Patent Number: 052727378
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to a cooling technique for a nuclear reactor installation, and more particularly to techniques suited for cooling and submerging a reactor core in the event of a coolant loss accident, and for removing decay heat produced at the reactor core, and for suppressing the increase of pressure within a containment vessel. In a convectional boiling reactor having an electric output of up to 1,100 MW, a large amount of steam, produced from a ruptured portion at the time of a coolant loss accident, is led into a pressure suppression pool, provided below a reactor pressure vessel, and is condensed there, thereby suppressing the increase of pressure within a primary containment vessel to below an allowable level. Then, an emergency core cooling system (ECCS), which comprises a high-pressure core spray system, a low-pressure core spray system, a low-pressure pouring system and an automatic depressurization system, is operated to pump up the water of the suppression pool to cool the reactor core. At this time, a residual heat removal system feeds, by a pump, the water of the suppression pool to a heat exchanger disposed outside of the containment vessel, thereby removing decay heat from the reactor core. On the other hand, in a small- to a medium-size boiling reactor having an electric output of up to 600 MW, in order to simplify the installation and to achieve a high safety, it has been proposed that an emergency core cooling system excludes the use of powered equipment, such as a pump, and instead employs a dual accumulator pouring system using a passive method in which gas pressure is beforehand applied to a water reservoir for pouring water to the reactor core under a pressure differential between the water reservoir and the reactor core so as to cool the reactor core upon emergency. Thus, the system heretofore used in the conventional reactor is omitted. Also, with respect to a small- to a medium-size reactor, Japanese Patent Unexamined Publication No. 63-191096 discloses a system in which the decay heat during a long cooling period after a coolant loss accident is removed by a passive method using a natural force. More specifically, an outer pool is provided around a primary containment vessel, and by utilizing a natural convection of a pressure suppression pool and the outer pool, with the surface of the containment vessel used as a heat transfer surface, the heat is transferred to the outer pool due to a temperature difference between the two pools so as to evaporate the pool water to thereby achieve the cooling. As described above, the containment vessel in the small- to the medium-size reactor is of such a construction that the steam of high temperature and pressure which has leaked into the primary containment vessel, is led into a pressure suppression chamber in the primary containment vessel so that the steam of high temperature and pressure is condensed by the pressure suppression pool within this chamber. The system used in the large-size reactor, along the prior art, requires auxiliary powered equipment, including a pump, a heat exchanger and an emergency power source, in order to cool the reactor core and also to remove decay heat produced at the core at the time of a coolant loss accident. Therefore, the construction of the plant becomes complicated, which poses a problem that attention is required so as not to lower the reliability when, for example, the power source is subjected to a malfunction. On the other hand, if the accumulator pouring system and the outer pool use as the safety equipment for the small- to the medium-size reactor, are adopted to the large-size reactor, the construction of the plant of the large-size reactor can be simplified; however, if they are merely adopted, the size of the primary containment vessel for accommodating the reactor pressure vessel and the pressure suppression chamber becomes excessively large in order to increase the area of heat radiation to the water of the outer pool so as to cope with a large power output. SUMMARY OF THE INVENTION It is a first object of this invention to provide an emergency core cooling system (ECCS) for rapidly cooling and submerging a core of a nuclear reactor in the event of a coolant loss accident, which is simple in construction, and can be automatically operated in a reliable manner without the need for any special power means. A second object of the invention is to provide a nuclear reactor installation in which decay heat produced from a reactor is efficiently removed to achieve a long-period cooling function. According to one aspect of the present invention, there is provided a nuclear reactor installation built within a building, the nuclear reactor installation comprising a reactor pressure vessel containing a reactor core which produces steam having a steam pressure; a concrete structure portion enclosing and holding the reactor pressure vessel; a dry well formed within the concrete structure portion and enclosing the reactor pressure vessel; a primary containment vessel enclosing the concrete structural portion; a pressure suppression chamber having a pressure suppression pool; and a vent tube communicating the dry well with the pressure suppression pool; the installation comprising an accumulator-type emergency core cooling system, a gravity-driven emergency core cooling system and an equalizing system for submerging said reactor core, which are mounted within the primary containment vessel, the equalizing system being adapted to feed water from the suppression pool to the reactor core so as to submerge the latter; wherein when the steam pressure in the reactor core decreases upon a coolant loss accident, the accumulator-type emergency core cooling system is operated to supply cooling water into the reactor pressure vessel to cool the reactor core; when the pressure in the reactor pressure vessel further decreases during the above cooling operation, the gravity-driven emergency core cooling system is operated to feed a large amount of cooling water into the reactor pressure vessel so as to allow the cooling water to overflow through a broken part into the lower portion of the dry well which is therefore filled with the cooling water, and then the cooling water is fed via the vent tube to the suppression pool in the suppression chamber to raise the water level of the suppression pool; and subsequently when the water level of the suppression pool becomes high as compared with the pressure in the reactor pressure vessel, the equalizing system is operated to supply the water from the suppression pool into the reactor pressure vessel to submerge and cool the reactor core. According to another aspect of the present invention, there is provided a nuclear reactor installation comprising: a reactor pressure vessel having a reactor core and an equalizing system for submerging the reactor core; a concrete structural portion enclosing and holding the reactor pressure vessel; a primary containment vessel made of steel enclosing the reactor pressure vessel and an operation floor; an outer pool surrounding the lower portion of the primary containment vessel; and an air-cooling system covering the upper portion of the primary containment vessel; wherein the upper wall of the concrete structural portion is connected air-tight to the inner periphery of the primary containment vessel; the operation floor is provided in that portion of the primary containment vessel which is disposed above the upper wall of the concrete structural portion; and the space of the operation floor is connected to that portion of the primary containment vessel which is disposed below the upper wall of the concrete structural portion through at least one pressure-responsive opening means which is opened when a pressure difference therebetween becomes higher than a predetermined level. With the construction of the present invention, when the pressure in the reactor pressure vessel decreases in the event of a coolant loss accident, the accumulator-type emergency core cooling system, the gravity-driven emergency core cooling system and the equalizing system are automatically operated sequentially. These systems are operated merely depending on the pressure difference between each of the emergency cooling systems and the reactor pressure vessel, without requiring any particular drive power, and therefore there can be provided the emergency core cooling system which is simple in construction and is reliable in operation. The reactor pressure vessel and the operation floor are contained in the primary containment vessel made of steel, and in the normal condition the pressure space containing the reactor pressure vessel is isolated from the space containing the operation floor, and in the case of emergency, the two spaces are communicated with each other by the opening means which is operated in accordance with the pressure difference between the two spaces. The upper portion of the primary containment vessel is air-cooled, and the lower portion thereof is water-cooled. Therefore, the entire surface of the primary containment vessel is cooled so as to remove decay heat of the reactor for a long period. With this construction, the cooling of the primary containment vessel is effected through the steel wall having a large heat radiation area and a good heat transfer efficiency, and therefore the decay heat can be removed in a reliable and efficient manner. Further, since the upper portion of the primary containment vessel suited for the air-cooling and the lower portion thereof suited for the water-cooling are cooled respectively by the air-cooling and the water-cooling, the cooling efficiency is enhanced, and this cooling construction can be simplified.