PURPOSE: To stably maintain a cooling stage over a long period of time by storing a reactor vessel into the water in a container in which water is filled and providing a decay heat cooler having an automatically operating valve to this reactor vessel.

CONSTITUTION: The automatically operating valve 29 of the decay heat cooler 22 is automatically opened by the pressure increase in the container or the pressure fluctuation or temp. fluctuation of the primary system when a rupture accident arises at the reactor vessel 2. The endothermic part 20 of the cooler 22 is positioned in the high-temp. part in the upper part of the reactor core 4 and the heat radiating part 21 thereof is positioned in water 19 which is a low-temp. part at this time; therefore, natural circulation (convection) arises in the inside water when the valve 29 is opened. The decay heat received in the endothermic part 20 is continuously radiated continuously into the water 19 in the heat radiating part 21. The water in the cooler 22 is stably circulated without boiling the water if the pressure in the endothermic part 20 of the cooler 22 is previously set high. The stable cooling state is thus maintained for the substantial time after the occurrence of the accident.