A suppression chamber provided with vent pipes whose upper ends are positioned above a core in a reactor pressure vessel (RPV) is formed in a primary containment vessel (PCV) surrounding the RPV. A steam discharge connection pipe having an open/close valve is connected to a core shroud surrounding the core in the RPV above the core. A feed water connection pipe having an open/close valve is communicated with a downcommer in the RPV. The steam discharge connection pipe and the feed water connection pipe are opened into an upper drywell in the PCV. After an occurrence of a severe accident accompanied by a station blackout, cooling water is collected in the PCV, and the cooling water is supplied to the core through the feed water connection pipe, and the cooling water discharged from the core and including steam is discharged to the upper drywell by the steam discharge connection pipe.
