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International audience; Powders and divided solids are widely used in industry as raw, intermediate or finished products in many fields foods, cosmetics, construction, pharmaceuticals, transport, electronics and nuclear energy. The improvement of the manufacturing processes requires control of processing steps based on an understanding of the phenomena involved (sintering, chemical reactivity, purity, etc.). Modelling and understanding these phenomena require data and characteristics that may be difficult to obtain. The present work mentions examples illustrating different physicochemical characterisation techniques suitable for analysis of uranium based powder samples (oxide, carbide, fluoride and metallic) in the raw state or after preparation (i.e. ion polishing) used in nuclear cycle. The first section discusses dimensional characterisation (particle size, morphology, porosity) by means of image analysis and preparation techniques SEM, TEM, FIB, electron and X ray tomography with possible 2D and 3D image analysis. Morphological data in the form of the powder size and shape can be obtained with new developments in 2D analysis of processed SEM images. Different raw grain of NIST powder, oxalate UNd, and Cu powder has been measured and compared with laser granulometer. A volumetric shape factor was determined for needle particle UNd.Closed porous squeletum of powder could be determined by SEM/FIB cutting and tomography calculation. An example of work in powder UO2 was showed (Fig 1).The second part of this work describes elemental chemical characterisation. Low-voltage EDS spectroscopy on polished powder cross sections is used to map the distribution of phases and to detect possible impurities in the solid grains and agglomerates. The contribution of this type characterisation to understanding the reaction mechanisms of phase transformations (especially solid-gas reactions) is illustrated for the hydro-fluorination of uranium dioxide. Sample preparation by mounting followed by ion polishing, optimisation of the EDS analysis parameters, and the contribution to solid-gas reaction models are discussed in this study. Complementary surface analyses as Auger spectrometry allows powder surface oxidation. An example was used on UAl powder and the detection of UO2 thin film (50 nm) by this ionic abrasive technique. The third section describes the characterisation of the chemical reactivity of powders and property mechanics. Chemical reactivity are performed by high-temperature in situ treatment in an environmental SEM, and allow observation of the powder morphology transformation. UC powder has been treat by high-temperature and we could see a growth of grain size. Property mechanics are performed by in situ treatment in an SEM FEG with plate of compressive force. In case of fundamental study in compressive nuclear fuel, simulant CeO2 powder has been tested in compressive test force with 15 mN. The result has been compared with DEM simulation (Fig. 2).This new analytical capacities allows strong supports to understand the reactivity mechanism of materials used in nuclear fuel fabrication processes.

International audience; Studies dedicated to the chemical durability of spent nuclear fuel are usually based on the overall inventory of elements present in solution under various dissolution conditions. However, these studies do not allow identifying and quantifying the role of the different phases present in the solid on the dissolution rates. Therefore, it is necessary to evidence separately the effect of the main fission products incorporated in the uranium dioxide structure or precipitated at grain boundaries on the evolution of the microstructure and the dissolution rates. First of all, different model compounds containing fission products were synthesized from oxalic and hydroxide precursors [1]. Uranium dioxide based compounds with lanthanides molar ratio in the range 1 to 10 mol. % were prepared (with 13 % La; 7.4 % Y; 25.6 % Ce; 12 % Pr; 42 % Nd). Divalent (Ba, Sr) or monovalent (Cs, Rb) cations were also incorporated separately in the UO2 matrix. Finally, samples of uranium dioxide containing 0.6 to 3 mol. % of precipitated platinoids (with 55 % Ru; 9.6 % Rh; 35.4 % Pd) were also obtained. Then, the starting precursors were converted into oxides, pelletized and sintered. The aim of these syntheses and heat treatments was to obtain pellets with fission products amounts, structure and microstructure representative of spent nuclear fuel. Different characterizations have been done. On the one hand, concerning Uranium dioxide based compounds with lanthanides, XRD analyzes allowed to identify the crystal lattice as the CaF2 fluorite structure Pm3 et773;m, characteristic of (+IV) uranium dioxide. On the other hand, concerning the other kinds of fission products, an additional phase was present in addition of the CaF2 fluorite structure Pm3 et773;m. Furthermore to control the total incorporated amount of fission products in the UO2 matrix, X-EDS analyzes on oxides and ICP-AES analyzes of oxides total dissolution were done. A multiparametric study of the dissolution was achieved in concentrate nitric acid in order to simulate the reprocessing conditions and to highlight the effect of the fission products on the overall dissolution rates. Furthermore, the microstructural evolution of the pellets during the dissolution was monitored by ESEM. The aim of the study was to identify the preferential dissolution zones and to allow a better understanding of the impact of the fission products on the dissolution mechanisms.

International audience; Powders and divided solids are widely used in industry as intermediate or finished products in many fields foods, cosmetics, construction, pharmaceuticals, transport, electronics and, of course, nuclear energy. Optimizing their use requires control of processing based on an understanding of the phenomena involved (sintering, chemical reactivity, purity, etc.). Modeling and understanding these phenomena require data and characteristics that may be difficult to obtain. The present work cites examples illustrating different physicochemical characterization techniques suitable for analysis of uranium based powder samples (oxide, carbide, fluoride and metallic) in the raw state or after preparation (ion polishing) used in nuclear cycle. The first section discusses dimensional characterization (particle size, morphology, porosity) by means of image analysis and preparation techniques SEM, TEM, FIB, electron and X ray tomography with possible 2D and 3D image analysis. Morphological data in the form of the powder size and shape can be obtained with new developments in 2D analysis of processed SEM images. The second part of this work describes elemental chemical characterization. Low-voltage EDS spectroscopy on polished powder cross sections is used to map the distribution of phases and to detect possible impurities in the solid grains and agglomerates. The contribution of this type characterization to understanding the reaction mechanisms of phase transformations (especially solid-gas reactions) is illustrated for the hydrofluorination of uranium dioxide. Sample preparation by mounting followed by ion polishing, optimization of the EDS analysis parameters, and the contribution to solid-gas reaction models are discussed in this study. Complementary surface analyses as Auger spectrometry allows powder surface oxidation. The third section describes the characterization of the chemical reactivity of powders and property mechanics. Chenical reactivity are performed by high-temperature in situ treatment in an environmental SEM, and allow observation of the powder morphology transformation. Property mechanics are performed by in situ treatment in an SEM FEG with plate of compressive force. The examples discussed concern materials used in nuclear fuel fabrication processes.

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International audience; A crystallization study of lithium niobium silicate glass with the molar composition 35Li$_2$O.25Nb$_2$O$_5$.40SiO$_2$ is reported using in situ techniques (x-ray diffraction, Raman spectroscopy and environmental electron microscopy) at high temperatures. Those techniques reveal that the surface crystallization begins at Tg with c-axis crystallites oriented perpendicular to the surface. Precipitation of nuclei, followed by growth and coalescence processes, induce the formation of particles and spherulites 0*5-4 μm diameter in size. A strong difference between surface and bulk crystallization has been observed with bulk crystallization yielding to 30 μm diameter size spherulites. Those original spherulite shapes could be the origin of the macroscopic bulk second harmonic generation (SHG) signal.