PURPOSE: To diffuse the heat of an atomic reactor when the reactor is in a transient operating state or an accident happens by providing a cooling water pool at a height sufficient to make cooling water to flow in the reactor against the internal pressure of the reactor maintained at a prescribed level.

CONSTITUTION: In a gravity-actuated cooling system pool 34, water 36 is stored by a quantity sufficient to fill up a pressure vessel to a level substantially higher than the upper ends of all fuel rods. When the coolant is lost or a main steam pipeline 16 is broken in the pressure vessel 12, a relief valve 20 is opened and, accordingly, steam flows out of the valve 20 and flows in the water 24 in a suppression pool 22. The water 24 initially has a temperature of about 100°F, but, as the steam flows in the water 24 and condenses, the steam pressures in the pipeline 16 and the vessel 12 drop and the temperature of the water 24 rises. While such a process continues, a proper reactor controller is actuated to reduce the quantity of heat generated in the vessel 12.