Helium chemistry in high temperature gas cooled reactors - chemical impurity behaviour in the secondary helium coolant of the HTTR

Japan Atomic Energy Agency (JAEA) constructed High Temperature Engineering Test Reactor (HTTR) in order to establish and upgrade the technology basis for the high-temperature gas-cooled reactors (HTGRs) and to develop the technology for high-temperature heat applications. The HTTR attained its maximum reactor-outlet coolant temperature of 950 Degrees C in 2004. The intermediate heat exchanger (IHX), which is a helium-helium heat exchanger and installed to the HTTR primary cooling system, is planned to be utilized for the hydrogen production system in the near future. Secondary coolant helium receives heat from primary helium at the heat exchanger pipes made by high-temperature super alloy Hastelloy XR. The secondary pressurized water cooler, which is a helium-water heat exchanger and installed to the secondary cooling system, will replace to the hydrogen production system. The thermochemical water-splitting IS process is a progressive candidate for the hydrogen production system. Chemistry control is necessary in order to avoid oxidation of core graphite and corrosions of high-temperature material of Hastelloy XR, e.g. carbon deposits, carburising and decarburising. The chemical impurity is reduced and monitored by helium purification system and helium sampling system installed primary and secondary helium cooling system, respectively. This report describes impurity behaviour of secondary coolant during the initial 950 operation of the HTTR