SiCOxidation

Description

The material SiCOxidation models oxidation, hydrogen production and corrosion of CVD (monolithic) silicon carbide (SiC) cladding.

At high temperatures ( 1473 K) SiC undergoes two distinct processes when exposed to steam. First silica is formed via parabolic rate kinetics through the following chemical reaction:

After oxidation occurs volatilization of the silica, which follows linear kinetics, occurs through: The parabolic and linear rate constants are computed by equations obtained from Terrani et al. (2014): where (mgcmh) is the parabolic rate consant, (mg-cmh) is the volatilization rate constant, is the water vapor partial pressure (MPa), is the temperature (K), is the gas velocity (m/s), and (J/mol-K) is the universal gas constant. The uncertainty in the exponential coefficients and the activation energies in the parabolic rate equation are 3.7 and 53000. For the linear rate equation the uncertainties are and 1.3 and 18000, respectively. The mass loss per unit area (mg/cm) is then computed through: where is the time (h). The oxide thickness can then be computed through the oxygen density present in the oxide. For silica (SiO) the density is 2650 kg/m. Therefore, the oxygen density is 1411.32 kg/m. This is equivalent to 1411.32 mg/cm. Thus, the oxide thickness is calculated by: where is the oxide thickness (cm) and is the density of the oxygen in the oxide (mg/cm). In BISON SI units are used and therefore internally to the code the units are changed such that the mass gain per unit area is in kg/m and the oxide thickness is in m.

The mass of hydrogen produced per unit area can be computed from the mass of SiC consumed and the oxidation chemical equation. The moles per unit area of SiC consumed is given by: where is the molar mass of SiC (44.11 g/mol). Then using the molar ratio in the balanced chemical equation for each mole of SiC consumed 3 moles of is produced. Therefore, the moles of is given by: Substituting for one obtains: Finally, the mass per unit area of hydrogen produced is then computed by multiplying by the molar mass of (2.016g/mol): where is the mass per unit area of hydrogen produced and is the molar mass.

SiC corrosion is dictated by the surface oxidation/formation of silica (rate limiting step) followed by dissolution in coolant. Different water chemistries lead to different observed behavior and literature work suggests that microstructure of the SiC will greatly affect corrosion resistance. Irradiation appears to accelerate corrosion via increasing the concentration of radicals in the coolant and introducing defects into the cladding. While data is very limited, and appears material dependent, it seems likely that irradiation may cause up to a magnitude of order increase in corrosion rates. Data on General Atomics (GA) prototype cladding has yielded similar out-of-pile autoclave corrosion rates as those observed in the literature. For corrosion rates under irradiation for GA prototype cladding material there are only two data points available, showing relatively large scatter. Until more irradiation data is available, a conservative corrosion rate estimate of 0.6 mg/dm/day is recommended (equivalent to 6.9444410 kg/m/s) GA (2020). This corresponds to a conservative irradiated SiC thickness loss due to corrosion of 8.28 microns/year.

Example Input Syntax

[Materials<<<{"href": "../../syntax/Materials/index.html"}>>>]
  [oxidation]
    type = SiCOxidation<<<{"description": "Computes the mass loss and hydrogen produced due to oxidation of CVD silicon carbide. Also computes corrosion of CVD silicon carbide", "href": "SiCOxidation.html"}>>>
    temperature<<<{"description": "The temperature variable."}>>> = temperature
    coolant_velocity<<<{"description": "The coolant velocity as a function of time (m/s)."}>>> = 1
    vapor_pressure<<<{"description": "The water vapor partial pressure as a function of time (Pa)."}>>> = 1e6
    boundary<<<{"description": "The list of boundaries (ids or names) from the mesh where this object applies"}>>> = right
    output_properties<<<{"description": "List of material properties, from this material, to output (outputs must also be defined to an output type)"}>>> = 'oxide_mass_loss thickness_consumed hydrogen_produced corrosion_mass_loss corrosion_thickness_consumed'
    outputs<<<{"description": "Vector of output names where you would like to restrict the output of variables(s) associated with this object"}>>> = all
  []
[]

Input Parameters

Input Files

• (test/tests/sic_oxidation/ad_oxidation.i)
• (test/tests/sic_oxidation/oxidation.i)

References