Dissolution of mixed oxide powders (U, Pu)O2

Le principal objectif de cette étude est d'acquérir des données de base sur la dissolution de composés (U,Pu)O2 en appui à la compréhension des phénomènes intervenant lors des étapes de dissolution des combustibles MOX des filières de réacteurs à eaux légères et à neutrons rapides sodium. Les études antérieures, en particulier sur des combustibles MOX non irradiés, ont mis en évidence la complexité d'une compréhension des mécanismes de dissolution par une approche directe. En effet, la dissolution dépend d'un grand nombre de paramètres, qui sont principalement les conditions chimiques d'attaque (acidité, température, ...), mais surtout les caractéristiques physico-chimiques de la pastille considérée (teneur en plutonium, homogénéité de la teneur en plutonium, microstructure, géométrie, ...), la majorité d'entre elles étant fortement dépendantes du procédé de fabrication utilisé. Pour éviter l'obtention de réponses moyennées du fait de la présence d'une hétérogénéité de la répartition en Pu au sein des pastilles, on se propose de réaliser une étude sur des poudres de composés monophasés de stœchiométrie parfaitement définie (U et Pu) et de morphologie parfaitement déterminée. Une approche par étapes a permis de déterminer les différents paramètres clé déterminant les cinétiques de dissolution de ces oxydes d’actinides (surface spécifique, taille des cristallites, teneur en Pu, activité des ions nitrate, température de dissolution, énergie d’activation).Une loi cinétique globale permettant de décrire les cinétiques de dissolution des oxydes U1-xPuxO2 a été établie à partir de 45 essais différents (avec 0≤x≤1). Elle décrit les cinétiques de dissolution obtenues à toutes teneurs en plutonium et morphologies de l’oxyde, dans des gammes de températures variant de 50 à 95°C et des intervalles d’acidités variant de 1,5 à 8,5 mol/L. Le modèle ainsi créé décrit assez précisément la cinétique de dissolution de n’importe quel oxyde U1-xPuxO2 alors qu’il existe plus de cinq ordres de grandeurs d’écart entre les cinétiques de dissolution de l’UO2 et du PuO2. Des essais complémentaires sur des composés monophasés ont été menés validant le caractère prédictif du modèle.The main objective of this study is to acquire data on the dissolution of (U, Pu)O2 compounds to support the understanding of the phenomena occurring during the dissolution steps of MOX fuels irradiated in light water or sodium fast reactors. Previous studies, in particular on unirradiated MOX fuel, have highlighted the complexity of understanding the dissolution mechanisms through a direct approach. Indeed, the dissolution depends on a large number of parameters, which are mainly chemical dissolution parameters (acidity, temperature…). But it also depends on the physico-chemical characteristics of the fuel pellets (plutonium content, homogeneity of the plutonium content, microstructure, geometry...), a majority of which being highly dependent on the manufacturing process used. To avoid getting averaged responses due to the presence of heterogeneity in the Pu distribution in pellets, it is proposed to carry out a study on single-phase compounds in the shape of powders characterized by a well-defined stoichiometry (U and Pu) and a perfectly determined morphology. A step approach allowed the determination of the key parameters controlling the dissolution kinetics of these actinide oxides (specific surface area, crystal size, Pu content, activity of nitrate ions, dissolution temperature).A global kinetics law describing the dissolution kinetics of U1-xPuxO2 oxides was established from 45 dissolution tests (with 0≤x≤1, [HNO3] and temperature ranging from 1.5 to 8.5M and from 50 to 95°C respectively). Despite the 5 orders of magnitude between dissolution kinetics of UO2 and PuO2, the model shows a good precision. Additional dissolution tests were conducted on different single-phase oxide powders to validate the predictive quality of this model

Impact of the plutonium content on dissolution kinetics of (U1−x_{1-x}Pux_x)O2±δ_{2±δ} powders

International audienceSeveral (U$_{1-x}$Pu$_x$)O$_{2±δ}$ powders with different morphologies and different plutonium contents were synthetized using a sol-gel route. All the powders were fully characterized to quantify their structural parameters. After understanding and verification of the morphological effects on mixed oxide dissolution, this study consists of quantification of effect of plutonium content on dissolution kinetics using nitric acid 8.5 M at 95 °C. Significant differences in dissolution kinetics were observed. Indeed, the plutonium content is a key parameter that has to be considered in studies of (U$_{1-x}$Pu$_x$)O$_{2±δ}$ dissolution kinetics. A kinetics law was established and validated for dissolutions within these particular experimental conditions

Effect of the Microstructural Morphology on UO2 Powders

International audienceSeveral UO2 powders with different morphologies were synthetized and characterized. Three different morphologies were synthesized thanks to sol gel process (big heap of about 200μm wide consisting of sintered crystallites) on the one hand, and to oxalic precipitations (one square platelet morphology and one hexagonal stick morphology) on the other hand. Significant differences in dissolution kinetics were observed. Therefore, the morphology of the powders was found to be a key parameter that has to be considered in studies of UO2 dissolution kinetics. The second part of the study consists in dissolving in nitric acid in in the same operating conditions three UO2 powders having different crystallites sizes. It was shown that dissolution kinetics is dependent on the morphology but also on the powder stoichiometry

Speciation of residual carbon contained in UO2_2

International audienceUO$_2$ powders were synthesized thanks to oxalic precipitation (platelet morphology) and sol-gel route and completely characterized. A secondary phase was found in addition of the oxide depending on the calcination atmospheres. This phase has been identified by Raman spectroscopy as graphitic material (i.e. carbon-based secondary compound) and quantified by thermogravimetric analyses. Its amount varies with the calcination atmosphere. The presence of this secondary phase has no significant effect on the lattice parameter and its specific surface area

Uranium removal from mining water using Cu substituted hydroxyapatite

International audienceIn 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