Investigation of tailored SiC/SiC composites for Sodium-cooled Fast Reactors
Résumé
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 CORRON$^2$
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.
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