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Highly reactive and nanosized Th1-x Yx O2-x/2 or Ce0.8 Ln0.2 O1.9 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.

International audience; Uranium dioxide containing 3 mol.% platinum group metals (PGMs) (Ru, Rh, Pd) was synthesized by hydroxide precipitation. The powders were converted to oxides, pelletized and sintered to prepare dense pellets of UO 2 incorporating PGMs particles. The characterization techniques performed revealed a microstructure similar to that of spent nuclear fuel (SNF). Dissolution tests in nitric acid demonstrated that in the presence of PGMs, the uranium dissolution rate was increased and the induction period was shortened. We used a new method based on the acquisition of Environmental Scanning Electron Microscopy (ESEM) micrographs recorded at three tilt angles and at different dissolution times. This method allowed the reconstruction of the topography of the solid/liquid interface. By monitoring the evolution of the solid/liquid interface during dissolution by means of 3D reconstructions, we were able to observe preferential dissolution zones in the vicinity of the PGMs particles and to determine microscopic dissolution rates for several regions of interest. PGMs particles were found mainly at the grain boundaries. In 0.1 M HNO 3 solution at 60˚C, the normalized dissolution rate for uranium at the grain boundaries reached R L (U) = (7 ± 1)×10-2 g.m-2 .d-1 , a value similar to the normalized dissolution rate determined for the whole image over the first 30 days of the experiment. This result showed that the dissolution occurred mainly at the UO 2 grain boundaries in the vicinity of PGMs particles. Furthermore, the 3D reconstructions of the solid/liquid interface were used to determine the evolution of the surface area of the pellet. By combining the weight losses determined at the macroscopic scale using the uranium concentrations in solution with the reactive surface area values, it was possible to estimate an effective normalized dissolution rate for the whole pellet.

International audience; An accurate method for the characterization of the topographic evolution of materials during dissolution at the submicrometric scale is reported. This method is based on the recording of Environmental SEM tilted images of a selected zone at the surface of a sample for various dissolution times. After each observation, a 3D surface reconstruction was made leading to a series of height maps of the selected zone of interest. One 3D reconstruction was compared to an AFM image of the same zone in order to estimate the accuracy of the heights determined using stereoscopic images. The maximum achievable resolution along the z-axis on the 3D reconstructions was found to be 38 nm. Several micro-structural parameters were extracted from the 3D-image series and their evolutions were related to the dissolution process. This method offers new insights for the monitoring of the changes occurring at the surface of a sample at the micrometer scale during dissolution or corrosion of the material.

International audience; The effect of metallic particles containing Platinum Group Metal (PGM) elements on the dissolution of sintered UO2 samples was examined through the development of multiparametric dissolution tests on sintered UO2 and UO2 + 3 mol.% PGM samples. This effect was found to be more important for CHNO3 < 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/UO2/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.

International audience; UO2 pellets were prepared by densification of oxides obtained from the conversion of the oxalate precursor. Then characterized in order to perform a multiparametric study of the dissolution in nitric acid medium. In this frame, for each sample, the densification rate, the grain size and the specific surface area of the prepared pellets were determined prior to the final dissolution experiments. By varying the concentration of the nitric acid solution and temperature, three different and successive steps were identified during the dissolution. Under the less aggressive conditions considered, a first transient step corresponding to the dissolution of the most reactive phases was observed at the solid/solution interface. Then, for all the tested conditions, a steady state step was established during which the normalised dissolution rate was found to be constant. It was followed by a third step characterized by a strong and continuous increase of the normalised dissolution rate. The duration of the steady state, also called “induction period”, was found to vary drastically as a function of the HNO3 concentration and temperature. However, independently of the conditions, this steady state step stopped at almost similar dissolved material weight loss and dissolved uranium concentration. During the induction period, no important evolution of the topology of the solid/liquid interface was evidenced authorizing the use of the starting reactive specific surface area to evaluate the normalised dissolution rates thus the chemical durability of the sintered pellets. From the multiparametric study of UO2 dissolution proposed, oxidation of U(IV) to U(VI) by nitrate ions at the solid/liquid interface constitutes the limiting step in the overall dissolution mechanism associated to this induction period.

Dissolution experiments have been performed in order to determine the solubility constant of coffinite, USiO4. Several assemblages of phases were used in under-saturated experiments performed in 0.1 mol L−1 HCl under Ar atmosphere, as well as in air. These samples were fully-characterized and were composed of either USiO4, solely, or USiO4 and additional oxide byproducts that resulted from the synthesis procedure. The solubility constant of coffinite was determined at 25 °C and 1 bar (log *KS°(USiO4, cr) = −5.25 ± 0.05), as well as the standard free energy of formation of coffinite (ΔfG°(298 K) = −1867.6 ± 3.2 kJ mol−1), which enables one to infer the relative stability of coffinite and uraninite as a function of groundwater composition. Geochemical simulations using PHREEQC 2 software and the Thermochimie data base indicate that coffinite precipitates at 25 °C under reducing conditions, at pH = 6, for H4SiO4(aq) concentration of 7 × 10−5 mol L−1 and U(OH)4(aq) concentration of 10−11 mol L−1. The ΔfG° value determined was used to calculate the standard free energy associated with the formation of coffinite from a mixture of uraninite and quartz. The value obtained (Δr,oxG° = 20.6 ± 5.2 kJ mol−1) indicates unambiguously that coffinite is less stable than the quartz + uraninite mixture at 25 °C. Geochemical simulations using PHREEQC 2 software indicate that coffinite precipitates in solutions supersaturated with respect to UO2(cr), but undersaturated with respect to UO2(am) in aqueous solutions with silica concentrations typical of groundwater. These favorable conditions during the formation of sedimentary uranium ore deposits, as well as slow dissolution kinetics, explain the common occurrence of coffinite.