INVESTIGATION OF TAILORED SIC/SIC COMPOSITES FOR SODIUM-COOLED FAST REACTORS

摘要

Increasing the sustainability and the safety of nuclear reactors require the development of new types of reactors (GEN-Ⅳ systems) which can work as breeder (producing more fuel that it consumes) and can offer the possibility of burning minor actinides to reduce the waste, such as the Sodium- (SFR) or the Gas-cooled (GFR) Fast Reactors. Therefore, there is a need to assess materials which can withstand these very harsh core conditions. In this aim, SiC/SiC composites are promising candidates thanks to their high decomposition temperature (＞2000°C), low swelling and creep under irradiation and good neutron transparency. A recent CEA patent has highlighted that SiC/SiC-based hexagonal tubes would increase the resistance to melting and, as a consequence, the safety of the SFR core. In this way, techniques have been developed to manufacture a SiC/SiC hexagonal tube with given dimensions, which has a relatively low level of porosities and a pseudo-ductile mechanical behavior (tolerance to deformation). Besides, the chemical compatibility between SiC and SiC/SiC composites towards liquid sodium and its impurities (in particular oxygen) was investigated. For this purpose, two sets of experiments were conducted in the CORRONa~2 facility (CEA). On the one hand, immersions up to 2000h in an oxygen-purified ([O]＜10ppm) liquid sodium heated up to the nominal temperature of a SFR (550°C) were carried out. On the other hand, oxygen was inserted in the liquid sodium to reach important oxygen quantities ([O] = 1000 ppm), well above the reference considered for incidental and transient states, to investigate the influence of this element on the SiC/SiC composites. Indeed, the SiC/SiC composites and their pyrocarbon interphase (employed to have a good linkage between the fiber and the matrix) can encounter active or passive oxidation at high temperatures. Mass assessments, SEM, XPS, X-ray tomography and tensile tests were conducted to characterize the sample properties before and after immersion. As a result, it was observed that there is no significant degradation of the material after exposure to either the oxygen-poor or -rich environments. Moreover, in some cases, an increase of the mechanical properties of SiC/SiC composites was observed.