Chemical impurity contained in the coolant helium of Very High Temperature Reactors (VHTRs) affects not only the deterioration of mechanical strength caused by decarburization of the high-temperature materials utilized at the heat transfer tubes of the Intermediate Heat Exchanger (IHX) but also the deterioration of heat transfer efficiency caused by carbon deposition, which could occur at the surface of the heat transfer tubes. Such deterioration results in the shortening of the lifetime of high-temperature equipment. Since the helium purification technology applied in past high-temperature gas-cooled reactors can only maintain the core integrity by limiting the oxidation of graphite, it is necessary to establish a control technology in order to maintain the mechanical strength as well as heat transfer efficiency of hightemperature equipment. In this study, carbon deposition that could occur at the surface of the heat transfer tubes of theIHX and decarburization of Hastelloy XR used at the heat transfer tubes were evaluated by referring to the actual chemistry data obtained by the HTTR. Also, the chemical composition to be maintained during a reactor operation was proposed by evaluating not only the core graphite oxidation but also carbon deposition and decarburization. It was identified that when the chemical composition could not be maintained adequately, the injection of 10 ppm carbon monoxide could effectively control the chemical composition towards the designated area at the chromium stability diagram where the stable oxidized layer would be generated sufficiently. The proposed chemistry control technology is expected to contribute to the economy of the purification systems of future VHTRs.
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