[en] Chemistry control is important for the helium coolant of high-temperature gas-cooled reactors (HTGRs) because impurities cause oxidation of the graphite applied to the core structure and corrosion of high-temperature materials utilized at the intermediate heat exchanger (IHX). This thesis describes the helium chemistry which should be maintained adequately during reactor operations not only for the safety and stability of operations, but also for the increase of economic competitiveness by reducing replacing times of IHXs and simplifying helium purification systems. In this paper the chemical impurity behaviour was clarified by the obtained chemistry data by the Japan's HTTR and the criteria of chemical impurities for the GTHTR300C, which can supply both of electricity and hydrogen for the future hydrogen society, was proposed with its control technologies. In this R and D, the chemical impurity behavior, purification abilities, emitted impurities from graphite and thermal insulator, and hydrogen permeation at the heat transfer tubes of the IHX during the 950degC operation of the HTTR were evaluated. Utilizing these results, radical reactions at the core were identified. Also, chemical impurity criteria were proposed to avoid structural degradation of the Hastelloy XR and thermophysical properties deterioration at the heat transfer tubes. As a result, active chemistry control technologies where the necessary chemistry impurity will be injected in response to the chemical balance at the core are proposed for the GTHTR300C. The proposing technology is expected to contribute economically to the purification systems of the future HTGRs. (author)