Patent Number: 046631160
Section: claims

1. In a nuclear reactor equipped with a cooling water circulation system which includes a water-feed conduit and a drain conduit to circulate the cooling water to and from a pressure vessel which surrounds a reactor core, the cooling water having a free liquid level maintained above the reactor core, the improvement characterized in that said drain conduit is inserted in the pressure vessel from the upper side toward the lower side relative to said free liquid level, a double tube consisting of an inner tube and an outer tube maintaining a gap therebetween is formed in the end portion of said drain conduit, said inner tube and outer tube being contiguous with said drain conduit, the opening at the lower end of the inner tube is located below said free liquid level but above said reactor core, the opening at the upper end of said outer tube is located above said free liquid level, and the opening at the lower end of said outer tube is located below the reactor core. 2. A nuclear reactor according to claim 1, wherein the lower end portion of the outer tube has the shape of a funnel which flares downwardly. 3. A nuclear reactor according to claim 1, wherein the lower end portion of the inner tube has the shape of a funnel which flares downwardly. 4. A nuclear reactor according to claim 1, wherein helical fins are provided in the outer tube at a position below the lower end of the inner tube. 5. A nuclear reactor according to claim 1, 2, 3 or 4, wherein the nuclear reactor is a boiling-water reactor, and the cooling water circulation system is of the type of recirculation to enhance the core efficiency. 6. A nuclear reactor according to claim 1, 2, 3 or 4, wherein the nuclear reactor is of the type of natural circulation, and the cooling water circulation system is of the type which purifies water. 7. A nuclear reactor according to claim 1, 2, 3 or 4, wherein the nuclear reactor is of the type of natural circulation, and the cooling water circulation system is of the spray type which controls pressure. 8. In a nuclear reactor equipped with a cooling water circulation system which includes a water-feed conduit means and a drain conduit means for circulating the cooling water to and from a pressure vessel which surrounds a reactor core, the cooling water having a free liquid level maintained above the reactor core, the improvement characterized in that said drain conduit means is inserted in the pressure vessel from the upper side toward the lower side relative to said free liquid level, said drain conduit means having an opening at a lower end thereof which is located below the reactor core, and at least one other opening at a position lower than said free liquid level and higher than the reactor core, said drain conduit means comprising a double tube including an inner tube and an outer tube maintaining a gap therebetween, said opening at the lower end of said drain conduit means located below the reactor core being provided by the opening at the lower end of said outer tube, and the at least one other opening at a position lower than said free liquid level and higher than the reactor core being provided by an opening at the lower end of said inner tube, said outer tube having an opening at the upper end thereof which is located above said free liquid level. 9. A nuclear reactor according to claim 8, wherein the lower end portion of said outer tube has the shape of a funnel which flares downwardly.