Thermomechanical compatibility of SiC/SiC composites as structural materials for Sodium-cooled Fast Reactors

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Corrosion of metallic materials in flowing liquid lead-bismuth

Corrosion tests of martensitic and austenitic steels were performed in a forced liquid eutectic Ph-Bi circulation loop. Experiments were carried out at 470°C and 600°C. Two oxygen concentrations in Pb-Bi were studied: $10^{-6}$ wt% and $10^{-7}-10^{-8}$ wt%. The results showed that at 470°C, all the tested steels have a satisfying corrosion behaviour for both oxygen contents. An oxide layer is formed on martensitic steels (T91 and EM10); its thickness depends on the oxygen content. It is constituted of an outer layer of magnetite and an inner (Fe, Cr)$_3$O$_4$ spinel layer. Austenitic steel 316L is protected by a very thin oxide layer ($<$ 1 $\mu$m). At 600°C, martensitic steels (T91 and EM10) undergo an important oxidation for both oxygen contents (after 1000h, the thickness of the oxide layer varies from 10-15 $\mu$m to 20-25 $\mu$m depending on the oxygen content). The oxide layer is constituted of (Fe, Cr)$_3$O$_4$ spinel and appears porous. Austenitic 316L undergoes severe dissolution at the lowest oxygen content in Ph-Bi and partial dissolution and oxidation at the highest oxygen concentration. Erosion phenomena were observed on all the steels

Evaluation of the chemical compatibility of SiC/SiC composite materials in nuclear environments

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Liquid metal embrittlement of an austenitic stainless steel in liquid sodium

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Corrosion of Fe-9Cr Steels in Sodium Fast Reactors Environments

The sodium cooled fast reactor is selected by France as the most mature GEN IV concept to be industrially developed by the year 2040. A collaborative research program has been established together with EDF and Areva NP. In this program, different innovations are being considered in the design of the reactor leading to the use of various environments apart from liquid sodium and water vapor encountered in the “classical” sodium fast reactors. As a matter of fact, considerations on the suppression of the water-sodium reaction risk led to the proposal of the use of alternative coolants such as Pb-Bi involving an intermediate circuit between the primary sodium and the steam generator. Other concepts involve the use of supercritical CO2 instead of water vapor in the energy conversion system. In all cases, structural materials encounter severe conditions regarding corrosion concerns: high temperatures and possibly aggressive chemical environments. In this paper, status of the research performed in CEA on the corrosion behavior of the structural material and especially Fe-9Cr steels is presented in the various environments: sodium (see paper by JL Courouau), Pb-Bi, water vapor and CO2. The materials studied are metallic materials: austenitic and ferrito-martensitic steels as well as ODS steels as an option for the cladding material. In the different environments studied, the scientific approach is identical, the objective being in all cases the understanding of the corrosion processes to establish recommendations on the chemistry control of the coolant and to predict the long term behavior of the materials by the development of corrosion models. First, the corrosion mechanisms are analyzed using dedicated experimental devices. As a matter of fact, the complex environments require also controlled, safe and precise experimental systems to perform long duration corrosion tests (several thousands of hours). Therefore, specific experiments, adapted to each corrosive medium, are carried out in the laboratory. For example: - laboratory scale loops are used for liquid metals corrosion studies (use of rotating cylinders to simulate high turbulent conditions), - thermogravimetric analyses are also used to perform gas corrosion studies in representative temperature and environmental conditions,… Then, multi-scale characterization of the materials studied is performed (FEG-SEM, EDX, XRD, GD-OES…), together with precise analyses of the environments tested (in situ measurements with specific probes, gas chromatography…). Corrosion mechanisms are then proposed and models developed, depending on the advancement and the maturity of the program

Effect of oxygen on liquid sodium embrittlement of T91 martensitic steel

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Investigation of tailored SiC/SiC composites for Sodium-cooled Fast Reactors

International audienceIncreasing the sustainability and the safety ofnuclear reactors require the development of new types ofreactors (GEN-IV systems) which can work as breeder(producing more fuel that it consumes) and can offer thepossibility of burning minor actinides to reduce the waste,such as the Sodium- (SFR) or the Gas-cooled (GFR) FastReactors. Therefore, there is a need to assess materialswhich can withstand these very harsh core conditions. Inthis aim, SiC/SiC composites are promising candidatesthanks to their high decomposition temperature(> 2000°C), low swelling and creep under irradiation andgood neutron transparency. A recent CEA patent hashighlighted that SiC/SiC-based hexagonal tubes wouldincrease the resistance to melting and, as a consequence,the safety of the SFR core. In this way, techniques havebeen developed to manufacture a SiC/SiC hexagonal tubewith given dimensions, which has a relatively low level ofporosities and a pseudo-ductile mechanical behavior(tolerance to deformation). Besides, the chemicalcompatibility between SiC and SiC/SiC compositestowards liquid sodium and its impurities (in particularoxygen) was investigated. For this purpose, two sets ofexperiments were conducted in the CORRON$^2$facility(CEA). On the one hand, immersions up to 2000h in anoxygen-purified ([O]<10ppm) liquid sodium heated up tothe nominal temperature of a SFR (550°C) were carriedout. On the other hand, oxygen was inserted in the liquidsodium to reach important oxygen quantities($[O]$ = $1000\ ppm$), well above the reference consideredfor incidental and transient states, to investigate theinfluence of this element on the SiC/SiC composites.Indeed, the SiC/SiC composites and their pyrocarboninterphase (employed to have a good linkage between thefiber and the matrix) can encounter active or passiveoxidation at high temperatures. Mass assessments, SEM,XPS, X-ray tomography and tensile tests were conductedto characterize the sample properties before and afterimmersion. As a result, it was observed that there is nosignificant degradation of the material after exposure toeither the oxygen–poor or –rich environments. Moreover,in some cases, an increase of the mechanical properties ofSiC/SiC composites was observed

Elaboration d'alliages a composition complexe par fabrication additive

International audienceLe plus souvent, dans les zones de contacts mobiles des centrales nucléaires on utilise un revêtement base cobalt dénommé Stellite®. Il a une bonne tenue à la corrosion et un excellent comportement en situation de frottement. Malheureusement, le cobalt est un élément qui s’active facilement sous flux neutronique par formation de cobalt 60 qui peut alors contaminer le reste du système. Malgré de nombreuses propositions de matériaux substituts sans cobalt (base fer ou nickel), aucune solution ne s’est révélée totalement équivalent en termes de propriétés tribologiques. Les alliages à composition complexe (ACC) sont une nouvelle possibilité de recherche de substituts des alliages base cobalt. A la différence des alliages classiques, les ACC ne possèdent pas d’élément majoritaire, ils sont composés de plusieurs éléments en forte proportion. Si leur microstructure ne comporte qu’une seule phase, l’alliage est dit « à haute entropie ». Afin de réaliser ces ACC, la technique de projection laser est retenue. La présence de plusieurs distributeurs de poudres permet de contrôler la composition des alliages et de la faire varier continument au cours de l’élaboration. Ce procédé de fabrication additive permet de générer rapidement différents ACC et donc d’étudier l’influence de certains éléments sur leurs microstructures et leurs propriétés. Une première sélection d’éléments est proposée en tenant compte du cahier des charges de l’application, des considérations métallurgiques (dont une analyse bibliographique) ainsi que des capacités et limites du procédé. Après définition d’un plan d’expérimentation, plusieurs ACC sont élaborés en faisant varier les paramètres du procédé (puissance, vitesse de déplacement, débit des poudres, …) et la proportion respective des éléments retenus (par exemple Ni, Fe, Cr, Mo, Mn, Al, Ti, V). Ces ACC sont ensuite analysés par microscopie optique, micro-dureté, MEB (électrons secondaires, EDS …), DRX et EBSD. Cela permet de retenir les ACC les plus intéressants pour poursuivre par des essais de frottement sur tribomètre pion (céramique)-disque sous gaz neutre. Une relation entre le comportement en tribologie et la microstructure est établie. Cette étude nous a permis de réaliser une exploration initiale de ces ACC ainsi que de mieux comprendre leur comportement en tribologie. L’étude se poursuit par des tests complémentaires (tenue à la corrosion, essais mécanique…) et l’étude de nouvelles composition

Towards a reliable determination of the intergranular corrosion rate of austenitic stainless steel in oxidizing media

International audienceThis paper studies the corrosion behaviour of non-sensitized stainless steels in oxidizing environments, where they can suffer intergranular corrosion. In this case, corrosion rate estimated by gravimetric measurement is not constant as a function of time. This paper proposes a quantitative modelling of the IGC kinetics. Two models were developed: the first one is based on the geometrical simulation of the groove penetration; the second one uses a semi-empirical approach based on the typical shape of the corrosion kinetics. Both models reproduce successfully the experimental corrosion kinetics observed for AISI 310L SS corroded in nitric acid containing oxidizing ions