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International audience; Despite the very high level of control and safety in Nuclear Power Plant, they are not exempt from device malfunction, human errors or even natural disasters, which may lead to nuclear incidents or accidents. When consequences involve a high degradation of the reactor core (melting), associated with the release of Fission Products (FPs) and other radioactive materials from the reactor core, it is classified as a Severe Accident (SA, TMI-2, Chernobyl and Fukushima for instance). These released FPs may be transported by air masses or water and thus cover extensive areas and affect all living beings due to their radiological effect and may as well act as poisons. The amount and isotopic composition of the radioactive material released from the core is called Source Term, and its assessment has been the main objective of several international research programs for more than thirty years. These research programs are commonly classified into two main groups, according to their approach (1) Integral programs, such as PHEBUS FP, studying the response of a whole nuclear core during a severe accident, in a reduced scale; (2) On the other hand, analytical programs studying the fuel and FPs when submitted to accidental conditions, by means of Separate-Effect Tests (SET). Examples of the latter are the HI/VI, VEGA, VERCORS and VERDON programs. As result from all the research programs an extensive experimental database has been generated. However, up to now, predicting correctly the FPs release from UO2 and/or MOX fuels in SA conditions is still a significant and very important challenge since there are many remaining uncertainties. In order to improve these estimations, the global fuel and FPs behavior during the accidental sequence must be better understood, and specific emphasis has to be put on mechanisms which promote FPs release and fuel relocation. One of the most useful ways to do that is to perform appropriated annealing treatments with representative thermal transients in order to measure the absolute level and kinetics of the released FPs. To understand the promoting mechanisms, theses FPs release measurements have to be coupled with the corresponding fuel micro-structural changes resulting from these thermal transient.To this end, since the last decade, CEA has set up two complementary research axes, aiming at reproducing conditions representative of nuclear severe accidents, using both high burn-up irradiated fuel samples and model materials. The first axis corresponds to the VERDON program and deals with commercial UO2 and MOX fuels irradiated in French PWR. Model materials (often called SIMFUELS) consist in natural UO2 doped with stable isotopes of FP in concentrations that match a targeted burn-up. Therefore, SIMFUELS are representative of irradiated nuclear fuels but without their radioactivity. The importance of such materials lies in the possibility of using powerful characterization techniques, such as X-ray Absorption Spectroscopy, which today are unavailable for large samples of irradiated nuclear fuels.The present paper, organized in four main parts, presents successively the experimental facilities available at the CEA Cadarache and Marcoule centers together with the corresponding RetD axes SA experimental VERDON laboratory and associated annealing test device as MERARG, Analytical and micro analysis laboratories by which all the pre- and post-test fuels examinations, supported by analytical development on simulated corium samples, are performedUse of SIMFUELS methodology.The last part of the paper focuses on results obtained with this general approach, with special emphasis on VERDON-1 test.

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; 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.

International audience; The confinement of high-level radioactive waste from spent nuclear fuel reprocessing is a key issue in the nuclear fuel cycle. In France, waste storage concept is based on four containment barriers including respectively glass matrix for radioactive waste immobilization, a primary stainless steel canister, a thick carbon steel overpack and finally a host rock. Numerous researches are carried out on nuclear glass to improve the properties of the confinement material. Depending on glass composition and process parameters crystals can appear in the matrix (Fig. 1). The characterization of these phases (range of micrometer size) and of the glass surrounding them is of great importance for a proper understanding of the glass properties and its long term performance. The composition of these phases can be characterized by means of electron microprobe (CAMECA SX 100) with a standard resolution electron beam. In order to improve the quantification of such small dimension inclusions, the use of FEG-EPMA CAMECA SX FiveFE has been tested to determine their chemical composition which is well adapted to micrometer range analysis. The improved analytical resolution obtained with the FEG-EPMA made it possible to optimize the analysis of the micro particles, and to determine their chemical composition. The nature of the crystal phases was confirmed by EBSD electronic diffraction analysis coupled with SEM and approved the crystalline phases of apatite and cerianite measured by EPMA. To study in a more precise way the glass matrix near crystals, thin lamellas were prepared by ionic cut by FIB (Fig. 2). A microstructural analysis by TEM was performed to characterize the glass matrix and the inclusions and the chemical composition was settled by EDS quantification. This thin lamella were analyzed by EPMA in transmission with the microprobe CAMECA SX 100 to get a more complete analysis of elements, including especially some light elements such as Boron. The results of TEM EDS and EPMA analyses in transmission are self-consistent and confirm a peculiar enrichment in Zr of the glass matrix near crystals.

International audience; All over the world, several nuclear facilities have been shut down few years ago and are now under dismantling or will be dismantled in the future.Dismantling project takes place according to three major phasesStudy and definition of the dismantling scenarioDismantling site operationsDecommissioning and final state characterization.Each step is lead in agreement with safety, security and waste management. This requires analytical tools appropriate to the radiological environment and aims. For instance, for safety, fissile products, explosives... will be researched and quantified while security requires quantification of radiological, carcinogenic, mutagenic and reprotoxic elements such as As, Se, Cr(IV) ... or even asbestos in order to follow more and more strengthened rules. For waste management, radiological and chemical quantification are necessary but must be completed by determining physical and chemical properties such as reactivity, stability or dissolution of the waste...Required level of knowledge, analytical deadline, uncertainties, limits of detection and matrix variety depend on the decommissioning phase.Setting up a dismantling scenario requires the most possible accurate knowledge of the initial source term present in the facility, in order to establish waste management flow-sheets, define dismantling intervention modes, and prepare the associated safety demonstrations. As history is often not sufficient, in-situ investigations and off-site laboratories are necessary to assign an initial waste and radiological inventory and propose some strategy operations.When a first scenario is chosen, more detailed samplings and analyses are required to fully prepare estimates of waste volumes and categories, and define and validate the extraction waste system would be used during the dismantling operation. As the definition of the dismantling plan takes shape, data must be more and more accurate and reliable in order to define the waste outlet. The scaling factors for the waste must be defined in order to assure the refunding of the waste. This includes complete characterizations of various matrixes, with high precision, low detection limit in a limited time of few months, as sketchy data could lead to fix inappropriate plan.Once site operations begin, it is necessary to control the safety operational conditions and waste production. Measurements requirement is under the safety threshold. Matrixes are often limited to filter. The most important factor is deadline, because operations delay is costly.Finally at the end of dismantling operations, analyses must be performed to demonstrate the successful clean-up of the work site. Analytical methods with low detection limit are required.Thus, characterization appears as a graded and iterative process and is a key issue for decommissioning and dismantling operations in term of efficiency and cost. In order to face this huge analytical demand, a strategy for sampling and characterization must be developed regarding the real needs. On site measurements are usually performed with existing hand-held instruments. A basic first level of knowledge is obtained without delay, feeding the dismantling scenario. On these bases, a sampling strategy permits to select some representative specimens to be more precise, and accurate analysis is performed on off-site laboratories introducing an additional difficulty the sampling on the site and sub-sampling in the laboratory itself. Whatever the level of sampling, aliquots are small considering the dimension of the dismantling area.This step is crucial. Sampling strategy must be optimized according to objectives (scaling factors measurement, waste level). But in the particular case of high radioactive level waste, due to security manipulation constraints, the number and mass of samples collected are limited. At the opposite for low level waste, large number of samples is collected meaning that large analytical method capacity is necessary.Optimization of the analytical plan is critical. The analytical strategy (including control of subsampling, analysis sequence according to analytical capability and performances, especially for pure beta emitters) developed for real case will be detailed.

International audience; The LECA-STAR is a CEA nuclear Hot Lab facility which is dedicated to RandD on irradiated fuels. It has been designed to analyze the behavior of a wide range of nuclear industrial and MTR fuel under irradiation. These capabilities are a key point in the fuel qualification/development cycle.The facility is divided in two parts, the LECA (Laboratoire d'Examens des Combustibles Actifs, 1964) and STAR (Station de Traitement, d'Assainissement et de Reconditionnement, 1994) which are dedicated to- Non-destructive and destructive examinations (PIE) and annealing tests on irradiated fuels of different types and geometry, in order to characterize and determine the fuel behavior. Collected data and mechanisms understanding are used to establish laws and models which allows predictive in service-behavior and performance.- Manufacturing of fuel rodlets from irradiated industrial rods for in-pile testing in MTR,- CEA irradiated spent fuel processing and packaging.Over 180 people work daily on the different processes and technical examinations tools available in the fifteen different hot cells (size up to 9m long) and in the LECA microanalysis laboratory which offers a unique and complementary combination of analytical equipment. The work of the staff is organized on the basis of the time slot of opening of the facility,( ie 6h-21h), in normal or shifted schedule either in round-the-clock operation, according to the function and type of activity.Inside the facility, the operating team insures the maintenance, operation and nuclearization of devices in compliance with the Safety and Security rules and is responsible for the deployment and supervision of the processes (design and manufacturing), with the aim to provide high availability for the activities of the experimental teams.Based on its current supply of services, and in view of the future CEA Jules Horowitz (JHR) reactor startup, the LECA-STAR launched actions to renew some of its equipment and launched an approach in order to upgrade its processes through innovative technologies.The presentation will aim at putting forward the prospects of the installation in connection with the future reactor JHR located on Cadarache site.

International audience; Absolute Mα x-ray production cross sections have been measured and calculated for heavy elements Au, Pb, Bi, and U for electron incident energies from the ionization threshold up to 38 keV. The experimental cross sections were deduced from M x-ray intensities emitted from ultrathin samples of the elements of interest deposited onto self-supporting C films. The measurements were performed using two electron microprobes and several wavelength-dispersive spectrometers. X-ray intensities were converted into x-ray production cross sections by using estimated values of the number of incident electrons, target thickness, and spectrometer efficiency. Experimental results are compared with calculated cross sections using different predictive formulae. Good agreement is found between the measured cross sections and the predictions from the distorted-wave Born approximation, which indicates that this approximation is suited for the calculation of M-shell ionization cross sections of heavy elements.

Measurements of absolute M-subshell X-ray production cross sections for element Th were made by electron impact for energies ranging from the ionization threshold up to 38 keV. Experimental data were obtained by measuring the X-ray intensity emitted from ultrathin Th films deposited onto self-supporting C backing films. The measurements were conducted with an electron microprobe using high-resolution wavelength dispersive spectrometers. Recorded intensities were converted into absolute X-ray production cross sections by means of atomic data and estimation of the number of primary electrons, target thickness, and detector efficiency. Our experimental X-ray production cross sections, the first to be reported for the M subshells of Th, are compared with X-ray production cross sections calculated with the mean of ionization cross sections obtained from the distorted-wave Born approximation. The Mα X-ray production cross section calculated is in excellent agreement with the measurements, allowing future use for standardless quantification in electron probe microanalysis.

International audience; The CEA contributes to the development of the reactors selected under the Generation IV International Forum, which will be commissioned in the middle of this century. For fast neutron reactors the French reference concept is the Sodium-cooled Fast Reactor (SFR); however, an alternative is the Gas-cooled Fast Reactor (GFR) using helium as heat-transfer fluid and operating at high temperatures (1000°C–1100°C). The fuel is constituted by a mixed uranium-plutonium carbide and the cladding consists of several layers of ceramic materials (SiC) and a metal liner. Thermochemical studies are conducted by the CEA to investigate the chemical compatibility of both fuel and cladding under nominal and accidental conditions (2000°C).In order to compile a thermodynamic database on the formation of liquid ternary phases at high temperatures, USiC samples have been prepared with various chemical compositions then heat treated within a range of 1630°C to 1920°C. Characterization of the formed phases requires the use of a variety of analytical tools, including optical microscopy, XRD, SEM, and EPMA.The measurement of light elements — particularly C but also Si — in the presence of uranium by microanalysis is still complex. We have identified some analytical key points to improve the methodology taking into account the phenomena that could bias the measurement, including:‒native oxidation of UC,‒spectral interference with U, C, O for carbon measurement,‒carbon contamination by the electron beam,‒sample preparation for conductivity.The method was validated on carbides measurement standards, before applying it USiC samples. The chemical compositions measured by means of a electron microprobe on the ternary phases formed at equilibrium seemed to be in good agreement with the phases measured by XRD, and were used to determine the USiC ternary diagram at 1800°C.

International audience