La spécialisation cognitive : les systèmes locaux de compétences.

National audienceLes notions de district industriel (Beccatini G., 1992), de système industriel local (Raveyre M.-F., Saglio J., 1984), de cluster (Jacobs D., De Jong M. W., 1992 ; Porter M., 2000) ou de pôles de compétitivité (Blanc C., 200 ?) désignent des ensembles d'entreprises spécialisées dans un même type de produit ou de filière et concentrées dans un espace restreint dont les relations associent concurrence et coopération. Parfois, ces firmes utilisent ou partagent des infrastructures spécialisées mises en place par des institutions publiques ou privées

MOX dopé chrome (optimisation du dopage et de l'atmosphère de frittage)

Dans un contexte d accroissement des marges de sûreté des réacteurs de générations II et III vis-à-vis des scénarios accidentels, des efforts importants de recherche sont consacrés à l amélioration de la microstructure du combustible MOX à l issue de son procédé de fabrication. Les deux caractéristiques microstructurales recherchées sont l accroissement de l homogénéité de répartition du plutonium et l augmentation de la taille de grain. Dans cette optique, une solution envisagée est l ajout lors du procédé de fabrication et sans modification de celui-ci, de sesquioxyde de chrome Cr2O3. Une précédente thèse sur le sujet a permis de proposer un modèle d homogénéisation de la répartition du plutonium suite à l ajout de Cr2O3. L auteur a souligné l importance de la formation du précipité PuCrO3 aux joints de grains lorsque la solubilité du chrome dans la matrice (U,Pu)O2 est atteinte. Cependant, les mécanismes d action du chrome n ont été étudiés que pour une atmosphère de frittage unique. Plusieurs points restent également à approfondir, notamment la solubilité du chrome et les conditions optimales de formation du précipité PuCrO3. Dans un premier temps, une étude de la spéciation du chrome solubilisé et précipité dans l oxyde mixte (U,Pu)O2 a été réalisée. Les techniques ayant permis d analyser directement le chrome sont la microsonde électronique et la spectroscopie d absorption des rayons X. Il a été montré que le degré d oxydation et l environnement du chrome solubilisé sont indépendants de la pression partielle d oxygène imposée lors du frittage et de la teneur en plutonium de l oxyde mixte. La nature des précipités et la solubilité du chrome dépendent, quant à eux, de la variable thermodynamique et de la teneur en Pu. Sur la base de ces résultats, un modèle de solubilité du chrome dans l oxyde mixte (U,Pu)O2-x a été construit. Ce modèle a été réalisé en fonction de la teneur en plutonium y de la solution solide (U1-yPuy)O2-x (y = 0,11 ; 0,275 et 1) et sur la gamme de potentiel d oxygène d intérêt pour le frittage du combustible (-445 kJ.mol -1< O2 < -360 kJ.mol -1). Outre l optimisation du dopage, ce modèle permet de définir les conditions optimales de formation du précipité PuCrO3 en fonction de la teneur en plutonium et de l atmosphère de frittage. Dans un second temps, nous avons regardé si les conditions d obtention du précipité PuCrO3 correspondaient à un accroissement de l homogénéité de répartition du plutonium et une taille de grains maximale. Pour ce faire, des échantillons fabriqués avec ou sans présence de chrome et frittés sous différentes atmosphères ont été étudiés. Il a été mis en évidence que la cinétique d interdiffusion U-Pu est complètement modifiée en présence de chrome. De plus, suite à l ajout de chrome, les conditions permettant d accroître la cinétique d interdiffusion U-Pu ne sont pas forcément associées à une taille de grain maximale.A partir de ces résultats, des préconisations pour la mise en œuvre industrielle sont proposées. Elles concernent le choix de l atmosphère de frittage et la teneur en chrome nécessaire à l optimisation de la microstructure.Optimal use of the Mixed Oxide (U,Pu)O2 nuclear fuel in pressurized water reactors is mainly limited by the behavior of gaseous fission produced during irradiation. Within the MOX microstructure, the probability of fission gas release is increased by the presence of rich localized plutonium areas exhibiting a higher local burn-up. A solution consists in optimizing plutonium distribution within the industrial product and promoting the crystalline growth of the fuel grains. For this purpose, addition of chromium sesquioxide during the manufacturing process is currently considered. A previous thesis has shown that the best results are obtained for a Cr addition slightly greater than the solubility limit of Cr in (U,Pu)O2. In order to explain the enhanced plutonium homogeneity, the author highlighted the formation of PuCrO3 precipitates at grain boundaries. A sintering model under reducing atmosphere, with chromium addition, was proposed. However, several points have to be more thoroughly investigated, especially regarding the solubility limit of chromium, as well as the optimal conditions of PuCrO3 precipitates formation. In a first part, speciation of solubilized and precipitated chromium in the mixed oxide (U,Pu)O2 is studied using electron probe microanalysis (EPMA) and X-ray absorption spectroscopy (XAS). It was shown that the oxidation state and the environment of soluble chromium within the (U,Pu)O2 matrix do not depend on the oxygen partial pressure during sintering, neither on the plutonium content of the mixed oxide. However, both chemical nature of the precipitates and chromium solubility depend on the thermodynamic variable and on the plutonium content.Based on these results, a chromium solubility model in the mixed oxide (U,Pu)O2-x was built using the law of mass action governing solubility equilibrium. This model is described as a function of the plutonium content (y) of the solid solution (U1-yPuy)O2-x (y = 0,11 ; 0,275 et 1) and in the oxygen potential range of interest for MOX fuel sintering (-445 kJ/mol < O2 < -360 kJ/mol). This thermodynamic model contributes to the optimization of the doping stage of fabrication and defines the optimal conditions of PuCrO3 precipitates formation.The aim of the second part is to verify if the thermodynamic conditions of PuCrO3 formation correspond to an optimal plutonium distribution and grain growth of the mixed oxide. Samples manufactured with and without Cr2O3 addition and sintered under various atmospheres were analyzed. It was shown that the U-Pu interdiffusion kinetics is completely modified with chromium addition. Morover, with chromium addition, sintering conditions which increase the U-Pu interdiffusion kinetics, don t necessarily correspond to optimal grain growth. Based on these results, recommendations for the industrial manufacturing process are proposed. They deal with the choice of the sintering atmosphere and doping concentration to obtain an optimized microstructure.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Influence of the microstructure of 2D-random heterogeneous media on the propagation of acoustic coherent waves

International audienceMultiple scattering of waves arises in all fields of physics either in periodic or random media. For random media the organization of the microstructure (uniform or non-uniform statistical distribution of scatterers) has effects on the propagation of coherent waves. Using a recent exact resolution method and different homogenization theories, the effects of the microstructure on the effective wavenumber are investigated over a large frequency range (ka between 0.1 and 13.4) and high concentrations. For uniform random media, increasing the configurational constraint makes the media more transparent for low frequencies and less for high frequencies. As a side but important result, we show that two of the homogenization models considered here appear to be very efficient at high frequency up to a concentration of 60%, in the case of uniform media. For non-uniform media, for which clustered and periodic aggregates appear, the main effect is to reduce the magnitude of resonances and to make network effects appear. In this case, homogenization theories are not relevant to make a detailed analysis

Deuterium adsorption on (and desorption from) SiC(0001)-(3×3), (√3×√3)R30°, (6√3×6√3)R30° and quasi-free standing graphene obtained by hydrogen intercalation

International audienceWe present a comparative high-resolution electron energy-loss spectroscopy study on the interaction of atomic hydrogen and deuterium with various reconstructions of SiC(0 0 0 1). We first show that on both the (3 × 3) and reconstructions, deuterium atoms only bind to silicon atoms, thereby confirming the silicon-rich appellation of these reconstructions. Deuterium passivation of the (3 × 3) is only reversible when exposed to atomic deuterium at a surface temperature of 700 K since tri- and dideuterides, necessary precursors for silicon etching, are not stable. On the other hand, we show that the deuteration of the is always reversible because precursors to silicon etching are scarce on the surface. Then, we demonstrate that hydrogen (deuterium) adsorption at 300 K on both the (buffer-layer) and the quasi-free-standing graphene occurs on carbon atoms justifying their carbon-rich appellation. Comparison of the deuterium binding in the intercalation layer of quasi-free-standing graphene with the deuterated surface provides some indication on the bonding structure at the substrate intercalation layer. Finally, by measuring C-H (C-D) vibrational frequencies and hydrogen (deuterium) desorption temperatures we suggest that partial sp2-to-sp3 rehybridization occurs for the carbon atoms of the buffer-layer because of the corrugation related to covalent bonding to the SiC substrate. In contrast, on quasi-free-standing graphene hydrogen (deuterium) atoms adsorb similarly to what is observed on graphite, i.e. without preferential sticking related to the underlying SiC substrate

Relation between static short-range order and dynamic heterogeneities in a nanoconfined liquid crystal

International audienceWe analyze the molecular dynamics heterogeneity of the liquid crystal 4-n-octyl-4'-cyanobiphenyl nanoconfined in porous silicon. We show that the temperature dependence of the dynamic correlation length wall, which measures the distance over which a memory of the interfacial slowing down of the molecular dynamics persists, is closely related to the growth of the short-range static order arising from quenched random fields. More generally, this result may also shed some light on the connection between static and dynamic heterogeneities in a wide class of condensed and soft matter systems

EntiMid : un middleware aux services de la maison

National audienceOne of the society challenges of tomorrow is the aging population. Often pointed as a possible solution for this issue, house automation can help elderly people stay at home as long as possible, and ease the coordination of the domain actors. In this context, this paper lists the requirements of such a system, showing that today's commercial solutions are not flexible enough to reach the necessary functional richness. According to these requirements, a set of properties a middleware should offer in order to be deployed at a city scale is identified. Then this paper describes the software architecture of a suitable middleware built over OSGi, respecting the listed properties. After that, an experience report of the deployment of this middleware, in the laboratory of the Université de Rennes 1 highlights the relevance of the proposed solution

Study of hydrogen isotopes behavior in tungsten by a multi trapping macroscopic rate equation model

International audienceDensity functional theory (DFT) studies show that in tungsten a mono vacancy can contain up to six hydrogen isotopes (HIs) at 300 K with detrapping energies varying with the number of HIs inthe vacancy. Using these predictions, a multi trapping rate equation model has been built and used to model thermal desorption spectrometry (TDS) experiments performed on single crystaltungsten after deuterium ions implantation. Detrapping energies obtained from the model to adjust temperature of TDS spectrum observed experimentally are in good agreement with DFTvalues within a deviation below 10%. The desorption spectrum as well as the diffusion of deuterium in the bulk are rationalized in light of the model results

Validation of a purely elastic model and a finite element model for a screw press

The increasing use of superalloys in the industry brings new issues to forging manufacturers concerning Finite Element analysis of the forming process. Once we consider high performing materials, significant differences can occur between numerical and experimental results. The lack of knowledge about materials and thermal exchanges explains a part of these differences but it has been shown that the machine behavior has a significant impact on the forging process. Press modelling is a way to quantify the energy dissipated in the machine structure and improve simulations. This study is a first step in the screw press modelling. Starting from an approximation of the press geometries, a purely static elastic (PSE) model is developed and a numerical simulation considering only uprights is performed. Uprights stiffness predictions from the PSE model and the FE simulation are compared and the PSE model is validated. Then, the press is approximated by a portal frame and FE simulation is realized to determined uprights stiffness by considering links effects and bending behaviors in the structure. Comparison with experimental results validates the approximation.région Grand Est Manoir Industrie Bouzonvill