Method for urgently cooling a core in high temperature gas reactor

Object: To bring the interior of a reactor vessel into communication with the lower gas prenum in a manner so as to be opened only in case of emergency at the lower end of core structure to form a natural convection circulation cooling system when forced convection circulation cooling system is inoperable, thereby preventing the core structure or the like from being damaged. Structure: In case of emergency, (1) the gas flow passage in the core is brought into communication with the gas flow passage in the reactor vessel at the bottom of the core so that heat in the core is transmitted to the reactor vessel by natural circulation of gases within the system; and (2) refrigerant is supplied to an indirect cooling layer disposed between the reactor vessel and the biological shield so that the heat transmitted from the reactor vessel to the indirect cooling layer by means of natural circulation is carried out of the system by means of natural circulation of the refrigerant. (Kamimura, M.)