Active chemistry control for coolant helium applying high-temperature gas-cooled reactors

Sakaba, Nariaki ; Hamamoto, Shimpei  ; Takeda, Yoichi*

Lifetime extension of high-temperature materials utilized, for instance, at the heat transfer tubes of the intermediate heat exchanger of high-temperature gas-cooled reactors is important because high-cost primary equipment will be continued its operation during reactor lifetime without replacing. Since lifetime of high-temperature materials almost depends upon the chemistry conditions in the coolant helium, it is necessary to establish an active chemistry control methods. This technology can maintain adequate chemical conditions during reactor operation. In the past, helium chemistry has been controlled by the passive chemistry control technology in which chemical impurity in the coolant helium removes as low concentration as possible, as does Japan's first high-temperature gas-cooled reactor HTTR. In this study, carbon deposition which could be occurred at the surface of the heat transfer tube and decarburization of the high-temperature material of Hastelloy XR were evaluated by referring the chemistry data obtained by the HTTR. In addition, the chemical composition to be maintained during the reactor operation in order to keep the structural integrity and thermal efficiency of the heat transfer tube was proposed by evaluating not only core graphite oxidation, but also carbon deposition and decarburization. It was also identified when the chemical composition could not keep adequately, injection of 10 ppm carbon monoxide could effectively control the chemical composition to the designated stable area where the high-temperature materials can keep their structural integrity beyond the assumed duration. The proposed active chemistry control technology is expected to contribute economically to the purification systems of the future very high-temperature reactors.