Abstract
In high-temperature gas-cooled reactors that use helium gas as a coolant, impurities in the helium coolant, such as H2, H2O, CO and CH4 molecules, react with the component materials of the reactors such as its heat transfer tubes. Theses reactions degrade the mechanical properties and shorten the lifetime of the components. In this study, tight-binding quantum chemical molecular dynamics simulations were employed to understand the chemical reactions caused by impurities (CH4 and O2) on Ni-Cr alloy surfaces. The molecules of the impurities were dissociated on the surface and dissociated oxygen and carbon atoms preferentially bonded with chromium atoms. Cr-C bonds were maintained during the simulation and consequently, chromium atoms were trapped by carbon atoms after the formation of Cr-C bonds. This result suggests that the carburized chromium atoms do not contribute significantly to the formation of oxide films because of the strong affinity of carbon for chromium. It is concluded, therefore, that the characteristics of the oxide film formed on the Ni-Cr alloy surface in a helium coolant are significantly affected by the chemical composition of the impurities in the coolant.
