Search

The preparation of synthetic (Zr,U)SiO4 solid solution is challenging, as the conventional high-temperature solid-state method limits the solubility of uranium (4 ± 1 mol%) in the orthosilicate phase due to its thermodynamic instability. However, these compounds are of great interest as a result of (Zr,U)SiO4 solid solutions, with uranium contents exceeding this concentration, being observed as corium phases formed during nuclear accidents. It has been identified that hydrothermal synthesis pathways can be used for the formation of the metastable phase, such as USiO4. The investigation carried out in this study has indeed led to the confirmation of metastable (Zr,U)SiO4 compounds with high uranium contents being formed. It was found that (Zr,U)SiO4 forms a close-to-ideal solid solution with uranium loading of up to 60 mol% by means of hydrothermal treatment for 7 days at 250 °C, at pH = 3 and starting from an equimolar reactant concentration equal to 0.2 mol L−1. A purification procedure was developed to obtain pure silicate compounds. After purification, these compounds were found to be stable up to 1000 °C under an inert atmosphere (argon). The characterisation methods used to explore the synthesis and thermal stability included powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) and Raman spectroscopies, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). [ABSTRACT FROM AUTHOR]

Hydrothermal conversion of actinide oxalates has recently gained attention as an innovative fabrication route for nuclear fuels but has remained mainly limited to tetra- or tri-valent cations. We report herein the reductive conversion of mixtures of uranyl and oxalate ions into UO2+x oxides under mild hydrothermal conditions (T = 250 °C). A multi-parametric study first led to specifying the optimal conditions in terms of pH, oxalate/U ratio and duration to provide a quantitative precipitation of uranium in the hyper-stoichiometric dioxide form with pH = 0.8; R = noxalate/nU = 3, and t = 72 hours. Particularly, pH was evidenced as a key parameter, with 3 different compounds obtained over a range of only 0.4 units. The mechanism leading to the formation of UO2+x was then investigated thanks to an in situ XANES study. Analysis of the supernatant showed that U(VI) was quickly reduced into U(IV) thanks to the presence of oxalates and/or their decomposition products in solution, following first-order kinetics. Tetravalent uranium was then hydrolysed, leading to the precipitation of UO2+x as the only crystalline phase. This study thus demonstrates that the hydrothermal conversion of actinide oxalates into oxides is an extremely versatile tool that can be implemented in a large variety of chemical systems, particularly in terms of the oxidation state of the cations initially present in solution. [ABSTRACT FROM AUTHOR]

Rare earth phosphates comprise a large family of compounds proposed as possible nuclear waste disposal forms. We report structural and thermodynamic properties of a series of rare earth rhabdophanes and monazites. The water content of the rhabdophanes, including both adsorbed and structural water, decreases linearly with increase in ionic radius of the rare earth. The energetics of the transformation of rhabdophane to monazite plus water and the enthalpy of formation of rhabdophane from the constituent oxides was determined by high temperature drop solution calorimetry. The former varies linearly with the ionic radius of the lanthanide, except for cerium. By combining the enthalpy of formation determined by high temperature drop solution calorimetry and the free energy of formation determined previously by solubility experiments, a complete set of thermodynamic data was derived for the rhabdophanes. They are thermodynamically metastable with respect to the corresponding monazites plus water at all temperatures under ambient pressure conditions. This conclusion strengthens the case for monazites being an excellent nuclear waste form.

Density functional theory plus Hubbard U (DFT+U) methodology was used to calculate the structures and energetic landscapes of CeSiO4, including its stetindite and scheelite phases from ambient pressure to ~24 GPa. To ensure accurate simulations of the high-pressure structures, assessments of strain-stress methods and stress-strain methods were conducted in prior, with the former found to have a better agreement with the experimental result. From DFT calculations the equation of states (EOS) of both stetindite and scheelite were further obtained, with the fitted bulk moduli being 182(2) GPa and 190.0(12) GPa, respectively. These results were found to be consistent with the experimental values of 177(5) GPa and 222(40) GPa. Furthermore, the calculated energetics suggest that the stetindite structure is more thermodynamically stable than the scheelite structure at a pressure lower than 8.35 GPa. However, the stetindite → scheelite phase transition was observed experimentally at a much higher pressure of ~15 GPa. A further phonon spectra investigation by the density functional perturbation theory (DFPT) indicated the Eg¹ mode is being softened with pressure and becomes imaginary after 12 GPa, which is a sign of the lattice instability. Consequently, it was concluded that the stetindite → scheelite transition is predominantly initiated by the lattice instability under high-pressure. [ABSTRACT FROM AUTHOR]

Coffinite, USiO4, is an important U(IV) mineral, but its thermodynamic properties are not well-constrained. In this work, two different coffinite samples were synthesized under hydrothermal conditions and purified from a mixture of products. The enthalpy of formation was obtained by high-temperature oxide melt solution calorimetry. Coffinite is energetically metastable with respect to a mixture of UO2 (uraninite) and SiO2 (quartz) by 25.6 ± 3.9 kJ/mol. Its standard enthalpy of formation from the elements at 25 °C is -1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was characterized by X-ray diffraction and by thermogravimetry and differential scanning calorimetry coupled with mass spectrometric analysis of evolved gases. Coffinite slowly decomposes to U3O8 and SiO2 starting around 450 °C in air and thus has poor thermal stability in the ambient environment. The energetic metastability explains why coffinite cannot be synthesized directly from uraninite and quartz but can be made by low-temperature precipitation in aqueous and hydrothermal environments. These thermochemical constraints are in accord with observations of the occurrence of coffinite in nature and are relevant to spent nuclear fuel corrosion.

Zircon-class ternary oxide compounds have an ideal chemical formula of ATO4, where A is commonly a lanthanide and an actinide, with T = As, P, Si, or V. Their structure (I41/amd) accommodates a diverse chemistry on both A- and T-sites, giving rise to more than 17 mineral end-members of five different mineral groups, and in excess of 45 synthetic end-members. Because of their diverse chemical and physical properties, the zircon structure-type materials are of interest to a wide variety of fields and may be used as ceramic nuclear waste forms and as aeronautical environmental barrier coatings, to name a couple. To support advancement of their applications, many studies have been dedicated to the understanding of their structural and thermodynamic properties. The emphasis in this review will be on recent advances in the structural and thermodynamic studies of zircon structure-type ceramics, including pure end-members [e.g., zircon (ZrSiO4), xenotime (YPO4)] and solid solutions [e.g., ErxTh1–x(PO4)x(SiO4)1–x]. Specifically, we provide an overview on the crystal structure, its variations and transformations in response to non-ambient stimuli (temperature, pressure, and radiation), and its correlation to thermophysical and thermochemical properties. [ABSTRACT FROM AUTHOR]

CO2 sequestration by reaction with abundant, reactive minerals such as olivine has often been considered. The most straightforward, direct process consists in performing the reaction at high temperature and CO2 pressure, in view to producing silica, magnesium and iron carbonates and recovering the traces of nickel and chromite contained in the feedstock mineral. Most of direct processes were found to have an overall cost far larger than the CO2 removal tax, because of incomplete carbonation and insufficient properties of the reaction products. Similar conclusions could be drawn in a previous investigation with a tubular autoclave. An indirect process has been designed for high conversion of olivine and the production of separate, profitable products e.g. silica, carbonates, nickel salts, so that the overall process could be economically viable: the various steps of the process are described in the paper. Olivine particles (120 μm) can be converted at 81% with a low excess of acid within 3 h at 95°C. The silica quantitatively recovered exhibits a BET area over 400 m2 g-1, allowing valuable applications to be considered. Besides, the low contents of nickel cations could be separated from the magnesium-rich solution by ion exchange with a very high selectivity.

Although ZrSiO4 is the most well-known compound in the zircon-structured family (space group I41/amd), the experimental conditions for preparing pure and well-crystallized phases that are doped with a tetravalent element via hydrothermal synthesis have never been clearly discussed in the literature. With the aim to answer this question, the experimental conditions of the preparation of ZrSiO4 and (Zr,Ce)SiO4 were investigated in order to synthesize well-crystallized and pure phases. A multiparametric study has been carried out using soft hydrothermal conditions with variables including reactant concentration, initial pH of the reactive medium, and duration of the hydrothermal treatment. Pure ZrSiO4 was obtained through hydrothermal treatment for 7 days at 250 °C, within a large acidity range (1.0 ≤ pH ≤ 9.0) and starting from CSi ≈ CZr ≥ 0.2 mol L−1. As hydrothermally prepared zircon structured phases can be both hydrated and hydroxylated, its annealed form was also studied after heating to 1000 °C. Based on these results, the synthesis of (Zr,Ce)SiO4 solid solutions was also investigated. The optimal hydrothermal conditions to acquire pure and crystallized phases were as follows: 7 days at 250 °C with initial pH = 1 and concentration of the reactants equal to 0.2 mol L−1. This led to Zr1−xCexSiO4 solid solutions with the incorporated Ce content up to 40 mol%. Samples were characterized using multiple methods, including laboratory and synchrotron PXRD, IR and Raman spectroscopies, SEM, and TGA. Moreover, it was found that these phases were thermally stable in air up to at least 1000 °C. [ABSTRACT FROM AUTHOR]

Homogeneous and heterogeneous U1-xCexO2 (with 0≤ x≤ 0.25) materials were prepared via wet and dry chemistry routes, respectively before being submitted to dynamic leaching experiments. The feeding solution containing 0.20 mmol.L−1 H2O2 was kept under air and renewed to guarantee the stability of H2O2 during the experiment. Normalized alteration rates were determined from U concentration in the leachates. For homogeneous (U,Ce)O2 materials, the dissolution rate was divided by a factor of 3 when increasing the Ce content from 0.08 to 0.25. Surface characterizations revealed that studtite precipitated all over UO2 pellet surface and only on the UO2 grains of heterogeneous U0.92Ce0.08O2 samples. The behaviour of this heterogeneous material was similar to that observed for (U,Pu)O2 in the same conditions, which revealed the reliability of cerium as a plutonium analogue. [ABSTRACT FROM AUTHOR]