Silicon Carbide Oxidation in High-Pressure Steam

Silicon carbide is a candidate cladding for fission power reactors that can potentially provide better accident tolerance than zirconium alloys. SiC has also been discussed as a host matrix for nuclear fuel. Chemical vapor-deposited silicon carbide specimens were exposed in 0.34-2.07MPa steam at low gas velocity (similar to 50cm/min) and temperatures from 1000 degrees C to 1300 degrees C for 2-48h. As previously observed at lower steam pressure of 0.15MPa, a two-layer SiO2 scale was formed during exposure to these conditions, composed of a porous cristobalite layer above a thin, dense amorphous SiO2 surface layer. Growth of both layers depends on temperature, time, and steam pressure. A quantitative kinetics model is presented to describe the SiO2 scale growth, whereby the amorphous layer is formed through a diffusion process and linearly consumed by an amorphous to crystalline phase transition process. Paralinear kinetics of SiC recession were observed after exposure in 0.34MPa steam at 1200 degrees C within 48h. High-pressure steam environments are seen to form very thick (10-100m) cristobalite SiO2 layers on CVD SiC even after relatively short-term exposures (several hours). The crystalline SiO2 layer and SiC recession rate significantly depend on steam pressure. Another model is presented to describe the SiC recession rate in terms of steam pressure when a linear phase transition k(l) governing the recession kinetics, whereby the reciprocal of recession rate is found to follow a negative unity steam pressure power law.