Helium chemistry in high-temperature gas-cooled reactors; Chemistry control for avoiding Hastelloy XR corrosion in the HTTR-IS system

Sakaba, Nariaki ; Hirayama, Yoshiaki*

The high-temperature gas-cooled reactor (HTGR) is capable of producing a massive quantity of hydrogen with no carbon dioxide emission during its production by a thermo chemical IS (Iodine-Sulphur) process. The HTTR (High Temperature Engineering Test Reactor), which is the first high-temperature gas-cooled reactor in Japan, will be connected to some heat utilization system in the near future. The thermo chemical IS process is one of the progressive candidates. The metallic material of the heat transfer tube of the intermediate heat exchanger (IHX) and liner in the concentric hot gas duct in the HTTR-IS system, which allows usage in high-temperature conditions, is the nickel-based high-temperature alloy Hastelloy XR. Since the coolant helium contains small amounts of impurities, it is necessary to control the chemical composition in order to minimize corrosion of the Hastelloy XR. Major corrosion phenomena of the Hastelloy XR are carburization, decarburization, oxidation, and carbon deposition depending upon the particular gas composition and its temperature. The carburization and decarburization phenomena can be restricted by controlling the carbon activity and oxygen partial pressure. This paper describes the effect of each coolant impurity for the carburization and decarburization. Also a chemical composition limit was evaluated to avoid the Hastelloy XR corrosion.