Patent Number: 
Section: claims

1. A drain sump of a nuclear reactor containment vessel having a containment vessel floor down below a reactor pressure vessel in the containment vessel, comprising:a heat-proof sump cover having a thickness to cover an upper part of the drain sump, the thickness allowing a top surface of the sump cover to be in a same surface as a top surface of the containment vessel floor; anda plurality of drain flow paths passing through the sump cover in a thickness direction to flow water therethrough and solidify a molten corium therein, the molten corium being produced in an unlikely event of a severe accident,wherein the drain sump is arranged inside the containment vessel floor, andwherein the top surface of the containment vessel floor comprises portions that are not part of the sump cover. 2. The drain sump according to claim 1, further comprising:transport piping buried in a pedestal wall of the reactor pressure vessel to transport drain water inside the drain sump to an outside of the containment vessel, the drain water being leakage water collected during plant operation; anda drain-water transport pump to be connected to the piping and located above a level which a surface of a molten corium settling on the containment vessel floor reaches, the molten corium being produced in an unlikely event of a severe accident. 3. The drain sump according to claim 1, further comprising:submersion piping being buried in a pedestal wall of the reactor pressure vessel and having two open ends,wherein one of the two open ends is opened to an internal space of the drain sump and the other of the two open ends is opened to the containment vessel above a level which a surface of a molten corium settling on the containment vessel floor reaches, the molten corium being produced in an unlikely event of a severe accident. 4. A drain sump of a nuclear reactor containment vessel having a containment vessel floor down below a reactor pressure vessel in the containment vessel, comprising:a heat-proof sump cover which covers an upper part of the drain sump, the sump cover having a top surface, the top surface being in a same surface as a top surface of the containment vessel floor;a sidewall; anda plurality of drain flow paths formed in the sidewall to flow water therethrough and solidify a molten corium therein, the drain flow paths having open ends which are opened to internal spaces of the containment vessel and the drain sump, the molten corium being produced in an unlikely event of a severe accident,wherein the drain sump is arranged inside the containment vessel floor, andwherein the top surface of the containment vessel floor comprises portions that are not part of the sump cover. 5. The drain sump according to claim 4, further comprising:transport piping buried in a pedestal wall of the reactor pressure vessel to transport drain water inside the drain sump to an outside of the containment vessel, the drain water being leakage water collected during plant operation; anda drain-water transport pump to be connected to the piping and located above a level which a surface of a molten corium settling on the containment vessel floor reaches, the molten corium being produced in an unlikely event of a severe accident. 6. The drain sump according to claim 4, further comprising:submersion piping being buried in a pedestal wall of the reactor pressure vessel and having two open ends,wherein one of the two open ends is opened to an internal space of the drain sump and the other of the two open ends is opened to the containment vessel above a level which a surface of a molten corium settling on the containment vessel floor reaches, the molten corium being produced in an unlikely event of a severe accident. 7. A drain sump of a nuclear reactor containment vessel having a containment vessel floor down below a reactor pressure vessel in the containment vessel, comprising:a heat-proof sump cover to cover an upper part of the drain sump, the sump cover having a top surface which is in a same surface as a top surface of the containment vessel floor;a heat-proof barrage surrounding an upper part of the drain sump to be formed on the containment vessel floor and support the sump cover; anda heat-proof gate provided to the barrage to be supported by supporting members including a low-melting point material, the supporting members being provided with a drain hole to flow water therethrough, the drain hole being closed up by a fall of the gate when a molten corium comes near the supporting members, the molten corium being produced in an unlikely event of a severe accident,wherein the drain sump is arranged inside the containment vessel floor. 8. The drain sump according to claim 7, further comprising:transport piping buried in a pedestal wall of the reactor pressure vessel to transport drain water inside the drain sump to an outside of the containment vessel, the drain water being leakage water collected during plant operation; anda drain-water transport pump to be connected to the piping and located above a level which a surface of a molten corium settling on the containment vessel floor reaches, the molten corium being produced in an unlikely event of a severe accident. 9. The drain sump according to claim 7, further comprising:submersion piping being buried in a pedestal wall of the reactor pressure vessel and having two open ends,wherein one of the two open ends is opened to an internal space of the drain sump and the other of the two open ends is opened to the containment vessel above a level which a surface of a molten corium settling on the containment vessel floor reaches, the molten corium being produced in an unlikely event of a severe accident. 10. A drain sump of a nuclear reactor containment vessel having a containment vessel floor down below a reactor pressure vessel in the containment vessel, comprising:a heat-proof sump cover having a thickness to cover an upper part of the drain sump and including heat-proof rubble, the thickness allowing a top surface of the sump cover to be in a same surface as a top surface of the containment vessel floor, the rubble flowing water therethrough and solidifying a molten corium therein, the molten corium being produced in an unlikely event of a severe accident,wherein the drain sump is arranged inside the containment vessel floor. 11. The drain sump according to claim 10, further comprising:transport piping buried in a pedestal wall of the reactor pressure vessel to transport drain water inside the drain sump to an outside of the containment vessel, the drain water being leakage water collected during plant operation; anda drain-water transport pump to be connected to the piping and located above a level which a surface of a molten corium settling on the containment vessel floor reaches, the molten corium being produced in an unlikely event of a severe accident. 12. The drain sump according to claim 10, further comprising:submersion piping being buried in a pedestal wall of the reactor pressure vessel and having two open ends,wherein one of the two open ends is opened to an internal space of the drain sump and the other of the two open ends is opened to the containment vessel above a level which a surface of a molten corium settling on the containment vessel floor reaches, the molten corium being produced in an unlikely event of a severe accident. 13. A drain sump of a nuclear reactor containment vessel having a containment vessel floor down below a reactor pressure vessel in the containment vessel, comprising:a heat-proof sump cover having a top surface to cover an upper part of the drain sump and an opening, the top surface being in a same surface as a top surface of the containment vessel floor, the opening being provided with a lid, the lid being supported by supporting members including a low-melting point material and having a drain hole to flow water therethrough, the supporting members melting to close up the drain hole when a molten corium comes near the supporting members, the molten corium being produced in an unlikely event of a severe accident,wherein the drain sump is arranged inside the containment vessel floor. 14. The drain sump according to claim 13, further comprising:transport piping buried in a pedestal wall of the reactor pressure vessel to transport drain water inside the drain sump to an outside of the containment vessel, the drain water being leakage water collected during plant operation; anda drain-water transport pump to be connected to the piping and located above a level which a surface of a molten corium settling on the containment vessel floor reaches, the molten corium being produced in an unlikely event of a severe accident. 15. The drain sump according to claim 13, further comprising:submersion piping being buried in a pedestal wall of the reactor pressure vessel and having two open ends,wherein one of the two open ends is opened to an internal space of the drain sump and the other of the two open ends is opened to the containment vessel above a level which a surface of a molten corium settling on the containment vessel floor reaches, the molten corium being produced in an unlikely event of a severe accident.