Volume interdiffusion coeffcient and uncertainty assessment for polycrystalline materials

A method has been developed in order to assess small volume interdiffusion coeffcients from experimental Electron Probe MicroAnalysis concentration profiles of polycrystalline materials by means of Boltzmann-Matano or den Broeder methods and their complementary Hall method. These methods have been used as tools for the investigation of the quasi-binary UO2/U(1-y)PuyO(2-z) interdiffusion, for which obtaining a solid solution in the bulk of grains is of major interest. In this paper uncertainties on the interdiffusion coefficient as a function of concentration have been computed for each method. Small volume cofficient measurements were enhanced by means of a small angle acquisition profile line with respect to the interdiffusion interface

Charge compensation mechanisms in Nd-doped UO2 samples for stoichiometric and hypo-stoichiometric conditions : Lack of miscibility gap

The evolution of the crystal lattice of samples made of UO2 doped with different concentrations of Nd in stoichiometric and hypo-stoichiometric conditions has been systematically studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The substitution of a trivalent cation for the U4+ initial position is responsible for creating local structural disorder and changes in the oxidation states. In this scenario, the lattice parameter is affected and the concentration of U5+ and formation of oxygen vacancies as well, since these are the mechanisms necessary to maintain the charge neutrality. The systematic oxidation of U4+ as predominant charge compensation mechanism over the formation of vacancies can be reduced by performing a thermal treatment under reducing conditions. This paper presents an experimental characterization of the uranium oxidation state mixture and local structure using XAS techniques in samples with chemical formula (U1-yNdy)O2-x, with y = 0.04, 0.17 and 0.25 (Peer reviewe

Développement du procédé de densification rapide appliqué au carbure de silicium

Les procédés d élaboration de Composites à Matrice Céramique (CMC) utilisés aujourd hui à l échelle industrielle sont longs et par conséquent coûteux. Dans ce contexte, le procédé de densification rapide ou procédé de caléfaction, jusque-là essentiellement connu pour l élaboration de carbone, permettant de réduire considérablement les durées d élaboration, apparaît intéressant. Cette étude est axée sur le développement du procédé de caléfaction dans le but d élaborer des carbures, matériaux connus pour leurs bonnes propriétés à haute température, et plus particulièrement du carbure de silicium (SiC). Dans cet objectif, un équipement de laboratoire, le mini-kalamazoo, a été mis au point, adapté et instrumenté de manière à répondre aux besoins de l étude. Les premiers essais ont été réalisés au moyen de méthyltrichlorosilane (MTS), précurseur largement connu pour la CVD/CVI du SiC. Les analyses des dépôts formés ont montré la présence de SiC mais aussi celle de carbone. Dans quelques cas spécifiques, du SiC pur peut être formé localement en début de caléfaction. Mis à part ces conditions particulières, l utilisation de MTS pur en tant que précurseur conduit à la présence inéluctable de carbone libre dans le dépôt de SiC. Plusieurs voies d amélioration ont alors été proposées et testées pour pallier cet excès de carbone. Certaines d entre elles se sont avérées efficaces et prometteuses, en particulier, l utilisation d un mélange de MTS et d un précurseur de silicium non carboné et l utilisation de précurseurs de SiC non chlorés, le CVD 4000 et l hexaméthyldisilane. Les vitesses de croissance de dépôt sont largement supérieures avec le procédé de caléfaction qu avec les moyens d élaboration aujourd hui employés. L ensemble des résultats obtenus valide l intérêt de la caléfaction en tant que procédé d élaboration du SiC et de nouveaux matériaux de type carbure.The current Ceramic Matrix Composites (CMC) manufacturing processes used at the industrial scale are slow and consequently expensive. In light of this, the fast densification process, also called the film-boiling process, essentially known to produce carbon deposit up to now, reduces significantly the processing time which seems to be promising. This study was focused on the film-boiling process development in order to manufacture carbides which are materials with good properties at high temperature, and especially to synthetize silicon carbide (SiC). In this aim, a laboratory-made equipment was developed, set-up and adapted to the needs of our study. The first tests were done with the methyltrichlorosilane (MTS), precursor widely used for SiC CVD/CVI. Characterizations of the deposits showed the formation of SiC but also the occurrence of carbon. Pure SiC can be locally obtained at the beginning of the film-boiling process in some specific experimental conditions. For most of the experiments, the use of pure MTS as precursor leads inevitably to the formation of free carbon in the SiC deposit. Several improvement routes were proposed and tested to remove this carbon excess. Some of the efficient and promising routes have consisted in the use of MTS mixed with a silicon precursor free of carbon and the use of two non-chlorinated SiC precursors, CVD 4000 liquid precursor and hexamethyldisilane. The deposit growth rates were significantly superior with the film-boiling process compared to the classical processes. All the data show that the film-boiling process is promising for the manufacturing of SiC and new carbide materials.ORLEANS-SCD-Bib. electronique (452349901) / SudocSudocFranceF

Synthèse par voie solide et frittage de céramiques à structure monazite (application au conditionnement des actinides mineurs)

Dans le cadre de la loi de 1991 concernant la gestion des déchets nucléaires en France, plusieurs études sont menées pour mettre au point des matrices cristallines spécifiques de conditionnement. La monazite, un orthophosphate de terre rare (TR3+) de formule TR3+PO4, est un minéral naturel contenant très souvent des quantités non négligeables de thorium et d uranium. Les qualités de résistance de ce matériau vis-à-vis des irradiations et de l altération aqueuse en font un candidat pour le conditionnement spécifique des actinides mineurs (Np, Am et Cm). Il est important désormais de vérifier la conservation de ces propriétés sur des matériaux de synthèse, ce qui sous-entend de parfaitement maîtriser toutes les étapes de l élaboration de pièces de monazite, de la synthèse des poudres jusqu à l élaboration par frittage de pastilles à microstructure contrôlée. C est dans ce cadre-là que s intègre le travail présenté dans ce document. La première partie de la thèse traite de l étude par voie solide de la synthèse de TR3+PO4 (TR = La3+ à Gd3+, Pu3+ et Am3+). Les réactions intervenant lors de la calcination des réactifs sont décrites dans le cas de monazites à un seul ou à plusieurs cations, permettant d établir un protocole de synthèse. L incorporation de cations tétravalents (Ce4+, U4+ et Pu4+) dans la structure monazite a également été étudiée. La deuxième partie traite de l élaboration de pièces de monazite à densité et microstructure contrôlées ainsi de leurs propriétés mécaniques et thermiques associées. L étude du broyage et du frittage, peu abordée jusqu alors, est présentée. Les résultats expérimentaux sont confrontés à des modèles théoriques afin d en déduire les mécanismes de densification et de grossissement de grains. Par la compréhension des différents phénomènes physico-chimiques se produisant lors des différentes étapes d élaboration (synthèse, broyage, frittage), ce travail a permis la mise au point d un protocole de fabrication de pièces de monazite TR3+PO4 à microstructure contrôlée. Les relations qui existent entre les différentes étapes du processus d élaboration du matériau ont pu être mises en évidence.In the framework of the French law of 1991 concerning the nuclear waste management, several studies are undertaken to develop specific crystalline conditioning matrices. Monazite, a rare earth (TR3+) orthophosphate with a general formula TR3+PO4, is a natural mineral containing significant amount of thorium and uranium. Monazite has been proposed as a host matrix for the minor actinides (Np, Am and Cm) specific conditioning, thanks to its high resistance to self irradiation and its low solubility. Its is now of prime importance to check the conservation of these properties on synthesized materials, which implies to master all the stages of the elaboration process, from the powder synthesis to the sintering of controlled microstructure pellets. This work can be divided into two main parts: The first part deals with the synthesis by high temperature solid state route of TR3+PO4 powders (with TR3+ = La3+ to Gd3+, Pu3+ and Am3+). The chemical reactions occurring during the firing of starting reagents are described in the case of monazite with only one or several cations. From these results, a protocol of synthesis is described. The incorporation of tetravalent cations (Ce4+, U4+, Pu4+) in the monazite structure was also studied. The second part of the present work deals with the elaboration of controlled density and microstructure monazite pellets and their related mechanical and thermal properties. The study of crushing and sintering is presented. For the first time, experimental results are confronted with theoretical models in order to deduce the densification and grain growth mechanisms. By the comprehension of the various physicochemical phenomena occurring during the various stages of the monazite pellets elaboration process (powder synthesis, crushing, sintering ), this work allowed the development of a protocol of elaboration of controlled microstructure monazite TR3+PO4 pellets. The determination of some mechanical and thermal properties could thus be carried out. The relations between the various stages of the material development process could be highlighted.LIMOGES-BU Sciences (870852109) / SudocSudocFranceF

Densification mechanisms of UO2 consolidated by spark plasma sintering

Despite the growing interest in the spark plasma sintering (SPS) of uranium dioxide, its sintering mechanisms have yet to be studied in great detail. Herein we propose a direct method to calculate the apparent activation energy for densification, Qact, and the stress exponent, n, for SPS of nearly stoichiometric UO2. A set of experiments performed at different heating rates (CHR) and different pressures levels allowed us to calculate Qact and n, respectively, though we were limited to a theoretical density between 50% to 75 %. The master sintering curve was employed as a complementary method to compare Qact. The average values were Qact =96 kJ/mol (CHR), Qact = 100 kJ/mol (MSC) and n = 1.4. We have therefore proposed grain boundary diffusion coupled with grain boundary sliding as the densification mechanism. The activation energy in SPS tends to be lower compared with that in other processes like conventional sintering (250−450 kJ/mol), creep (350−550 kJ/mol) and hot pressing (222 kJ/mol and 480 kJ/mol). This decrease could be due to the effect of the electric field combined with the higher heating rates, typical of SPS.JRC.G.I.3-Nuclear Fuel Safet

Densification mechanisms of UO2_2 consolidated by spark plasma sintering

International audienceDespite the growing interest in the spark plasma sintering (SPS) of uranium dioxide, its sintering mechanisms have yet to be studied in great detail. Herein we propose a direct method to calculate the apparent activation energy for densification, $Q_{act}$, and the stress exponent, $n$, for SPS of nearly stoichiometric UO$_2$. A set of experiments performed at different heating rates (CHR) and different pressures levels allowed us to calculate $Q_{act}$ and $n$, respectively, though we were limited to a theoretical density between 50% to 75 %. The master sintering curve was employed as a complementary method to compare $Q_{act}$. The average values were $Q_{act}$ =96 kJ/mol (CHR), $Q_{act}$ = 100 kJ/mol (MSC) and n = 1.4. We have therefore proposed grain boundary diffusion coupled with grain boundary sliding as the densification mechanism. The activation energy in SPS tends to be lower compared with that in other processes like conventional sintering (250−450 kJ/mol), creep (350−550 kJ/mol) and hot pressing (222 kJ/mol and 480 kJ/mol).This decrease could be due to the effect of the electric field combined with the higher heating rates, typical of SPS

Amélioration de la conductivité thermique des composites à matrice céramique pour les réacteurs de 4ème génération

RésuméAbstractBORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Ceramic processing of uranium–plutonium mixed oxide fuels (U1−yPuy)O2 with high plutonium content

International audienceThe ternary thermodynamic U–Pu–O system has been studied for decades for MOX fuel applications but the phase diagram is still not precisely described mostly in the UO 2 –PuO 2 –Pu 2 O 3 sub-system. Furthermore, uranium–plutonium mixed oxides containing high amounts of plutonium are now being considered within the scope of future nuclear reactors. Within this framework, obtaining homogeneous mixed oxides by powder metallurgy is paramount. The studied process is based on UO 2 and PuO 2 co-milling and applied to compounds with high Pu content. The objective of this study is obtaining microstructures free of local heterogeneities in the U–Pu distribution which are not suitable for research studies. Furthermore, in case of prospective irradiation application, local high Pu concentrations lead to “hot spots” in the material influencing the fission.

Evolution of the perovskite phase in UO2-based samples under conditions representative of a severe nuclear accident by XANES

International audienceTo understand the fission products behaviour during a Nuclear Severe Accident (NSA), simulated high burnup UO2 fuels produced by sintering at high temperature (SIMFUEL) were submitted to thermal treatments in conditions representative of the beginning of a Pressurized Water Reactor (PWR) severe accident. The samples made of UO2 doped with eleven fission products (FP) surrogates were thus thermally treated from 400 ​°C up to 1000 ​°C at different oxygen potentials (up to −290 ​kJ ​mol(O2)−1 at 1000 ​°C). The samples were characterized before and after the heat treatments by Secondary Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDX) and X-ray Absorption Spectroscopy (XAS) at the MARS beamline, SOLEIL Synchrotron and at the FAME-UHD beamline, ESRF. Initially the SIMFUEL is constituted of a UO2 matrix, a metallic phase composed of Mo, Pd, Ru, Rh and Tc, and an oxide phase with perovskite structure depicted as (Ba, Sr)(Zr, U, RE)O3 where RE stands for Rare Earths. Around 1000 ​°C, it was found that Mo, initially contained in the metallic inclusions, oxidizes into MoO2. In the same conditions, a part of the oxidized Mo interacts with the perovskite phase containing Ba to form BaMoO4. These results will be useful to validate the theoretical models used in calculation codes and to assess thermodynamic databases, which are one of the main sources of errors when calculating a NSA sequence involving molten fuel