[en] In a pressure suppression type container, a device for reducing hydrogen gas concentration is disposed to a bent tube which communicate from a dry well to a pressure suppression pool. That is, hydrogen gas generated in the container upon occurrence of a primary circuit pipeline rupture is effectively decreased by disposing catalyst type hydrogen absorbers in the midway of a pipeline or a suction portion of the dry well. Further, the catalyst type hydrogen absorbers are disposed so that the atmosphere is effectively brought into contact with the absorbers by natural convection in a space portion due to temperature elevation of water when FP in the reactor is released into the pressure suppression pool water, and hydrogen and oxygen are generated by water radiolysis in the pressure suppressing pool water. Further, the catalyst type hydrogen absorbers are caused to have an optional angle relative to the direction of convection. As a result, if an accident should occur, the flow of atmosphere in the space of the reactor container can be utilized effectively, thereby enabling to reduce generated hydrogen gas effectively. (N.H.)