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The authors thank the technicians from DHA-Atalante facility who strongly participated to this experiment: Loïck Chauvin,Nathalie Vaissières, Manuel Delaville and Maxime Fagard.; International audience; The formation of studtite has been studied during the oxidative dissolution of a MIMAS MOX fuel disc in aerated water enriched in $^{18}$O under a gamma radiation source, coupling Raman spectroscopy and solution analyzes. The use of isotopic labeling allowed following the reactions responsible for the precipitation of studtite. At the beginning of the experiment, different $^{18}$O enrichments in the uranyl and peroxide bonds of the studtite were observed. While the uranyl bond was primarily enriched in $^{18}$O, the peroxide bond contained large amounts of $^{16}$O. This result suggests an oxygen contribution coming from different radiolytic species for each bond: H$_2$O$_2$ and radicals. The comparison with a UO$_2$ sample leached in similar conditions ruled out a role of the Pu alpha self-irradiation in this different behavior. Yet, the influence of the MOX MIMAS heterogeneous microstructure and chemistry is observed with the preferential dissolution of the UO$_2$ grains, the Pu-rich areas being much more stable with regard to the dissolution. In addition, the studtite first precipitates preferentially on the Pu-poor areas of the sample before covering the entire surface, including the plutonium-enriched aggregates, at the end of the experiment.

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.

Studtite and metastudtite are uranyl peroxides formed on nuclear fuel in water through the reaction with H2O2 produced by the radiolysis of water. However, it is unclear how the two types of uranyl peroxides are generated and distributed on the surface of nuclear fuel. Here, we used Raman imaging technique to exemplify distribution data of the two uranyl peroxides formed on the surface of a U3O8 pellet through immersion in a H2O2 aqueous solution. As a result, we observed that studtite and metastudtite are heterogeneously distributed on the U3O8 surface. No clear correlation between the distributions of studtite and metastudtite was observed, suggesting that the two uranyl peroxides are independently generated on the surface of U3O8. We anticipate that this Raman imaging technique could reveal how these uranyl peroxides are generated and distributed on the surface of the nuclear fuel debris in the Fukushima-Daiichi nuclear power plants.

The dissolution behavior of uranyl peroxide studtite, [(UO2)(O2)(H2O)2](H2O)2, was examined under a wide range of alkaline aqueous environments with and without the addition of hydrogen peroxide. In the absence of added H2O2, studtite dissolved in aqueous solutions with a tetraethylammonium hydroxide to uranium molar ratio greater than 0.5, and the resulting species in solution characterized by Raman spectroscopy and electrospray ionization mass spectrometry (ESI-MS) is the uranyl peroxide nanocluster U24, [(UO2)(O2)(OH)]2424–. This is the first demonstration of the formation of uranyl peroxide nanoclusters from studtite in a solution lacking additional hydrogen peroxide. In similar systems containing added hydrogen peroxide (0.01 M – 1.0 M), studtite dissolved in solutions with a TEAOH to uranium ratio greater than 0.1, and the resulting uranyl peroxide species in solution was U28, [(UO2)(O2)1.5]2828–.

Mixed-phase and low-symmetry systems are widely observed among uranium oxide materials. Amorphous-U2O7 forms during the calcination of studtite, a uranyl peroxide mineral. Using a genetic algorithm search for stable crystalline phases, we have identified a potentially stable phase of U2O7 that shares structural features with experimentally observed amorphous U2O7 samples. The crystalline structure is expected to undergo a solid–solid phase change around 12 GPa.

A digital colour image may be composed of hundreds of thousands of pixels, every pixel exhibiting a single colour. Each colour can be described as a combination of red, green and blue (RGB) components, of discrete values between 0-255. The RGB data contained within the pixels of an image could, therefore, be used to quantitatively establish the colour of nuclear material powders from digital images, particularly for use in nuclear forensics applications, where there is a need for consistent, objective analysis. This paper sets out a standard method for the photography and analysis of digital images of uranium oxide powder, for the objective quantification of colour by mean RGB values. Eight heat treated (up to 550°C) powder samples of studtite ([(UO2)(O2)(H2O)2]·2H2O) were photographed at room temperature and analysed by the RGB method. Hue, saturation and value of the coloured samples were obtained alongside mean RGB values, both of which were used to successfully determine the heating temperatures of unknown specimens of studtite.

International audience; Obtaining bulk materials ordered at the nanoscale using nanoparticles as building blocks is a new challenge for scientists and is of primary interest in some research areas. In this study, we developed a simple one-pot route towards nanostructured hybrid materials based on uranium oxide through a non-hydrolytic condensation process under mild solvothermal conditions (T = 160 °C) using ditopic organic linkers as structuring agents. The obtained materials present a sand rose morphology composed of agglomerated nanometric sheets with a lamellar substructure. Nanometric sheets are composed of inorganic fringes consisting of a studtite phase [(UO2)O2(H2O)2]·2H2O that alternates with organic fringes formed by the structuring organic linker. A multiparametric study based on SAXS experiments and TEM analyses has highlighted the control of structural parameters by varying some synthesis conditions. The interlamellar distances are tunable from 1.60 nm to 1.94 nm through the length of the structuring linker and the inter-sheet distance from 1.85 nm to 2.10 nm through synthesis temperature. This efficient one-pot route appears to be very promising for the production of tunable multiscale nanostructured hybrid materials and can be implemented in many research areas, paving the way for various applications.

The minerals studtite, [UO2(η2-O2)(H2O)2]·2H2O, and metastudtite, [UO2(η2-O2)(H2O)2], are uranyl peroxide minerals that are major oxidative alteration phases of UO2 under conditions of geological storage. The dehydration of studtite has been studied using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy. XPS of the U 4f region shows small but significant differences between studtite and metastudtite, with the 4f binding energy of studtite being the highest reported for a uranyl mineral studied by this technique. Further information about the changes in the electronic structure was elucidated using U M4-edge high-energy resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy, which directly probes f orbital states. The transition from the 3d to 5fσ* orbital is sensitive to variations in the U═Oaxial bond length and to changes in the bond covalency. We report evidence that the covalence in the uranyl fragment decreases upon dehydration. Photoluminescence spectroscopy ...

Computational modeling is an important aspect of the research on nuclear waste materials. In particular, atomistic simulations, when used complementary to experimental efforts, contribute to the scientific basis of safety case for nuclear waste repositories. Here we discuss the state-of-the-art and perspectives of atomistic modeling for nuclear waste management on a few cases of successful synergy of atomistic simulations and experiments. In particular, we discuss here: (1) the potential of atomistic simulations to investigate the uranium oxidation state in mixed valence uranium oxides and (2) the ability of cementitious barrier materials to retain radionuclides such as 226Ra and 90Sr, and of studtite/metastudtite secondary peroxide phases to incorporate actinides such as Np and Am. The new contribution we make here is the computation of the incorporation of Sr by C-S-H (calcium silicate hydrate) phases., MRS Advances (2020)

A thorough understanding of the aqueous uranium VI (UVI) chemistry in alkaline, sodium containing solutions is imperative to address a wide range of critical challenges in environmental engineering, including nuclear waste management. The aim of the present study was to characterise experimentally in more detail the control of pH on the removal of UVI from aqueous alkaline solutions through particle formation and on subsequent transport through porous media. We conducted first static batch experiments in the pH range between 10.5 and 12.5 containing 10 ppm UVI in 0.1 M NaCl solutions and examined the particles formed using filtration, dynamic light scattering, transition electron microscopy and X-ray powder diffraction. We found that at pH 10.5 and 11.5, between 75 and 96 % of UVI was removed from the solutions as clarkeite and studtite over a period of 48 h, forming particles with hydrodynamic diameters of 640 ± 111 nm and 837 ± 142 nm, respectively and representing aggregates of 10′s nm sized crystals randomly orientated. At pH 12.5, the formation of particles >0.2 μm became insignificant and no UVI was removed from solution. The mobility of UVI in these solutions was further studied using column experiments through quartz sand. We found that at pH 10.5 and 11.5, UVI containing particles were immobilised near the column inlet, likely due physical immobilisation of the particles (particle straining). At pH 12.5, however, UVI quantitatively eluted from the columns in the filter fraction

The effects of radiation on a variety of uranyl peroxide compounds were examined using γ-rays and 5 MeV He ions, the latter to simulate α-particles. The studied materials were studtite, [(UO2)(O2)(H2O)2](H2O)2, the salt of the U60 uranyl peroxide cage cluster, Li44K16[(UO2)(O2)(OH)]60·255H2O, the salt of U60Ox30 uranyl peroxide oxalate cage cluster, Li12K48[{(UO2)(O2)}60(C2O4)30]·nH2O, and the salt of the U24Pp12 (Pp = pyrophosphate) uranyl peroxide pyrophosphate cage cluster, Li24Na24[(UO2)24(O2)24(P2O7)12]·120H2O. Irradiated powders were characterized using powder X-ray diffraction, Raman spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, and UV-vis spectroscopy. A weakening of the uranyl bonds of U60 was found while studtite, U60Ox30, and U24Pp12 were relatively stable to γ-irradiation. Studtite and U60 are the most affected by α-irradiation forming an amorphous uranyl peroxide as characterized by Raman spectroscopy and powder X-ray diffraction while U60Ox30 and U24Pp12 show minor signs of the formation of an amorphous uranyl peroxide.

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.

The interplay between thermodynamics and mechanical properties in the transformation of studtite, (UO2)(O2)(H2O)2·2H2O, into metastudtite, (UO2)(O2)(H2O)2, two important corrosion phases observed on the surface of uranium dioxide exposed to water, is revealed using density functional perturbation theory. Phonon calculations within the quasi-harmonic approximation predict that the standard entropy change for the (UO2)(O2)(H2O)2·2H2O → (UO2)(O2)(H2O)2 + 2H2O reaction is ΔS0 = +80 J·mol–1·K–1 for the production of water in the liquid state and +389 J·mol–1·K–1 for water vapor. Similar to bulk H2O(l), the bulk modulus of (UO2)(O2)(H2O)2·2H2O increases with temperature, contrasting with (UO2)(O2)(H2O)2 which features the typical Anderson–Gruneisen temperature dependence of oxide solids. Upon removal of interstitial H2O in studtite, the most important changes in the shear modulus, the parameter limiting the mechanical stability, arise in the planes normal to chain propagation directions. The present findings ha...

At the back-end of the nuclear fuel cycle, spent nuclear fuel (SNF) may be processed in solution to form a precipitate of uranyl peroxide tetrahydrate, also known as studtite ([(UO2)(O2)(H2O)2]•2H2O). The physical and chemical properties of studtite are influenced by the solution processing conditions employed, thus, the processing history of the precipitate may be elucidated from these characteristics. This is useful in the field of nuclear forensics, where the provenance of intercepted illicit nuclear material is of interest. Studtite is often calcined to ≥500 °C to form alpha uranium trioxide (α-UO3), an oxide prevalent in waste storage. In this study, the crystal structure of UO3 is analysed, with a discussion on the α/α’-UO3 C2mm symmetry. The morphological and structural signatures of both studtite and UO3 are investigated, with respect to a matrix of solution processing conditions and the thermal treatment of studtite.

International audience; The purpose of this work is to determine how long UF$_4$ in micro-particles can be detected after release in the environment, what the degradation products are and what are the parameters which mostly lead to degradation of UF$_4$. For this, Raman spectra of UF$_4$ μm-sized particles (typical diameters ∼5 μm) stored in various conditions (humid or dry air/argon, without or with UV light, 20 °C, 40 °C, 80 °C) were monitored by micro-Raman spectrometry over three months. The characteristic spectral signature of UF$_4$ followed here is the very intense and broad fluorescence peak which covers the 700—1100 cm-1 region when a visible laser at 514 nm is used. This study shows that persistence of the UF$_4$ signature in μm-sized particles released in the environment is strongly dependent on ambient conditions: typically a few days when submitted to both humid air or argon atmosphere and only intense UV light, a few weeks when exposed to a humid air or argon atmosphere in the dark, till at least several months and most probably several years, when kept in a dry atmosphere. Reaction of UF4 with water vapor ultimately leads to the formation uranyl hydroxide (schoepite) whereas exposure to both water vapor and UV light produces mainly uranyl peroxide (studtite).

International audience; In this study, the behavior of alpha irradiation-induced defects in UO2, when exposed to different interfaces, is investigated. Raman spectroscopy is used to measure the formation kinetics of irradiation defects in UO2 leached under oxidizing water environment and the data are then compared to a reference UO2/Ar system. The results reveal that the presence of either aerated water or inert argon gas modifies the formation kinetics of irradiation defects. The UO2 alteration in aerated water leads to the precipitation of secondary phases in the form of studtite and water chemical analysis reveals that the UO2 dissolution mechanism proceeds without the formation of an oxidized UO2 layer.

For the first time the effect of hydrogen peroxide on the dissolution of electrodeposited uranium oxide films on 316L stainless steel planchets (acting as simulant uranium-contaminated metal surfaces) has been studied. Analysis of the H2O2-mediated film dissolution processes via open circuit potentiometry, alpha counting and SEM/EDX imaging has shown that in near-neutral solutions of pH 6.1 and at [H2O2] ⩽ 100 μmol dm−3 the electrodeposited uranium oxide layer is freely dissolving, the associated rate of film dissolution being significantly increased over leaching of similar films in pH 6.1 peroxide-free water. At H2O2 concentrations between 1 mmol dm−3 and 0.1 mol dm−3, formation of an insoluble studtite product layer occurs at the surface of the uranium oxide film. In analogy to corrosion processes on common metal substrates such as steel, the studtite layer effectively passivates the underlying uranium oxide layer against subsequent dissolution. Finally, at [H2O2] > 0.1 mol dm−3 the uranium oxide film, again in analogy to common corrosion processes, behaves as if in a transpassive state and begins to dissolve. This transition from passive to transpassive behaviour in the effect of peroxide concentration on UO2 films has not hitherto been observed or explored, either in terms of corrosion processes or otherwise. Through consideration of thermodynamic solubility product and complex formation constant data, we attribute the transition to the formation of soluble uranyl–peroxide complexes under mildly alkaline, high [H2O2] conditions – a conclusion that has implications for the design of both acid minimal, metal ion oxidant-free decontamination strategies with low secondary waste arisings, and single step processes for spent nuclear fuel dissolution such as the Carbonate-based Oxidative Leaching (COL) process.

The (U,Pu)O 2 matrix behavior of an irradiated MIMAS-type (MIcronized MASter blend) MOX fuel, under radiolytic oxidation in aerated pure water at pH 5–5.5 was studied by combining chemical and radiochemical analyses of the alteration solution with Raman spectroscopy characterizations of the surface state. Two leaching experiments were performed on segments of irradiated fuel under different conditions: with or without an external γ irradiation field, over long periods (222 and 604 days, respectively). The gamma irradiation field was intended to be representative of the irradiation conditions for a fuel assembly in an underwater interim storage situation. The data acquired enabled an alteration mechanism to be established, characterized by uranium (UO 2 2+ ) release mainly controlled by solubility of studtite over the long-term. The massive precipitation of this phase was observed for the two experiments based on high uranium oversaturation indexes of the solution and the kinetics involved depended on the irradiation conditions. External gamma irradiation accelerated the precipitation kinetics and the uranium concentrations (2.9 × 10 −7 mol/l) were lower than for the non-irradiated reference experiment (1.4 × 10 −5 mol/l), as the quantity of hydrogen peroxide was higher. Under slightly acidic pH conditions, the formation of an oxidized UO 2+ x phase was not observed on the surface and did not occur in the radiolysis dissolution mechanism of the fuel matrix. The Raman spectroscopy performed on the heterogeneous MOX fuel matrix surface, showed that the fluorite structure of the mainly UO 2 phase surrounding the Pu-enriched aggregates had not been particularly impacted by any major structural change compared to the data obtained prior to leaching. For the plutonium, its behavior in solution involved a continuous release up to concentrations of approximately 3 × 10 −6 mol L −1 with negligible colloid formation. This data appears to support a predominance of the +V oxidation state for plutonium in solution under highly oxidizing conditions. Furthermore, the Raman spectroscopy monitoring of the sample surface oxidation states did not point to any significant effect from the high Pu content of the aggregates (10–15%) and therefore did not indicate a better aggregate stability under radiolysis compared to the mainly UO 2 matrix. This is because acidic pH conditions do not favor the development of oxidized layers on a fuel surface, with the exception of secondary phases.

14 pags., 11 figs., 11 tabs., The thermodynamic properties of uranyl containing materials, including dehydrated schoepite, metastudtite, studtite, soddyite, rutherfordine, and γ-UO3 were studied. These materials are among the most important secondary phases arising from corrosion of spent nuclear fuel under the final geological disposal conditions, and γ-UO3 is the main oxide of hexavalent uranium. The crystal structures of the dehydrated schoepite and metastudtite were determined by means of density functional theory using a new norm-conserving pseudopotential for uranium atom. The resulting structural properties and X-ray powder patterns were found to be in very good agreement with the experimental data. By using the optimized structures of these materials, as well as of those obtained in previous works for studtite and soddyite, the thermodynamic properties of dehydrated schoepite, metastudtite, studtite, and soddyite were determined, including specific heats, entropies, enthalpies and Gibbs free energies. Finally, the computed thermodynamic properties of these materials together with those reported previously for rutherfordine and γ-UO3 were used to determine their enthalpies and free energies of formation and their variation with temperature. The theoretical results for rutherfordine and γ-UO3 are shown to be in excellent agreement with experimental information, even at high temperatures (up to 700 and 900 K, respectively). The results for dehydrated schoepite are in reasonable agreement with the experimental values obtained from the extrapolation of the measured values for UO2·0.77H2O and UO2·0.85H2O to UO2·1H2O, and in good agreement with previous theoretical data. The corresponding temperature dependent functions and the associated reaction constants for metastudtite, studtite, and soddyite, for which there are no experimental data to compare with, were predicted. The overall thermodynamic data obtained in this work by a theoretical approach can be used for the improvement and extension of the nuclear thermodynamic databases for modeling uranium containing systems and its dynamical behavior under different geochemical environments., This work was supported by ENRESA in the project: No. 079000189 “Aplicación de técnicas de caracterización en el estudio de la estabilidad del combustible nuclear irradiado en condiciones de almacenamiento” (ACESCO) and Project FIS2013-48087-C2-1-P. Supercomputer time by the CETA CIEMAT, CTI-CSIC, and CESGA centers are also acknowledged. This work has been carried out in the context of a CSIC−CIEMAT collaboration agreement: “Caracterización experimental y teórica de fases secundarias y óxidos de uranio formados en condiciones de almacenamiento de combustible nuclear”. We also want to thank Dr. Rafael Escribano for reading the document and many helpful comments

12 pags., 6 figs., 4 tabs., The thermodynamic properties of uranyl-containing materials, including dehydrated schoepite, metastudtite, studtite, soddyite, rutherfordine, and γ-UO3, determined by means of density functional theory using a new norm-conserving pseudopotential for uranium atom in previous works, were used to obtain the enthalpies and Gibbs free energies of eight reactions involving these materials and its variation with the temperature. The first five reactions represent the formation of the first five considered materials in terms of the corresponding oxides, and the remaining reactions are the transformations of rutherfordine into dehydrated schoepite, studtite into metastudtite, and uranium trioxide into triuranium octoxide, respectively. The experimental values of the enthalpies of these reactions, which are only known at the standard state (temperature of 298.15 K and pressure of 1 bar), were reproduced accurately by these calculations, the errors being 2.5, 2.5, 0.2, 0.0, 12.3, −1.1, 0.9, and 4.0 kJ·mol–1, respectively. These calculations predict that soddyite is stable with respect to the corresponding oxides at all the temperatures considered (280–500 K), and that studtite and metastudtite are unstable. However, dehydrated schoepite and rutherfordine are stable for temperatures lower than 462.5 ± 16.7 and 513.7 ± 35.8 K, respectively, and become unstable for higher temperatures. The relative stability of the uranyl peroxide hydrates, studtite and metastudtite, with respect to γ-UO3, dehydrated schoepite, rutherfordine and soddyite in the presence of water and hydrogen peroxide was also studied by considering the corresponding reactions. The experimental values of the enthalpies of two of these reactions, those relating studtite and metastudtite with γ-UO3 in the presence of water and hydrogen peroxide, which are only known at the standard state, were theoretically very well reproduced (within 0.1 kJ·mol–1). From the results, we conclude that metastudtite and studtite become stable phases at low temperatures in the presence of water and small amounts of hydrogen peroxide. However, the stability of the studtite phase decreases rapidly when the temperature increases. Finally, the relative stability of these phases in the presence of very high concentrations of hydrogen peroxide was evaluated. The results show that in this case, occurring under very intense radiation fields causing the radiolysis of most of the water present, studtite is by far the most stable phase within the full range of temperature studied., This work was supported by ENRESA in the project: No. 079000189 “Aplicación de técnicas de caracterización en el estudio de la estabilidad del combustible nuclear irradiado en condiciones de almacenamiento” (ACESCO) and Project FIS2013-48087-C2-1-P. Supercomputer time by the CETACIEMAT, CTI-CSIC and CESGA centers are also acknowledged. This work has been carried out in the context of a CSIC−CIEMAT collaboration agreement: “Caracterización experimental y teórica de fases secundarias y óxidos de uranio formados en condiciones de almacenamiento de combustible nuclear”.

18 pags., 11 figs, 10 tabs. -- Open Access funded by Creative Commons Atribution Licence 3.0, The structure, thermodynamic and mechanical properties of becquerelite mineral, Ca(UO)O(OH)·8HO, were studied by means of theoretical solid-state calculations based on density functional theory using plane waves and pseudopotentials. The positions of the hydrogen atoms in the unit cell of becquerelite mineral were optimized theoretically since it was not possible to determine them from X-ray diffraction data by structure refinement. The structural results, including the lattice parameters, bond lengths and X-ray powder pattern, were found to be in excellent agreement with their experimental counterparts. The fundamental thermodynamic properties of becquerelite mineral, including specific heat, entropy, enthalpy and Gibbs free energy, were then computed by performing phonon calculations at the computed optimized structure. Since the experimental values of these properties are unknown, their values were predicted. The values obtained for the isobaric specific heat and entropy of becquerelite at the temperature of 298.15 K were 148.4 and 172.3 J K mol, respectively. The computed thermodynamic properties were combined with those of the corresponding elements in order to obtain the enthalpy and Gibbs free energy of formation as a function of temperature. The availability of these thermodynamic properties of formation allowed to determine the enthalpies and free energies and associated reaction constants of a series of reactions involving becquerelite and other uranyl containing materials. Futhermore, knowledge of these properties permitted the study of the thermodynamic stability of becquerelite with respect to a rich set of secondary phases of spent nuclear fuel, including dehydrated schoepite, schoepite, metaschoepite, studtite, metastudtite, rutherfordine and soddyite under different conditions of temperature. Becquerelite is shown to be highly stable in the presence of hydrogen peroxide. It is the second most stable phase under intermediate hydrogen peroxide concentrations (after schoepite), and the fourth most stable phase under high hydrogen peroxide concentrations (after studtite, schoepite and metaschoepite). Finally, the equation of state and elastic properties of this mineral, unknown to date, were determined. The crystal structure of becquerelite was found to be stable mechanically and dynamically. Becquerelite can be described as a brittle material exhibiting large anisotropy and large compressibility in the direction perpendicular to the sheets characterizing the structure of this layered uranyl containing material. The dependence of the elastic properties of becquerelite with respect to the strain orientation is shown to be analogous to that of schoepite mineral. The calculated bulk modulus is also very similar to that of schoepite, B ∼ 31 GPa., This work was supported by ENRESA within the project: No. 079000189 and project FIS2013-48087-C2-1-P. Supercomputer time by the CETACIEMAT and CESGA centers are also acknowledged. This work has been carried out in the context of a CSIC – CIEMAT

The mechanical properties and stability of studtite, (UO2)(O2)(H2O)2·2H2O, and metastudtite, (UO2)(O2)(H2O)2, two important corrosion phases observed on spent nuclear fuel exposed to water, have been investigated using density functional perturbation theory. While (UO2)(O2)(H2O)2 satisfies the necessary and sufficient Born criteria for mechanical stability, (UO2)(O2)(H2O)2·2H2O is found to be mechanically metastable, which might be the underlying cause of the irreversibility of the studtite to metastudtite transformation. According to Pugh's and Poisson's ratios and the Cauchy pressure, both phases are considered ductile and shear modulus is the parameter limiting their mechanical stability. Debye temperatures of 294 and 271 K are predicted for polycrystalline (UO2)(O2)(H2O)2·2H2O and (UO2)(O2)(H2O)2, suggesting a lower micro-hardness of metastudtite.

Metastudtite, (UO 2 )O 2 (H 2 O) 2 , is one of two known natural peroxide minerals, but little is established about its thermodynamic stability. In this work, its standard enthalpy of formation, −1,779.6 ± 1.9 kJ/mol, was obtained by high temperature oxide melt drop solution calorimetry. Decomposition of synthetic metastudtite was characterized by thermogravimetry and differential scanning calorimetry (DSC) with ex situ X-ray diffraction analysis. Four decomposition steps were observed in oxygen atmosphere: water loss around 220 °C associated with an endothermic heat effect accompanied by amorphization; another water loss from 400 °C to 530 °C; oxygen loss from amorphous UO 3 to crystallize orthorhombic α-UO 2.9 ; and reduction to crystalline U 3 O 8 . This detailed characterization allowed calculation of formation enthalpy from heat effects on decomposition measured by DSC and by transposed temperature drop calorimetry, and both these values agree with that from drop solution calorimetry. The data explain the irreversible transformation from studtite to metastudtite, the conditions under which metastudtite may form, and its significant role in the oxidation, corrosion, and dissolution of nuclear fuel in contact with water.

The knowledge of the characteristics of the U(VI)-peroxide chemical system is of importance because of the relevance of the uranyl peroxide solid phases studtite (UO2O2·4H2O) or metastudtite (UO2O2·2H2O) as uranyl secondary solid phases during the leaching of the spent nuclear fuel as well as to the formation in alkaline waters of soluble nanoscale uranyl peroxo cage clusters. In this work, the solubility of studtite was determined for the first time as a function of pH (from 3 to 11) by means of studtite undersaturation experiments. The results obtained showed a V-shaped solubility curve, which was modelled considering the uranyl complexes formation constants (database of the Nuclear Energy Agency, NEA) and recently published formation constants of uranium hydroxo-peroxo-complexes. The best fit of the model to the experimental solubility data was obtained with log Ks0 = −2.7 ± 0.2 for the studtite solubility reaction: UO 2 O 2 · 4 H 2 O ( s ) + 2 H + ⇆ UO 2 2 + + 4 H 2 O + H 2 O 2 On the other hand, the so-called immersion methodology was used in order to determine the pHpzc of studtite, which resulted to be 4.0 ± 0.2. This value would corroborate a sorption mechanism based on electrostatic interactions described for previously published sorption data of cations (Cs+, Sr2+) on studtite.

© 2014, Akadémiai Kiadó, Budapest, Hungary. The sorption of Se(IV) and Se(VI) on uranium peroxide has been studied considering the sorption kinetics, the sorption isotherms and the effect of pH. Selenium sorption on studtite is fitted with a pseudosecond order reaction model; in addition, two different mechanisms seem to influence the sorption process: micropore diffusion and intra-particle diffusion. Both selenium(IV) and selenium(VI) are sorbed on studtite through a monolayer coverage. Sorption is higher at acidic pH than at alkaline pH. This behaviour is consistent with the chemical speciation of selenium in solution and with the acid–base properties of the solid.

Incorporation reactions play an important role in dictating immobilization and release pathways for chemical species in low-temperature geologic environments. Quantum-mechanical investigations of incorporation seek to characterize the stability and geometry of incorporated structures, as well as the thermodynamics and kinetics of the reactions themselves. For a thermodynamic treatment of incorporation reactions, a source of the incorporated ion and a sink for the released ion is necessary. These sources/sinks in a real geochemical system can be solids, but more commonly, they are charged aqueous species. In this contribution, we review the current methods for ab initio calculations of incorporation reactions, many of which do not consider incorporation from aqueous species. We detail a recently-developed approach for the calculation of incorporation reactions and expand on the part that is modeling the interaction of periodic solids with aqueous source and sink phases and present new research using this approach. To model these interactions, a systematic series of calculations must be done to transform periodic solid source and sink phases to aqueous-phase clusters. Examples of this process are provided for three case studies: (1) neptunyl incorporation into studtite and boltwoodite: for the layered boltwoodite, the incorporation energies are smaller (more favorable) for reactions using environmentally relevant source and sink phases (i.e., ΔErxn(oxides) > ΔErxn(silicates) > ΔErxn(aqueous)). Estimates of the solid-solution behavior of Np5+/P5+- and U6+/Si4+-boltwoodite and Np5+/Ca2+- and U6+/K+-boltwoodite solid solutions are used to predict the limit of Np-incorporation into boltwoodite (172 and 768 ppm at 300 °C, respectively); (2) uranyl and neptunyl incorporation into carbonates and sulfates: for both carbonates and sulfates, it was found that actinyl incorporation into a defect site is more favorable than incorporation into defect-free periodic structures. In addition, actinyl incorporation into carbonates with aragonite structure is more favorable than into carbonates with calcite structure; and (3) uranium incorporation into magnetite: within the configurations tested that preserve charge neutrality (U6+ → 2Fe3+oct/tet or U4+ → Fe2+oct), uranium incorporation into magnetite is most favorable when U6+ replaces octahedral Fe3+ with charge balancing accomplished by an octahedral Fe3+ iron vacancy. At the end of this article, the limitations of this method and important sources of error inherent in these calculations (e.g., hydration energies) are discussed. Overall, this method and examples may serve as a guide for future studies of incorporation in a variety of contexts.

The formation of uranyl peroxide phases was identified as a corrosion product of spent fuel by Hanson et al [1]. The subsequent analysis of this phase showed that metastudtite retained 241Am, 237Np and 239Pu [2]. In this study, the retention of radionuclide Pu4+ and An3+, released from the spent fuel matrix into studtite structure, has been evaluated by the precipitation of studtite from uranyl dissolution with variable concentrations of REE (Th, Nd, Sm and Eu). Three different precipitation conditions parameters were studied: media of synthesis, time of synthesis and REE concentration. Synthesized phases were characterized by XRD and the cell parameter was calculated. The REE incorporation was determined by ICP-MS analysis. The results showed that studtite could incorporate 63% of Th in solution during its precipitation. Changes in the “a” cell parameter were identified. The results suggest that studtite coprecipitated with REE could play a role as a limiting for the REE mobility.

Understanding UO2 matrix dissolution is of major importance for the safety assessment of a future deep geological repository. Oxidative dissolution of UO2 and SIMFUEL pellets have been extensively studied in HCO3- solutions, while less is known about systems with no or very low HCO3- concentrations. The aim of this work is to elucidate the oxidative dissolution of UO2 and SIMFUEL pellets in HCO3- free solutions by studying the dissolution of U (VI) and consumption of H2O2 over time. The results are compared with previous experiments performed in HCO3- solutions. The oxidative dissolution rate is higher for the UO2 pellet in HCO3- compared to the other systems. It is evident that the kinetics of the reaction with H2O2 is qualitatively different for SIMFUEL in comparison with pure UO2. For the UO2 pellet in pure water, the presence of a secondary phase (meta) studtite, on the surface of the pellet is confirmed by Raman spectroscopy. The kinetic impact of the secondary phase is evaluated in separate UO2 powder experiments. The (meta) studtite (surface) precipitation leads to a slower release of uranium into the solution. Numerical simulations using experimentally determined rate constants are used to evaluate a simple mechanism of surface precipitation. The numerical results are in fair agreement with the experimental observations given certain criteria. In addition, the γ-radiation induced dissolution of UO2 and SIMFUEL pellets were investigated in pure water, and compared with HCO3- systems. Also here the dissolution rate of uranium is higher for UO2 in HCO3- compared to pure water, while for SIMFUEL longer irradiation times are needed to observe any difference between pure and HCO3- containing water.