PURPOSE: To prevent the loss of pooled water in a gravity drop type core cooling system (GDCS) tube, and ensure the effective use thereof for core cooling by closing a filling valve according to a signal from a differential pressure detector at the pressure vessel inlet of the GDCS tube.

CONSTITUTION: A differential pressure detector 16, a normally closed filling valve 17 and a normally closed nonreturn valve 10 are provided in a plurality of GDCS tubes so laid as to lead GDCS pooled water to a reactor pressure vessel 11 containing a core 12. If a fracture 18 takes place on the tubes 15, while a loss of coolant accident (LOCA) happens to take place, causing the valve 17 to open and water 14 in a pool 13 to flow to the pressure vessel 11 via the tubes 15, the detector 16 detects differential pressure ≤ α (i.e. differential pressure evaluated as a reverse flow to the direction of the pool 13. α is a specific value determined with consideration of a vibration or the like during normal operation.) in the tubes 15, and the valve 17 is closed. In this case, the filling valve 17 in the sound tube 15 continues to be open and, therefore, water is continuously fed to the pressure vessel 11.