Formation of studtite during the oxidative dissolution of UO2 by hydrogen peroxide: a SFM study

Chronicles of plutonium peroxides: spectroscopic characterization of a new peroxo compound of Pu(IV)

Although hydrogen peroxide (H2O2) has been highly used in nuclear chemistry for more than 75 years, the preparation and literature description of tetravalent actinide peroxides remain surprisingly scarce. A new insight is given in this topic through the synthesis and thorough structural characterization of a new peroxo compound of Pu(IV).

Superoxide Radicals in Uranyl Peroxide Solids: Lasting Signatures Identified by Electron Paramagnetic Resonance Spectroscopy

U(VI) peroxide phases (studtite and meta-studtite) are found throughout the nuclear fuel cycle and exist as corrosion products in high radiation fields. Peroxides are part of a family of reactive oxygen species (ROS) that include hydroperoxyl and superoxide species and are produced during alpha radiolysis of water. While U(VI) peroxides have been thoroughly investigated, the incorporation and stability of ROS species within studtite have not been validated. In the current study, electron paramagnetic resonance (EPR) spectroscopy was used to identify the presence of free radicals within a series of U(VI) peroxide samples containing depleted, highly enriched, and natural uranium. Density functional theory calculations indicated that the predicted EPR signals matched well with a superoxide (O2 -⋅) species incorporated into the studtite structure, confirming the presence of ROS in the material. Further analysis of samples that were synthesized between 1945 and 2023 indicated that there is a correlation between the radical signal and the product of specific activity multiplied by age of the sample.

Micrometric drilling of (meta-)studtite square platelets formed by pseudomorphic conversion of UO2 under high-frequency ultrasound

Pseudomorphic transformations are related to chemical conversions of materials while conserving their shape and structural features. Structuring ceramic shapes this way can be used to tailor the physico-chemical properties of materials that can benefit particular applications. In the context of spent nuclear fuel storage interacting with radiolysis products, the sonochemical behavior of powdered UO2 was investigated in dilute aqueous solutions saturated with Ar/(20 %)O2 (20 °C). Optimized parameter settings enabled the complete conversion of UO2 micrometric platelets into uranyl peroxide precipitates, referred to as (meta-)studtite [(UO2(O2)(H2O)2)xH2O] with x = 2 or 4. While the most acidic conditions yielded elongated crystal shapes in agreement with a dissolution/reprecipitation mechanism, softer conditions allowed the pseudomorphic transformation of the platelet shape oxide suggesting a complex formation mechanism. For specific conditions, this unprecedented morphology was accompanied with the formation of a hole in the platelet center. Investigations revealed that the formation of the drilled polymorphs is related to a perfect blend of H+, in-situ generation of H2O2 and high-frequency ultrasound, and is most probably related to the sono-capillary effect. These insights pave the way for new sonochemical approaches dedicated to the preparation of material polymorphs tailoring specific structural properties.

Capture and Stabilization of the Hydroxyl Radical in a Uranyl Peroxide Cluster

Here we describe the synthesis and characterization of a new uranyl peroxide cluster (UPC), U60 Ox30 *, which captures and stabilizes oxygen-based free radicals for more than one week. These radical species were first detected with a nitroblue tetrazolium colorimetric assay and U60 Ox30 * was characterized by single crystal X-ray diffraction as well as infrared (IR), Raman, UV-Vis-NIR, and electron paramagnetic resonance (EPR) spectroscopies. Identification of the free radicals present in U60 Ox30 * was done via room temperature solid and solution state X-band EPR studies using spin trapping methods. The spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was definitive for identifying the free radicals in U60 Ox30 *, which are hydroxyl radicals (⋅OH) that are stable for up to ten days that also persist upon addition of the metalloenzymes catalase and superoxide dismutase. Addition of the spin trapping agent α-(4-pyridyl N-oxide)-N-tert-butylnitrone (POBN) further confirmed the radicals were oxygen based, and deuteration experiments showed that the origin of the free radicals was from the decomposition of H2 O2 in water. These results demonstrate that highly oxidizing species such as the ⋅OH radical can be stabilized in UPCs, which alters our understanding of the role of free radicals present in spent nuclear fuel.

On the Stability of Uranium Carbide in Aqueous Solution-Effects of HCO3 - and H2O2

Uranium carbide (UC) is a candidate fuel material for future Generation IV nuclear reactors. As part of a general safety assessment, it is important to understand how fuel materials behave in aqueous systems in the event of accidents or upon complete barrier failure in a geological repository for spent nuclear fuel. As irradiated nuclear fuel is radioactive, it is important to consider radiolysis of water as a process where strongly oxidizing species can be produced. These species may display high reactivity toward the fuel itself and thereby influence its integrity. The most important radiolytic oxidant under repository conditions has been shown to be H₂O₂. In this work, we have studied the dissolution of uranium upon exposure of UC powder to aqueous solutions containing HCO₃ ^- and H₂O₂, separately and in combination. The experiments show that UC dissolves quite readily in aqueous solution containing 10 mM HCO₃ ^- and that the presence of H₂O₂ increases the dissolution further. UC also dissolves in pure water after the addition of H₂O₂, but more slowly than in solutions containing both HCO₃ ^- and H₂O₂. The experimental results are discussed in view of possible mechanisms.

Thermodynamics and Chemical Behavior of Uranyl Superoxide at Elevated Temperatures

Understanding the alteration mechanisms of UO₂-based nuclear fuel has a range of practical implications for both short- and long-term storage of spent fuel rods and environmental ramifications for the mobility of radioactive material at the Chernobyl and Fukushima sites. The major identified alteration phases on the surface of nuclear waste are analogues of schoepite UO₃·2H₂O, studtite UO₂(O₂)·4H₂O, rutherfordine UO₂CO₃, and čejkaite Na₄UO₂(CO₃)₃. While α-radiolysis has been shown to cause the ingrowth of uranyl peroxide alteration phases, the prevalence of uranyl carbonate phases on solid waste forms has not been mechanistically explained to date, especially since the alteration chemistry is largely affected by the high temperatures of the spent nuclear material. Herein, we demonstrate the first mechanistic link between the formation of the uranyl superoxide (KUPS-1) phase, its reactivity at temperature ranges relevant to the spent nuclear fuel (40-350 °C), and its thermodynamic transformation into a potassium uranyl carbonate mineral phase, agricolaite K₄[UO₂(CO₃)₃], using thermogravimetric analysis, calorimetry, vibrational spectroscopy, and powder X-ray diffraction techniques. The thermodynamics data reveal the metastability of the uranyl superoxide KUPS-1 phase through decomposition of the hydrogen peroxide within the solid-state lattice. Increasing the temperature does not result in the breakdown of the superoxide anion bound to the uranyl cation but instead enhances its reactivity in the presence of CO₂ gas, resulting in potassium carbonate phases at intermediate temperatures (150 °C) and in uranyl carbonate phases at higher temperatures (350 °C).

Ultrasonically assisted conversion of uranium trioxide into uranium(vi) intrinsic colloids

Under oxidizing conditions, the corrosion of spent nuclear fuel may lead to the leaching of radionuclides including soluble uranyl-based species. The speciation of the generated chemical forms is complex and the related potential formation of colloidal species appears surprisingly poorly reported in the literature. Their formation could however contribute significantly to the mobility of radionuclides in the environment. A better knowledge in the speciation and reactivity of these species appears particularly relevant. This study describes the preparation and characterization of intrinsic uranium(vi) colloids from amorphous and crystalline UO₃ in pure water assisted by 20 kHz ultrasound. In the presence of carbon monoxide preventing the sonochemical formation of hydrogen peroxide, ultrasonic treatment boosts the conversion of UO₃ powder into (meta-)schoepite precipitates and yields very stable and notably concentrated uranium(vi) nanoparticles in the liquid phase. Using HR-TEM, SAXS and XAS techniques, we confirmed that the colloidal suspension is composed of quasi-spherical nanoparticles measuring ca. 3.8 ± 0.3 nm and exhibiting a schoepite-like crystallographic structure. The proposed method demonstrates the possible formation of environmentally relevant U(vi) colloidal nanoparticles appearing particularly interesting for the preparation of reference systems in the absence of added ions and capping agents.

Isolation and Reactivity of Uranyl Superoxide

The high radiation field associated with spent nuclear fuel (U^IV O₂ ) 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 U^VI O₂ ²⁺ and its effects on the resultant alteration phases is unclear. We report the first example of a U^VI 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 U^VI 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.

Meta-studtite stability in aqueous solutions. Impact of HCO3-, H2O2 and ionizing radiation on dissolution and speciation

Two uranyl peroxides meta-studtite and studtite exist in nature and can form as alteration phases on the surface of spent nuclear fuel upon water intrusion in a geological repository. Meta-studtite and studtite have very low solubility and could therefore reduce the reactivity of spent nuclear fuel toward radiolytic oxidants. This would inhibit the dissolution of the fuel matrix and thereby also the spreading of radionuclides. It is therefore important to investigate the stability of meta-studtite and studtite under conditions that may influence their stability. In the present work, we have studied the dissolution kinetics of meta-studtite in aqueous solution containing 10 mM HCO3-. In addition, the influence of the added H2O2 and the impact of γ-irradiation on the dissolution kinetics of meta-studtite were studied. The results are compared to previously published data for studtite studied under the same conditions. 13C NMR experiments were performed to identify the species present in aqueous solution (i.e., carbonate containing complexes). The speciation studies are compared to calculations based on published equilibrium constants. In addition to the dissolution experiments, experiments focussing on the stability of H2O2 in aqueous solutions containing UO22+ and HCO3- were conducted. The rationale for this is that H2O2 was consumed relatively fast in some of the dissolution experiments.

Time-dependent surface modification of uranium oxides exposed to water plasma

Thin UO₂ films exposed to water plasma under UHV conditions have been shown to be interesting models for radiation induced oxidative dissolution of spent nuclear fuel. This is partly attributed to the fact that several of the reactive oxidizing and reducing species in a water plasma are also identified as products of radiolysis of water. Exposure of UO₂ films to water plasma has previously been shown to lead to oxidation from U(iv) to U(v) and (vi). In this work we have studied the dynamics of water plasma induced redox changes in UO₂ films by monitoring UO₂ films using X-Ray photoelectron Photoemission (XPS) and Ultra-Violet Photoemission (UPS) spectroscopy as a function of exposure time. The surface composition in terms of oxidation states obtained from U4f7/2 peak deconvolution could be retraced along the exposure time, and compared to the valence band. The spectral analysis showed that U(iv) is initially oxidized to U(v) which is subsequently oxidized to U(vi). For extended exposure times it was shown that U(vi) is slowly reduced back to U(v). UPS data show that, unlike the U(v) formed on the surface upon oxidation of U(iv), the U(v) formed upon reduction of U(vi) is localized in the bulk of the film. It also displays a different reactivity than the initially formed U(v). The experiments can be reproduced using a simple kinetic model describing the redox processes involved.

Americium incorporation into studtite: a theoretical and experimental study

Studtite, [UO₂(η²-O₂)(H₂O)₂]·2H₂O, and metastudtite, [UO₂(η²-O₂)(H₂O)₂], are important phase alterations of UO₂ in a spent nuclear fuel repository and have previously been shown to react with Np(v). In this work we extend the study to Am(v) on a tracer scale and show spectroscopic evidence that the Am is incorporated into the structure of studtite as Am(iii). A computational study on the possible mechanisms for the incorporation of Np and Am shows that protonation of the -yl oxygen is the favoured route and the calculated incorporation energies are large and positive. The results suggest that Am is less favoured compared to Np but energetically more favoured to incorporate both actinide ions into metastudtite rather than studtite. Finally, we have shown that once incorporated, Am readily leaches into water but spectroscopic measurements suggest subtle changes in the structure of studtite.

Dissolution of studtite [UO2(O2)(H2O)4] in various geochemical conditions

This study determined the dissolution rate of studtite, (UO₂)O₂(H₂O)₄, which can be formed by reaction between H₂O₂ and UO₂²⁺ that leaks from spent nuclear fuel (SNF) in deep geological repositories. The batch dissolution experiments were conducted using synthesized studtite under different solution conditions with varying pHs and concentrations of HCO₃^- and [H₂O₂] in synthetic groundwater. The experimental results suggested that carbonate ligand and H₂O₂ in groundwater accelerated the dissolution of studtite and uranium (U) release. Above 10^-5 M of H₂O₂ initial concentration, the released uranium concentration in solution decreased, possibly as a result of reprecipitation of studtite due to reaction between uranium and H₂O₂. The results will be useful to assess the comprehensive transport of uranium from both nuclear waste and SNF stored in deep geological repositories.

Monitoring bromide effect on radiolytic yields using in situ observations of uranyl oxide precipitation in the electron microscope

During electron microscopy observations of uranium-bearing phases and solutions in a liquid cell, the electron beam induced radiolysis causes changes in the chemistry of the system. This could be useful for investigating accelerated alteration of UO₂ and can be also used to monitor radiolytic effects. Low concentrations of bromide in aqueous solutions are known to reduce the generation rate of H₂O₂ during radiolysis and increase H₂ production. We deduced the presence of radiolytic H₂O₂ by monitoring the formation of a uranyl peroxide solid from both solid UO₂ and a solution of ammonium uranyl carbonate at neutral pH. Additionally, the effect of bromine on water radiolysis was investigated through chemical modelling and in situ electron microscopy. By measuring the contrast in the electron microscopy images it was possible to monitor H₂O₂ formation and diffusion from the irradiated zone in agreement with the models.

During electron microscopy observations of uranium-bearing phases and solutions in a liquid cell, the electron beam induced radiolysis causes changes in the chemistry of the system.

Study of the thermal stability of studtite by in situ Raman spectroscopy and DFT calculations

The design of a safe spent nuclear fuel repository requires the knowledge of the stability of the secondary phases which precipitate when water reaches the fuel surface. Studtite is recognized as one of the secondary phases that play a key-role in the mobilization of the radionuclides contained in the spent fuel. Thereby, it has been identified as a product formed under oxidation conditions at the surface of the fuel, and recently found as a corrosion product in the Fukushima-Daiichi nuclear plant accident. Thermal stability is one of the properties that should be determined due to the high temperature of the fuel. In this work we report a detailed analysis of the structure and thermal stability of studtite. The structure has been studied both by experimental techniques (SEM, TGA, XRD and Raman spectroscopy) and theoretical DFT electronic structure and spectroscopic calculations. The comparison of the results allows us to perform for the first time the Raman bands assignment of the whole spectrum. The thermal stability of studtite has been analyzed by in situ Raman spectroscopy, with the aim of studying the effect of the heating rate and the presence of water. For this purpose, a new cell has been designed. The results show that studtite is stable under dry conditions only at temperatures below 30°C, in contrast with the higher temperatures published up to date (~130°C). Opposite behaviour has been found when studtite is in contact with water; under these conditions studtite is stable up to 90°C, what is consistent with the encounter of this phase after the Fukushima-Daiichi accident.

Products of in Situ Corrosion of Depleted Uranium Ammunition in Bosnia and Herzegovina Soils

Hundreds of tons of depleted uranium (DU) ammunition were used in previous armed conflicts in Iraq, Bosnia and Herzegovina, and Serbia/Kosovo. The majority (>90%) of DU penetrators miss their target and, if left in the environment, corrode in these postconflict zones. Thus, the best way to understand the fate of bulk DU material in the environment is to characterize the corrosion products of intact DU penetrators under field conditions for extended periods of time. However, such studies are scarce. To fill this knowledge gap, we characterized corrosion products formed from two intact DU penetrators that remained in soils in Bosnia and Herzegovina for over seven years. We used a combination of X-ray powder diffraction, electron microscopy, and X-ray absorption spectroscopy. The results show that metaschoepite (UO₃(H₂O)₂) was a main component of the two DU corrosion products. Moreover, studtite ((UO₂)O₂(H₂O)₂·2(H₂O)) and becquerelite (Ca(UO₂)₆O₄(OH)₆·8(H₂O)) were also identified in the corrosion products. Their formation through transformation of metaschoepite was a result of the geochemical conditions under which the penetrators corroded. Moreover, we propose that the transformation of metaschoepite to becquerelite or studtite in the DU corrosion products would decrease the potential for mobilization of U from corroded DU penetrators exposed to similar environments in postconflict areas.

Preparation and characterisation of uranium oxides with spherical shapes and hierarchical structures

One of the first reports on shape-controlled uranium oxides with hierarchical structures and their mechanism of formation.

Radiation Effects on Materials Used in Geological Repositories for Spent Nuclear Fuel

Safe long-term storage of radioactive waste from nuclear power plants is one of the main concerns for the nuclear industry as well as for governments in countries relying on electricity produced by nuclear power. A repository for spent nuclear fuel must be safe for extremely long time periods (at least 100 000 years). In order to ascertain the long-term safety of a repository, extensive safety analysis must be performed. One of the critical issues in a safety analysis is the long-term integrity of the barrier materials used in the repository. Ionizing radiation from the spent nuclear constitutes one of the many parameters that need to be accounted for. In this paper, the effects of ionizing radiation on the integrity of different materials used in a granitic deep geological repository for spent nuclear fuel designed according to the Swedish KBS-3 model are discussed. The discussion is primarily focused on radiation-induced processes at the interface between groundwater and solid materials. The materials that are discussed are the spent nuclear fuel (based on UO2), the copper-covered iron canister, and bentonite clay. The latter two constitute the engineered barriers of the repository.

Uranyl peroxide enhanced nuclear fuel corrosion in seawater

The Fukushima-Daiichi nuclear accident brought together compromised irradiated fuel and large amounts of seawater in a high radiation field. Based on newly acquired thermochemical data for a series of uranyl peroxide compounds containing charge-balancing alkali cations, here we show that nanoscale cage clusters containing as many as 60 uranyl ions, bonded through peroxide and hydroxide bridges, are likely to form in solution or as precipitates under such conditions. These species will enhance the corrosion of the damaged fuel and, being thermodynamically stable and kinetically persistent in the absence of peroxide, they can potentially transport uranium over long distances.

Cesium sorption on studtite (UO2O2·4H2O)

One of the mechanisms that may decrease the mobility of cesium released from spent fuel in a high level nuclear waste repository (HLNW) is its sorption onto uranyl-containing alteration phases formed on the spent fuel surface such as studtite (UO2O2·4H2O). The results obtained in this work show that sorption is a very fast process; cesium in solution is sorbed in less than one hour at pH 5. Sorption as a function of initial concentration in solution was also studied between initial cesium concentrations ranging from 7.6×10−9 mol dm−3to 1.0×10−3 mol dm−3. The data have been modelled considering a Freundlich isotherm, withKFandnvalues of 10±1, and 1.4±0.1, respectively (r2=0.998). Sorption is very dependent on ionic strength, suggesting that cesium sorbs onto studtite by forming an outer-sphere complex involving electrostatic interactions. Sorption is observed to be very low at acidic pH, while relatively high at alkaline pH (i.e., almost 60% of the total cesium concentration in solution is sorbed at pH>9). The results point to the importance of sorption processes on uranyl alteration phases on the retention of radionuclides.

UO2dissolution in the presence of hydrogen peroxide at pH>11

The dissolution of non irradiated UO2was studied in the presence of hydrogen peroxide (10−4 mol dm−3) at alkaline pH (11, 11.5, 12, and 13). Both hydrogen peroxide and uranium concentration in solution were determined as a function of time. The H2O2consumption was modelled considering a pseudo first order reaction, the rate constants obtained were 0.126±0.004 h−1, 0.126±0.003 h−1, 0.078±0.002 h−1, and 0.056±0.002 h−1at pH 11, 11.5, 12, and 13, respectively. The uranium concentrations measured at the end of the experiments were close to the solubility of sodium uranate (Na2U2O7). X-ray photoelectron spectroscopy showed that the surface of the solid was more oxidized at lower pH, indicating that at this pH the limiting step of the oxidative dissolution process is the dissolution of the U(VI) formed on the surface. At more alkaline pH values, the rate limiting step would be the oxidation of the UO2surface.

Influence of β radiation on UO2dissolution at different pH values

In this work we studied the effect of external β radiation (90Sr-90Y source with an activity of 7 mCi) on the dissolution rate of non-irradiated UO2as a chemical analogue of spent nuclear fuel (SF). The experiments were carried out at three different pH values inside a glove-box in nitrogen atmosphere to avoid oxygen contamination. The MAKSIMA code was used to model the generation of radiolytic products, both molecular species and radicals.

The formation of hydrogen peroxide in solution was observed in the experiments, as was predicted by the MAKSIMA code. When H2O2could be quantified, its concentration was within the range predicted by this code. In addition, the application to our data of an empirical model for UO2dissolution in the presence of H2O2gave similar results.

The UO2dissolution rates obtained in this work were similar to the UO2corrosion rates determined electrochemically and under γ irradiation with a similar dose rate. On the other hand, they were always lower than the ones obtained with "fresh" spent nuclear fuel, probably because in this case the dose rates to the solution are higher and, in addition, more than just β radiation is emitted by the fuel.

Expanding the Crystal Chemistry of Uranyl Peroxides: Synthesis and Structures of Di- and Triperoxodioxouranium(VI) Complexes

Four compounds containing tri- and diperoxodioxouranium(VI) complexes have been synthesized under ambient conditions and structurally characterized. The crystal structures of Na4(UO2)(O2)3(H2O)12 (monoclinic, P21/c, a=6.7883(6) A, b=16.001(2) A, c=16.562(2) A, beta=91.917(2) degrees, V=1797.9(3) A3, Z=4) and Ca2(UO2)(O2)3(H2O)9 (orthorhombic, Pbcn, a=9.576(3) A, b=12.172(3) A, c=12.314(2) A, V=1435.4(6) A3, Z=4) contain clusters of triperoxodioxouranium(VI). These clusters are bonded through a network of H bonding to H2O groups and in the Ca compound by bonds to Ca2+ cations. In the crystal structure of Na2Rb4(UO2)2(O2)5(H2O)14 (orthorhombic, Pbcm, a=6.808(2) A, b=16.888(6) A, c=23.286(8) A, V=2677.5(16) A3, Z=4), triperoxodioxouranium(VI) polyhedra share a peroxide edge, forming dimers of polyhedra of composition (UO2)2(O2)5(6-). Adjacent dimers are linked through bonding to Rb+ cations and by H bonds to H2O groups. The crystal structure of K6[(UO2)(O2)2(OH)]2(H2O)7 (orthorhombic, Pcca, a=15.078(8) A, b=6.669(4) A, c=23.526(13) A, V=2366(2) A3, Z=4) contains diperoxodioxouranium(VI) polyhedra that include two OH groups. These polyhedra share an OH-OH edge, forming dimers of composition (UO2)2(O2)4(OH)2(6-). The dimers are linked by bonds to K+ cations and by H bonding to H2O groups.

Quantifying hydrogen peroxide in iron-containing solutions using leuco crystal violet

Hydrogen peroxide is present in many natural waters and wastewaters. In the presence of Fe(II), this species decomposes to form hydroxyl radicals, that are extremely reactive. Hence, in the presence of Fe(II), hydrogen peroxide is difficult to detect because of its short lifetime. Here, we show an expanded use of a hydrogen peroxide quantification technique using leuco crystal violet (LCV) for solutions of varying pH and iron concentration. In the presence of the biocatalyst peroxidase, LCV is oxidized by hydrogen peroxide, forming a colored crystal violet ion (CV+), which is stable for days. The LCV method uses standard equipment and allows for detection at the low microM concentration level. Results show strong pH dependence with maximum LCV oxidation at pH 4.23. By chelating dissolved Fe(II) with EDTA, hydrogen peroxide can be stabilized for analysis. Results are presented for hydrogen peroxide quantification in pyrite-water slurries. Pyrite-water slurries show surface area dependent generation of hydrogen peroxide only in the presence of EDTA, which chelates dissolved Fe(II). Given the stability of CV+, this method is particularly useful for field work that involves the detection of hydrogen peroxide.