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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]

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]

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 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]

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]

The standard solubility constant of TPHPH, Th 2 (PO 4 ) 2 (HPO 4 )·H 2 O was evaluated at 298 K by performing dissolution experiments of a synthetic sample in different nitric acid solutions at a constant ionic strength of 1 mol L −1 . The average value of the solubility constant and the associated expanded uncertainty U( log 10 K s , 0 ∘ (298 K), 0.95 level of confidence) were evaluated to log 10 K s , 0 ∘ (TPHPH, 298 K) = −(71.2 ± 1.3), while the Δ f G ∘ (TPHPH, 298 K) value reached −(5200 ± 23) kJ·mol −1 . From the experiments performed at different temperatures, the standard molar enthalpy as well as the standard molar entropy of the dissolution reaction was determined. The solubility of TPHPH was found to be retrograde over the investigated range of temperatures (298–363) K. The values of the standard enthalpy of formation and the standard molar entropy of TPHPH are equal to: Δ f H ∘ ( TPHPH ) = - ( 5640.5 ± 16.0 ) kJ · mol - 1 and S m = (298 ± 68) J K −1 mol −1 , respectively. [ABSTRACT FROM AUTHOR]

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