Silicon Carbide Oxidation in High‐Pressure Steam

Abstract

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°C to 1300°C for 2-48 h. As previously observed at lower steam pressure of 0.15 MPa, a two-layer SiO 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°C within 48 h. High-pressure steam environments are seen to form very thick (10-100 lm) 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 l governing the recession kinetics, whereby the reciprocal of recession rate is found to follow a negative unity steam pressure power law.

Effects of water vapor on the oxidation and the fracture strength of SiC layer in TRISO fuel particles

“…Due to the complexity of the solutions, it was traditionally expressed by using following equations . A brief summary is shown in Equation :where C 1 and C 2 are the intermediate coefficients, μ is the intermediate variable, ν is the Poisson ratio, r is the outer diameter of shell, δ is the thickness of shell, F is the applied load, r 0 is the radius of the circular contact area, σ membrane is the maximum membrane stress, σ bending is the maximum bending stress and is the local fracture strength of the specimen, which presents the maximum tensile stress at the inner surface of the shell.…”

“…In this study, the SiO 2 volatilization can be negligible. Then the oxidation of SiC was expected to follow the parabolic law, which is expressed as:where x is the oxide thickness; t is the oxidation time; k p is the parabolic rate. By fitting the SiO 2 thickness with the oxidation time using Equation , the parabolic rates were obtained and summarized in Table .…”

“…The TRISO particles were placed on the platinum crucible in the middle of the hot zone. The TRISO particle samples which adhered on the platinum crucible especially at high temperature were excluded in the performance characterization to avoid any effects of interaction between the platinum and SiC . To check if any SiO 2 volatilization occurred during the high‐temperature steam oxidation, tests were carried out to measure the weight change in the SiO 2 bulk sample (α‐cristobalite, 20 mm×10 mm×5 mm) after steam oxidation using a balance with an accuracy of 0.01 mg. For comparison, parallel experiments were conducted in dry air at 1473 K.…”

Isothermal Oxidation Comparison of Three Ni-Based Superalloys

“…The tendency of scale formation in the alloys is also a function of the shape of a specimen and the surface-to-volume ratio [110] [111]. For example, if non-uniform oxidation takes place and if a specimen has finite dimensions, constraints are imposed on the system, particularly at edges and corners.…”

MoSi2 Oxidation in 670–1498 K Water Vapor

“…18,21 It is expected that starting at temperatures greater than 1473 K MoSi 2 will not passivate during oxidation under water vapor environments. SiO 2 is not identified in the 1498 K sample surface, though Mo 5 Si 3 is detected, as shown in Fig.…”