PURPOSE: To perform the operation as a fluid hydraulic vacuum breaker by connecting a liquid reservoir and riser by a duct arranged under a predetermined distance from the surface of the reservoir, and increasing the area of the reservoir specific magnification as large as that of the riser.

CONSTITUTION: A liquid reservoir 60 is arranged in a dry well 14, a riser 64 is arranged in a wet well 18, and connected to the reservoir 60 via a release port 66. The area of the reservoir 60 is 25 times or more as large as that of the riser. The static head of the reservoir 60 is decided according to the distance D from the surface of the reservoir 60 to the duct disposed down. At the stage of the blowdown after an accident, when the water level in the riser 64 is raised so that the differential pressure between the wells 14 and 18 exceeds the head of the riser 64, the air flows from the well 18 to the well 14, the pressures become equivalent to prevent the damage of the storage, thereby performing the operation of a fluid pressure vacuum breaker.