PURPOSE: To improve a reliability of a rector cooling system without enlargement of a facility, by providing a pressure suppression pool placed at a space above a nuclear reactor containment, and a valving mechanism and a spray mechanism connected to the pressure suppression pool.

CONSTITUTION: In case of a rapture of a nuclear reactor pressure vessel 102, a coolant 103 in the vessel 102 flows out into a nuclear reactor containment 101 through the rapture 121. With this flowing out, a pressure inside the containment 101 increases, an inert gas in the containment 101 is stored into a room in the pressure suppression pool 107 via a bent tube 113 and a pressure in the pool 107 increases. And the coolant 103 flowing out from the rapture 121 is being stored into a reactor cavity 123 and the pressure in the containment 101 decreases to be lower than the pressure in the pool 107, resulting in vacuum breaking valve's 111 action. On the other hand, with a pressure high signal of the containment 101 and a pressure low signal of the pressure vessel 102, an open/close valve 110 is opened and therewith a pool water 108 flows into the containment 101 and cools down the containment 101.