Compatibility of CVD SiC and SiCf/SiC Composites with High Temperature Helium Simulating Very High Temperature Gas-Cooled Reactor Coolant Chemistry

The chemical compatibility aspects of CVD β-SiC and SiC_f/SiC composites with a VHTR specific helium coolant were examined. The specimens were exposed to helium gas containing 20 Pa H₂, 5 Pa CO, 2 Pa CH₄, and 0.02–0.1 Pa H₂O, which is an expected VHTR coolant chemistry. Oxidation tests were carried out at 900 and 950 °C for up to 250 h. β-SiC and SiC_f/SiC composites had an excellent compatibility with the expected VHTR helium coolant environment. The oxidation of β-SiC as a matrix material of the SiC_f/SiC composite reacted in a passive oxidation regime owing to the presence of water vapor. A condensed version of the oxide SiO₂ formed at an early stage of oxidation and the growth of this oxide layer was very limited as the oxidation time increased up to 250 h. The recession of the pyrolytic carbon interphase of SiC_f/SiC composite could not be observed in the test range.