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

Actinide mixed oxide conversion by advanced thermal denitration route

International audienceIn the framework of generation IV development for nuclear reactors, actinide mixed oxides are considered for multirecycling plutonium fuels and for transmutation targets of minor actinides. In this context, new processes are being developed for either the synthesis of mixed uranium-plutonium oxide compounds for MOx fuel or uranium-americium target fabrications. The main purposes are to simplify and step up industrial processes as well as to decrease actinide dust dispersion, and liquid effluent and gaseous releases. Among options for conversion route, a novel and patented advanced thermal denitration in presence of organic additives was established successfully to synthesize UO$_{2+δ}$, U$_{0.55}$Pu$_{0.45O2±δ}$, and U$_{0.9}$Am$_{0.1}$O$_{2-δ}$ oxides. Here, we describe the different intermediate steps of this process together with the characterization of the oxides obtained. The data highlight several advantages of this new route for actinide conversion

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

PROCÉDÉ DE PRÉPARATION D'UN MATÉRIAU ACTIF SPÉCIFIQUE POUR LA PRODUCTION D'HYDROGÈNE OU DE MONOXYDE DE CARBONE VIA LA DISSOCIATION DE L'EAU OU DU DIOXYDE DE CARBONE

L’invention a trait à un procédé de préparationd’un matériau actif utile pour la production d’hydrogènevia la dissociation d’eau ou de monoxyde de carbone viala dissociation du dioxyde de carbone, lequel matériauactif comprend des microsphères de cérine, ledit procédécomprenant les étapes suivantes :a) une étape de mise en contact d’une solution aqueusecomprenant du cérium sous forme ionique et, lorsque lacérine à préparer est une cérine substituée, le ou lesautres éléments métalliques sous forme ionique, avecune résine échangeuse d’ions, moyennant quoi le cériumet le ou les éventuels autres éléments métalliquess’échangent avec les ions mobiles de la résine ;b) une étape de traitement thermique de ladite résinesous atmosphère oxydante pour former le matériau actif

Influence of milling on structural and microstructural properties of cerium oxide: Consequence of the surface activation on the dissolution kinetics in nitric acid

International audienceCeria (CeO2) is known as a refractory oxide for dissolution in nitric acid, since the leaching reaction is thermodynamically unfavorable, except when it is complexed by nitrates but with very slow kinetics. To enhance dissolution, surface activation was achieved using high-energy milling. With the mechanically-activated cerium oxide, leaching in nitric acid reached 36%. The mechanical activation of the solid caused structural and microstructural changes (particle size, specific surface area, crystallite size, lattice strain, defects…). After one hour, the cleavage induced by energetic milling generated two populations: nanoparticles and grains containing defects like dislocations. Beside crystallite size and micro-strain evaluation using X-ray diffraction, cerium oxidation state was measured by Electron Energy-Loss Spectroscopy (EELS) analyses while linear defects were pictured by Transmission Electron Microscopy (TEM) observations. On one hand, it was found that the nanoparticles formed during milling process greatly enhance the dissolution reaction by the creation of Ce3+ thin layers of a few nanometer depth on their surfaces. On the other hand, it is shown that dislocations represent another way to increase the kinetics by activation energy. In conclusion, dissolution rate's growth can be due to different parameters like the leaching of the smallest particles, the presence of reduced oxidation state on nanoparticles and some highly reactive sites concentrating structural defects such as dislocation nodes. Finally, as ceria is also well known to be a safe analogue of PuO2, especially for dissolution studies, a solution for improving the dissolution of ceria would probably also be useful for dissolving the oxides rich in Pu

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

Vitamin C boosts ceria-based catalyst recycling

International audienc

Impact of uranium carbide organics treated by prolonged boiling and electrochemical oxidation upon uranium and plutonium solvent extraction

International audienceAbstract The dissolution of uranium or uranium-plutonium carbide fuel in nitric acid leads to ~50% carbon evolved as carbon dioxide, the remainder remains in the solution as soluble organics. These dissolved organic molecules interfere with the solvent extraction of uranium and plutonium by complexing to the actinide ions and decreasing the efficiency of their extraction. Experiments reported here describe two series of experiments assessing the uranium carbide dissolution liquor treatment by prolonged boiling and electrochemical oxidation. Plutonium losses to aqueous and solvent raffinates are observed for untreated liquors, highlighting that mineralisation of dissolved organics is necessary to reduce the complexing effects of organic acids to an extent that permit efficient operation of a solvent extraction process both in the first solvent use (considered here) and for maintaining solvent quality during industrial solvent reuse in the highly active cycle. Solution carbon analysis and 30% TBP solvent extraction batch tests of uranium and plutonium originating from dissolved uranium carbide liquors untreated and after treatment are compared. These experiments demonstrate the reprocessing of uranium carbides by direct dissolution coupled to a mineralisation process, can achieve near quantitative uranium and high plutonium recoveries (99.9%)