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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]

Monazite (Ln3+PO4) and related solid solutions are a well-known source of rare earth elements on earth. They may also accommodate large amounts of thorium and uranium without sustaining damage to the structure by self-irradiation. Such observations led to monazite-type structures being proposed as a potential host matrix for sequestering long-lived radionuclides produced during the nuclear fuel cycle and/or plutonium and americium from dismantled nuclear weapons. Monazite has two main advantages as a matrix for the containment of radioactive waste (or "radwaste"). The first is a highly flexible structure that permits accommodation of high concentrations of actinides. The incorporation of trivalent elements may be achieved by direct synthesis of An3+PO4 (An3+ = plutonium, Pu to einsteinium, Es), while tetravalent cation incorporation requires coupled substitutions, either on the anionic site (leading to monazite-huttonite solid solution) or on the cationic site (monazite-cheralite solid solution). Various methods developed for the preparation of such compounds are summarized here, as well as the experimental conditions required for the production of sintered pellets, with a particular focus on plutonium-bearing compositions. The second highly favorable property of monazite is its high chemical durability. Several experimental procedures developed to determine normalized leaching rates are reviewed, as well as results obtained from natural and synthetic monazite. Potential phases formed during dissolution were considered because they also partially control the concentration of actinides in the media. A preliminary list for such phases of interest, as well as corresponding thermodynamic data, is presented. [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]

Abstract: Three new crystal structures, isotypic with β-Zr2O(PO4)2, have been resolved by the Rietveld method. All crystallize with an orthorhombic cell (S.G.: Cmca) with a =7.1393(2)Å, b =9.2641(2)Å, c =12.5262(4)Å, V =828.46(4)Å3 and Z =8 for Th(OH)PO4; a =7.0100(2)Å, b =9.1200(2)Å, c =12.3665(3)Å, V =790.60(4)Å3 and Z =8 for U(OH)PO4; a =7.1691(3)Å, b =9.2388(4)Å, c =12.8204(7)Å, V =849.15(7)Å3 and Z =4 for Th2O(PO4)2. By heating, the M(OH)PO4 (M=Th, U) compounds condense topotactically into M2O(PO4)2, with a change of the environment of the tetravalent cation that lowers from 8 to 7 oxygen atoms. The lower stability of Th2O(PO4)2 compared to that of U2O(PO4)2 seems to result from this unusual environment for tetravalent thorium. [Copyright &y& Elsevier]

Abstract: 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 β form of the thorium phosphate diphosphate (β-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°C, it transforms into an α form of TPD. This transformation is due to the complete condensation of hydrogen phosphate groups HPO4 into P2O7 entities. By heating α-TPD above 950°C it transforms into its well-known β form. The α 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 α form. The main effect of these PO4 and P2O7 units is to give the system a channel structure and the water enters in them. [Copyright &y& Elsevier]

Abstract: In the field of the immobilisation of high-activity-level and long-life radwaste (HAVL) for a deep underground repository, several phosphate matrices were already proposed as good candidates to delay the release of actinides in the near-field of such disposal. Among them, thorium phosphate–diphosphate (TPD), monazites/brabantites, britholites, and TPD/monazite composites were extensively studied. The synthesis of samples doped with actinides (Th, U...) through wet and dry chemistry methods then their complete characterisation are reported. Their chemical durability is also examined. These materials appear as promising matrices to immobilise tetravalent and/or trivalent actinides. To cite this article: N. Dacheux et al., C. R. Chimie 7 (2004). [Copyright &y& Elsevier]

NMR study of materials for immobilisation of nuclear waste. We present an NMR investigation on thorium phosphate hydrogen phosphate (TPHP), which is a precursor for thorium phosphate diphosphate (TPD), that is one of the four retained matrix for the study of the actinide immobilisation. As XRD cannot work, it is particularly difficult to define the TPHP chemical formula: Th2(PO4)2HPO4 H2O or Th4(PO4)4P2O7 3 H2O. In this contribution, we show that a local technique as NMR can bring an answer to this problem. To cite this article : J. Emery et al., C. R. Chimie 7 (2004). [Copyright &y& Elsevier]

Some thorium phosphates have been synthesised: caesium–thorium phosphate, barium–thorium phosphate, thorium fluoride phosphate, thorium sulphate phosphate, and thorium silicate phosphate, starting from aqueous solutions. Their chemical composition was established by Electron Probe Microanalysis (EPMA) and/or by Particle Induced X-ray Emission (PIXE). The X-rays diffraction patterns and/or the infrared spectra are given for these series of thorium phosphate. [Copyright &y& Elsevier]

Applies three procedures using two alpha liquid scintillation spectrometers and two scintillation cocktails for the determination of alpha-emitter low activities. Determination of the limit of detection, the resolution, the separation factor, and the Fischer coefficient; Isotopic measurements.

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]

Determines activities of thorium, uranium, plutonium, americium, and curium at very low levles by a new ... liquid scintillation system (PERALS). How the limit of detection for these nuclides was determined; Study methodology; Results showing the validity of the PERALS system; Reasons why a process for separation of the five actinides as quickly and easily as possible was developed; What the study concluded.

This work is the first attempt to prepare Nd1−xCaxUxPO4 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 Nd1−xCaxUxPO4·nH2O 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-UP2O7 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. [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]

• An in situ Raman monitoring of isotopically-labeled studtite growth was performed. • The link between studtite formation mechanism and radiolysis effects was described. • The preservation of the peroxide bond between H 2 O 2 and studtite was demonstrated. • The oxidation mechanism of uranium dioxide is detailed. Studtite [(UO 2)(O 2)(H 2 O) 2 ]·2(H 2 O) is a secondary phase precipitating during the alteration of uranium-bearing materials. The mechanistic link between the formation of uranyl and peroxide bonds in the solid and the nature of the oxidizing species produced by water radiolysis remains to be elucidated. In order to improve our understanding of these mechanisms, an original experimental methodology in the presence of 18O isotopes and Raman spectroscopy has been developed. It appears that there is a direct chemical relationship between the peroxo ligands inside studtite and the peroxide entities of H 2 O 2 molecule into the solution. The link between H 2 O 2 formation mechanism in solution by the radiolysis of water, the nature of the radiation and the isotopy of the peroxo ligands inside studtite has been described thanks to a coherent set of experimental data. For the uranyl UO 2 2+ ions, the characterization of its isotopy allows to specify the mechanism of oxidation at the UO 2 /water interface. The isotopic configurations observed for the uranyl ion inside studtite, can be explained by assuming an oxidation mechanism of UO 2 involving both a simple transfer of electrons by interaction with H 2 O 2 and the incorporation of oxygen atoms from the solution into the fluorite structure via OH° radicals. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, the influence of the working atmosphere on the sinterability and chemical durability of Nd-doped UO 2 mixed oxides was investigated. To this end, the starting powder was first prepared by a hydroxide co-precipitation route, resulting in a nano-sized granulometry combined with a high specific surface area. The powders were then converted to oxides by heating and sintered in pellet form at 1600°C under an argon or reducing (Ar-4 %H 2) atmosphere. The use of argon or reducing atmosphere resulted in very different densification pathways and final microstructures. The reducing sintering atmosphere hindered the uranium (IV) oxidation that could occur at high temperature, leading to the formation of U 3 O 8 , as was the case when working under argon atmosphere. Regarding the microstructure of the sintered pellets, the use of an argon sintering atmosphere resulted in an average grain size ten times larger than that of a reducing sintering atmosphere, while macroscopic properties such as relative density, porosity and homogeneity of cation distribution at the pellet scale remained the same. Nevertheless, a slight local enrichment of Nd at the grain boundaries was observed for the pellet sintered under Ar-4 %H 2. In a second step, the study of the chemical durability of these sintered samples showed a significant influence of the sintering atmosphere on the dissolution kinetics and mechanism. These differences could be related to the microstructural properties of the pellets, i.e. the average grain size and the occurrence of grain boundaries. The cation distribution in the pellets could also influence their chemical durability, such as local Nd enrichment, the formation of defects in the oxygen sublattice and the presence of a different fraction of U(V) depending on the sintering atmosphere, as shown by HERFD-XANES measurements. The use of reducing or argon sintering atmospheres could even direct the charge compensation mechanisms that occur into the solid, thereby simultaneously affecting the sinterability and chemical durability of the samples. [Display omitted] [ABSTRACT FROM AUTHOR]

This work aims to investigate the behavior of Mo in UO 2± x for being an abundant fission product with a high fission yield and a complex speciation linked to its interaction with the fuel and other fission product elements. UO 2 -based model compounds containing different Mo contents (between 0 and 15 mol%) were synthesized by a wet-chemistry route using hydroxide precipitation. The recovered powders were converted to oxides, pelletized, and sintered to obtain densified pellets of UO 2 incorporating Mo. PXRD analyses and Rietveld refinement calculations indicate that Mo has an almost negligible solubility in the fluorite structures of both UO 2 and UO 2+ x samples. SEM, TEM, and EDX characterizations of the produced UO 2 + Mo pellets revealed the homogeneous distribution of nanosized metallic Mo particles of spherical geometry inside and outside the UO 2 grains and throughout the whole sample pellets, whatever the amount of Mo added, thus confirming that the solubility of Mo in the fluorite structure is way below 0.6 mol% Mo in accordance with the PXRD results. The microstructural properties of the UO 2 + Mo pellets, including density, porosity, Mo particle size distribution, and UO 2 grain size variation with Mo content, were also determined. The addition of Mo to UO 2 reduced the UO 2 grain size as compared to UO 2 grains in pure pellets, and Mo thus plays an inhibiting role in the UO 2 grain growth during sintering. The produced UO 2 + Mo pellets exhibit a microstructure similar to that of the real spent nuclear fuel, except that the UO 2 grains are smaller. The Mo metallic nanoparticles in these simplified UO 2 -based model compounds of controlled microstructure could be harnessed as surrogates of the Mo-rich ε -phase metallic nanoparticles in the real spent nuclear fuel for future studies. [ABSTRACT FROM AUTHOR]

The AnP2O7 diphosphates (An = Th, U, Np, Pu) have been synthesized by various routes depending on the stability of the AnIV cation and its suitability for the unusual octahedral environment. Synchrotron and X-ray diffraction, thermal analysis, Raman spectroscopy, and 31P nuclear magnetic resonance reveal them as a new family of diphosphates which probably includes the recently studied CeP2O7. Although they adopt at high temperature the same cubic archetypal cell as the other known MP2O7 diphosphates, they differ by a very faint triclinic distortion at room temperature that results from an ordering of the P2O7 units, as shown using high-resolution synchrotron diffraction for UP2O7. The uncommon triclinic-cubic phase transition is first order, and its temperature is very sensitive to the ionic radius of AnIV. The conflicting effects which control the thermal variations of the P-O-P angle are responsible for a strong expansion of the cell followed by a contraction at higher temperature. This inversion of expansion occurs at a temperature significantly higher than the phase transition, at variance with the parent compounds with smaller MIV cations in which the two phenomena coincide. As shown by various approaches, the P-Ob-P linkage remains bent in the cubic form. [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]

Abstract: High temperature X ray diffraction performed on recently discovered orthorhombic Th2O(PO4)2 shows a continuous linear thermal contraction (−1.6×10−6 °C−1) in 20–600°C range and a near-zero expansion at higher temperatures resulting from a dual structural deformation involving oxygen oscillations and inter-cations repulsions. Although similar mechanisms were observed in isotypic Zr2O(PO4)2 (+1.5×10−6 °C−1) and U2O(PO4)2 (−1.4×10−6 °C−1), those observed in Th2O(PO4)2 are particularly intense because of the high ionic radius of tetravalent thorium. [Copyright &y& Elsevier]

To underline the potential links between the crystallization state and the microstructure of powdered cerium—neodymium oxides and their chemical durability, several CeIV1-xNdIIIxO2-x/2 mixed dioxides were prepared in various operating conditions from oxalate precursors and then leached. The powdered samples were first examined through several physicochemical properties (crystallization state and associated crystallite size, reactive surface area, porosity…). The dependence of the normalized dissolution rates on various parameters (including temperature, nitric acid concentration, crystallization state) was examined for pure CeO2 and Ce1-xNdxO2-x/2 solid solutions (with x = 0.09 and 0.16). For CeO2, either the partial order related to the proton activity (n = 0.63) or the activation energy (EA = 37 kJ∙mol-1) suggested that the dissolution was mainly driven by surface reactions occurring at the solid—liquid interface. The chemical durability of the cerium-neodymium oxides was also strongly affected by chemical composition. The initial normalized dissolution rates were also found to slightly depend on the crystallization state of the powders, suggesting the role played by the crystal defects in the dissolution mechanisms. On the contrary, the crystallite size had no important effect on the chemical durability. Finally, the normalized dissolution rates measured near the establishment of saturation conditions were less affected, which may be due to the formation of a gelatinous protective layer at the solid/liquid interface. [ABSTRACT FROM AUTHOR]

Abstract: The AXO4 monazite-type compounds form an extended family that is described in this review in terms of field of stability versus composition. All the substitution possibilities on the cationic and anionic sites leading to the monazite structure are reported. The phosphate, vanadate, chromate, arseniate, sulphate and silicate families are described and the unit-cell parameters of pure compounds and solid solutions are gathered. The stability limits of the monazite-type structure are discussed versus several models generally correlated with geometric criteria. The effects of physico-chemical parameters such as pressure, temperature and irradiation on the monazite-type structure stability are also discussed. The structural relationships between the monazite structure and the related structures (zircon, anhydrite, barite, AgMnO4, scheelite and monoclinic BiPO4, CaSeO3, rhabdophane and SrNp(PO4)2) are described. [Copyright &y& Elsevier]

Abstract: We report the synthesis of thorium diphosphate ThP2O7 and its study by Raman spectroscopy and Rietveld analysis. This compound has been found to be closely related to the zirconium diphosphate type, with space group Pa-3 and a=8.7601(3)Å. No superstructure was observed. The metastability of ThP2O7 appears to stem from the six-fold oxygen environment of ThIV, a unique case in the structural chemistry of this cation. [Copyright &y& Elsevier]

The dissolution of ThO2 powdered samples was examined in various conditions of pH and concentration of anions in the leachate. The first part of this paper describes the influence of pH on the dissolution of ThO2 in both nitric and hydrochloric media. The partial order relative to the proton concentration and the apparent normalized dissolution rate constants were determined. The second part of the paper describes the influence of other ligands such as perchlorate, chloride, sulfate, and hydrogen peroxide on the dissolution kinetics (at pH 1). An increase of RL was observed correlatively with the increase of complexing affinity of the ligand with Th. While nitric and hydrochloric media, which are weakly complexing, lead to RL values with the same order of magnitude as those for perchlorate media, the presence of sulfate or peroxide in the leachate significantly enhances the dissolution of ThO2. Consequently, the dissolution mechanism can be explained by the weakening of ThO bonds through the formation of surface complexes at the solid/liquid interface, which enhance the detachment and thus accelerate the global dissolution. In addition, the dissolution of Th0.87Pu0.13O2 solid solution was also examined. The increase of the dissolution kinetics of Th0.87Pu0.13O2, in comparison with that of ThO2, is considered to be caused by the presence of hydrogen peroxide formed by radiolysis of the leachate. Moreover, the redox properties of plutonium in acidic media, like disproportionation of Pu(IV) and Pu(V) and reduction of Pu(VI) and Pu(IV) in Pu(III) by H2O2, probably increase the dissolution of plutonium. [ABSTRACT FROM AUTHOR]

Abstract: Unlike thorium which incorporation is easy and quantitative in the britholite structure, that of tetravalent uranium appears rather difficult even after heating at high temperature. Indeed, only about 3.5wt% of uranium (instead of 10.3wt% expected) are incorporated in the britholite structure when using a manual grinding of the initial mixture while it is increased to 5.9wt% when performing mechanical grinding/heating cycles. The optimized grinding and heating conditions can be fixed to 15min at 30Hz and to 1400°C for 6h, respectively. All the samples prepared at 1400°C are found to be composed by (Nd,U)-britholite and calcium uranate CaU2O5+y which formation results from that of CaUO4 above 800°C consequently to the direct reaction between UO2 and CaO. The incorporation of tetravalent uranium in (Nd,U)-britholites begins simultaneously to the transformation of CaUO4 into CaU2O5+y above 1100°C. Due to these redox reactions, the incorporation of uranium remains partial even though it is significantly increased at 1400°C and mainly proceeds through diffusion phenomena. Two main methods can be used to improve significantly the incorporation of uranium in (Nd,U)-britholites. The first one deals with the compaction of the powdered initial mixture prior to perform the heating treatment at 1400°C. The second one is based on the simultaneous incorporation of tetravalent thorium and uranium, leading to the formation of (Nd,Th,U)-britholites. Like for uranium, such methods could be of significant interest in the field of the incorporation of other tetravalent actinides, e.g. neptunium or plutonium, which could be also stabilized in several oxidation states. [Copyright &y& Elsevier]

Abstract: β-Thorium phosphate diphosphate (β-TPD), considered as a very promising radwaste storage material, was obtained from thorium phosphate hydrogenphosphate hydrate (TPHPH) precursor through dehydration and hydrogen phosphate condensation. The structures of TPHPH, intermediate α-thorium phosphate diphosphate (α-TPD) and its hydrate (α-TPDH) have been resolved ab initio by Rietveld analysis of their synchrotron diffraction patterns. All were found orthorhombic (space group Cmcm) and similarly composed of [ThPO4]4 4+ slabs alternating with disordered layers hosting either [HPO4·H2O]2 4− (TPHPH), [P2O7·2H2O]4− (α-TPDH), or [P2O7]4− (α-TPD), unlike the 3D structure of β-TPD. The diphosphate groups of α-TPD and α-TPDH are strongly bent. The irreversible transition to the final β-TPD consists in a shearing of the slabs and a reduction of the interslabs cavities that explains the stability of this high-temperature form. [Copyright &y& Elsevier]

Abstract: In the field of the immobilization of tri- and tetravalent minor actinides, apatites and especially britholites were already proposed as good candidates. In order to simulate tetravalent minor actinides, the incorporation of thorium, through dry chemical routes, was studied in britholite samples of general formula Ca9Nd1−x Th x (PO4)5−x (SiO4)1+x F2. The study showed that the incorporation of thorium was effective whatever the thorium reagent used or the grinding conditions considered. Nevertheless, it appeared necessary to use mechanical grinding (30Hz, 15min) before heating treatment (T =1400°C, 6h) to improve the reactivity of powders and the sample homogeneity. In these conditions, the incorporation of thorium in the britholite structure occurred above 1100°C. The heating treatment at 1400°C led to single phase and homogeneous compounds. This work also underlined the necessity to prefer the coupled substitution instead of (Nd3+, F−)⇔(Th4+, O2−) in order to prepare pure and single phase samples in all the range of composition examined. [Copyright &y& Elsevier]

The sintering of uranium oxide powders prepared by hydrothermal conversion of oxalates was studied. A dilatometric study first showed that among the synthesis conditions, only the pH largely impacted the densification, notably through the powders morphology. Three samples (single crystals – pH = 1, microspheres – pH = 2 and nano‑powders – pH = 8) were further selected to study their sintering behaviour. Densitometric and granulometric data then allowed establishing sintering maps. For single crystals and microspheres, densities above 96 %TD were obtained at 1700 °C. In contrast, only 2–8 hours at 1500 °C were needed to yield comparable results for nano-powders. These observations confirm that the hydrothermal conversion of oxalates can be considered as a promising route for the synthesis of actinide oxides, which can subsequently be sintered directly. [ABSTRACT FROM AUTHOR]

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 10−2 mol L−1. Successive filtration processes allowed evaluation of their size, which was found to be smaller than 6 nm. Their structural characterization by XAS evidenced that their structure was similar to those identified for the other tetravalent actinide-silicate colloidal systems like thorium, uranium and neptunium. Their formation could explain the increase of plutonium solubility usually observed in alkaline silicate-rich solutions and could affect the plutonium mobility as a result in contaminated sites or in other environmental permeable media. [ABSTRACT FROM AUTHOR]

We provide new experimental data on monazite, xenotime and U–Th-bearing cheralite solubility in slightly peralkaline to peraluminous granitic melts using dissolution and reverse (i.e. recrystallization after dissolution) experiments in water-saturated and flux-bearing (P + F + Li) granitic melts, at 800 °C and 200 MPa. Although a positive correlation between rare earth element (REE) solubility and melt peralkalinity is confirmed, monazite solubilities reported here are much lower than the values previously published. We suggest that the presence of elevated phosphorus concentrations in our melts depresses monazite solubility, principally because phosphorus complexes with Al and alkali, which normally depolymerize the melt through the formation of non-bridging oxygens. The new solubility data provide an explanation for the very low REE concentrations generally encountered in phosphorus-bearing peraluminous granites and pegmatites. This accounts for the compatibility of REE in peraluminous systems, as the early crystallization of REE-bearing minerals (mainly monazite and zircon) leads to progressive REE depletion during liquid differentiation. In addition, dissolution and reverse experiments on U–Th-bearing cheralite–monazite display liquid–liquid immiscibility processes in our slightly peralkaline glass. The immiscible liquid forms droplets up to 10 µm in diameter and hosts on average 35 wt% P2O5, 25–30 wt% F, 22 wt% Al2O3, 4 wt% CaO, 5 wt% Na2O, 2 wt% La2O3, and 12 wt% ThO2 + UO2. We believe that the droplets formed during the runs and may have coalesced to larger droplets during quenching. We suggest that liquid–liquid immiscibility is a possible mechanism of REE concentration in highly fluxed melts and should be considered in natural systems where REE are extremely concentrated (up to thousands of µg g–1) in magmatic rocks. [ABSTRACT FROM AUTHOR]

• Evidencing of phase separation behaviors of using thermosensitive and flocculation properties. • Investigation of sorption properties in acidic Gd/Th/U solutions. • Highlighting the high selectivity for thorium and uranium. In this work, the investigation of thermosensitive behavior of P(CPAAm6C) highlighted the formation of particles with an average radius of about 1940 nm in nitric acid. While h P(CPAAm6C) does not exibit thermosensitive behavior, it was shown this polymer had floculation properties. The selective sorption properties of two valuable carbamoylmethylphosphonated-based polymers regarding to Th/U/Gd mixtures were studied. The influence of the total concentration of cations on the sorption of each cation was investigated. In the case of P(CPAAm6C), sorption experiments evidenced that the Q max (Th) should be higher than 1.5 mmol·g−1. Sorption results with h P(CPAAm6C) evidenced that Q max (U) should be higher than 2.75 mmol·g−1. Moreover, the results highlighted the different selectivities between the carbamoylmethylphosphonate and carbamoylmethylphosphonic diacid functions. The phase separation behavior and selectivity will be of great interest for the design of a future environmental-friendly process for hydrometallurgical treatments of rare earth ores. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 UO2. Thus, the behaviour of UO2 in SNF must be understood and evaluated under the weathering conditions of geologic disposal, which extend to periods of hundreds of thousands of years. There is ample evidence from nature that many uranium deposits have experienced conditions for which the formation of coffinite, USiO4, has been favoured over uraninite, UO2+x, during subsequent alteration events. Thus, coffinite is an important alteration product of the UO2 in SNF. Here, we present the first evidence of the formation of coffinite on the surface of UO2 at the time scale of laboratory experiments in a solution saturated with respect to amorphous silica at pH = 9, room temperature and under anoxic conditions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Brannerite (UTi2O6) is among the major uranium-bearing minerals found in ore deposits, however as it has been long considered as a refractory mineral for leaching it is currently disregarded in ore deposits. Brannerite is found in a variety of geological environments with the most common occurrences being hydrothermal and pegmatitic. On the basis of scanning electron microscopy observations coupled with electron probe micro-analyses and laser ablation inductively-coupled plasma mass spectrometer analyses, this study describes the morphological features and the major- and trace-element abundances of brannerite samples from five hydrothermal and five pegmatitic localities across the world. Mineral compositions are also compared with observations from transmission electron microscopy and Raman spectrometry showing that brannerite is amorphous. Significant results include the definition of substitution trends and REE patterns, which are characteristics of either an occurrence or genetic type (hydrothermal and pegmatitic). Hence, in combination, it is possible to obtain reliable constraints for establishing a geochemical classification of brannerite. Inferred fingerprints have direct implications for forensic science and the exploration industry; they also contribute to a better understanding of metallogenic processes and to optimising the extraction of uranium. [ABSTRACT FROM AUTHOR]

The direct sintering of Nd 1-2x Ca x Th x PO 4 ·nH 2 O rhabdophanes (x = 0 – 0.1) was achieved for the first time resulting in homogeneous high-density monazite-cheralite pellets. Thanks to their large specific surface area, rhabdophane precursors led to a lower shrinkage temperature compared to samples synthesized through solid-state reactions. The associated activation energy was found between 360 ± 90 and 530 ± 90 kJ.mol−1, depending on the thorium incorporation. Meanwhile, sintering map was built up, showing that densification predominated for T ≤ 1200 °C. Conversely, the complete densification took place at 1400 °C concomitantly to grain growth and elimination of open porosity. Moreover, Th-Ca coupled substitution seemed to inhibit both densification and grain growth as the average grain size dropped down one order by one order of magnitude between x = 0.0 and x = 0.1. However, these differences did not affect microhardness, which reached 4.9 ± 0.8 GPa whatever the chemical composition tested. [ABSTRACT FROM AUTHOR]

New stability constants for NpV and PuV carbonates have been established by coupling capillary electrophoresis (CE) and ICP‐MS at the ionic strength of 0.202 mol/kgwater in NaClO4 and at various temperatures. The thermodynamic data [variation of the molar enthalpy, ΔrHm(T°) and entropy, ΔrSm(T) of reaction] associated with the formation of the AnO2CO3– (An = Np, Pu), AnO2(CO3)23–, and AnO2(CO4)35– complexes were determined. A drastic decrease in molar entropy was observed between the second and third complexes, suggesting important structural changes. Ab initio molecular dynamics showed that the 1:3 complex is formed with two carbonate ligands located at a shorter distance and one at a longer distance, suggesting that the closer ligands are coordinated in a bidentate fashion to the plutonyl(V) moiety whereas the third ligand is monodentate. The variation of the electrophoretic mobilities as function of charge/size ratio confirms unambiguously that the triscarbonate complex of pentavalent plutonium structurally differs from that of hexavalent plutonium where all carbonates anions are bidentate. [ABSTRACT FROM AUTHOR]

• Sorption using a new carbamoylmethylphosphonic acid functionalized polymer. • Investigation of sorption efficiency for gadolinium ions in various operating conditions. • Mechanism of sorption combining solvation and ion-exchange. • Selective sorption of Gd(III) ions in Gd/Ni mixtures. • Selective flocculation. Sorption properties of a new hydrosoluble polymer containing carbamoylmethylphosphonic diacid ( h cmp) moieties were investigated. This polymer was obtained by hydrolysis of the poly(diethyl-6-(acrylamido)hexylcarbamoylmethylphosphonate) (P(CPAAm6C)). The study allowed the determination of an experimental maximum capacity Q max equal to 1.5 mmol⋅g−1 which is three times higher than the one obtained for P(CPAAm6C), as previously described (i.e. 0.6 mmol⋅g−1). Experimental data were fitted with the Langmuir equation and parameters Q max L and K L , were found to be 1.72 ± 0.067 mmol⋅g−1 and 0.302 ± 0.063 L⋅mmol−1, respectively. Additionally, this capacity depends on different parameters (concentration of metals and sorbent, pH, and ionic strength). The study proved that the h P(CPAAm6C) was selective of gadolinium in Gd/Ni mixtures. Additionally, the precipitation of the polymer was observed simultaneously to the Gd complexation. The results described in this paper proved that the h P(CPAAm6C) is a very promising hydrosoluble material for the purification of lanthanide elements or in the field of nuclear fuel reprocessing by using new solvent free and low energy consuming processes. [ABSTRACT FROM AUTHOR]

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. This study allows the determination of optimized conditions for the hydrothermal synthesis of pure CeSiO4 (A-Ce2Si2O7 or Ce4.67(SiO4)3O as starting precursors, nitric medium, pH = 7, 7 days at 150 °C). The in situ low oxidation rate of Ce(iii) into Ce(iv) was a key parameter to consider in order to avoid the presence of CeO2 in the final mixtures. [ABSTRACT FROM AUTHOR]

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(O2)(SO4)(H2O)2 compound, which crystallizes in a new structure type in the space group Pna21 of the orthorhombic crystal system. Particularly, the peroxide ligand coordinates to the Th cations in an unusual μ3‐η2:η2:η2 bridging mode, forming an infinite 1D chain decorated with sulfato ligands exhibiting simultaneously monodentate and bidentate coordination modes. [ABSTRACT FROM AUTHOR]

Even though CeSiO4 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 CeSiO4 could not be obtained starting from Ce(iv) reactants. An optimal set of conditions was defined to prepare single phase samples. Pure CeSiO4 was obtained through a hydrothermal treatment at 150 °C using a starting mixture of 1 mol L−1 Ce(iii) nitrate and Na2SiO3 solutions and by adjusting the initial pH to 8. The chemical limitations observed during the synthesis of CeSiO4 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. [ABSTRACT FROM AUTHOR]

Abstract Coffinite (USiO 4) is a common uranium-bearing mineral of roll-front uranium deposits. This mineral can be identified by the visible near infrared (Vis-NIR) portable field spectrometers used in mining exploration. However, due to the low detection limits and associated errors, the quantification of coffinite abundance in the mineralized sandstones or sandy sediments of roll-front uranium deposits using Vis-NIR spectrometry requires a specific methodological development. In this study, the 1135 nm absorption band area is used to quantify the abundance of coffinite. This absorption feature does not interfere with NIR absorption bands of any other minerals present in natural sands or sandstones of uranium roll-front deposits. The correlation between the 1135 nm band area and coffinite content was determined from a series of spectra measured from prepared mineral mixtures. The samples were prepared with a range of weighted amounts of arenitic sands and synthetic coffinite simulating the range of uranium concentration encountered in roll-front uranium deposits. The methodology presented in this study provides the quantification of the coffinite content present in sands between 0.03 wt% to 1 wt% coffinite with a detection limit as low as 0.005 wt%. The integrated area of the 1135 nm band is positively correlated with the coffinite content of the sand in this range, showing that the method is efficient to quantify coffinite concentrations typical of roll-front uranium deposits. The regression equation defined in this study was then used as a reference to predict the amount of natural coffinite in a set of mineralized samples from the Tortkuduk uranium roll-front deposit (South Kazakhstan). Highlights • Field Vis-NIR spectroscopy easily detects coffinite in arenitic to arkosic sands. • Coffinite can be quantified for concentrations between 0.03 and 1.0 wt%. • Coffinite represents the main uranium-bearing mineral of studied roll-front deposit. [ABSTRACT FROM AUTHOR]

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 (234U/238U, 235U/238U and 236U/238U) 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. The 236U/238U ratios were also corrected for the presence of 235U hydrides and tailing of the 238U+ peak. Results: 236U/238U ratios were successfully measured in micrometer‐sized particles from the NBS U050 certified standard material with a 236U/238U ratio of ~5 × 10−4. The analysis of 77 natural uranium sub‐μm‐sized particles yielded a very good trueness with respect to the expected 234U/238U and 235U/238U ratios, while the measurement errors for single particles ranged from −2.7% to +2.1% for 235U/238U and from −17% to +33% for the 234U/238U ratios. Their relative combined standard uncertainties ranged from 3.3% to 32.8% and from 0.4% to 4.0% for 234U/238U and 235U/238U ratios, respectively. In addition, extremely low detection limits, in the attogram range, were achieved. Conclusions: This study demonstrates that coupling of a ns‐laser ablation system with a MC‐ICP mass spectrometer allows measurements of the isotopic composition in natural uranium particles of a few hundreds of nm with very good trueness, average combined standard uncertainties of ~1% for 235U/238U ratios and 12% for 234U/238U ratios, and detections limits of a few ag for minor isotopes. [ABSTRACT FROM AUTHOR]

Abstract A new method based on the precipitation of uranium(IV) and titanium(IV) hydroxide precursors was developed to prepare pure brannerite UTi 2 O 6 samples. In fact, U(IV) dissolved in HCl (6 mol L−1) was mixed to Ti (IV) alkoxyde before a basification step with an excess of NH 4 OH to obtain a highly reactive nanometric (U,Ti)(OH) 4 powder. The obtained powder was then dried under vacuum, pressed into pellets and finally fired at 1300 °C. This method led to the formation of pure brannerite in contrast to previous reported protocols, which showed the formation of impurities such us UO 2 and TiO 2. The refined unit cell parameters of UTi 2 O 6 led to a = 9.8113(2) Å, b = 3.7681(1) Å, c = 6.9232(1) Å, β = 118.94(1)° and V = 223.9(1) Å3. Highlights • Single phase brannerite UTi 2 O 6 samples have been prepared at 1300 °C under argon atmosphere. • One-step synthesis of brannerite has been developed from uranium-titanium hydroxide precursor. • XRD and spectroscopic characterization of the samples have been performed. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Hybrid materials are synthesized with sulfonate ligands. • A pillared strategy is used to extend the network. • Structure have been solved by single crystal X-Ray diffraction. • Materials have been tested in a coin cell as electrode material. Abstract The syntheses of sulfonate-based metal-organic frameworks (MOFs) following or not a pillared strategy using a linear N-donor ligand have been successfully developed. These MOFs based on Co, Ni or Mn present a layered structure separated by a solvent molecule of DMF or by a bridging ligand (4,4′-BiPyridine – bpy). Due to the presence of transition metals on the coordination nodes bridged by ligands (bpy), these materials have been tested as negative electrodes materials in a coin cell. It reveals that materials have capacities up to 350 mAh·g−1 and good electrochemical stability over 50 charge/discharge cycles. This strategy may shed light on designing new MOF-based electrode materials with the use of organic ligands structured in a hybrid material. [ABSTRACT FROM AUTHOR]

Abstract In order to study the effects of cationic homogeneity on the life cycle of Th 1- x U x O 2 ceramics, including sintering and reprocessing (dissolution) steps, five different ways of preparation were set up, going from the most homogenous oxalic co-precipitation to a mechanical mixture of the parent oxides. Dilatometric experiments evidenced a better sintering capability for the most homogenous compounds obtained through wet chemistry methods while dry chemistry routes led to poor density values (between 80 and 90 %TD). However, the introduction of an additional mechanical grinding step prior to the powders sintering systematically led to the homogenization of the systems. Improved homogeneity also provide a better chemical durability associated with the congruent release of thorium and uranium in solution during dissolution tests of Th 0.5 U 0.5 O 2 samples. However, heterogeneous samples led to incongruent behaviors that can be lowered by introducing a grinding step before the sintered samples preparation. Since the impact of the cationic homogeneity must be followed carefully during dissolution, in operando observations of evolving solid/solution interface by ESEM were performed. They allowed imaging the preferential dissolution of uranium-enriched zones and confirmed the significant impact over dissolution rate of the presence of chemical heterogeneities at the interface. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Sorption using a new polymer combining both sorption and thermosensitive properties. • Investigation of sorption efficiency for gadolinium ions in various operating conditions. • Mechanism of extraction by solvation effect with amido-phosphonate sites. • Selective sorption of Gd(III) ions in Gd/Ni mixtures. Abstract The sorption properties of gadolinium (Gd) by an original carbamoylmethylphosphonate (cmp)-based acrylamide polymer combining both chelating and thermosensitive properties namely poly(diethyl-6-(acrylamido)hexylcarbamoyl-methylphosphonate) (P(CPAAm6C)) are presented for the first time. For pH = 1 and at 20 °C, the maximum sorption capacity reached 0.6 mmol·g−1 for the highest Gd concentrations. Different parameters influencing the sorption of Gd were studied as pH, ionic strenght and temperature. The temperature was shown to influence both the sorption and the stability of the complex cmp-Gd formed by coordination mode with C O and P O functions. The selectivity of the cmp group for Gd was demonstrated in Gd/Ni mixture and shows its thermosensitive polymer is very promising for the development of selective sorption processes. [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]