Abstract: A reactor containment vessel comprises typically a reactor pressure vessel housed in a dry well of the reactor containment vessel, a vent passage through which steam in the dry well is introduced into coolant accommodated in a pressure suppression chamber of the reactor containment vessel, a closed space formed at a position lower than the level of the normal liquid surface of the coolant, a first passage having an inlet opened into the pressure suppression chamber at a level higher than that of the normal liquid surface of the coolant and an outlet opened into the closed space, and a second passage communicating between the closed space and the dry well through a counter flow preventing arrangement for checking the flow directed toward the closed space, and therefore, noncondensable gas and liquid accumulated in the pressure suppression chamber are discharged into the dry well the pressure in which is higher than that in the pressure suppression chamber by making use of a difference in water head increased d

Abstract: An emergency reactor core cooling structure comprises a flow passage provided on an upper core plate so as to extend downward therefrom into the core. The flow passage has an orifice member at a lower portion thereof. The orifice member has such a portion that the cross-sectional area of the flow passage is made minimal at the orifice member. The flow passage causes the cooling water held on the upper plate by vapor flowing up on an accident to flow down therethrough, whereby the core is cooled rapidly.