Beyond the classical kinetic model for chronic graphite oxidation by moisture in high temperature gas-cooled reactors

Four grades of nuclear graphite were oxidized in helium with traces of moisture and hydrogen in order to evaluate the effects of slow oxidation by moisture on graphite components in high temperature gas cooled reactors. Kinetic analysis showed that the Langmuir-Hinshelwood (LH) model cannot consistently reproduce all results. In particular, at high temperatures and water partial pressures, oxidation was always faster than the LH model predicts. It was also found empirically that the apparent reaction order for water has a sigmoid-type variation with temperature which follows the integral Boltzmann distribution function. This suggests deviations from the LH model are apparently caused by activation with temperature of graphite reactive sites, which is probably rooted in specific structural and electronic properties of graphite. A semi-global kinetic model was proposed, whereby the classical LH model was modified with a temperature-dependent reaction order for water. This new Boltzmann-enhanced Langmuir-Hinshelwood (BLH) model consistently predicts oxidation rates over large ranges of temperature (800–1100 °C) and partial pressures of water (3–1200 Pa) and hydrogen (0–300 Pa). The BLH model can be used for modeling chronic oxidation of graphite components during life-time operation in high- and very high temperature advanced nuclear reactors.