Abstract: A drain sump of a reactor containment vessel having a containment vessel floor down below a reactor pressure vessel, and includes a heat-proof sump cover and two or more drain flow paths. The drain sump is arranged inside the containment vessel floor. The heat-proof sump cover has a thickness, and covers an upper part of the drain sump. The thickness allows a top surface of the sump cover to lie in the same surface as a top surface of the containment vessel floor. The drain flow paths pass through the sump cover in a thickness direction to flow water therethrough and solidify a molten corium therein. The molten corium is produced in the unlikely event of a severe accident.

Abstract: A transient mitigation system for a reactor includes: a reactor core isolation cooling system; and a high pressure flooder system including a high pressure flooder pump and a flooder valve. When the water level of the reactor water in the reactor decreases to a first predetermined water level, the reactor core isolation cooling system is activated to supply the cooling water into the reactor. When the water level in the reactor decreases to the first predetermined water level, the high pressure flooder pump is activated. When the water level in the reactor decreases to a second predetermined water level lower than the first predetermined water level, the flooder valve is opened to inject the cooling water into the reactor.

Abstract: A drain sump of a reactor containment vessel having a containment vessel floor down below a reactor pressure vessel, and includes a heat-proof sump cover and two or more drain flow paths. The drain sump is arranged inside the containment vessel floor. The heat-proof sump cover has a thickness, and covers an upper part of the drain sump. The thickness allows a top surface of the sump cover to lie in the same surface as a top surface of the containment vessel floor. The drain flow paths pass through the sump cover in a thickness direction to flow water therethrough and solidify a molten corium therein. The molten corium is produced in the unlikely event of a severe accident.