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Dicesium molybdate (Cs2MoO4)׳s thermal expansion and crystal structure have been investigated herein by high temperature X ray diffraction in conjunction with Raman spectroscopy. This first crystal-chemical insight at high temperature is aimed at predicting the thermostructural and thermomechanical behavior of this oxide formed by the accumulation of Cs and Mo fission products at the periphery of nuclear fuel rods in sodium-cooled fast reactors. Within the temperature range of the fuel׳s rim, Cs2MoO4 becomes hexagonal P63/mmc, with disordered MoO4 tetrahedra and 2D distribution of Cs–O bonds that makes thermal axial expansion both large (50≤α l ≤70 10−6 °C−1, 500–800°C) and highly anisotropic (α c −α a =67×10−6 °C−1, hexagonal form). The difference with the fuel׳s expansion coefficient is of potential concern with respect to the cohesion of the Cs2MoO4 surface film and the possible release of cesium radionuclides in accidental situations. [ABSTRACT FROM AUTHOR]

Abstract: While uranium-based mixed oxides are interesting materials for several concepts of nuclear reactors, the effects of trivalent elements in the fluorite structure and the redox reactions at the solid/solution interface during the dissolution of such compounds remain widely unknown. In order to underline the influence of those parameters, various dissolution experiments were carried out on different compositions (i.e. U0.75Ce0.25O2, U0.75Th0.25O2, Th0.75Nd0.25O1.875 and U0.75Ln0.25O1.875 with Ln = Nd, Gd). These tests were performed on sintered pellets in nitric acid solutions (from 10−2 M to 4 M) at different temperatures (22°C–90°C) under dynamic conditions. Therefore, a multiparametric study of the dissolution kinetics was then achieved in order to determine the partial order of dissolution reaction regarding to H3O+ activity and the activation energy. The obtained results gave evidence of the strong dependency of the dissolution rate with the nitric acid concentration. In fact, for concentrations higher than 2 M, the dissolution process is almost controlled by the oxidation of U(IV) to U(VI). On the contrary the effect of oxygen vacancies became predominant for acid concentrations lower than 0.5 M. Under these conditions, the systems containing trivalent elements exhibited the lowest chemical durability. The partial order of the reaction regarding to the H3O+ activity reached n = 2.1 for U0.75Th0.25O2 and was found to be constant over the entire acidity range. A variation of the partial order of the dissolution reaction regarding to H3O+ was observed along the nitric acid concentration range for U0.75Ln0.25O1.875 solid solutions, that underlines the predominance of different controlling reactions in the dissolution mechanism: surface-controlling reaction involving H3O+ at low pH (i.e. n < 1) for ≤ 0.5 M, and uranium oxidation (i.e. n > 1) for ≥ 1 M. [Copyright &y& Elsevier]

Highlights: [•] Solubility of synthesized meta-torbernite (H3O)0.4Cu0.8(UO2)2(PO4)2⋅7.6H2O. [•] Solubility constant and thermodynamic data of formation were determined. [•] Comparison with a natural sample: same steady state reached for both samples. [•] Simulations performed under environmentally relevant conditions using the thermodynamic data obtained. [ABSTRACT FROM AUTHOR]

Abstract: The dissolution of uranium carbide (UC) in nitric acid media is considered by means of power ultrasound (sonication) or magnetic stirring. The induction period required to initiate UC dissolution was found to be dramatically shortened when sonicating a 3M nitric solution (Ar, 20kHz, 18Wcm−2, 20°C). At higher acidity, magnetic stirring offers faster dissolution kinetics compared to sonication. Ultrasound-assisted UC dissolution is found to be passivated after ∼60% dissolution and remains incomplete whatever the acidity which is confirmed by ICP–AES, LECO and SEM–EDX analyses. In general, the kinetics of UC dissolution is linked to the in situ generation of nitrous acid in agreement with the general mechanism of UC dissolution; the nitrous acid formation is reported to be faster under ultrasound at low acidity due to the nitric acid sonolysis. The carbon balance shared between the gaseous, liquid, and solid phases is strongly influenced by the applied dissolution procedure and HNO3 concentration. [Copyright &y& Elsevier]

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]

In order to reduce the radiotoxicity of nuclear fuel waste, the transmutation of americium in U1− x Am x O2± δ dedicated fuels is considered. A convenient route to produce such fuels is reactive sintering from a UO2+ δ /AmO2− δ green pellet, i.e., a single heat treatment during which both the densification and the formation of the U1− x Am x O2± δ solid solution occur. The mechanisms of such sintering are however barely known and require experimental data. In this aim, the densification through reactive sintering of a UO2+ δ /AmO2− δ sample was monitored by dilatometry. The obtained results were compared to those reported for the formation of the U1− x Am x O2± δ solid solution monitored by in situ high-temperature X-ray diffraction. To assess the use of Ce as a substitute of Am, similar dilatometric studies were also carried out on UO2+ δ /CeO2 pellets. Obtained results show that the use of a reactive sintering causes a delay in the densification process associated to the competition between solid solution formation and densification, which yields limitations in pellet final densities. The importance of redox behavior of Am (or Ce) on the achievement of solid solution formation and densification are also discussed, especially based on discrepancies in densification behavior between UO2+ δ /AmO2− δ and UO2+ δ /CeO2. [ABSTRACT FROM AUTHOR]

Experiments on the solubility of intermediate members of the Th1-xUxSi04 solid solution were carried out to determine the impact of Th--U substitutions on the thermodynamic properties of the solid solution and then allow extrapolation to the coffinite end member. The ion activity products in solutions equilibrated with Th1-xUxSiO4, (0 < x < 0.5) were determined by dissolution experiments conducted in 0.1 moI-LT1 HCl under Ar atmosphere at several temperatures ranging from 298 to 346 K. For all experiments, dissolution was congruent, and a constant composition of the aqueous solution was reached after 50--200 days of dissolution. The solubility product of thorite was determined (log *Ks,ThSio4= -5.62 ± 0.08) whereas the solubility product of coffinite was estimated (log *Ks, usio4= --6.1 ± 0.2). The stoichiometric solubility product of ThiUSK reached a maximum value for x = 0.45 ± 0.05. In terms of the standard Gibbs free energy of dissolution, solid solutions dissolve more spontaneously than the end members. The standard Gibbs free energy associated with the formation o£ thorite, coffinite, and intermediate members of the series were then evaluated. The standard Gibbs free energies of formation were found to increase linearly with tbe uranium mole fraction. Our data at low temperature clearly show that uranothorite solid solutions with x > 0.26, thus coffinite, are less stable than the mixture of binary oxides, which is consistent with qualitative evidence from petrographic studies of uranium ore deposits. [ABSTRACT FROM AUTHOR]

The crystal structure of the cheralite-CaTh(PO)-has been revisited by neutron diffraction and its behaviour under high pressure investigated by X-ray diffraction up to 36 GPa. The neutron diffraction data at ambient pressure gave a more accurate determination of the Ca/Th cation position than previous XRD data, taking advantage that the neutron scattering lengths of calcium and thorium are of same order of magnitude. The nuclear density distribution was also determined using the maximum entropy method (MEM) confirming that the two cations are not located at the same position in the unit cell but are slightly displaced from one another along a specific direction in order to minimize the electrostatic repulsion with the surrounding phosphorus atoms. At high pressure, the compound did not show any phase transition or amorphization. From the evolution of the unit-cell volume as a function of the pressure, the zero-pressure bulk modulus B and its pressure derivative B have been determined by fitting the experimental compressibility curve to the Birch-Murnaghan equation of state. The results are B = 140(2) GPa and B = 4.4(4) GPa. [ABSTRACT FROM AUTHOR]

Abstract: Sintering of CeO2 is studied in situ by high temperature scanning environmental microscopy (HT-ESEM) at T =1400°C. The morphological modifications of a single grains population are recorded for 6h. Kinetic parameters are extracted from image series. The local grain growth determined from the single population studied in situ is compared to the general grain growth obtained by classical ex situ technique. Using HT-ESEM for sintering study is validated. The grain boundary velocities range between 0 and 5μmh−1, with a mean value of about 1μmh−1. The migration of the intragranular surface pores is described. Their velocities range between 0.4 and 1.2μmh−1 and depend on pore diameters: the smaller the pore, the faster the pore velocity. The time required to fill a pore that arises at the sample surface is determined as a function of pore diameter. The time for pore elimination dependence with the pore diameters is also established. [Copyright &y& Elsevier]

Abstract: The crystal structure of (UO2)4(pdzdc)3(OH)2(H2O)5·11H2O (pdzdc =pyridazine-3,6-dicarboxylate) consists of two distinct dinuclear motifs [(UO2)2N4O11] and [(UO2)2N2O10] with eight-fold and seven-fold coordinations for the uranium centers, respectively. The uranium atoms having the same environment are bridged through a μ 2-hydroxo group to each other. The dimeric units are connected to each other through a carboxylate arm from a pdzdc molecule to generate tetranuclear building blocks, which are further assembled into a three-dimensional network by hydrogen bonding. Preliminary investigation of fluorescence property of the complex is presented as well as Raman and Infrared spectroscopies. [Copyright &y& Elsevier]

(U2O)(PO4)2 is related to ultra-low expansion β-(Zr2O)(PO4)2 ceramics, but shows a continuous thermal contraction. High-temperature neutron diffraction has allowed to follow accurately the thermal variations of its cell edges and to give a structural explanation to the phenomenon: like in β-(Zr2O)(PO4)2, the dilatometric anomaly arises simultaneously from a contractive push–pull effect due to Coulombic repulsions and from a libration of the PO4 and UO7 polyhedra, but in the present case, the second mechanism predominates. The size of the tetravalent cation appears as a key parameter in monitoring the thermal expansion of ceramics of this family. [Copyright &y& Elsevier]

In order to gain insights on the kinetics of the dissolution of Gen(IV) MOX fuels, sintered pellets of Th 0.25 U 0.75 O 2 were prepared as surrogates. Particularly, the impact of nitrogen species on the kinetics of dissolution of Th 1- x U x O 2 in nitric acid must be evaluated. In this work, various experiments were performed to determine the influence of nitrate ions and nitrous acid on the dissolution rate and mechanism of Th 0.25 U 0.75 O 2 in nitric acid. It appeared that the addition of 3 mol L−1 NH 4 NO 3 to the 1 mol L−1 HNO 3 solution led to a strong increase of the dissolution rate. Thus, it seemed that the greatest impact on the dissolution kinetics during the induction period was associated to undissociated HNO 3 instead of free nitrate ions. What is more, addition of a constant HNO 2 concentration of 2 × 10−3 mol L−1 to 1 mol L−1 HNO 3 induced a strong increase of the dissolution rate. However, the dissolution kinetics remained much faster in 4 mol L−1 HNO 3 , which was associated to the presence of an equivalent concentration of HNO 2 in the solution. It then appeared that HNO 2 species was not the prevailing species responsible for the catalysis of the dissolution kinetics of Th 0.25 U 0.75 O 2. However, microstructural evolutions of the pellets dissolved in the presence of HNO 2 showed the existence of small intra-granular corrosion pits covering the entire surface of the grains. That confirmed the highly oxidizing nature of HNO 2 regarding to tetravalent uranium. Nevertheless, the presence of large cavities also suggested the existence of heterogeneous progress of the dissolution, associated to the existence of preferential dissolution zones consequently to the accumulation of reactive species in confined areas. Such reactive species might be HNO 2 , but more probably another unknown catalyst present at equilibrium with HNO 2. • The impact of nitrogen species on the dissolution kinetics of Th 0.25 U 0.75 O 2 pellets has been underlined. • Undissociated HNO 3 has the greatest impact on the dissolution kinetics during the induction period. • HNO 2 is not the prevailing species responsible for the catalysis of the dissolution kinetics. • The formation of large cavities in the pellets underlines heterogeneous progress of the dissolution at the microscopic scale. [ABSTRACT FROM AUTHOR]

Focuses on radon emanation, a method used to identify radium isotopes in mineral drinking waters. Concerns raised about radium in the environment since it was discovered in 1898; Discussion on harmfulness associated with radium isotopes; Highlight of an experiment conducted on radium isotopes; Problems associated with the use of the radon emanation method.

• UO 2 single crystals with (100), (110) and (111) surfaces were dissolved in 2 M HNO 3. • Two successive kinetic regimes were identified during dissolution. • The impact of the initial surface orientation on the dissolution rate was evaluated. • During the first kinetic regime, the dissolution occurred mainly at surface defect sites. • The second kinetic step was attributed to a catalysed dissolution mechanism. UO 2 single crystals with (100), (110) and (111) oriented faces were dissolved in 2 mol.L−1 HNO 3 at room temperature. The evolution of the topography of the surface was monitored and reliable dissolution rates corresponding to the three crystallographic orientations were determined under controlled hydrodynamic and chemical conditions. The dissolution tests of UO 2 polished single crystals showed two different kinetic steps. During the first uncatalysed kinetic regime, the enhanced reactivity of the surface at defect sites was demonstrated. The second kinetic step was attributed to a catalysed dissolution mechanism involving species produced at the solid/solution interface during the first step. [ABSTRACT FROM AUTHOR]

Rare-earth phosphates adopting the xenotime (REPO 4 ; RE = Tb − Lu & Y, Sc) structure are proposed as a potential matrix for the confinement of minor actinides. Minor actinides (e.g., Np, Am, Cm) undergo a radioactive decay process in which high-energy recoil atom (70–100 keV) and energetic alpha particles (4.5–5.8 MeV) are produced. In this study, the impact of these energetic decay products on the structure of xenotime-type ErPO 4 has been investigated via high energy dual ion-beam irradiation of ErPO 4 ceramics. Au2+ (1.5 MeV) and He+ (160 keV) ions were used to simulate the effects of recoil atom and α-particles, respectively. Multiple experiments were carried out in which the Au2+ and He+ ions with varying ion-fluences (ions/cm2) and ion-flux (ions/cm2/s) were implanted sequentially (Au2+ followed by He+ irradiation) and simultaneously (Au2+ + He+ irradiation) into ErPO 4 ceramics. Sequential ion-irradiation experiments have shown that the xenotime structure was amorphized by Au2+ ions at a relatively lower ion-fluence (5 × 1013 ions/cm2) in comparison to the monazite structure. Upon irradiation of the amorphous ErPO 4 with He+ ions, recrystallization of the amorphous xenotime due to α-particles was not observed. However, simultaneous ion-irradiation experiments on ErPO 4 showed that the amorphization of the xenotime structure was prevented upon deposition of higher amounts of electronic energy (E electronic) in the lamella. Likewise monazite samples, the α-healing mechanism was also experimentally demonstrated in synthetic xenotime samples. Image 1 • Ion beam irradiation performed using Au2+ (recoil atom) and He+ (α-particle) ions. • Structural response of dual-ion beam irradiated ErPO 4 was studied using in-situ TEM. • Xenotime structure remained intact upon simultaneous (Au2+ + He+) ion-irradiation. • α-healing of amorphous xenotime is less efficient than the monazite structure. • Electron beam-induced recrystallization of amorphous xenotime was observed. [ABSTRACT FROM AUTHOR]

• Cu-substituted Hap samples with controlled copper content were synthesized. • Cu-Hap samples were used as sorbent materials for immobilizing uranyl. • Cu-Hap samples were contacted with U-spiked solutions and real mine waters. • The mechanism of immobilization was investigated. • Precipitation of the uranyl-phosphate phase meta-torbernite was evidenced. In this study, synthetic copper substituted hydroxyapatite (Cu-Hap), Cu x Ca 10-x (PO 4) 6 (OH) 2 were prepared by co-precipitation method and were used as reactive materials in batch experiments to immobilize uranyl. The limit of incorporation of Cu into a single-phased Cu-Hap reached x Cu ≤1.59. The synthetic Cu-Hap samples obtained with various Cu contents were contacted with synthetic uranyl doped solutions and with real mining waters showing various pH and chemical compositions. A fast and strong decrease of the uranium concentration was observed, followed by the establishment of an equilibrium after 1–4 days of contact with the solutions. Examination of the solid phase after uranium uptake was performed using a combination of techniques. Depending on the composition of the solution and the copper content of the Cu-Hap, various mechanisms of uranium removal were observed. Based on the experimental results and geochemical simulations, it appeared that the main interest for using Cu-Hap is to enlarge the domain of water compositions for which the precipitation of meta-torbernite, (H 3 O) 0.4 Cu 0.8 (UO 2) 2 (PO 4) 2 ·7.6 H 2 O is the predominant mechanism associated to the uranium removal, especially for pH > 6.7 where carbonate uranium species are predominant. [ABSTRACT FROM AUTHOR]

The effect of metallic particles containing Platinum Group Metal (PGM) elements on the dissolution of sintered UO 2 samples was examined through the development of multiparametric dissolution tests on sintered UO 2 and UO 2 + 3 mol.% PGM samples. This effect was found to be more important for C HNO3 < 1 mol L−1 whereas it was lowered for higher nitric acid concentrations due to the preponderant oxidation of U(IV) by nitrate ions. Increase of the normalised dissolution rate of the ceramics was associated to the presence of PGM elements along with the decrease of the duration of the induction period (the stronger effect being observed for ruthenium). Simultaneously, the decrease of the apparent activation energy associated to the reaction of dissolution supported the existence of catalytic effect. This latter was connected to redox reaction between the metallic particles and nitrate ions either in solution or at the PGM/UO 2 /solution interface. The involved reactions induced the formation of nitrogen species close to the PGM bearing particles. Among them, nitrous acid played an important role due to its strong oxidative power regarding to U(IV). Additionally, no synergistic effect was evidenced when mixing the three PGM elements. • Sintered UO 2 samples doped with PGM (Pd, Rh, Ru) have been dissolved in nitric acid solutions. • Increase of the normalised dissolution rate and decrease of the duration of the induction period have been observed in the presence of PGM bearing ε metallic particles. • Catalytic reaction involving nitrogen species formed close to the PGM bearing ε metallic particles has been suggested. • No synergistic effect was evidenced when mixing the three PGM elements. [ABSTRACT FROM AUTHOR]

Highlights from the article: Peroxide ligands coordinate to Th cations with a rare SB 3 sb - SP 2 sp : SP 2 sp : SP 2 sp bridging mode, forming infinite 1D chains decorated simultaneously with monodentate and bidentate sulfato ligands. Peroxide, spectroscopic techniques, structure elucidation, sulfate, thorium.

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 2+ 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 = n oxalate / n U = 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 UO 2+ 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 UO 2+ 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.

The preparation of synthetic (Zr,U)SiO 4 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 4 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 4 . The investigation carried out in this study has indeed led to the confirmation of metastable (Zr,U)SiO 4 compounds with high uranium contents being formed. It was found that (Zr,U)SiO 4 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).

Density functional theory plus Hubbard U (DFT+ U ) methodology was used to calculate the structures and energetic landscapes of CeSiO 4 , 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 E g 1 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.

This work is the first attempt to prepare Nd 1- x Ca x U x PO 4 monazite-cheralite with 0 < x ≤ 0.1 by a wet chemistry method. This method relies on the precipitation under hydrothermal conditions ( T = 110 °C for four days) of the Nd 1- x Ca x U x PO 4 · n H 2 O rhabdophane precursor, followed by its thermal conversion for 6 h at 1100 °C in air or Ar atmosphere. The optimized synthesis protocol led to the incorporation of U and Ca in the rhabdophane structure. After heating at 1100 °C for 6 h in air, single-phase monazite-cheralite samples were obtained. However, α-UP 2 O 7 was identified as a secondary minor phase in the samples heated under Ar atmosphere. The U speciation in the samples converted in an oxidising atmosphere was carefully characterized using synchrotron radiation by combining HERFD-XANES and XRD. These results showed the presence of a minor secondary phase containing hexavalent uranium and phosphate with a stoichiometry of U : P = 0.78. This highly labile uranyl phosphate phase incorporated 21 mol% of the uranium initially precipitated with the rhabdophane precursor. This phase was completely removed by a washing protocol. Thus, single-phase monazite-cheralite was obtained through the wet chemistry route described in this work with a maximum U loading of x = 0.08.

Although ZrSiO 4 is the most well-known compound in the zircon-structured family (space group I 4 1 / 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 ZrSiO 4 and (Zr,Ce)SiO 4 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 ZrSiO 4 was obtained through hydrothermal treatment for 7 days at 250 °C, within a large acidity range (1.0 ≤ pH ≤ 9.0) and starting from C Si ≈ C Zr ≥ 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)SiO 4 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 Zr 1- x Ce x SiO 4 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.

Although uranium-cerium dioxides are frequently used as a surrogate material for (U,Pu)O 2-δ 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 2±δ sintered samples prepared by a wet-chemistry route. HERFD-XANES spectroscopy led to determining accurately the O/M ratios (with M = U + Ce). Under a reducing atmosphere ( p O 2 ≈ 6 × 10 -29 atm at 650 °C), the oxides were found to be close to O/M = 2.00, while the O/M ratio varied with the sintering conditions under argon ( p O 2 ≈ 3 × 10 -6 atm at 650 °C). They globally appeared to be hyperstoichiometric (i.e., O/M > 2.00) with the departure from the dioxide stoichiometry decreasing with both the cerium content in the sample and the sintering temperature. Nevertheless, such a deviation from the ideal O/M = 2.00 ratio was found to generate only moderate structural disorder from EXAFS data at the U-L 3 edge as all the samples retained the fluorite-type structure of the UO 2 and CeO 2 parent compounds. The determination of accurate lattice parameters owing to S-PXRD measurements led to complementing the data reported in the literature by various authors. These data were consistent with an empirical relation linking the unit cell parameter, the chemical composition, and the O/M stoichiometry, showing that the latter can be evaluated simply within a ± 0.02 uncertainty.

The effect of annealing on structural and thermochemical properties of a thorite-xenotime solid solution Th 1- x Er x (SiO 4 ) 1- x (PO 4 ) x 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. During annealing, the structure lost water and exsolved some thorianite phases. The thermodynamic parameters did not change much after annealing, suggesting that xenotime was not a low-temperature metastable phase but rather a stable structure able to withstand elevated temperatures regardless of the thorium content. The solid solution exhibited subregular behavior with the Margules function W ( x ) = (73.1 ± 20.1) - (125.7 ± 49.8)· x .

Orthosilicates adopt the zircon structure types ( I 4 1 /amd ), consisting of isolated SiO 4 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 4 ) and coffinite (USiO 4 ) can be formed under hydrothermal conditions despite both being thermodynamically metastable. Water has been hypothesized to play a significant role in stabilizing and forming these orthosilicate phases, though little experimental evidence exists. To understand the effects of hydration or hydroxylation on these orthosilicates, in situ high-temperature synchrotron and laboratory-based X-ray diffraction was conducted from 25 to ∼850 °C. Stetindite maintains its I 4 1 /amd symmetry with increasing temperature but exhibits a discontinuous expansion along the a- axis during heating, presumably due to the removal of water confined in the [001] channels, which shrink against thermal expansion along the a -axis. Additional in situ high-temperature Raman and Fourier transform infrared spectroscopy also confirmed the presence of the confined water. Coffinite was also found to expand nonlinearly up to 600 °C and then thermally decompose into a mixture of UO 2 and SiO 2 . A combination of dehydration and dehydroxylation is proposed for explaining the thermal behavior of coffinite synthesized hydrothermally. Additionally, we investigated high-temperature structures of two coffinite-thorite solid solutions, uranothorite (U x Th 1- x SiO 4 ), which displayed complex variations in composition during heating that was attributed to the negative enthalpy of mixing. Lastly, for the first time, the coefficients of thermal expansion of CeSiO 4 , USiO 4 , U 0.46 Th 0.54 SiO 4 , and U 0.9 Th 0.1 SiO 4 were determined to be α V = 14.49 × 10 -6 , 14.29 × 10 -6 , 17.21 × 10 -6 , and 17.23 × 10 -6 °C -1 , respectively.

Hydrothermal conversion of thorium oxalate, Th(C 2 O 4 ) 2 · n H 2 O, 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 4+ 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. All the ThO 2 · n H 2 O samples prepared presented amounts of residual carbon and water in the range 0.2-0.3 wt % and n ≈ 0.5, respectively. A combined FTIR, PXRD, and EXAFS study showed that these impurities mainly consisted of carbonates trapped between elementary nanosized crystallites, rather than substituted directly in the lattice, which generated a tensile effect over the crystal lattice. The presence of carbonates at the surface of the elementary crystallites could also explain their tendency to self-assembly, leading to the formation of spherical aggregates. Hydrothermal conversion of oxalates could then find its place in different processes of the nuclear fuel cycle, where it will provide an interesting opportunity to set up dustless routes leading from ions in solution to dioxide powders in a limited number of steps.

Zircon (ZrSiO 4 , I 4 1 / amd ) 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. Thus, we have completed a thermodynamic study of cerium orthosilicate, stetindite (CeSiO 4 ), a surrogate of PuSiO 4 . For the first time, the standard enthalpy of formation of CeSiO 4 was obtained by high temperature oxide melt solution calorimetry to be -1971.9 ± 3.6 kJ/mol. Stetindite is energetically metastable with respect to CeO 2 and SiO 2 by 27.5 ± 3.1 kJ/mol. The metastability explains the rarity of the natural occurrence of stetindite and the difficulty of its synthesis. Applying the obtained enthalpy of formation of CeSiO 4 from this work, along with those previously reported for USiO 4 and ThSiO 4 , we developed an empirical energetic relation for actinide orthosilicates. The predicted enthalpies of formation of AnSiO 4 are then determined with a discussion of future strategies for efficiently immobilizing Pu or minor actinides in the zircon structure.

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. Through in situ small and wide angle X-ray scattering (SWAXS) measurements, colloids with a few nanometers in size were first evidenced at a low reaction time. These colloids have elongated shapes and finally tend to aggregate after their size has reached 10 nm. Once aggregated, the system goes through a maturation step, ending with the emergence of nanocrystallites as thorite zircon structures. This maturation step is longer when the reaction temperature is decreased which highlights the kinetic considerations. These results have potential implications on the paragenesis of Th mineral deposits and also in the behaviour of Th and, by analogy, tetravalent actinides in the environment. The significant characteristics of this work are that Th-silicate colloids were demonstrated at low temperatures and a near neutral pH with long-term stability and a morphology in favor of high mobility in groundwater. If these species are formed in more diluted media, this could be problematic owing to the spreading of Th and, by analogy, other tetravalent actinides in the environment.

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). These results shed new light on the potential behavior of plutonium in reductive environments, highlighting the representative nature of cerium surrogates when studying plutonium under such conditions and providing some important pieces of information regarding plutonium chemistry in silicate solutions.

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(C 2 O 4 ) 2 · n H 2 O 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 2+ x /U 4 O 9 mixtures exhibiting a global O/U ratio evaluated as 2.38 ± 0.10 from U M 4 -edge HERFD-XANES experiments. Higher temperatures (220-250 °C) led the oxide stoichiometry to decrease down to 2.13 ± 0.04 which corresponds to a lower fraction of U 4 O 9 in the mixture. Additionally, increasing the temperature of the hydrothermal treatment efficiently improved the elimination of residual carbon species and water. Hydrothermal conversion of U(C 2 O 4 ) 2 · n H 2 O also led to a drastic modification of the powders morphology. With this aim, pH tuning could be used to shift from bipyramidal aggregates (up to pH 1) to microspheres (2 ≤ pH ≤ 5) and then to nanometric powders (pH > 5). Finally, a kinetics study showed that uranium oxides can be obtained from the hydrothermal decomposition of oxalate within only few hours. If the samples collected early during the treatment always presented the characteristic XRD lines of UO 2+ x /U 4 O 9 fluorite-type structure, then they were found to be strongly oxidized (O/U = 2.65 ± 0.14) which suggested the existence of a U(VI)-bearing amorphous secondary phase. The latter further tended to reduce through time. Hydrothermal conversion then probably proceeds as a two-step mechanism composed by the oxidative decomposition of uranium(IV) oxalate followed by the reduction of uranium by organic moieties and its hydrolysis. It appears as an easy and efficient way to yield highly pure uranium oxide samples in solution.

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 2 and ThO 2 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. Particularly, the first oxygen shell of the probed An(iv) evidences an analogous behaviour for both Pu and Th oxides. This observation suggests that the often observed and controversial splitting of the Pu-O shell on the Fourier transformed EXAFS signal of the PuO 2 samples is attributed to a local structural disorder driven by a nanoparticle surface effect rather than to the presence of PuO 2+ x species., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Even though CeSiO 4 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 4 could not be obtained starting from Ce(iv) reactants. An optimal set of conditions was defined to prepare single phase samples. Pure CeSiO 4 was obtained through a hydrothermal treatment at 150 °C using a starting mixture of 1 mol L -1 Ce(iii) nitrate and Na 2 SiO 3 solutions and by adjusting the initial pH to 8. The chemical limitations observed during the synthesis of CeSiO 4 suggested that the formation of this phase may result from the slow in situ oxidation of a Ce(iii) silicate complex during the hydrothermal treatment.

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.

Multiparametric study of the hydrothermal synthesis of thorite, ThSiO 4 , 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 4 were prepared in weakly basic reactive media and at temperatures over 150 °C, for thorium and silicate concentrations higher than 8 × 10 -3 mol L -1 and carbonate concentrations of at least 8 × 10 -2 mol L -1 . Although the synthesis of thorite in carbonate media was already described in the literature, this study gives new insights to explain the key role of carbonate ions in the preparation of thorite. Especially, beyond their simple role of pH buffer, carbonate ions are involved in the formation of thorium-carbonate complexes at high pH, increasing the apparent solubility of thorium in weakly basic media. The presence of carbonate ions has an important impact not only on the domain of formation of thorite but also on the morphology of the silicate phase.

A multiparametric study of the hydrothermal synthesis of ThSiO 4 , thorite, was performed with the aim of determining the most efficient conditions to form single-phase samples. Among the experimental parameters examined, significant effects were found for the concentration of reactants in the starting mixture, pH of the reactive media, and temperature of the hydrothermal process. Such parameters affected both the rate of formation of thorite and the morphology of the final products synthesized. Precipitation of pure ThSiO 4 was obtained over a wide range of pH on going from C HNO3 = 0.3 mol L -1 to pH 9.1 with a yield of over 95%. Temperatures higher than 160 °C favor the formation of thorite. Finally, thorium and silicon concentrations above 2.1 × 10 -3 mol L -1 are required to obtain pure thorium silicate.

Highly reactive and nanosized Th 1-x Y x O 2-x/2 or Ce 0.8 Ln 0.2 O 1.9 mixed oxides were prepared through the initial precipitation of hydroxide precursors which were further dried under vacuum. Whatever the chemical system investigated, the characterization of the powdered samples evidenced a rapid aging process leading to hydrated oxides. The thermal behavior of these samples was further investigated and first showed a two-step dehydration process, with the successive departure of adsorbed and constitutive water, both yielding a drastic drop of the powders' reactivity (i.e. decrease of the specific surface area). Sintering experiments were then undertaken by starting directly from raw powders and revealed very rapid densification kinetics. Highly densified pellets (above 95 %TD) with a fine grain microstructure were obtained after only 1 hour of heat treatment at 1600 °C. This easy and versatile process of precipitation, that can be followed by direct densification of the powders, then appears as a promising option for the elaboration of homogenous ceramic electrolytes., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Pure powdered compounds with a general formula Th 1-x Er x (SiO 4 ) 1-x (PO 4 ) x belonging to the zircon-xenotime family were successfully synthesized under hydrothermal conditions (250 °C, 7 days) as recently reported for the preparation of coffinite. Therefore, a thorough, combined PXRD, EDX, EXAFS, Raman, and FTIR analysis showed the formation of a solid solution in agreement with Vegard's law. Moreover, the examination of the local structure shows that the Th-O distances remain close to those found in ThSiO 4 , whereas the Er-O distances show a significant decrease from 2.38(14) to 2.34(7) Å when increasing the erbium content from x = 0.2 to x = 1. The variation of the local structure also affects the PO 4 3- groups that are surely distorted in the structure.

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.

With through space and through bond experiments in two-dimensional NMR we analyze the transformation from the thorium phosphate-hydrogen phosphate hydrate (TPHPH) to the beta form of the thorium phosphate diphosphate (beta-TPD) in relation with the phosphorus networks. These techniques are complementary: the through space coupling gives an insight on the dipolar phosphorus networks while the through bond coupling is particularly efficient in the detection of the P2O7 groups. With these experiments we show that in a first step, by heating the precursor TPHPH above 250 degrees C, it transforms into an alpha form of TPD. This transformation is due to the complete condensation of hydrogen phosphate groups HPO4 into P2O7 entities. By heating alpha-TPD above 950 degrees C it transforms into its well-known beta form. The alpha form is characterized by a hygroscopic behavior: some water molecules are present near the P2O7 groups that makes non-equivalent their phosphorus nuclei. PO4 dipolar networks are always present in the alpha form. The main effect of these PO4 and P2O7 units is to give the system a channel structure and the water enters in them.