PURPOSE: To loosen the impact of vented tubes at the accident time, by means of allowing to be ejecting holizontally to a plenum structure existing at the lower position than the lowr part of a container by setting up the vented tubes at the lower part of a reactor container, and of allowing the small tubes set up at the vented tubes to be immersed into a pool water.

CONSTITUTION: A plenum structure 8 is set up at the lower position than the lower part of a reactor container 1 around this vessel, thereby a pool water is stored. A plurality of vented tubes 10 are set up at the lower part of the vessel 1, and are allowed to be ejected holizontally to the plenum structure 8, and guide members 13 fixed at the ceiling of the plenum structure 8 and inserting sliding panels are mounted via connecting arms 12. Further, a plurality of small tubes 14 are set up at the vented tubes 10, and are immersed in a pool water 9, and at the accident time, the air flow or the flow of steam pressure in the vented tube 10 is discharged. the impact force entered into the vented tubes 10 can be loosened by the sliding to the outside of the sliding panels.