Reductive hydrothermal conversion of uranyl oxalates into UO monitored by XANES analyses.

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 UO oxides under mild hydrothermal conditions ( = 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.

Hydrothermal synthesis of (Zr,U)SiO: an efficient pathway to incorporate uranium into zircon.

The preparation of synthetic (Zr,U)SiO 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)SiO 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 USiO.

Phonon softening induced phase transition of CeSiO: a density functional theory study.

Density functional theory plus Hubbard U (DFT+) methodology was used to calculate the structures and energetic landscapes of CeSiO, 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.

Incorporation of U(IV) in monazite-cheralite ceramics under oxidizing and inert atmospheres.

This work is the first attempt to prepare NdCaUPO monazite-cheralite with 0 < ≤ 0.1 by a wet chemistry method. This method relies on the precipitation under hydrothermal conditions ( = 110 °C for four days) of the NdCaUPO·HO rhabdophane precursor, followed by its thermal conversion for 6 h at 1100 °C in air or Ar atmosphere.

How hydrothermal synthesis improves the synthesis of (Zr,Ce)SiO solid solutions.

Although ZrSiO is the most well-known compound in the zircon-structured family (space group 4/), the experimental conditions for preparing pure and well-crystallized phases that are doped with a tetravalent element hydrothermal synthesis have never been clearly discussed in the literature. With the aim to answer this question, the experimental conditions of the preparation of ZrSiO and (Zr,Ce)SiO 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.

Influence of Sintering Conditions on the Structure and Redox Speciation of Homogeneous (U,Ce)O Ceramics: A Synchrotron Study.

Although uranium-cerium dioxides are frequently used as a surrogate material for (U,Pu)O nuclear fuels, there is currently no reliable data regarding the oxygen stoichiometry and redox speciation of the cations in such samples. In order to fill this gap, this manuscript details a synchrotron study of highly homogeneous (U,Ce)O sintered samples prepared by a wet-chemistry route. HERFD-XANES spectroscopy led to determining accurately the O/M ratios (with M = U + Ce).

Time-resolved laser-induced fluorescence spectroscopy and chemometrics for fast identification of U(VI)-bearing minerals in a mining context.

We evaluated the potential of time-resolved laser-induced fluorescence spectroscopy (TRLFS) combined with chemometric methods for fast identification of U(VI)-bearing minerals in a mining context. We analyzed a sample set which was representative of several environmental conditions. The set consisted of 80 uranium-bearing samples related to mining operations, including natural minerals, minerals with uranium sorbed on the surface, and synthetic phases prepared and characterized specifically for this study.

Correction: Formation of plutonium(IV) silicate species in very alkaline reactive media.

Correction for 'Formation of plutonium(IV) silicate species in very alkaline reactive media' by Paul Estevenon , , 2021, , 12528-12536, DOI: 10.1039/D1DT02248B.

Formation of plutonium(IV) silicate species in very alkaline reactive media.

Studying the speciation of Pu(IV) in very alkaline and silicate ion rich reactive media allowed identification of the formation of plutonium(IV)-silicate colloidal suspensions which were stable for months. These colloids were stabilized in aqueous solution for pH > 13 and for concentrations around 10 mol L. Successive filtration processes allowed evaluation of their size, which was found to be smaller than 6 nm.

Effect of Annealing on Structural and Thermodynamic Properties of ThSiO-ErPO Xenotime Solid Solution.

The effect of annealing on structural and thermochemical properties of a thorite-xenotime solid solution ThEr(SiO)(PO) was assessed. The samples synthesized at low temperatures and stored at room temperature for 2 years retained their tetragonal structures. This structure was also maintained after heating to 1100 °C.

The Role of Water and Hydroxyl Groups in the Structures of Stetindite and Coffinite, MSiO (M = Ce, U).

Orthosilicates adopt the zircon structure types (4), consisting of isolated SiO tetrahedra joined by A-site metal cations, such as Ce and U. They are of significant interest in the fields of geochemistry, mineralogy, nuclear waste form development, and material science. Stetindite (CeSiO) and coffinite (USiO) can be formed under hydrothermal conditions despite both being thermodynamically metastable.

Hydrothermal Conversion of Thorium Oxalate into ThO·HO Oxide.

Hydrothermal conversion of thorium oxalate, Th(CO)·HO, into thorium dioxide was explored through a multiparametric study, leading to some guidelines for the preparation of crystallized samples with the minimum amount of impurities. As the formation of the oxide appeared to be operated through the hydrolysis of Th after decomposition of oxalate fractions, pH values typically above 1 must be considered to recover a solid phase. Also, because of the high stability of the thorium oxalate precursor, hydrothermal treatments of more than 5 h at a temperature above 220 °C were required.

Thermodynamics of CeSiO: Implications for Actinide Orthosilicates.

Zircon (ZrSiO, 4/) can accommodate actinides, such as thorium, uranium, and plutonium. The zircon structure has been determined for several of the end-member compositions of other actinides, such as plutonium and neptunium. However, the thermodynamic properties of these actinide zircon structure types are largely unknown due to the difficulties in synthesizing these materials and handling transuranium actinides.

study of the synthesis of thorite (ThSiO) under environmental representative conditions.

Thorite, (ThSiO4) with a zircon type structure, is one of the most abundant natural sources of thorium on Earth. Generally, actinides are known to form nanoparticles in silicate medium, though no direct link between those colloids and the crystalline form of thorite was evidenced until now. Here we show the formation of thorite from colloids and nanocrystalline structures under experimental conditions close to environmental pH and temperature.

Coffinite formation from UO.

Most of the highly radioactive spent nuclear fuel (SNF) around the world is destined for final disposal in deep-mined geological repositories. At the end of the fuel's useful life in a reactor, about 96% of the SNF is still UO. Thus, the behaviour of UO in SNF must be understood and evaluated under the weathering conditions of geologic disposal, which extend to periods of hundreds of thousands of years.

The formation of PuSiO under hydrothermal conditions.

Attempts to synthesize plutonium(iv) silicate, PuSiO4, have been made on the basis of results recently reported in the literature for CeSiO4, ThSiO4, and USiO4 under hydrothermal conditions. Although it was not possible to prepare PuSiO4via applying the conditions reported for thorium and uranium, an efficient method of PuSiO4 synthesis was established by applying the conditions optimized for the CeSiO4 system. This method was based on the slow oxidation of plutonium(iii) silicate reactants under hydrothermal conditions at 150 °C in hydrochloric acid (pH = 3-4).

Hydrothermal Conversion of Uranium(IV) Oxalate into Oxides: A Comprehensive Study.

Within the development of future nuclear reactors, wet chemistry routes have been investigated for the fabrication of advanced oxide fuels. In this frame, a multiparametric study focused on the hydrothermal conversion of uranium(IV) oxalate U(CO)·HO into uranium oxides was undertaken in order to unravel the effects of temperature, pH, and kinetics. For pH ≤ 1, the lowest temperatures explored (typically from 180 to 200 °C) led to stabilized UO/UO mixtures exhibiting a global O/U ratio evaluated as 2.

Probing the local structure of nanoscale actinide oxides: a comparison between PuO and ThO nanoparticles rules out PuO hypothesis.

Actinide research at the nanoscale is gaining fundamental interest due to environmental and industrial issues. The knowledge of the local structure and speciation of actinide nanoparticles, which possibly exhibit specific physico-chemical properties in comparison to bulk materials, would help in a better and reliable description of their behaviour and reactivity. Herein, the synthesis and relevant characterization of PuO and ThO nanoparticles displayed as dispersed colloids, nanopowders, or nanostructured oxide powders allow to establish a clear relationship between the size of the nanocrystals constituting these oxides and their corresponding An(iv) local structure investigated by EXAFS spectroscopy.

Formation of CeSiO from cerium(iii) silicate precursors.

Although the preparation of CeSiO4 has been already reported, the formation of pure cerium silicate from aqueous precursors appears as a challenge. An innovative way of synthesis has been identified in this study, allowing the formation of CeSiO4 after hydrothermal treatment starting from Ce(iii) silicate precursors. Among the experimental parameters examined, significant effects were found according to the nature of the precursor and of the reactive media considered, the pH of the reactive media and the temperature of the hydrothermal process.

Preparation of CeSiO from aqueous precursors under soft hydrothermal conditions.

Even though CeSiO was synthesized one time through a hydrothermal treatment, the conditions leading to its formation remain largely unknown. In order to define the optimized conditions of synthesis, a multiparametric study was developed by varying the pH of the solution, the temperature, and the nature of the reactants and of the complexing ions in solution. This study highlighted that CeSiO could not be obtained starting from Ce(iv) reactants.

Deciphering the Crystal Structure of a Scarce 1D Polymeric Thorium Peroxo Sulfate.

The preparation and structural characterization of an original Th peroxo sulfate dihydrate, crystallizing at room temperature in the form of stable 1D polymeric microfibres is described. A combination of laboratory and synchrotron techniques allowed solution of the structure of the Th(O )(SO )(H O) compound, which crystallizes in a new structure type in the space group Pna2 of the orthorhombic crystal system. Particularly, the peroxide ligand coordinates to the Th cations in an unusual μ -η :η :η bridging mode, forming an infinite 1D chain decorated with sulfato ligands exhibiting simultaneously monodentate and bidentate coordination modes.

Thermodynamics and Stability of Rhabdophanes, Hydrated Rare Earth Phosphates REPO · n HO.

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.

Determination of the isotopic composition of single sub-micrometer-sized uranium particles by laser ablation coupled with multi-collector inductively coupled plasma mass spectrometry.

Rationale: A multi-collector inductively coupled plasma (MC-ICP) mass spectrometer coupled to a UV ns-laser ablation (LA) system was used to measure uranium isotopic ratios ( U/ U, U/ U and U/ U) in single uranium particles of various sizes and isotopic compositions, including home-made sub-micrometric natural uranium particles of narrow size distribution (415 ± 60 nm).

Methods: The LA-ICP mass spectrometer was operated in wet plasma conditions thanks to simultaneous injection of the laser aerosol and water vapor through a desolvating nebulizer. The isotopic ratios were corrected for mass bias and gain factors between detectors.

Impact of Carbonate Ions on the Synthesis of ThSiO under Hydrothermal Conditions.

Multiparametric study of the hydrothermal synthesis of thorite, ThSiO, was performed with the aim to determine the most efficient conditions to form single phase thorite samples. Among the experimental parameters investigated, temperature of the hydrothermal process, concentration of carbonate ions, thorium and silicon reactants, and pH of the reactive media significantly affect the composition of the final system obtained. Single phase samples of ThSiO were prepared in weakly basic reactive media and at temperatures over 150 °C, for thorium and silicate concentrations higher than 8 × 10 mol L and carbonate concentrations of at least 8 × 10 mol L.