure (XANES) measurements in the high-energy resolution fluorescence detection (HERFD) mode at the uranium M4 edge for the UIII and UIV halides, namely UX3 and UX4 (X = F, Cl, Br, I). The spectral shapes of these two series exhibit clear differences, which we explain using electronic structure calculations of the 3d-4f resonant inelastic X-ray scattering (RIXS) process. To understand the changes observed, we implemented crystal field models with ab initio derived parameters and investigated the effect of reducing different contributions to the electron-electron interactions involved in the RIXS process. Our analysis shows that the electron-electron interactions weaken as the ligand changes from I to F, indicative of a decrease in ionicity both along and between the UX3 and UX4 halide series.

Uranium oxidation states in zircon and other accessory phases

Zircon and other U-bearing accessory phases are important time-capsules for studying the evolution of Earth and other planetary bodies as these minerals can record both temporal and compositional information regarding their host rocks. In silicate melts, uranium can occur in either the UIV, UV, or UVI valence state and its redox sensitive nature could, in principle, allow for information on magma oxygen fugacity (ƒO2) to be gleaned from U-bearing phases provided they can incorporate multivalent U during crystallization. Currently, however, little is known regarding the details of how U is speciated in these minerals.In this study, we conducted conventional X-ray absorption near-edge structure (XANES) spectroscopy at the U M4-edge on a set of natural zircon (n = 140), titanite (n = 9), apatite (n = 7), baddeleyite (n = 7), and garnet (n = 2) samples to determine the oxidation state of U in these crystals. We also collected U L3-edge spectra for select zircon samples to investigate the bonding environment of U using extended X-ray absorption fine structure (EXAFS) analysis. The effects of crystallographic orientation and radiation damage on zircon U M4-edge spectra are found to be minimal compared to the magnitude of the peak shifts associated with U oxidation state. We find that titanite and garnet contain only tetravalent U, while zircon, apatite, and baddeleyite can contain U of variable valence. Of these phases, zircon shows the greatest variability, with white-line energy, Ewl (i.e., peak absorbance) covering a range of >2.0 eV between grains: i.e., the entire energy range expected between pure UIV and pure UVI species. Moreover, a correlation is observed between the Ewl of zircon U M4-edge spectra (i.e., relative proportions of UIV, UV, UVI) and the ƒO2 of their host rocks. Our results thus establish U oxidation states in zircon as a powerful new tracer of magma redox. Since XANES is non-destructive and can be performed in situ, this technique can be utilized alongside other microanalytical methods (e.g., LA-ICPMS, SIMS) to further expand the breadth of information that can be extracted from single mineral grains.

Metal-Halide Covalency, Exchange Coupling, and Slow Magnetic Relaxation in Triangular (CpiPr5)3U3X6 (X = Cl, Br, I) Clusters.

The actinide elements are attractive alternatives to transition metals or lanthanides for the design of exchange-coupled multinuclear single-molecule magnets. However, the synthesis of such compounds is challenging, as is unraveling any contributions from exchange coupling to the overall magnetism. To date, only a few actinide compounds have been shown to exhibit exchange coupling and single-molecule magnetism. Here, we report triangular uranium(III) clusters of the type (CpiPr5)3U3X (1-X; X = Cl, Br, I; CpiPr5 = pentaisopropylcyclopentadienyl), which are synthesized via reaction of the aryloxide-bridged precursor (CpiPr5)2U2(OPhtBu)4 with excess Me3SiX. Spectroscopic analysis suggests the presence of covalency in the uranium-halide interactions arising from 5f orbital participation in bonding. The dc magnetic susceptibility data reveal the presence of antiferromagnetic exchange coupling between the uranium(III) centers in these compounds, with the strength of the exchange decreasing down the halide series. Ac magnetic susceptibility data further reveal all compounds to exhibit slow magnetic relaxation under zero dc field. In 1-I, which exhibits particularly weak exchange, magnetic relaxation occurs via a Raman mechanism associated with the individual uranium(III) centers. In contrast, for 1-Br and 1-Cl, magnetic relaxation occurs via an Orbach mechanism, likely involving relaxation between ground and excited exchange-coupled states. Significantly, in the case of 1-Cl, magnetic relaxation is sufficiently slow such that open magnetic hysteresis is observed up to 2.75 K, and the compound exhibits a 100-s blocking temperature of 2.4 K. This compound provides the first example of magnetic blocking in a compound containing only actinide-based ions, as well as the first example involving the uranium(III) oxidation state.

U(V) Stabilization via Aliovalent Incorporation of Ln(III) into Oxo-salt Framework.

Pentavalent uranium compounds are key components of uranium's redox chemistry and play important roles in environmental transport. Despite this, well-characterized U(V) compounds are scarce primarily because of their instability with respect to disproportionation to U(IV) and U(VI). In this work, we provide an alternate route to incorporation of U(V) into a crystalline lattice where different oxidation states of uranium can be stabilized through the incorporation of secondary cations with different sizes and charges. We show that iriginite-based crystalline layers allow for systematically replacing U(VI) with U(V) through aliovalent substitution of 2+ alkaline-earth or 3+ rare-earth cations as dopant ions under high-temperature conditions, specifically Ca(UVIO2)W4O14 and Ln(UVO2)W4O14 (Ln=Nd, Sm, Eu, Gd, Yb). Evidence for the existence of U(V) and U(VI) is supported by single-crystal X-ray diffraction, high energy resolution X-ray absorption near edge structure, X-ray photoelectron spectroscopy, and optical absorption spectroscopy. In contrast with other reported U(V) materials, the U(V) single crystals obtained using this route are relatively large (several centimeters) and easily reproducible, and thus provide a substantial improvement in the facile synthesis and stabilization of U(V).

Ab initio study of the adsorption of O, O2, H2O and H2O2 on UO2 surfaces using DFT+U and non-collinear magnetism

In order to model correctly the corrosion of spent nuclear fuel under disposal conditions, it is important to understand its behavior in the presence of oxidants. To advance in this direction, we consider the oxidation of UO2. We investigate computationally the adsorption of various species on its three most stable surfaces: (111), (110), and (100), with emphasis on incorporating a full non-collinear PBE+U approach. Various species, namely O, O2, H2O and H2O2 are considered due to their relevance for the oxidation of UO2. The dissociation energy and an estimate for the dissociation barrier for O2 were obtained, using the preferred adsorption configurations of O and O2. The adsorption configurations for H2O in our study compare well with previous studies that used collinear approximations, both in terms of relative stability of configurations and bond lengths. Differences in adsorption energies were found, which may be important for reaction kinetics. Dissociative reactions in which the water molecule splits in hydrogen and hydroxyl occur only on one of the three surfaces. The hydrogen further reacts with a surface oxygen to also form a hydroxyl group. Not surprisingly, we find that H2O2 binds more strongly to the three surfaces than water (lower formation energy), and similar to H2O adsorption, dissociative reactions may occur. The dissociated hydrogen reacts with a surface oxygen to form a hydroxyl group and the hydroperoxyl molecule binds with a surface uranium. Our study, which includes a detailed study of electron transfer, magnetic structure and the preferred adsorption configurations, gives insight into the uranium oxidation states and the influence of surface geometry on adsorption. The findings contribute to a more comprehensive understanding of the early stages of UO2 oxidation.

Exploring the Valence Diversity of Uranium by Flux Growth of Uranium Silicate under Inert Atmosphere.

The attributes of good solubility and the redox-neutral nature of molten salt fluxes enable them to be useful for the synthesis of novel crystalline actinide compounds. In this work, a flux growth method under an inert atmosphere is proposed to explore the valence diversity of uranium, and a series of five uranium silicate structures, [K3Cl][(UVIO2)(Si4O10)] (1), Cs3[(UVO2)(Si4O10)] (2), K2[UIV(Si2O7)] (3), K8[(UVIO2)(UVO2)2(Si8O22)] (4), and Cs6[UIV(UVO)2(Si12O32)] (5), were synthesized using different metal halide salt and feeding U/Si ratios. Crystal structure analysis reveals that the utilization of argon atmosphere that helps to avoid possible oxidation of low-valence uranium generates a variety of oxidation states of uranium including U(VI), U(V), U(IV), mixed-valence U(V) and U(VI), and mixed-valence U(IV) and U(V). Characterization of physicochemical properties of representative compounds shows that all these uranium silicate compounds have bandgaps among the range of 2.0-3.4 eV, and mixed-valence uranium silicate compounds have relatively narrower bandgaps. Density functional theory calculations on formation enthalpies, lattice energies, and bandgaps of all five compounds were also performed to provide more structural information about these uranium silicates. This work enriches the library of variable-valence uranium silicate compounds and provides a feasible way to produce novel actinide compounds with intriguing properties through the flux growth method that might show potential application in relevant fields such as storage media for nuclear waste.

Pyridine Diphosphonate Ligand for Stabilization of Tetravalent Uranium and Neptunium in Aqueous Medium under Aerobic Conditions.

Though uranium is usually present in its +6 oxidation state (as uranyl ion) in aqueous solutions, its conversion to oxidation states such as +4 or +5 is a challenging task. Electrochemical reduction and axial oxo activation are the preferred methods to get stable unusual oxidation states of uranium in an aqueous medium. In previous studies, dicarboxylic acid has been used to stabilize UO2+ in aqueous alkaline solutions. In the present work, a diphosphonate ligand was chosen due to its higher complexing ability compared to that of the carboxylate ligands. Neptunium complexation studies with 2,6-pyridinediphosphonic acid (PyPOH) indicated the formation of different species at different pH values and the complexation facilitates disproportionation of NpO2+ to Np4+ and NpO22+ at pH 2. Hexavalent actinides form insoluble complexes in aqueous media at pH = 2, as confirmed by UO22+ complexation studies. The in situ complexation-driven precipitation resulted in conversion to pure Np4+ in aqueous media as the Np4+-PyPOH complex. A strong complexing ability of the PyPOH ligand toward the Np4+ ion is also seen for the stabilization of the electrochemically generated U4+ in aqueous medium under aerobic conditions. The U4+-PyPOH complex was found to be stable for 3 months. Raman, UV-vis, fluorescence, and cyclic voltametric studies along with density functional theory (DFT) calculations were done to get structural insights into the PyPOH complexes of actinides in different oxidation states.

Controlled and sequential single-electron reduction of the uranyl dication.

A flexible tripodal pyrrole-imine ligand (H3L) has been used to facilitate the controlled and sequential single-electron reductions of the uranyl dication from the U(VI) oxidation state to U(V) and further to U(IV), processes that are important to understanding the reduction of uranyl and its environmental remediation. The uranyl(VI) complexes UO2(HL)(sol) (sol = THF, py) were straightforwardly accessed by the transamination reaction of H3L with UO2{N(SiMe3)2}2(THF)2 and adopt 'hangman' structures in which one of the pyrrole-imine arms is pendant. While deprotonation of this arm by LiN(SiMe3)2 causes no change in uranyl oxidation state, single-electron reduction of uranyl(VI) to uranyl(V) occurred on addition of two equivalents of KN(SiMe3)2 to UO2(HL)(sol). The potassium cations of this new [UVO2(K2L)]2 dimer were substituted by transmetalation with the appropriate metal chloride salt, forming the new uranyl(V) tetra-heterometallic complexes, [UVO2Zn(L)(py)2]2 and [UVO2Ln(Cl)(L)(py)2]2 (Ln = Y, Sm, Dy). The dimeric uranyl(V)-yttrium complex underwent further reduction and chloride abstraction to form the tetrametallic U(IV) complex [UIVO2YIII(py)]2, so highlighting the adaptability of this ligand to stabilise a variety of different uranium oxidation states.

Unusual redox stability of pentavalent uranium with hetero-bifunctional phosphonocarboxylate: insight into aqueous speciation.

The +5 state is an unusual oxidation state of uranium due to its instability in the aqueous phase. As a result, gaining information about its aqueous speciation is extremely difficult. The present work is an attempt in that direction and it provides insight into the existence of a new pentavalent species in the presence of hetero-bifunctional phosphonocarboxylate (PC) chelators, other than the carbonate ion, in the aqueous medium. The aqueous chemistry of pentavalent uranium species with three environmentally relevant PCs was probed using electrochemical and DFT methods to understand the redox energy and kinetics of conversion of the U(VI)/U(V) couple, stability, structure, stoichiometry, binding modes, etc. Interestingly, pentavalent uranium complexes with PCs are quite persistent over a wide range of pH starting from acidic to alkaline conditions. The PC chelators block the cation-cation interaction (CCI) of U(V) through strong hetero-bidentate chelation and intermolecular hydrogen bonding (IMHB) interactions which stabilize the pentavalent metal ion against disproportionation. For uranyl species in the presence of PCs, acting as chelators, CV plots were obtained at varying pH values from 2 to 8. The obtained results indicate an irreversible single redox peak involving U(VI) to U(V) conversion and association of a coupled chemical reaction with the electron transfer step. ESI-MS studies were performed to understand the speciation effect on the U(VI)/U(V) redox couple with varying pH. Speciation modelling of U(V) with the PC ligands was carried out, which indicated that the U(V) is redox stable in nearly 47% of the pH region in the presence of the PCs as compared to the carboxylate-based chelators. The free energy and reduction potential of the U(V) complexes and the reduction free energy and disproportionation free energy for the U(VI)/U(V) couple were determined by DFT computations in the presence of the PCs. In situ spectroelectrochemical spectra were recorded to provide evidence for the existence of U(V) species with PCs in the aqueous medium and to acquire its absorption spectra. The present study is highly significant for understanding the coordination chemistry of pentavalent uranium species, accurate modelling of uranium, and isolation of U(V).

X-ray diffraction and Raman spectroscopic studies on ThO2−UO2 solid solutions

In view of the vast resources of thorium in India, thorium-based fuel cycle has been envisaged as one of the most promising options to meet sustainable energy requirements. In this direction, ThO2−233UO2 and ThO2−UO2 (low-enriched) mixed oxide fuels with < 30% UO2 have been proposed for the Advanced Heavy Water Reactor. In the present work, phase relations in the Th1-xUxO2+δ (x ≤ 0.3) system under reducing and oxidizing conditions have been investigated by means of X-ray diffraction and Raman spectroscopic studies. Diffraction studies confirmed the formation of single-phase fluorite-type solid solutions; however, the samples synthesized in air exhibited slightly lower lattice parameters than those of their reduced counterparts, attributed to oxidation of U4+. Raman spectroscopic studies indicate the formation of various defects upon uranium substitution in parent ThO2, depending on synthesis conditions. It also highlights the existence of more disorder in the air-synthesized samples as compared to their reduced counterparts, plausibly due to the presence of a variety of oxidation states of uranium and associated oxygen anions interstitials. Thermal annealing of the solid solutions under air at T ≥ 1873 K results in heavy uranium evaporation loss, which has been established by XRD and XRF studies.

Electrochemical Behavior and Reduction of in LiCl-KCl Molten Salt

In this work, we explored the electrochemical behavior and reduction process of uranyl ions in molten LiCl-KCl eutectic at 773 K. Cyclic voltammetry (CV) and square wave voltammetry (SWV) results showed that the reduction of ions on the inert W electrode was a three-step process: (1) (2) and (3) Electrolysis experiments further confirmed this reduction mechanism that ions were reduced to UO2 on the molybdenum electrode by applying a constant potential of −1.00 V vs Ag/AgCl and subsequently to uranium metal at a more negative potential of −2.35 V vs Ag/AgCl. In addition, ions could be thoroughly reduced to uranium metal through a 4-h constant current electrolysis at −18 mA cm−2. Electronic absorption spectroscopy (EAS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) respectively illustrated that the oxidation state of uranium was unchanged and uranium concentration gradually decreased during the electrolysis. Finally, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the phase composition and microstructure of deposited products. Nano-sized UO2 and U metal particles were successfully obtained by constant potential and current electrolysis. The results of this work further reveal the electrochemical behavior and reduction mechanism of ions in molten LiCl-KCl, providing a guiding ideology for the pyrochemical reprocessing of oxide spent fuels.

Accessing five oxidation states of uranium in a retained ligand framework

Understanding and exploiting the redox properties of uranium is of great importance because uranium has a wide range of possible oxidation states and holds great potential for small molecule activation and catalysis. However, it remains challenging to stabilise both low and high-valent uranium ions in a preserved ligand environment. Herein we report the synthesis and characterisation of a series of uranium(II–VI) complexes supported by a tripodal tris(amido)arene ligand. In addition, one- or two-electron redox transformations could be achieved with these compounds. Moreover, combined experimental and theoretical studies unveiled that the ambiphilic uranium–arene interactions are the key to balance the stabilisation of low and high-valent uranium, with the anchoring arene acting as a δ acceptor or a π donor. Our results reinforce the design strategy to incorporate metal–arene interactions in stabilising multiple oxidation states, and open up new avenues to explore the redox chemistry of uranium.

Distinguishing secondary uranium mineralizations in uranium ore using LIBS imaging

The main aim of this work is to demonstrate the potential of LIBS as a complementary technique to electron probe microanalysis (EPMA) for distinguishing and characterizing uranium mineralizations. Combining both methods can help estimate uranium oxidation states and monitor the possible mobilization of uranium in the environment by detecting oxygen and hydrogen using LIBS. It was confirmed that the LIBS signal of oxygen is proportional to oxygen content and that the strength of the oxygen signal is closely related to the oxidation state of uranium. The second assumption that the hydrogen signal is closely related to water (or hydroxyl group) content was also confirmed by detecting a stronger hydrogen signal from the presumed secondary mineralization. In contrast, hydrogen was not found in uraninite and quartz.When superimposed, images obtained with LIBS and EPMA show a clearly visible contrast between primary and secondary uranium mineralizations. Images of uranium obtained with the two techniques match perfectly, while the LIBS image of oxygen confirms the presence of an oxidized form of secondary uranium minerals (uranophane). The LIBS image of hydrogen clearly shows mineral phases containing water or a hydroxyl group, confirming that uranophane and other associated minerals contain greater amounts of hydrogen (water).

Phase evolution in the UO2–CeO2 system under oxidizing and reducing conditions: X-ray diffraction and spectroscopic studies

The solid-solution formation mechanism and defect dynamics in U1-xCexO2±δ solid solutions have been investigated by means of X-ray diffraction (XRD) analysis, Raman spectroscopy, and X-ray absorption near-edge structure (XANES) spectroscopy. Diffraction studies have revealed the formation of single-phase solid solutions with a fluorite-type structure at x ≤ 0.2 and a single-phase fluorite-type structure with broadened diffraction peaks, attributed to inhomogeneity in the O/M ratio, at 0.3 ≤ x ≤ 0.8 under reducing conditions. Oxidizing conditions result in the formation of single-phase solid solutions with fluorite-type structure at x ≥ 0.4. Upon the formation of Ce3+ in solid solutions at x ≤ 0.2 under reducing conditions, charge neutrality is achieved through oxygen vacancy creation and formation of higher oxidation states of uranium through a charge compensation mechanism. Lattice parameter–composition relationships for fluorite-type phases synthesized under oxidizing conditions show a decrease in lattice parameters as compared to their stoichiometric counterparts due to the oxidation of U4+ to U5+/U6+. XANES studies provide direct experimental evidence of the presence of Ce3+ in solid solutions, despite synthesis under oxidizing conditions.

Thermodynamic Assessment of CrCl2 with NaCl–KCl–MgCl2–UCl3–UCl4 for Molten Chloride Reactor Corrosion Modeling

The formation of corrosion species in molten salt reactor systems is driven by the salt redox condition, which can be indicated by the uranium oxidation state ratio (U4+/U3+). In chloride salts, chromium is well known to have the highest tendency to deplete from alloy surfaces; however, no available thermodynamic database suitably represents the effect of trivalent and tetravalent uranium chloride on the corrosion potential. In this work, we extend the Molten Salt Thermal Properties Database–Thermochemical with Gibbs energy models suitable for application to alloy corrosion in chloride molten salt-fueled nuclear reactors. The work required the development of fully constrained thermodynamic models, utilizing the modified quasi-chemical model in the quadruplet approximation for the melt and a single sublattice model for the solid solution phases allowing accurate estimations of thermodynamic properties even in systems for which few thermodynamic data are available. The effort included differential scanning calorimetry (DSC) measurements to find the previously unreported phase equilibria of the CrCl2–UCl3 system and to confirm the equilibria of the MgCl2–UCl3 system. Elemental period correlations allowed the estimation of thermodynamic values for compounds in the NaCl–CrCl2 and KCl–CrCl2 systems allowing a well-informed description of CrCl2 behavior in the compositions of technical interest for the Na–K–Mg–U3+–U4+ chloride salt.

Phase relations and lattice parameter trends in Nd3+−substituted UO2 system under reducing and oxidizing conditions

While Am-substituted UO2 solid solutions have been considered as advanced fuels for heterogeneous transmutation of americium, U-substituted Am2O3 with cubic structure has been envisaged as a promising candidate for radioisotope heat unit and radioisotope thermoelectric generator. Considering Nd2O3 as a surrogate for Am2O3, a series of Nd3+-substituted UO2 samples with the general formula U1-xNdxO2±δ (0 ≤ x ≤ 1, δ ≥ 0) have been investigated for phase relations under reducing and oxidizing conditions. In this direction, samples have been synthesized by a solid-state route and thoroughly characterized by X-ray diffraction and Raman spectroscopy. XRD studies revealed the formation of fluorite-type solid solution at x ≤ 0.7 and a biphasic phase field consisting of C-type (U,Nd)O2−δ phase and A-type Nd2O3 at x ≥ 0.8 in reducing conditions. In the fluorite-type regime, due to the aliovalent Nd3+-substitution in UO2, the charge neutrality was achieved primarily through oxygen vacancy creation as evident from lattice parameter trends as well as Raman spectroscopic investigation that exhibit Raman band due to oxygen vacancies. The reduced samples have a high tendency to get oxidized even during storage under ambient conditions leading to a decrease in lattice parameters, which can be attributed to the formation of the higher oxidation state of uranium (U5+/U6+) following a charge compensation mechanism. Synthesis in the oxidizing conditions yielded a biphasic phase field at x ≤ 0.2, a single-phase fluorite-type phase field at 0.3 ≤ x ≤ 0.7, a biphasic mixture of fluorite-type (U,Nd)O2−δ and trigonal Nd6UO12 phases at x = 0.8 and another biphasic mixture of Nd6UO12 and Nd2O3 phases at x = 0.9. The transition of reduced single-phase fluorite-type solid solution at x ≤ 0.2 to a biphasic mixture upon oxidation has been probed by a high-temperature XRD.

Influence of groundwater composition on the reductive precipitation of U(VI) on corroding iron foil surfaces

In order to assess the disposal of spent nuclear fuel in a deep geological nuclear waste repository, the interactions between U(VI) and corroded iron present in the canister material are of importance. It is important to correctly model the fate of the oxidatively dissolved uranium in order to correctly estimate radium releases from the canister in the long term. The release of radionuclides into the environment depends on the dissolution of the UO2 matrix which is dependent on the redox conditions at the fuel surface. The effect of metallic iron on the reduction of U(VI) was studied under anoxic conditions using synthetic groundwaters with different compositions, chosen to investigate the influence of calcium-uranyl-carbonato complexes on the thermodynamics and kinetics of U(VI) reduction on anoxically corroding iron. The corrosion products formed on the iron surface were investigated using SEM-EDS and XPS to identify elemental composition and oxidation states of uranium and iron on the surface. The iron foils efficiently reduced U(VI) to U(IV) causing its significant sorption and precipitation on the iron foil surfaces in the form of U(IV).

Direct electrochemical dissolution of metallic uranium into deep eutectic solvent

Deep eutectic solvents (DESs) could make uranium recycling processes greener than using conventional solvents, owing to their inherent advantages such as being biodegradable and non-toxic. However, the chemistry of uranium ions in DES systems, particularly the mechanism of direct anodic dissolution of uranium metal, remains unclear. The anodic dissolution of uranium metal in choline chloride (ChCl)–ethylene glycol (EG) DES was conducted by applying a constant current. The oxidation state of uranium in ChCl-EG DES was uranium(IV) when compared to concentration and applied charge. Absorption spectra showed 8 coordinated U(IV) centered complexes formed in the ChCl-EG DES, regardless of the uranium source (i.e. uranium chloride or metallic uranium anodic dissolution). The in situ spectroelectrochemistry during potentiostatic electrolysis, cyclic voltammograms, and absorption spectra of different times of potentiostatic electrolysis showed U(IV) oxidized to U(V), and then the U(V) disproportionated into U(IV) and U(VI) species. Understanding fundamental uranium speciation in DES systems and electrochemical properties could facilitate the development of an eco-friendly nuclear fuel cycle and nuclear uranium recycling process using green DESs.

Route to Chemical Accuracy for Computational Uranium Thermochemistry.

Benchmark spinor-based relativistic coupled-cluster calculations for the ionization energies of the uranium atom, the uranium monoxide molecule (UO), and the uranium dioxide molecule (UO2) and for the bond dissociation energies of UO and UO2 are reported. The accuracy of these calculations in the treatments of relativistic, electron-correlation, and basis-set effects is analyzed. The intrinsic convergence of the computed results and the favorable comparison with the experimental values demonstrate the unique applicability of the spinor representation of quantum-chemical methods to open-shell uranium-containing atomic and molecular species with uranium oxidation states ranging from U(0) to U(V).