Corrosion of SiC/SiC in GFR conditions. Bibliographic data

In order to answer to the objectives of sustainable development, of safety, and of economic competitiveness involved by the fourth generation of nuclear systems (reactors or fuel cycle), CEA studies fast neutrons reactors. In this work, the bibliographic data relative to the resistance of SiCf/SiC composites under neutral gas and temperature between 900-1500 C are discussed. The atmosphere to be considered is hypothetical; it can be thought that inevitably the heat carrier will be polluted by some gases coming from leaks or removal of structures or fuel: O2, N2 and H2O or the products of their reaction with reactor materials, eventually CH4, H2, CO, CO2...If no article has examined the behaviour of SiCf/SiC in very weakly oxidizing environment, a very well documented literature deals with some aspects of this questions. Two points appears critical: 1)Under some gas mixtures, silicon oxide, which gives to SiC these good properties at high temperature, does not form, either because the partial pressure in oxidant is inadequate, or because SiO2 reacts with impurities. In these conditions, the oxidation of SiC produces a gaseous suboxide SiO and induces the volatilization of the ceramics. The noxious consequences on the bulky SiC mechanical resistance are established. 2)The composites contain a graphite or a boron nitride interphase which improves their mechanical resistance while facilitating the sliding of fibers in the matrix. If gases have access to the interphase (at the level of defects, porosity,...), C or BN can be oxidized. The graphite oxidation products are gaseous; those of BN are volatile species in presence of steam. The interphase oxidation velocity depends on the oxidant partial pressure and decreases when the oxidant amount decreases. Nevertheless, there are no kinetic barrier and it is expected that the interphase disappears progressively reducing then the tensile strength of the composite. Before studying the behaviour of complex SiCf/SiC materials in GFR environment, the resistance of each component at very high temperature in an helium atmosphere polluted by oxidants traces and eventually reducing gases is investigated. In these conditions, corrosion appears preoccupying: in one hand, SiO can be formed if the oxidant partial pressure is insufficient or in presence of reactive compounds with SiO2, on the other hand, C or BN can be volatilized if oxidants have access to the interphase. These two phenomena affect the mechanical properties of the composite. (O.M.)