Note on Graphite Oxidation by Oxygen and Moisture

Characterization of Porosity Development in Oxidized Graphite using Automated Image Analysis Techniques

“…Although on different grades of nuclear graphite, the results reported here for PCEA graphite correlate well with previous observations from the ORNL group [10] on development of non-uniform density profiles in oxidized specialty grade graphite (R4-650). The results are also in very good agreement with recent theoretical estimates advanced by ORNL researchers [13,14] on penetration depth of the oxidant gas at quasi-steady state oxidation conditions. These results confirm one more time the validity of the chemically reactive oxidation zone proposed by Wichner and Ball (ORNL) [17].…”

“…In continuation of these efforts directed at modeling the oxidation process, an analytical procedure for calculation of penetration depth of the oxidant during transient oxidation conditions was recently proposed with help from Dr. Robert Wichner's external collaboration [13]. A short letter containing the results has been very recently accepted for publication [14].…”

Oxidation of PCEA nuclear graphite by low water concentrations in helium

“…Using the linear form, Eq. (9), of the kinetic equation, Wichner et al [15,16] derived an analytical model for estimation of oxidant penetration depth during transient oxidation by water vapor. In general, when oxidation rate is faster, but the supply of oxidant is limited (as in normal operating conditions) the penetration depth is expected to narrow.…”

“…The latter is the chemical oxidation rate, whose parameters are now available for graphite H-451 [10] and PCEA (this work). Using simple assumptions on the diffusion coefficient, the model developed by Wichner et al [15,16] shows that faster reaction rates and low oxidant supply rates cause large concentration gradients and steep oxidant depletion profiles under the exposed surface. According to this model, the effect of the oxidation temperature on the penetration depth X b where the oxidant concentration drops from the external surface value to a certain fraction b follows the approximate relationship X b % exp(E o /RT) where E o is the activation energy in the linearized rate Eq.…”

“…He concluded that gasification of graphite elements will be confined at the coolant-graphite interface, and therefore corrosion by steam should not affect normal operation of HTGR, provided steam concentration is kept below 0.01 Pa. This conclusion was reiterated by more recent estimates, all based on the same graphite-specific parameters reported by GA for reaction of H-451 grade with water [12][13][14][15][16].…”