[en] Purpose: In a reactor, which uses helium gas as a coolant, to mount emergency cut-off valves in an inlet and outlet of an intermediate heat exchanger in a secondary cooling system so that in case of failure in the cooling system, pressure in the primary cooling system is synchronized with pressure in the secondary cooling system to thereby prevent the coolant barrier of an intermediate heat exchanger from failure. Constitution: The emergency cut-off valves disposed in the inlet and outlet of the intermediate heat exchanger in the secondary cooling system are immediately closed, for example, when some point of the secondary cooling system has failed. In this case, to compensate for a reduced portion of coolant pressure in the secondary cooling system due to the time lag until the emergency cut-off valves are closed, the other valve is opened at the same time when said point fails and a coolant bomb is directly connected to the secondary cooling system so that stand-by coolant within the tank is fed into the secondary cooling system. (Yoshino, Y.)