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

Physico-chimie et modélisation du frittage des combustibles d'oxydes d'actinides

This report gives a synthesis of the work I have carried out or to which I have numerically contributed to from 1996 up to 2012 in the Department of Plutonium Uranium and minor Actinides in Cadarache CEA Center. Their main goal is the study and the modeling of the sintering process of nuclear fuels which is the unifying thread of this document. Both in order to take into account the physical andchemical features of the actinide bearing oxide material and in order to combine the different transport phenomena leading to sintering, a sub-granular scale model is under development. Extension to a varying chemical composition as well as exchanges with the gaseous phase are foreseen. A simulation on a larger scale (pellet scale) is ongoing in the framework of a PhD thesis. Validation means have been tested with (U,Pu) O2 material on the scale of the pellet (Small Angle Neutron Diffusion), on the scale of powder granules (X-Ray High Resolution Micro-Tomography) and with CeO2 at the 'Institut de Chimie Séparative' in Marcoule on a single crystal scale (Environmental Scanning Electron Microscope). The required microstructure homogeneity for nuclear fuels has led to a campaign of experimental studies about the role of Cr2O3 as a sintering aid. Whole of these studies improve our understanding of fuel sintering and hence leads to an improved mastering of this process.Ce document synthétise les travaux que j'ai menés ou auxquels j'ai contribué en tant que support numérique entre 1996 et 2012 au Service Plutonium Uranium et Actinides mineurs du CEA Cadarache. Ces travaux sont essentiellement orientés vers l'étude et la modélisation du frittage du combustible nucléaire qui servent de fil conducteur au mémoire. La modélisation du frittage a été abordée à une échelle sub-granulaire afin d'une part de prendre en compte les spécificités physico-chimiques des oxydes d'actinides qui constituent le matériau et d'autre part de pouvoir combiner les divers phénomènes de transport à l'origine du frittage. Le modèle se limite actuellement au cas d'une composition chimique constante à l'échelle sub-granulaire, sans échanges avec la phase gaz. Sonextension à une composition variable est prévue. Des travaux sont en cours vers une simulation à l’échelle d'un volume représentatif de la pastille. Des moyens de validation à l'échelle de la pastille (Diffusion de Neutrons aux Petits angles), à l'échelle des granulés de poudre (microtomographie X Haute Résolution) ont été testés sur l'oxyde (U,Pu)O2 ; d'autres sont en cours d'étude à l'Institut de Chimie Séparative de Marcoule à l'échelle du grain monocristallin sur CeO2 (Microscopie Electronique à Balayage Environnemental). L'homogénéité requise pour la microstructure du combustible a amené à étudier expérimentalement le mode d'action de certains adjuvants de frittage tels que Cr2O3. L'ensemble des travaux permet d'améliorer la compréhension que l'on peut avoir du frittage et, de là, à pouvoir en assurer une meilleure maîtrise

Hydrothermal Reactivity of Mixed-Layer Kaolinite/Smectite: Experimental Transformation of High-Charge to Low-Charge Smectite

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Assessing the oxygen stoichiometry during the sintering of (U, Pu)O2 fuel

International audienceDiffusion phenomena occurring in ceramics such as (U, Pu)O2 during sintering are affected by the oxygen content in the atmosphere. The latter sets the nature and the concentration of point defects which govern diffusion mechanisms in the bulk of the material. The oxygen partial pressure, pO2, of the sintering gas in equilibrium with mixed oxide (MOX) pellets needs to be precisely controlled; otherwise it may induce a large dispersion in the critical parameters for fuel manufacturing (Gauche, 2013; Matzke, 1987). It is crucial to understand the relation between the sintering atmosphere and the fuel throughout the thermal cycle. In this study, the oxygen potential of the sintering gas was monitored by measuring the oxygen partial pressure (pO2) at the outlet of a dilatometer by means of a zirconia probe. Coupling the thermal cycle with an outlet gas pO2 measurement makes it possible to identify different redox phenomena. Variations in the oxygen stoichiometry can be determined during the sintering of (U, Pu)O2, as well as can its final O/M. Our results make it possible to recommend a sintering atmosphere and sintering thermal cycle in order to obtain an O/M ratio that is as close as possible to the target value

Positron Annihilation Spectroscopy Characterisation of Non−Stoichiometric Uranium Dioxide

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Novel approaches for the in situ study of the sintering of nuclear oxide fuel materials and their surrogates

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Contribution to the study of fission products release from nuclear fuels in severe accident conditions: effect of the pO 2 on Cs, Mo and Ba speciation

International audienceThe objective of this work is to experimentally investigate the effect of the oxygen potential on the fuel and FP chemical behaviour in conditions representative of a severe accident. More specifically, the speciation of Cs, Mo and Ba is investigated. These three highly reactive FP are among the most abundant elements produced through 235U and 239Pu thermal fission and may have a significant impact on human health and environmental contamination in case of a light water reactor severe accident. This work has set out to contribute to the following three fields: providing experimental data on Pressurized Water Reactor (PWR) MOX fuel behaviour submitted to severe accident conditions and related FP speciation; going further in the understanding of FP speciation mechanisms at different stages of a severe accident; developing a method to study volatile FP behaviour, involving the investigation of SIMFuel samples manufactured at low temperature through SPS. In this paper, a focus is made on the impact of the oxygen potential towards the interaction between irradiated MOX fuels and the cladding, the interaction between Mo and Ba under oxidizing conditions and the assessment of the oxygen potential during sintering

From in Situ HT-ESEM Observations to Simulation: How Does Polycrystallinity Affects the Sintering of CeO2 Microspheres?

International audienceThe in situ observation of the first stage of sintering of cerium dioxide microspheres was performed using an environmental scanning electron microscope at high temperature (HT-ESEM). The associated morphological modifications were described quantitatively for systems constituted by two single crystal grains, on the one hand, and by two polycrystalline particles, on the other hand. Particularly, the in situ HT-ESEM observations, and subsequent image analysis with homemade image process software, led to assess the evolution of several parameters of interest during isothermal heat treatments, such as neck size, particles radii, dihedral angles between the spheres, and distance between the grains centers. It was then possible to evaluate the activation energies associated with the neck formation for both systems studied, then to identify the different mechanisms involved. The diffusion process operating during the first stage of sintering was also pointed out. Furthermore, the comparison of the results obtained from polycrystalline particles and single crystals, and their confrontation with data coming from numerical computation, led to assess the influence of polycrystallinity on the sintering kinetics. For all the conditions tested, sintering degree was found to be enhanced for polycrystalline particles, mainly because of the contribution of the mechanical rearrangement of crystallites during the neck’s elaboration and of the existence of diffusion paths within the particles. On this basis, polycrystallinity should be considered during numerical computations in order to provide predictive models for the first step of sintering