Corrosion behavior of heat resistant alloys for gas seal elements of VHTR in helium environment

The corrosion behavior of the heat resistant alloys for the gas seal elements used for the fixed reflectors to improve the thermal flow characteristics in the core of the high tempeerature gas-cooled experimental reactor was examined in helium. As the results, the following facts were clarified. In Hastelloy X and Incoloy 800, carburization was observed below 900 deg C. Especially in the state of contact with graphite, carburization was promoted more than the case of non-contact. In Incoloy MA 956, decarbonization and carburization were suppressed below 800 deg C in helium, but in the state of contact with graphite, carburization was observed. When Al2O3 was coated on the surfaces of test pieces by chemical vacuum evaporation, oxidation, decarbonization and carburization were suppressed. The Al2O3 film coated on Hastelloy X by chemical vacuum evaporation showed sufficient reliability against thermal cycles and repeated bending. In the high temperature gas-cooled experimental reactor, the crossflow of helium through the gaps between graphite blocks lowers the thermal flow characteristics, therefore, the use of the gas seal elements made of 0.4 mm metallic sheets is considered. It is required to examine the reaction with the impurities in helium and the reaction in contact with graphite. (Kako, I.)