Stability analysis of SiO2/SiC coatings on matrix graphite for HTR-10 fuel elements

The stability behaviours of SiC coatings and SiO2/SiC coatings in helium with little impurities are studied by HSC Chemistry 4.1. It is found that in helium with a low partial pressure of oxidative impurities, the critical stable temperature of SiC (Tcp) increases with the partial pressure of oxidative impurities; but the critical stable temperature of SiO2 (Tcs) depends on both the partial pressure of impurities and the ratio of the amount of impurities to the amount of SiO2, and Tcs of SiO2 increases with the partial pressure of oxidative impurities while it decreases with the ratio of the amount of impurities to the amount of SiO2. The influence of other impurities on Tcp of SiC in He–O2 is further studied and it is found that the introduction of CO2, H2O and N2 increases Tcp of SiC in He–O2 while the introduction of H2, CO and CH4 decreases Tcp of SiC He–O2. When there exist both oxidative impurities and reductive impurities, their effect of a kind of impurities on Tcs of SiO2 can be suppressed by the other kind of impurities. In HTR-10 operation atmosphere, SiO2/SiC coatings could keep stable at higher temperature than SiC coatings, so SiO2/SiC coatings should be more suitable to improve the oxidation resistance of graphite in HTR-10 operation atmosphere compared with SiC coatings.

► The stability of SiO2/SiC coatings was studied by thermodynamic analysis.
► The analyzed conditions are similar to the service condition of graphite used in HTR.
► SiC coatings are not stable in HTR-10 operation conditions in thermodynamics.
► SiO2/SiC coatings should be suitable to improve the oxidation resistance of graphite in HTR.