Abstract
The Very-high-temperature Reactor (VHTR) is one of the Gen-IV reactors. Helium is used as the coolant of a VHTR and the core outlet temperature can achieve 700-950°C, which might reach up to 1000°C for higher efficiency in the future. At such high temperature, the impurities from original coolant or a loss-of-coolant accident (LOCA) may have a significant impact on the performance of structural materials. As the promising candidate materials for Intermediate Heat Exchanger (IHX), chromia formers Inconel 625 and Hastelloy C4 alloys and alumina former HR-224 alloy should be tested in further studies to prove their corrosion resistant ability. In this research, three alloys were tested at 850°C and 950°C in helium environments with various concentrations of O2 and H2O. In general, the oxidation rates of all the alloys increased with the increasing temperature, and Inconel 625 alloy showed the highest mass gain among the alloys at 950℃. Continuous chromium oxide layer with internal alumina was formed on the surface of Inconel 625 and Hastelloy C4 alloys in all conditions, while the alumina scale spalled and other oxides could be found on the surface of HR-224 alloy under specific conditions. Further analyses and oxidation mechanisms of these alloys would be presented in this study.