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In an advanced nuclear fuel cycle, partitioning and transmutation (P&T) is a key strategy to reduce spent nuclear fuel’s radiotoxicity and heat generation. Long-lived minor actinides (MA) are partitioned from spent nuclear fuel and subsequently converted into fuel or targets materials for use in fast reactor systems, where the actinides are fissioned to short-lived radionuclides. The sol-gel route via internal gelation is a process that is currently explored for the production of such MA containing transmutation fuel, by converting an actinide-containing solution into a homogeneous precipitate with spherical geometry. Those precursors are thermally treated and can be used as particle fuel or compressed into fuel pellets. The advantages of the process are to avoid handling of fine powder and to facilitate automation for the production of nuclear fuel precursors. Within this study, the structural changes in Nd- and Ce- doped ammonium diuranate (ADU) gels, prepared by internal gelation during the conversion to U1-yLnyO2±x were investigated. Nd and Ce molar metal fractions up to 30mol% were introduced, acting as surrogates for the actinides Am and Pu. Nd was used in its trivalent oxidation state, while both trivalent and tetravalent cerium were tested. The dried Ln-doped ADU gels were characterised by optical microscopy and X-ray powder diffraction (XRD). Those precursors were thermally treated in two individual steps: (1) calcination under oxidising conditions at a maximum temperature of 900°C, followed by (2) sintering in reducing atmosphere at a maximum temperature of 1600°C (10h). The behaviour of the dried gels during both thermal treatment steps was studied in-situ by high temperature scanning electron microscopy (HT-SEM) and thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC) and evolved gas analysis mass spectrometry (EGA-MS). The material prepared with the trivalent dopant precursors behaved significantly different during the decomposition in air compared to mixtures prepared with Ce(IV), which indicates different crystallinities and/or initial compositions. The calcined products were identified as (Ln-doped) α−U3O8 for dopant contents up to 10mol%. For higher Ln contents, a more complex mixed-phase behaviour was found. The introduction of a reducing atmosphere led to a sudden mass loss. Particle shrinkage occurred significantly later, pointing out that the shrinkage observed within the sintering is controlled by a general densification of the material and insignificantly by the volume change due to the transition of the orthorhombic (Ln-doped) α−U3O8 phase to the cubic (Ln-doped) UO2±x phase. The sintered products were identified as single phase solid solutions according to the formula U1-yLnyO2±x for Nd contents up to 30mol% and Ce contents up to 20mol%. In this contribution we will report and discuss the experimental results.

International audience; The effect of several impuritites on the life-cycle of yttria-doped thoria electrolyte was evaluated in this study. All the oxide exhibited the single-phase fluorite-type structure expected for such materials, while the variation of the unit cell parameter attested for the incorporation of the impurities in the lattice. Doping with 0.2, 0.5 or 1 wt.% of Al, Si or Zr only slightly affected the morphology of the powders, as well as the microstructure of the pellets obtained after sintering, even if a deleterious effect, leading to the embrittlement of the samples, was observed for high silicon contents. The presence of silicon and zirconium in the ceramic further modified its electrical properties. The grain conductivity was always found to decrease. Impedance spectroscopy also revealed a strong modification of the signal assigned to grain boundaries, which evidenced that impurities partially migrated during the sintering step. Finally, corrosion tests undertaken in liquid sodium at 500°C showed that the pellets were fragilized by the presence of silicon or high zirconium contents, possibly through the formation of Na2MO3 ternary oxides at the grain boundaries. From a general point of view, silicon seems to be the most harmful impurity that should be especially checked during the fabrication processes.

International audience; During operational conditions of pressurized water reactors (PWR) nuclear fuel is highly irradiated. This process induces many modifications of the initial properties, such as porosity, thermal conductivity or the apparition of a particular structure named High Burn-up Structure (HBS), characterized by sub micrometric grain size and high population of closed pores, among others (1). This work focuses on the evolution of the microstructure of simulant materials to model nuclear fuel under conditions similar to those found in the HBS. The chosen system to study was U-Nd-O, being Nd used as dopant since it is the most abundant lanthanide fission products. At low temperature and low concentrations of Nd, the system is partially miscible and presents a region where it is biphasic. This region is formally called as miscibility gap. Previous studies carried out on the U-Nd-O ternary system highlighted that the miscibility gap at room temperature exists when the concentration of Nd is higher than 9 at% (2,3). With rising temperature, the solubility increases and the biphasic system becomes monophasic. The objective of this paper is to analyze the system when it is submitted to a strain energy that appears when the initially homogeneous phase at elevated temperature crosses a biphasic domain at a lower temperature (enters the miscibility gap). Thus, the most salient feature of this study will be to observe if this internal strain would produce dislocations which would contribute to the formation of the HBS. In the current work, three different samples with 4, 17 and 25 at% of Nd were studied. The evolution of the crystalline structure in temperature and the presence of dislocations at room temperature as a consequence of phase separation are analyzed through means of High Temperature-X-Ray Diffraction (HT-XRD), High-Temperature Scanning Electron Microscopy (HT-SEM), and Transmission Electron Microscopy (TEM). Experimental results are confronted to thermodynamic modelling calculated through the CALPHAD method using the Thermo-Calc code. In this paper a first approach on the evolution of the microstructure as a function of the concentration of Nd is also proposed.

ISBN 978-3-940237-50-7 / Oral Presentations 4: Chemicals, Polymers, Bio(macro)molecules & Biocomposites, Life science, Organic & Functional complex compounds; International audience; Among the processes Chat could be used to synthesize actinide oxides for nuclear fuel rcycling purpose, one well-known method is the thermal treatment of actinide oxalates under controlled atmosphere. In fact, the actinide is first stabilized by a precipitation step, then the actinide oxalate is calcined in order to obtain an oxide powder to be shaped and sintered during the fabrication process. The transformation of actinide oxalate into the final oxide consists in a succession of transformations modifying the characteristics of the solid such as composition, specific surface area, granulometry, impurity content,... The thermal treatment leads to a complex evolution of the solid phase with some gaseous releases. A better understanding of gas-gas and gas-solid reactions is essential to be able to predict the final characteristics of the oxide according to the thermal parameters. In this work we used differential scanning calorimetry and thermogravimetric analysis with evolved gas analysis in order to investigate the thermal behavior of uranium oxalate in various atmospheres. Contrary to previous studies, a set of solids characterizations (morphological, textural and structural) was used to unambiguously indentify the reaction products. Among the seven stages involved during the global reaction, the kinetics of two of them were specifically investigated. The experimental kinetic data in isobaric and isothermal conditions are explained thanks to kinetic modelling based on the characterizations of solid and heterogeneous kinetics approach. This allows us to obtain predictive model which includes the effect of temperature and partial pressure of some gases of interest, as well as the shape and size of the solid phases.

International audience; Dans le cadre des études menées pour le développement des réacteurs électronucléaires degénération future (Gen IV), des modifications de la teneur en plutonium du combustible ainsiqu’une augmentation du flux de neutrons rapides sont envisagées. Le procédé de frittage de lacéramique oxyde qu’est le combustible, étape essentielle de sa fabrication, doit donc être optimisé.Il est ainsi nécessaire d’approfondir la compréhension des différents stades gouvernant ceprocédé conduisant d’une poudre à un matériau dense par le biais d’un traitement thermique.Le frittage est généralement décrit comme la succession de trois étapes successivementcaractérisées par la formation de ponts entre les grains, l’élimination de la porosité ouverte etl’élimination de la porosité fermée. Les étapes intermédiaire et finale sont connues et décrites dansla littérature, à la fois par des approches expérimentales et numériques. Le premier stade estquant à lui principalement décrit par modélisation, des premiers travaux sur les oxydes de cériumet de thorium n’ayant été reportés que récemment.L’objectif de ce travail de thèse concerne donc la compréhension du premier stade du frittagedu dioxyde d’uranium UO$_2$ et de solutions solides associées (UO$_2$-CeO$_2$ par exemple). Pour celades études in situ par MEBE-HT$^1$ et MET-HT$^2$ ont été réalisées, conduisant à des observationspermettant de caractériser quantitativement les phénomènes régissant le 1er stade du frittage. Lavariation des conditions opératoires a en outre permis de mettre en évidence le rôle de différentsparamètres tels que la taille et la morphologie des particules étudiées, ainsi que la température etl’atmosphère de frittage