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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]

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]

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]

The simultaneous incorporation of thorium and uranium(IV) in the rare-earth orthophosphates from a mixture of powdered AnO2, (NH4)(H2PO4), and Ca(HPO4)·2H2O was successfully performed, leading to the formation of single-phase Th-brabantite (Ca0.5Th0.5PO4), U-brabantite (Ca0.5U0.5PO4), and (Th,U)-brabantites (Ca0.5Th0.5− yU yPO4). The repetition of several cycles composed of a grinding step (30 Hz, 15 min) and of a heat treatment ( T=1200°C, 10 h, Ar atmosphere) allowed preparation of single-phase and homogeneous compounds. The complete characterization of the samples by the means of X-Ray powder diffraction, electron probe microanalyses, and μ-Raman spectrometry did not reveal either the presence of minor phases or the partial oxidation of uranium(IV) into uranyl. This study also evidenced the formation of a complete Ca0.5Th0.5− yU yPO4 solid solution with the associated linear decrease of a, b, and c unit cell parameters and β increase versus the uranium mole loading. Even if the samples were found to be single phase, the significant improvement of the cation distribution in the materials was reached by the mean of the repetition of grinding/heating cycles. The same solid-state process was applied with success to the preparation of homogeneous and single-phase (Th,U)-monazite/brabantite solid solutions with general formula Ln1−2 xCa xTh x− yU yPO4 (with 0.1≤ x≤0.4 and y= x/5). [ABSTRACT FROM AUTHOR]

The dissolution of Th1-xCexO2 solid solutions samples prepared by thermal conversion of oxalate precursors was studied by varying independently several parameters (such as chemical composition, leachate acidity, leaching temperature, firing temperature, and crystallization state). The relative effects of these parameters on the normalized dissolution rate were examined. Either the obtained partial order related to the proton activity (n = 0.50 ± 0.01) or the activation energy (EA = 57 ± 6 kJ.mol-1) suggested that the dissolution was mainly driven by surface reactions occurring at the solid/liquid interface. Conversely to that observed for Th1-xUxO2 and Ce1-xNdxO2-x/2 solid solutions, the chemical composition did not induce strong modifications of the chemical durability of the leached samples. While the initial normalized dissolution rate slightly depended on the elimination of crystal defects for firing temperatures below 800 °C, it was mainly independent of the crystallite size (T ⩾ 900 °C). The role of crystal defects (then that of preparation conditions) appeared thus important to consider since they contributed to modifications of the normalized dissolution rates of the same order of magnitude than that of the leachate acidity. [ABSTRACT FROM AUTHOR]

The chemical durability of Nd-doped UO 2 samples was studied using an innovative macro-/microscopic dual approach method. The starting precursor was first prepared by a hydroxide co-precipitation route leading to a nano-sized powder associated to high specific surface area. The powder was then converted into U 0.8 Nd 0.2 O 2±y solid solution by calcination. After shaping of the oxide powder into the pellet form through uniaxial pressing at 500 MPa, the pellets were sintered at 1600 °C under inert (Ar) or reducing (Ar-4%H 2) atmosphere. At the macroscopic scale, multiparametric dissolution tests were carried out under static conditions in 2 mol.L−1 HNO 3 at 60 °C. Simultaneously, an operando monitoring of the solid/liquid interface evolution during dissolution was also performed by ESEM at the microscopic scale, in order to follow the consequences of the dissolution on the ceramics microstructure. This dual approach was validated by comparing the calculated normalized dissolution rates obtained for each series of experiments. Changing the sintering atmosphere did not appear to significantly affect the dissolution kinetics of Nd-doped UO 2 samples at the macroscopic scale whereas notorious differences in the dissolution mechanisms have been highlighted at the microscopic scale by ESEM monitoring solid/liquid interface. [Display omitted] • Dense U0.8Nd0.2O2 ± y pellets were prepared under inert or reducing atmosphere. • Nd incorporation in UO2 significantly impacts the material chemical durability. • The pellets presented similar dissolution kinetics at the macroscopic scale. • Solid liquid interfaces evolved differently at the microscopic scale. • Thermal history of the ceramics impacted significantly the chemical durability. [ABSTRACT FROM AUTHOR]

Abstract In order to gain insights into the kinetics of dissolution of Gen(IV) MOX fuels, sintered pellets of Th 0.25 U 0.75 O 2 were prepared as surrogates. The study of the kinetics of dissolution was achieved to determine the impact of the incorporation of 25 mol % of thorium in UO 2 on the rate and on the mechanism of dissolution. Especially, various acidic media were used to evidence changes occurring in the rate of the limiting step and to establish specific rate laws. For most of the investigated dissolution media the dissolution reaction was found to be congruent. The homogeneity of the Th 0.25 U 0.75 O 2 solid/solutions allowed both elements to be recovered at the same rate. The way of synthesis by oxalate co-precipitation, then conversion in mixed oxide, allowed the quantitative dissolution of the elements. In aerated hydrochloric acid solutions, the kinetics of the overall dissolution reaction appeared to be controlled by adsorption and desorption of protons on activated surface sites. The dissolution rate determined in aerated sulfuric acid solution was enhanced compared to hydrochloric acid solution of the same acidity. This result was interpreted in terms of formation of sulfate ions surface complexes that favor the detachment of the actinides from the mixed oxide solid solution. In nitric acid solutions, the dissolution of the Th 0.25 U 0.75 O 2 pellets followed several successive steps. During this first steady state period, the kinetics of the overall dissolution reaction appeared to be controlled by the oxidation of U(IV) by HNO 3 at the solid/solution interface. A rate law was established by adding a redox contribution to the proton-promoted surface contribution defined for aerated hydrochloric acid solutions. Then, an increase of the normalized dissolution rate was observed, which was attributed to the simultaneous increase of the specific surface area of the pellet and of the HNO 2 concentration in solution. Highlights • Dissolution kinetics of Th 0.25 U 0.75 O 2 pellets has been investigated in various acid solutions. • Four successive dissolution steps have been identified in nitric acid. • The impact of the evolving reactive surface area has been highlighted. • The particular role of nitrogen species has been evidenced. [ABSTRACT FROM AUTHOR]

Thorium incorporation in the rhabdophane structure as Pr 1-2x Ca x Th x PO 4 ·nH 2 O solid solutions was successfully achieved and resulted in the preparation of a low temperature precursor of the monazite-cheralite type Pr 1-2x Ca x Th x PO 4 . The rhabdophane compounds are considered as potential neoformed phases in case of release of actinides from the phosphate-based ceramic wasteforms envisaged to host radionuclides in the back-end of the nuclear fuel cycle. A multiparametric study was thus undertaken to specify the wet chemistry conditions (starting stoichiometry, temperature, heating time) leading to single phase Pr 1-2x Ca x Th x PO 4 ·nH 2 O powdered samples. The excess of calcium appeared to be a prevailing factor with a suggested initial Ca:Th ratio being equal to 10. Similarly, the recommended heating time should exceed 4 days while the optimal temperature of synthesis is 110 °C. Under these conditions, the stability domain of Pr 1-2x Ca x Th x PO 4 ·nH 2 O ranged from x = 0.00 to x = 0.15. After heating at 1100 °C under air during 6 h, rhabdophane-type samples were fully converted into the highly durable Pr 1-2x Ca x Th x PO 4 cheralite ceramic wasteform. [ABSTRACT FROM AUTHOR]

The long-term behavior of Th 0.87 Pu 0.13 O 2 was examined in nitric acid concentrations. The normalized dissolution rates after 3380 days, range from (1.4 ± 0.2) × 10 −6 g m −2 d −1 in 5 M HNO 3 down to (3.2 ± 0.4) × 10 −8 g m −2 d −1 in 10 −3 M HNO 3 , which confirms the high chemical durability of this solid solution. The amounts of plutonium measured in solution lead to 0.9% and 2.1% of dissolved solid in 1 M and 5 M HNO 3 , respectively. In such conditions, the time required to reach the full dissolution of the material varies from 430 years (5 M HNO 3 ) to 18,000 years (10 −3 M HNO 3 ). Moreover, the partial order related to the proton activity ( n = 0.45 ± 0.03) suggests that the dissolution is mainly driven by surface reactions occurring at the solid/liquid interface. The characterization of the leached samples by SEM shows small microstructural modifications (i.e. detachment of crystallites) and the absence of neoformed phase while from PXRD, the unit cell parameter and crystallite size are not significantly affected. [ABSTRACT FROM AUTHOR]

Monazite-cheralite ceramics are a promising waste form for actinides. To elucidate the long-term behavior of this matrix in aqueous solutions, this study measured thermodynamic data for Th-rhabdophanes Ln 1-2 x Ca x Th x PO 4 ·nH 2 O (with Ln = Pr, Nd; x = 0–0.15) and the associated anhydrous monazite-cheralites Ln 1-2 x Ca x Th x PO 4. Solubility experiments at 298 K and high temperature oxide melt solution calorimetry were combined for calculation of Δ G f ° , Δ H f ° and S m ° of Th-rhabdophanes and associated monazite-cheralites. Standard solubility constants were employed in a geochemical simulation using the PHREEQC software, the results of which confirmed the high chemical stability of the monazite-cheralite phases and supported their use as a specific conditioning matrix for the long-term immobilization of actinides. • Pure Th-rhabdophanes and associated monazite-cheralites were synthesized. • Solubility experiments were performed at low temperature and various pH conditions. • Samples were submitted to high-temperature oxide melt solution calorimetry. • A complete set of thermodynamic data of was determined. • Simulations of the long-term behavior of the monazite-cheralite was achieved. [ABSTRACT FROM AUTHOR]

High-temperature oxide melt solution calorimetric measurements were completed to determine the enthalpies of formation of the uranothorite, (USiO4)x-(ThSiO4)1-x, solid solution. Phase-pure samples with x values of 0, 0.11, 0.21, 0.35, 0.71, and 0.84 were prepared, purified, and characterized by powder X-ray diffraction, electron probe microanalysis, thermogravimetric analysis and differential scanning calorimetry coupled with in situ mass spectrometry, and high-temperature oxide melt solution calorimetry. This work confirms the energetic metastability of coffinite, USiO4, and U-rich intermediate silicate phases with respect to a mixture of binary oxides. However, variations in unit cell parameters and negative excess volumes of mixing, coupled with strongly exothermic enthalpies of mixing in the solid solution, suggest short-range cation ordering that can stabilize intermediate compositions, especially near x = 0.5. [ABSTRACT FROM AUTHOR]

Highlights: [•] Purification of Th1−xU x SiO4 uranothorites from oxide mixture was investigated. [•] Repetition of centrifugation steps was discarded due to poor recovery yields. [•] Successive washings in acid and basic media allowed the elimination of oxide secondary phases. [•] Structural and microstructural characterization of the purified samples was provided. [ABSTRACT FROM AUTHOR]

A complete Th1-xUx(C2O4)2·2H2O solid solution was prepared by mild hydrothermal synthesis from a mixture of hydrochloric solutions containing cations and oxalic acid. The crystal structure has been solved from twinned single crystals for x = 0.5, and 1 with monoclinic symmetry, space group C2/c, leading to unit cell parameters of am ≈10.5 A, b vs ≈8.5 A, and cm ≈9.6 Å. The crystal structure consists of a two-dimensional arrangement of actinide centers connected through bis-bidentate oxalate ions forming squares. The actinide metal is coordinated by eight oxygen atoms from four oxalate entities and two water oxygen atoms forming a bicapped square antiprism. The connection between the layers is assumed by hydrogen bonds between the water molecules and the oxygen of oxalate of an adjacent layer. Under these conditions, Ihe unit cell contains two independent oxalate ions. From high-temperature μ-Raman and X-ray diffraction studies, the compounds were found 10 undergo a transition to an orlhorhombic form (space group Ccca). The major differences in the structural arrangement concern the symmetry of uranium, which decreases from C2 to D2, leading to a unique oxalate group. Consequently, the vs(C-O) double band observed in the Raman spectra recorded al room temperature turned into a singlet. This transformalion was then used to make the phase transition temperature more precise as a function of the uranium content of the sample. [ABSTRACT FROM AUTHOR]