Patent Number: 046631160
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear reactor of the type in which the cooling water has a free liquid level in the pressure vessel such as in the boiling-water reactor, so that even when a break has developed in the drain conduit in a system which circulates the cooling water into the pressure vessel in the reactor, the cooling water is prevented from escaping, and the cooling water is allowed to remain in sufficiently large amounts in the pressure vessel. 2. Description of the Prior Art A conventional boiling-water reactor is equipped, with a cooling water recirculation system which consists of recirculation conduits recirculation pumps, and jet pumps, in order to supply large amounts of cooling water to a reactor core to enhance the core efficiency. From the necessity to take out the cooling water of a relatively low temperature from the pressure vessel of the reactor, an intake of the drain conduit in the cooling water recirculation system is opened in the pressure vessel at a position below the reactor core. In such a nuclear reactor, if breakage develops in the drain conduit, the cooling water gushes out through a broken portion since the pressure is high in the pressure vessel, the water level in the pressure vessel drops quickly to the position of intake of the conduit, and the reactor core burns without the cooling water. Another nuclear reactor in which the cooling water has a free liquid level in the pressure vessel of nuclear reactor can be represented by a nuclear reactor of the type of natural circulation. In the nuclear reactor of this type, provision is usually made of a purifying system or a spray system for pressure control between the interior and the exterior of the pressure vessel to circulate the cooling water while purifying the cooling water or controlling the pressure in the pressure chamber. In the former system, the intake of the drain conduit is opened in the lower portion of the pressure vessel so that precipitates deposited on the lower portion of the pressure vessel can be removed. In the latter system, also, the intake of the drain conduit is opened in the lower portion of the pressure vessel so that cold cooling water can be taken out. Even in such a nuclear reactor, if the drain conduit breaks, the water level in the pressure vessel may drop to the level of intake of the drain conduit, and the reactor core may burn without the cooling water. SUMMARY OF THE INVENTION The object of the present invention therefore is to provide a nuclear reactor equipped with a cooling water circulation system which circulates the cooling water to and from a pressure vessel which surrounds a reactor core, maintaining a free liquid level of cooling water above the reactor core, wherein even when the conduit is broken, the level of cooling water in the pressure vessel does not drop to the position of an intake even though the intake of the drain conduit in the cooling water circulation system is located below the reactor core, and the cooling water remains in the pressure vessel in amounts sufficient to avoid such an occurrence that the reactor core burns without the cooling water. To accomplish the above-mentioned object according to the present invention there is provided 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 wherein 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. According to the present invention, there is provided 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 wherein said drain conduit is inserted in the pressure vessel from the upper side toward the lower side relative to said free liquid level, the opening at the lower end of said drain conduit is located below the reactor core, and a lateral hole is formed in said drain conduit at a position lower than said free liquid level but higher than the reactor core.