Silicon Carbide Oxidation In High-Pressure Steam

摘要

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 0.34-2.07 Mpa steam at low gas velocity (～50 cm/min) and temperatures from 1000℃ to 1300℃ for 2-48 h. As previously observed at lower steam pressure of 0.15 MPa, a two-layer SiC_2 scale was formed during exposure to these conditions, composed of a porous cristobalite layer above a thin, dense amorphous SiO_2 surface layer. Growth of both layers depends on temperature, time, and steam pressure. A quantitative kinetics model is presented to describe the SiO_2 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 0.34 Mpa steam at 1200℃ within 48 h. High-pressure steam environments are seen to form very thick (10-100 μrn) cristobalite SiO_2 layers on CVD Sic even after relatively short-term exposures (several hours). The crystalline SiO_2 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_1 governing the recession kinetics, whereby the reciprocal of recession rate is found to follow a negative unity steam pressure power law.