Alkali Uranyl Borates: Bond Length, Equatorial Coordination and 5f States

Three uranyl borates, UO2B2O4, LiUO2BO3 and NaUO2BO3, have been prepared by solid state syntheses. The influence of the crystallographic structure on the splitting of the empty 5f and 6d states have been probed using High Energy Resolved Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS) at the uranium M4-edge and L3-edge respectively. We demonstrate that the 5f splitting is increased by the decrease of the uranyl U-Oax distance, which in turn correlates with an increased bond covalency. This is correlated to the equatorial coordination change of the uranium. The role of the alkalis as charge compensating the axial oxygen of the uranyl is discussed

Applications of the double-pulse laser-induced breakdown spectroscopy (LIBS) in the collinear beam geometry to the elemental analysis of different materials.

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Alkali uranyl borates: bond length, equatorial coordination and 5f states

International audienceThree uranyl borates, UO2B2O4, LiUO2BO3 and NaUO2BO3, have been prepared by solid state syntheses. The influence of the crystallographic structure on the splitting of the empty 5f and 6d states have been probed using High Energy Resolved Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS) at the uranium M4-edge and L3-edge respectively. We demonstrate that the 5f splitting is increased by the decrease of the uranyl U-Oax distance, which in turn correlates with an increased bond covalency. This is correlated to the equatorial coordination change of the uranium. The role of the alkalis as charge compensating the axial oxygen of the uranyl is discussed

X Rays methods applied to corium material

International audienceDue to their intrinsic properties, X-rays are at the basis of several methods of material investigation. In the case of crystals, X-ray scattering and diffraction allow structural and microstructural analysis. Whether the material under consideration is amorphous or crystallized, elemental analysis can be done through the measurement of the energies and intensities of the X-ray emission characteristic lines of the elements constituting a material. The detection of these X-rays emission lines allows local chemical analysis and mapping at the micrometric scale that can be performed by Electron Probe Micro-Analysis (EPMA).The lecture will give an example of the application of X-ray methods for the elementary and structural analysis of the corium, a complex material coming from the melting of the core of nuclear reactor at a very high temperature (2500-2800 K). Studies were carried out on prototypical corium according to the U1-x ZrxO2y solid solution obtained by Fuel Coolant Interaction experiments performed in the KROTOS facility at CEA Cadarache. The fragmented or exploded corium particles typical size of 200 µm or less than 50 µm have been studied to determine their microstructure, elemental composition and structural state. EPMA was used to determine with high accuracy the local variation of the composition. A specific methodology will be presented and results will be compared with the determination of the cationic composition fluctuations determined through coupled Rietveld refinement of X-rays and neutrons diffraction patterns recorded at the MARS beamline at the SOLEIL synchrotron and at the D2B beamline of the Institute Laue Langevin respectively.Moreover, micro texture analyses were performed through SEM/EBSD observations and Laue microdiffraction experiments done on the BM32 beamline at the ESRF synchrotron. We clearly evidenced intra-crystalline local cationic composition fluctuations that we were able to be associated to the different cooling process and the particles sizes

Etudes des distributions de compositions de la solution solide U1‐x ZrxO2y(Corium) lors de différents processus de refroidissements / solidifications

International audienceLe corium, formé en cas d’accidents grave au sein de réacteurs nucléaires, est un matériaucomplexe issu de la fusion partielle ou totale à très haute température (T° 2000°C) d'un cœur deréacteur nucléaire et de son interaction avec les différentes barrières : gaines en zircaloy, cuve enacier, béton de l’enceinte de confinement. La connaissance de l’état solide du corium estfondamentale tant du point de vue de la progression de l’accident grave et de sa modélisation que dupoint de vue du démantèlement de réacteurs accidentés comme ceux de Fukushima. Les régimes desolidification de ce matériau à partir d'un état liquide peuvent varier en fonction des conditions derefroidissement qui peut être rapide, en cas d’interaction avec l’eau, ou lent, en cas d’interaction avecle béton de l’enceinte d’un réacteur.Les coriums sont constitués pour une large part d’oxyde d’uranium et de zirconium. Cesoxydes forment une solution solide totale U1‐x ZrxO2y [1]. Ce travail vise à mettre en évidence lesdifférents régimes de solidification et de formation des solutions solides de type U1‐xZrxO2y, horséquilibre ou à l’équilibre thermodynamique en fonction des régimes de refroidissement. Pour celades échantillons ont été prélevés à différentes étapes de l’interaction entre un jet de coriumprototypique liquides à T° 2600 K et l’eau dans l’installation PLINIUS/KROTOS du CEA de Cadarache. Al’issue de ce type d’interaction, des débris de différentes tailles sont formés. Ces solides se présententalors sous forme de poudre avec des granulométries différentes selon les processus de fragmentationet explosion au contact de l’eau.Nous avons étudié ces différents matériaux par diffraction des rayons X sur la ligne MARS dusynchrotron SOLEIL. Les diagrammes de diffraction obtenus présentent un très fort élargissement desraies alors que la taille des cristaux observés par MEB/EBSD est micrométrique. En accordavec des études antérieures [2,3], l’élargissement observé est lié à une distribution de compositioncationique U/Zr qui varie suivant les différents régimes de solidification. Chaque pic de diffraction peutêtre traité comme une somme de pics élémentaires correspondant à une composition, et donc unedistance inter‐réticulaire, donnée. Nous avons ainsi déterminé, par cette approche établieinitialement par C.R. Houska [4,5], pour chaque échantillon une distribution de composition. Les diagrammes de diffraction ont été modélisés de façon globale à l’aide du logiciel MAUD [6]. Laposition angulaire de chaque pic a permis de remonter à la composition cationique via la valeur duparamètre de maille et la loi de Végard du système choisi [3]. Une forte corrélation entre lescaractéristiques de la distribution de compositions cationiques et les régimes de solidification imposésau corium a pu être identifiée. Cette distribution cationique engendre une hétérogénéité du solide,dont il va falloir déterminer l’échelle et le degré dans les études futures.[1] D. Jacquemainl. (2013). EDP. ISBN : 978-2-7598-0972-1.[2] P. Buisson. (1999). Thèse université joseph Fourier, Grenoble (France).[3] P. Piluso, G. Trillon, C. Journeau. (2005). J. Nucl. Mat. 344, 259–264.[4] C.R. Houska. (1975). Thin Solid Films, 25, 451.[5] C. R. Houska. (1970). J. Appl. Phys, 41, 69.[6] L. Lutterotti, R. Ceccato, R. Dal Maschio, E. Pagani. (1998). Mater. Sci. Forum, 87, 278–281

Study of the local compositions fluctuation in U1-xZrxO2 y solid solutions (Corium) during differentcooling / solidification processes

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Experimental phase diagram study of the Fe–Ni–Te system

International audienceThe Fe–Ni–Te alloys have been experimentally studied using Isothermal heattreatments with SEM-EDS and WDS, XRD, and DTA in order to add phasediagram data for a thermodynamic assessment. This is for the application offission product-induced corrosion of the stainless steel cladding of nuclear fuelpins, where Te is a key element. Phase diagram data is presented at 700 Cand 800 C, together with the solidus and liquidus temperatures across theentire system

Cationic local composition fluctuations in rapidly cooled nuclear fuel melts

X-ray spectroscopy of lithium is very difficult, even impossible, with wavelength dispersive spectrometers commonly deployed on scanning electron microscopes or electron microprobe analyzers. This is due to the absence of crystals and lack of efficient periodic multilayer for this spectral range, around 50 eV. To address this issue, we propose using a Be/Si/Al multilayer having a period of about 29 nm. The multilayer was deposited by magnetron sputtering and its reflectivity measured as a function of the glancing angle in the spectral range of the Li K emission and as a function of the incident energy up to ~200 eV. This characterization demonstrates that the designed multilayer is suitable to efficiently perform spectroscopy in the range of the Li K emission in terms of reflectance (0.32 at 51.5 eV), bandwidth (around 3.5 eV) and rejection of high order diffracted radiation

Crystal field effects on the photoemission spectra in Cr2O3 thin films: from multiplet splitting features to the local structure

International audienceChanges in the shape of X-ray photoemission (XPS) spectra can be related to changes in the local structure of a transi-tion metal. By combining Crystal Field Multiplet calculations and well-controlled molecular beam epitaxy growth of alpha-Cr2O3(0001) thin films on alpha-Al2O3(0001) substrates, we prove that it possible to link the features of Cr 2p XPS spectra with local distortions of CrO6 octahedra and d-orbitals reorganization. Hence, we show that the splitting of the Cr 2p3/2 envelope is related to the degeneracy of the t2g orbital triplet, which corresponds to a fully relaxed structure. Conversely, the broad unstructured Cr 2p3/2 envelope relies on splitting of t2g orbitals and it is the fingerprint of large trigonal distor-tions of CrO6 octahedra. Then, using the Cr 2p XPS as a structural tool for -Cr2O3, we show that the Cr2O3 protective layer formed by oxidation of polycrystalline Ni30Cr alloy exhibits in-plane strains at early oxidation stages and grows preferentially along the c-axis

TRANSIENT DEM SIMULATION OF STARCH GRANULE SUSPENSION UNDERGOING THERMAL TREATMENT

International audienceDuring hydrothermal treatment of starch suspension, the starch granules swell with kinetics depending on temperature evolution. Variability of swelling characteristics between starch granules was also observed. A semi-mechanistic and semistochastic model for individual starch granule swelling was developed and coupled with a discrete element (DEM) model. The DEM model takes into account the normal elastic forces exerted between the granules when they come in contact and optimally occupy the available volume. The evolution of particle volume fractions, size dispersion, the fraction of overlap volume and elastic energy per unit volume were analysed in the case of chemically modified waxy maize starch. It appears that for this particular starch all the jamming indicators increase rapidly for a volume fraction of about 0.65