Study on Chemical Kinetics of Carbon Migration in Superalloys in the Non-Pure Helium Environment in High Temperature Gas Cooled Reactors

The primary coolant in the high temperature gas cooled reactor (HTGR) contains impurities of low content, which will cause serious corrosion of the superalloys in the HTGR operating at the ultra-high temperature. In particular, the carbon migration between the superalloys and non-pure helium has a great influence on the material performance. This study explores the chemical kinetics principle of the carbon migration in the non-pure helium environment, from which the theoretical criteria for material decarburization and carburization are gained. Also, according to the chemical thermodynamics and kinetics principles, this study calculates the oxygen partial pressure and carbon activity in the non-pure helium environment, and indicates that a high partial pressure ratio of CH4 to H2O may lead to serious carburization of the alloys. On this basis, this study presents a widely used carbon migration model, “chromium stable phase diagram”, analyzes the chromium activity calculation method, and summarizes the recommended values. Finally, the corrosion behavior of the 10 MW HTGR (HTR-10) designed by Tsinghua University under the actual operating conditions is obtained by calculation based on the chromium stable phase diagram.