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Dans le cadre de ce travail, nous nous sommes intéressés à un concept innovant d’empoisonnement homogène des neutrons par insertion de terres rare (erbium et gadolinium) dans les gaines en alliage de zirconium pour les réacteurs à eau pressurisée. L’étude des équilibres entre phases des alliages erbium–zirconium et gadolinium–zirconium est indispensable comme préalable à la mise en oeuvre industrielle de ce procédé prometteur d’empoisonnement. Ce travail a consisté à déterminer expérimentalement le diagramme de phases du système erbium–zirconium. Nous avons, par le biais de différentes caractérisations, obtenu des données diagrammatiques. Avec celles-ci, nous proposons un nouveau tracé du diagramme de phases. Celui-ci est radicalement différent de celui disponible dans la littérature. Nous avons modélisé le système par l’approche CALPHAD. Nous avons également déterminé les limites de solubilité des solutions solides terminales du système gadolinium–zirconium. Les données obtenues expérimentalement sont en accord avec le tracé expérimental de la littérature et avec le modèle thermodynamique disponible. Afin de prendre en compte l’oxydation des gaines en service, nous nous sommes également intéressés aux systèmes erbium–oxygène–zirconium et gadolinium–oxygène–zirconium. Le premier système a fait l’objet d’une étude expérimentale. Nous avons mis en place un procédé de synthèse par métallurgie des poudres, incluant la synthèse de celles-ci à partir de métaux massifs. La caractérisation des échantillons ternaires nous a permis de proposer deux coupes isothermes (800°C et 1100°C). Pour le système gadolinium–oxygène–zirconium, nous avons prédit les équilibres entre phases à différentes températures à l’aide de calculs effectués à partir d’une base de données que nous avons construite avec les modèles thermodynamiques de la littérature des systèmes oxygène–zirconium, gadolinium–zirconium et sesquioxyde de gadolinium–zircone. Enfin, nous avons travaillé avec des alliages erbium–zirconium fabriqués en milieu industriel. Nous nous sommes intéressés à leurs propriétés mécaniques en traction, en lien avec leur microstructure. Nous avons mis en évidence l’effet durcissant de l’erbium, notamment à 325‰°C / This work is a contribution to the development of innovating concepts for fuel cladding in pressurized water nuclear reactors. This concept implies the insertion of rare earth (erbium and gadolinium) in the zirconium fuel cladding. The determination of the phase equilibrium in the systems is essential prior to the realisation of such a promising solution. This study consisted in the experimental determination of the erbium–zirconium phase diagram. For this, we used many different techniques to get diagram data like solubility limits and solidus, liquidus or invariant temperatures. With these data, we were able to give a new diagram, very different from the literature one. With the experimental data we collected, we also assessed the diagram, using the CALPHAD approach. In this work, we also determined the solubility limits of the gadolinium–zirconium system. Those limits had never been determined before, and the values we obtained are in excellent agreement with the experimental and with the assessed diagrams. Because these alloys are subjected to oxygen diffusion throughout their life, we focused our attention on the erbium–oxygen–zirconium and gadolinium–oxygen–zirconium system. The first system has been investigated experimentally.We used many different synthesis techniques, and we finally have opted for a powder metallurgy one. As raw material, we fabricated powder from erbium and zirconium bulk metals using hydrogen absorption/desorption. With the formed ternary pellets, we investigated the phase equilibria at 800°C and 1100°C. With the obtained data, we propose two isotherms at those two temperatures. For the gadolinium–oxygen–zirconium system, we calculated the phase equilibria at temperatures ranging from 800°C to 1100‰°C, using a homemade database compiled from literature assessments of the oxygen–zirconium, gadolinium–zirconium et gadolia-zirconia systems. We also determined the mechanical properties, in connexion with the microstructure, of industrial quality alloys in order to identify the influence of erbium content. We highlighted the hardening influence of erbium at 325‰°C

A otimização de combustíveis nucleares para uso em reatores a água pressurizada pode ser obtida pelo aumento da taxa de queima do combustível. Para isso, no entanto, é necessário levar em conta o aumento na reatividade inicial no reator, causada pelo maior enriquecimento do combustível. Esse problema, por sua vez, pode ser contornado por meio da introdução dos chamados venenos queimáveis diretamente nas pastilhas combustível de UO2. Alguns elementos do grupo das terras-raras possuem propriedades físicas e químicas que os tornam apropriados para esse uso dentro de reatores. Para caracterizar a microestrutura do combustível UO2 utilizado em reatores a água pressurizada dopado de érbio, pastilhas de UO2-Er2O3 foram preparadas, com teor de Er2O3 variando de 1,0 a 9,8 wt%, e analisadas por difração de raios X (DRX) para determinar se houve a formação de solução sólida pelo composto e determinar a variação do parâmetro de rede da solução em função da concentração de érbia. Apesar da análise por DRX ter mostrado que todo o érbio se incorporou à rede de UO2, ela também evidenciou a emergência de uma segunda fase, de estrutura do tipo fluorita, e cuja fração mássica aumenta em função do teor de érbia, enquanto seu parâmetro de rede diminui. Esses resultados são compatíveis com o fenômeno de segregação de defeitos, que consiste na formação de microdomínios segregados da rede principal nos quais há uma concentração maior dos defeitos i.e. são regiões mais ricas em érbio. Assim, a análise por DRX mostrou que houve formação de solução sólida de (U,Er)O2, mas que são necessários ajustes nos parâmetros de sinterização para que seja obtida uma solução monofásica. / Optimization of nuclear fuel for use in pressurized water reactors can be achieved by obtaining higher burnups. This, however, requires the excess reactivity caused by increasing the fuels enrichment to be taken into account, which can be done by introducing burnable absorbers into the UO2 fuel pellets themselves. Some of the rare earth elements have thermal and mechanical properties that make them appropriate for use inside the reactor. In order to characterize the microstructure of erbium-doped UO2 fuel, sintered UO2-Er2O3 pellets were prepared, with Er2O3 content ranging from 1.0 to 9.8wt%, and analyzed by X-ray diffraction to determine whether the composite formed solid solutions and, if so, evaluate the lattice parameter as a function of erbia concentration. While XRD analysis showed the Er2O3 completely dissolved in the UO2 powder, it also evidenced the emergence of a second fluorite-type phase, whose phase fraction increases and lattice parameter decreases with increasing erbia concentration. Analysis of the diffraction patterns showed this emerging phase has the same crystalline structure as the host lattice, but with a smaller lattice parameter. These results are compatible with the phenomenon of defect segregation, which consists in the formation of microdomains with a higher concentration of defects i.e. rare-earth richer regions. Thus, XRD analysis showed the formation of (U,Er)O2 solid solution, but such that there are still adjustments in the sintering parameters that need to be made in order to achieve a single-phase solid solution.

A otimização de combustíveis nucleares para uso em reatores a água pressurizada pode ser obtida pelo aumento da taxa de queima do combustível. Para isso, no entanto, é necessário levar em conta o aumento na reatividade inicial no reator, causada pelo maior enriquecimento do combustível. Esse problema, por sua vez, pode ser contornado por meio da introdução dos chamados venenos queimáveis diretamente nas pastilhas combustível de UO2. Alguns elementos do grupo das terras-raras possuem propriedades físicas e químicas que os tornam apropriados para esse uso dentro de reatores. Para caracterizar a microestrutura do combustível UO2 utilizado em reatores a água pressurizada dopado de érbio, pastilhas de UO2-Er2O3 foram preparadas, com teor de Er2O3 variando de 1,0 a 9,8 wt%, e analisadas por difração de raios X (DRX) para determinar se houve a formação de solução sólida pelo composto e determinar a variação do parâmetro de rede da solução em função da concentração de érbia. Apesar da análise por DRX ter mostrado que todo o érbio se incorporou à rede de UO2, ela também evidenciou a emergência de uma segunda fase, de estrutura do tipo fluorita, e cuja fração mássica aumenta em função do teor de érbia, enquanto seu parâmetro de rede diminui. Esses resultados são compatíveis com o fenômeno de segregação de defeitos, que consiste na formação de microdomínios segregados da rede principal nos quais há uma concentração maior dos defeitos i.e. são regiões mais ricas em érbio. Assim, a análise por DRX mostrou que houve formação de solução sólida de (U,Er)O2, mas que são necessários ajustes nos parâmetros de sinterização para que seja obtida uma solução monofásica. / Optimization of nuclear fuel for use in pressurized water reactors can be achieved by obtaining higher burnups. This, however, requires the excess reactivity caused by increasing the fuels enrichment to be taken into account, which can be done by introducing burnable absorbers into the UO2 fuel pellets themselves. Some of the rare earth elements have thermal and mechanical properties that make them appropriate for use inside the reactor. In order to characterize the microstructure of erbium-doped UO2 fuel, sintered UO2-Er2O3 pellets were prepared, with Er2O3 content ranging from 1.0 to 9.8wt%, and analyzed by X-ray diffraction to determine whether the composite formed solid solutions and, if so, evaluate the lattice parameter as a function of erbia concentration. While XRD analysis showed the Er2O3 completely dissolved in the UO2 powder, it also evidenced the emergence of a second fluorite-type phase, whose phase fraction increases and lattice parameter decreases with increasing erbia concentration. Analysis of the diffraction patterns showed this emerging phase has the same crystalline structure as the host lattice, but with a smaller lattice parameter. These results are compatible with the phenomenon of defect segregation, which consists in the formation of microdomains with a higher concentration of defects i.e. rare-earth richer regions. Thus, XRD analysis showed the formation of (U,Er)O2 solid solution, but such that there are still adjustments in the sintering parameters that need to be made in order to achieve a single-phase solid solution.

L’excès de réactivité neutronique dans les centrales nucléaires est compensé par des sys-tèmes actifs de contrôle du réacteur : acide borique et barres de contrôle. L’apport d’antiréactivité peut se faire passivement avec des poisons consommables, c'est-à-dire des absorbants de neutrons, en particulier du gadolinium (Gd).Dans le cadre d’une augmentation de l’enrichissement en U²³⁵ et de réduction de l’utilisation d’acide borique, cette thèse a pour objectif d’optimiser la distribution du ga-dolinium dans des céramiques d’UO₂ afin d’obtenir un apport optimisé d’antiréactivité dans un Réacteur à Eau sous Pression.Dans ce sens, le travail est orienté à trouver des nouveaux matériaux riches en gadolinium. Le diagramme de phase U-O-Gd a donc été exploré dans le domaine des fortes teneurs en Gd. Deux phases cubiques ont été trouvées et caractérisées : les phases C1 et C2. En vue d’une application industrielle, la phase C1 a été retenue comme candidate pour l’ajout du Gd dans les pastilles d’UO₂.La distribution optimale de cette phase C1 dans les assemblages de combustible nucléaire a été étudiée avec le code de calcul neutronique APOLLO2.8. Des études paramétriques ont été réalisées. Ces études neutroniques ont aboutit à un concept performant de pastille empoisonnée. Finalement, des pastilles prototype ont été fabriquées en laboratoire suivant ce concept. L’ensemble des résultats obtenus montre qu’un concept de pastille avec un dépôt superficiel neutrophage de phase C1 est une manière d’apporter de l’antiréactivité de manière optimisée dans le cadre de cycles longs. Ceci pourrait potentiellement être appliquée à l‘échelle industrielle. Un brevet a été déposé en ce sens. / Reactivity excess in Nuclear Power Plants is controlled by reactor’s active systems: boric acid dilution and control rods. Alternatively, negative reactivity insertion can be made in a passive way using burnable poisons, i.e. neutron absorbers, this is the case of gadolinium (Gd).In the industrial framework of U²³⁵ enrichment increase and boric acid restraint, the goal of this thesis is to optimize the distribution of gadolinium in UO₂ ceramics to obtain a high-performance provision of negative reactivity in Pressurized Water Reactors.In this sense, the work is focus on new gadolinium-rich materials. Thus, U-Gd-O phase diagram was explored in the field of high Gd contents. Two cubic phases were found and characterized: the C1 and C2 phases. With the aim of an industrial application, C1 phase was selected as candidate for Gd addition into UO₂ pellets.The optimal distribution of C1 phase within a nuclear fuel assembly was studied using APOLLO 2.8 neutron transport code. Parametrical calculations were performed. These neutronic studies have ends in a successful “concept of poisoned pellet”.Finally, some prototype pellets following this concept were made in laboratory to proof it feasibility.All the obtained results shows that the proposed concept of a neutrophage C1-phase coating on UO₂ pellets is a convenient way to reduce reactivity excess within the framework of long irradiation cycles. This concept could be potentially applied in industrial scale. Consequently a patent application process was initiated.