Patent Number: 054917301
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to a cooling system for a primary containment vessel (PCV) in a nuclear power plant, and more particularly to a passive cooling system with no active components (e.g. pumps) which system has an improved performance. Passive cooling systems for a primary containment vessel in a nuclear power plant are less subjected to a malfunction because they do not employ active components such as pumps, and therefore are high in reliability. One type of such passive cooling system is a condenser-type heat removal system disclosed, for example, in Japanese Patent Unexamined Publication Nos. 4-98198 and 4-136794. In such a cooling system, a cooling water pool is provided outside a pressure boundary of a primary containment vessel, and a condenser (heat exchanger) is provided within this cooling water pool, which condenser is connected to a main steam line from the Reactor Pressure Vessel or to a gas-phase space in the primary containment vessel. In this technique, when a loss of coolant accident (LOCA), which must be taken into consideration in designing the Nuclear Power Plant, occurs, steam produced in a reactor pressure vessel is led to the condenser, and is condensed there. At this time, uncondensed steam which has not been condensed in the condenser is discharged, together with noncondensable gas introduced into the condenser from the gas-phase space of the primary containment vessel, into a suppression pool within the primary containment vessel through a gas vent pipe, and this uncondensed steam is condensed in the water of this suppression pool such that the heat produced by this condensation is accumulated as sensible heat in the suppression pool. Thus, a pressure increase of the primary containment vessel at the time of an accident is suppressed. With respect to this condenser-type heat removal system, a thermal-hydraulic behavior by the uncondensed steam discharged into the suppression pool through the gas vent pipe is described in "Proc. of Fifth International Topical Meeting on Reactor Thermal Hydraulics, Vol II (September 1992) pages 547-555". Japanese Patent Unexamined Publication No. 63-1995 discloses another conventional arrangement in which there is provided a steam discharge device for discharging steam from a reactor pressure vessel into a pool water via a discharge pipe (vent pipe) and a safety relief valve, and a discharge portion of the discharge pipe (vent pipe) is constituted by a horizontal pipe having a plurality of holes or opening through which the steam is discharged into the pool water. The steam is discharged from the discharge holes in a fine manner, and the discharge holes are dispersed at such a density that steam bubbles from the adjacent holes can not easily be combined together. The above Japanese Patent Unexamined Publication No. 63-1995 does not describe the combination of this steam discharge device with a condenser-type heat removal system. In the conventional condenser-type heat removal system, a non-condensable gas, charged during a normal operation in the gas-phase space in the primary containment vessel surrounding the reactor pressure vessel, flows together with steam, into a condenser at the time of an accident when a cooling system is operated, thereby degrading the condensation capability. In order to deal with the uncondensed steam which has not been condensed as a result of the degraded condensation capability, there is provided a gas vent pipe extending from a heat exchanger (condenser) into a suppression pool to introduce the uncondensed steam into the water in the suppression pool for condensation. In the prior art technique, however, heat of the uncondensed steam discharged into the water of the suppression pool is presumed to be absorbed as sensible heat of the pool water at a depth to which the gas vent pipe is submerged, and no consideration has been given to a thermal-hydraulic behavior in the water of the suppression pool. Referring to the thermal-hydraulic behavior in the suppression pool described in the above-mentioned literature, the uncondensed steam discharged from the gas vent pipe into the suppression pool is condensed by the water of this pool. However, since an amount of the uncondensed steam flowing into the suppression pool is small, condensation is finished after only a part of the pool water disposed in the close vicinity of the outlet of the gas vent pipe is made hot. The hot water produced by this condensation moves upward to the suppression pool surface while forming a thin thermal boundary layer (having a thickness of about 10.about.15 cm) along the gas vent pipe. Moreover, the region where the water is made hot in the vicinity of the outlet of the gas vent pipe is small, and therefore the volume of that portion of the suppression pool water which produces buoyancy by the high temperature is small. As a result, flow can not be induced in the suppression pool water, and the bulk water except for vicinity of the gas vent pipe remains stagnant. Furthermore, in the suppression pool water, there is not provided any cooling means for inducing a downward flow from the water surface of the suppression pool. Therefore, the hot water rising along the gas vent pipe will not be mixed with the bulk water in the suppression pool. Namely, since the condensation region is small, a relatively great temperature rise of the suppression pool water due to the condensation occurs in a localized portion of the suppression pool, and the hot water moves upward to the surface of the suppression pool water without being sufficiently mixed with the surrounding bulk and the hot water is accumulated in the vicinity of the surface of the pool water upon arrival at this water surface. As a result, in the event of an accident, only an upper layer of the suppression pool water near to the water surface thereof is made hot. In other words, the water of the suppression pool is not sufficiently effectively used as a heat absorption source. Under the circumstances, the following problems arise in view of a pressure behavior of the primary containment vessel at the time of an accident, as well as the strength of this containment vessel which should withstand this pressure behavior. Namely, if only the upper layer of the suppression pool water near to the water surface thereof becomes hot as described above, this hot water causes the temperature of the gas-phase space (wetwell) at the upper portion of the suppression pool to rise, thereby increasing a steam partial pressure in this space. The pressure within the primary containment vessel at the time of an accident corresponds to the sum of the partial pressure of the noncondensable gas and the steam partial pressure in the wetwell. Therefore, an increase of the steam partial pressure means a pressure increase in the primary containment vessel at the time of an accident, and the strength of the primary containment vessel must be increased in order to withstand this pressure. Alternatively, it is necessary to increase a heat transfer area of the heat exchanger mounted in the cooling water pool provided at the upper portion of the primary containment vessel so that the uncondensed steam which will cause the steam partial pressure to increase will not flow into the suppression pool. In this case, in conformity with the increase of the heat transfer area of the heat exchanger, the capacity of the cooling water pool disposed at the high level must be increased, and the strength of the primary containment vessel must be increased with a view to increasing a seismic strength. Either of the above measures results in an increased strength of the primary containment vessel, and the structure needs to be increased in strength, for example, by increasing the thickness of the wall of the containment vessel, and therefore the cost of the plant is increased. SUMMARY OF THE INVENTION With the above problems in view, it is a primary object of this invention to provide a cooling system for a primary containment vessel in a nuclear power plant which system utilizes a condenser-type heat removal system, in which uncondensed steam discharged to a suppression pool is prevented from making only a surface layer of the water of the suppression pool hot, and the pool water can be sufficiently used as a heat absorption source by uniforming the suppression pool temperature, and the pressure within the primary containment vessel at the time of an accident is reduced, thereby enhancing a reliability thereof, and a design strength of the primary containment vessel can be reduced. Another object of the invention is to provide a component part used in such a cooling system. To achieve the primary object of the invention, according to a first aspect of the invention, there is provided a cooling system for a primary containment vessel in a nuclear power plant which system includes a gas vent which extends from a heat exchanger of a condenser-type heat removal system into water of a suppression pool water to be communicated with the water of the suppression pool; wherein means for discharging a vent fluid in a dispersing manner is mounted on a portion of the gas vent disposed in the water of the suppression pool. According to a second aspect of the invention, there is provided a cooling system for a primary containment vessel in a nuclear power plant which system includes a gas vent which extends from a heat exchanger of a condenser-type heat removal system into water of a suppression pool to be communicated with the water of the suppression pool; wherein means for forming a gas-to-liquid contact surface at which a vent fluid and the water of the suppression pool are contacted with each other is mounted on that portion of the gas vent disposed in the water of the suppression pool to extend horizontally. According to a third aspect of the invention, there is provided a cooling system for a primary containment vessel in a nuclear power plant which system includes a gas vent which extends from a heat exchanger of a condenser-type heat removal system into water of a suppression pool to be communicated with the water of the suppression pool; wherein means for restraining upward movement of a vent fluid is provided on a portion of the gas vent disposed in the water of the suppression pool to extend horizontally to an extent greater than a flow area of the gas vent, the restraining means including communication holes communicated with the water of the suppression pool and disposed below a level where a maximum gas-to-liquid contact area is formed, and the communication holes being distributed over a range larger than the flow area of the gas vent. According to a fourth aspect of the invention, there is provided a cooling system according to the third aspect, in which the restraining means comprises horizontal pipes connected to and communicated with the gas vent in a plurality of directions, each of the pipes having a closed distal end, and each of the pipes having openings which are in free communication with the suppression pool, and are disposed below a horizontal plane in which axes of the pipes lie. According to a fifth aspect of the invention, there is provided a cooling system according to the third aspect, in which the restraining means comprises a baffle or baffles extending generally horizontally from an outer periphery of the gas vent with an outer peripheral edge portion thereof directed generally downwardly. According to a sixth aspect of the invention, there is provided a cooling system according to the fifth aspect, in which the baffles are mounted on the outer periphery of the gas vent in a multi-stage manner in a vertical direction, and the higher the level of said baffles the larger the extent thereof. According to a seventh aspect of the invention, there is provided a cooling system for a primary containment vessel in a nuclear power plant which system includes a gas vent which extends from a heat exchanger of a condenser-type heat removal system into water of a suppression pool to be communicated with the water of the suppression pool; wherein a plurality of horizontal pipes are connected to and communicated with a portion of the gas vent, which is disposed in the water of the suppression pool, in a plurality of different directions, each of the pipes having a plurality of openings which are in communication with the water of the suppression pool to be arranged in a dispersed manner below a horizontal plane in which axes of the pipes lie, and a portion of each of the pipes disposed above the horizontal plane being closed. To achieve another object of the invention, according to an eighth aspect of the invention, there is provided a gas vent pipe comprising a main pipe portion, and a plurality of horizontal pipes provided on a lower end portion or a portion near said lower end portion of said main pipe portion in communication therewith in different directions, said each horizontal pipes having a plurality of dispersedly distributed opening at a portion thereof below a horizontal plane which includes therein axes of said horizontal pipes, said each horizontal pipes being closed at a portion thereof above said horizontal plane. According to a ninth aspect of the invention, there is provided a gas vent pipe comprising a baffle or baffles provided on a lower end portion or a portion near said lower end portion of said gas vent pipe, said baffle or baffles being lowered at the outwardly extending edge or edges thereof. According to a tenth aspect of the invention, there is provided a gas vent pipe comprising a plurality of baffles provided vertically in a multi-stage manner on a lower end portion or a portion near said lower end portion of said gas vent pipe, said baffle or baffles being lowered at the outwardly extending edge or edges thereof, and the higher the level of said baffles the larger the extent of the outwardly extending edge or edges of said baffle or baffles. To achieve the primary object of the invention, according to an eleventh aspect of the invention, there is provided a cooling system for a primary containment vessel in a nuclear power plant, comprising: a pressure boundary; PA1 a reactor pressure vessel containing a core; PA1 a drywell in which the reactor pressure vessel is mounted; PA1 a suppression pool separated from the drywell by a partition wall; PA1 a vent tube for communicating the drywell with water of the suppression pool; PA1 a pool disposed at a level above the core; PA1 an injection pipe for communicating water of the pool via a check valve to a portion of reactor pressure vessel disposed at a level below the pool; the reactor pressure vessel, the drywell, the suppression pool, the vent tube, the pool, and the injection pipe being provided within the pressure boundary; PA1 a cooling water pool provided outside the pressure boundary and pool having an exhaust passage leading to the atmosphere; PA1 a heat exchanger of a condenser-type heat removal system provided in water of the cooling water pool; PA1 a pipe for introducing the atmosphere of the drywell into the heat exchanger at the time of an accident; PA1 a return line of condensate produced in the heat exchanger, to the pool; PA1 a gas vent pipe for discharging an uncondensed fluid from the heat exchanger into the water of the suppression pool at a level above a discharge port of said vent tube; and PA1 means mounted on a portion of the gas vent pipe disposed in the water of the suppression pool for restraining upward movement of the uncondensed fluid, the restraining means being extended horizontally to an extent greater than a flow area of the gas vent pipe, the restraining means including communication holes communicated with the water of the suppression pool, the communication holes being disposed below a level, at which a maximum gas-to-liquid contact area is formed, and the communication holes being distributed over a range wider than the flow area of the gas vent pipe. According to a twelfth aspect of the invention, there is provided a cooling system according to the eleventh aspect, in which there is provided an additional heat exchanger for cooling the water of the suppression pool at a region of the water where the gas vent pipe is submerged. According to a thirteenth aspect of the invention, there is provided a cooling system according to the twelfth aspect, in which the range of cooling effected by the additional heat exchanger is extended to the wetwell disposed above the surface of the water of the suppression pool. According to a fourteenth aspect of the invention, there is provided a cooling system for a primary containment vessel in a nuclear power plant which system includes a gas vent extending from a heat exchanger of a condenser-type heat removal system into a suppression pool; wherein cooling means is provided in the suppression pool for cooling the water of the suppression pool at a region of the water where the gas vent is submerged. According to a fifteenth aspect of the invention, there is provided a cooling system according to the fourteenth aspect, in which the range of cooling effected by the cooling means is extended to a wetwell disposed above the surface of the water of the suppression pool. According to a sixteenth aspect of the invention, there is provided a cooling system according to the fourteenth aspect or the fifteenth aspect, in which the cooling means comprises a heat exchanger, cooling water which receives heat from said heat exchanger, and a cooling water pool containing therein said cooling water and having a exhaust pipe communicating with the atmosphere. According to a seventeenth aspect of the invention, there is provided a cooling system according to the fourteenth aspect, the fifteenth aspect or the sixteenth aspect, in which the gas vent is constituted by the gas vent pipe of the eighth aspect, the ninth aspect or the tenth aspect. According to the first aspect, uncondensed steam flows from the heat exchanger through the gas vent to be discharged into the suppression pool from the dispersingly-discharging means. The dispersingly-discharging means is mounted horizontally, and therefore at the time of this discharge, the extent of dispersing of the discharged fluid is enlarged horizontally, so that the region where the water of the suppression pool is mixed with the uncondensed steam is enlarged. Even if the outlet of the gas vent extending from the heat exchanger is disposed in the water of the suppression pool, the temperature of the water surface of the suppression pool can be lowered, so that the pressure within the primary containment vessel is decreased. According to the second aspect, uncondensed steam from the heat exchanger is blown through the gas vent into the gas-to-liquid contact surface forming means. In this gas-to-liquid contact surface forming means, the gas phase of the uncondensed steam is contacted with the liquid phase (or the water of the suppression pool in the gas-to-liquid contact surface forming means) at a horizontally-extending gas-to-liquid contact surface, and the uncondensed steam is subjected to promoted condensation to be discharged into the water of the suppression pool. Furthermore, the heat is released from the gas-to-liquid contact surface forming means to the water of the suppression pool on the outer side and with this heat releasing effect, the fluid lowered in temperature is discharged from the gas-to-liquid contact surface forming means into the water of the suppression pool. With this arrangement, even if the outlet of the gas vent extending from the heat exchanger is extended in the water of the suppression pool, the temperature of the water surface of the suppression pool can be lowered, so that the pressure within the primary containment vessel is decreased. According to the third aspect, uncondensed steam fed from the heat exchanger through the gas vent is caught by the horizontally-extending means for restraining upward movement of a vent fluid, and is contacted with the liquid phase (or the water of the suppression pool) at the gas-to-liquid contact surface in the restraining means, so that the uncondensed steam is condensed. Furthermore, the heat is released from the restraining means to the water of the suppression pool on the outer side of this restraining means, and with this heat releasing effect, the fluid lowered in temperature is discharged from the restraining means into the water of the suppression pool over a wide range. With this arrangement, even if the outlet of the gas vent extending from the heat exchanger is extended in the water of the suppression pool, the temperature of the water surface of the suppression pool can be lowered, so that the pressure within the primary containment vessel is decreased. According to the fourth aspect, in addition to the effects of the third aspect, uncondensed steam enter the horizontal pipes from the gas vent. Each of the horizontal pipes has the openings which are in communication with the water of the suppression pool and are disposed below the horizontal plane in which the axes of the pipes lie. Therefore, the uncondensed steam is caught as a gas phase in an upper portion of the interior of each pipe disposed above the above-mentioned horizontal plane, and therefore is prevented from upward movement, while the water of the suppression pool is present as a liquid phase in a lower portion of the interior of the pipe below the above horizontal plane. In each of the horizontal pipes, the uncondensed steam is contacted with the water of the suppression pool to be condensed, and besides is lowered in temperature by the release of heat from each horizontal pipe to the exterior thereof. The condensed steam (condensate) is discharged as cold (low-temperature) liquid into the water of the suppression pool through the openings over a wide range. With this arrangement, even if the outlet of the gas vent extending from the heat exchanger is extended in the water of the suppression pool, the temperature of the water surface of the suppression pool can be lowered, so that the pressure within the primary containment vessel is decreased. According to the fifth aspect, in addition to the effects of the third aspect, the uncondensed steam discharged from the gas vent is restrained by the baffle from upward movement in the suppression pool, and is condensed through the gas-to-liquid contact to be lowered in temperature. The condensed fluid turns around the outer peripheral edge of the baffle to be dispersed into the suppression pool. With this arrangement, even if the outlet of the gas vent extending from the heat exchanger is extended in the water of the suppression pool, the temperature of the water surface of the suppression pool can be lowered, so that the pressure within the primary containment vessel is decreased. According to the sixth aspect, the effect achieved by the fifth aspect is repeated several times until the uncondensed steam reaches the water surface of the suppression pool. According to the seventh aspect, uncondensed steam enters each of the horizontal pipes from the gas vent. Each of the horizontal pipes has the openings which are in communication with the water of the suppression pool and are disposed below the horizontal plane in which the axes of the pipes lie. Therefore, the uncondensed steam is caught as a gas phase in an upper portion of the interior of each pipe disposed above the above-mentioned horizontal plane, and therefore is prevented from upward movement, while the water of the suppression pool is present as a liquid phase in a lower portion of the interior of the pipe below the above horizontal plane. In each of the horizontal pipes, the uncondensed steam is contacted with the water of the suppression pool to be condensed, and besides is lowered in temperature by the release of heat from each horizontal pipe to the exterior thereof. The condensed steam (condensate) is dispersingly discharged as cold (low-temperature) liquid into the water of the suppression pool through the openings over a wide range. With this arrangement, even if the outlet of the gas vent extending from the heat exchanger is extended in the water of the suppression pool, the temperature of the water surface of the suppression pool can be lowered, so that the pressure within the primary containment vessel is decreased. According to the eighth aspect, the fluid to be condensed, such as steam gas, which has flowed through the gas vent pipe, is caught in an upper portion of the interior of each horizontal pipe disposed above the horizontal plane in which the axes of the horizontal pipes lie. This uncondensed fluid is contacted with the water of the suppression pool disposed at a lower portion of the interior of each horizontal pipe, so that the uncondensed fluid is subjected to promoted condensation. Furthermore, the condensation of the fluid is promoted by the release of heat from the horizontal pipe to the exterior thereof. The fluid thus condensed and lowered in temperature is dispersingly discharged into the water of the suppression pool from the openings. According to the ninth aspect, the fluid to be condensed, such as steam gas, which has flowed through the gas vent pipe, is discharged from the gas vent pipe, and is restrained by the baffle from upward movement in the suppression pool. The uncondensed steam is condensed through gas-to-liquid contact to be lowered in temperature. The condensed fluid turns around the outer peripheral edge of the baffle to be dispersed into the water of the suppression pool. According to the tenth aspect, the effect achieved by the ninth aspect is repeated several times until the uncondensed steam reaches the water surface of the suppression pool. According to the eleventh aspect, when steam within the reactor pressure vessel, produced as a result of heating water by the core, is discharged into the drywell because of an accident, the pressure within the drywell increases abruptly, so that the thus discharged steam is blown into the suppression pool through the vent tube to be condensed. At the same time, the discharged steam is introduced into the heat exchanger through the pipe to be condensed. The condensate in the heat exchanger flows into the pool through a return line of condensate, and this condensate in this pool is injected through the injection pipe into the reactor pressure vessel by the gravity force. Thus, the condensate is reused as cooling water for the core. The uncondensed steam in the heat exchanger flows, as part of the uncondensed fluid, through the gas vent, and is discharged into the suppression pool. The uncondensed steam is caught by and restrained by the upward movement restraining means from upward movement, and is subjected to promoted condensation through the gas-to-liquid contact with the water of the suppression pool and also through the release of heat from the upward movement restraining means. Then, the condensed steam is dispersed into the water of the suppression pool over a range larger than the flow area of the gas vent to be mixed with the water of the suppression pool over a wide range. With this arrangement, even if the outlet of the gas vent extending from the heat exchanger is extended in the water of the suppression pool, the temperature of the water surface of the suppression pool can be lowered, so that the pressure within the primary containment vessel is decreased. With the lapse of time after an accident initiation, when the pressure difference between the drywell and the wetwell becomes small, the discharge from the vent tube whose outlet is disposed at a greater depth of water is stopped, while the discharge into the suppression pool from the gas vent whose outlet is disposed at a smaller depth of water continues. As a result, the cooling function as well as the suppression function is maintained for a long period of time after the accident. According to the twelfth aspect, in addition to the effects of the eleventh aspect, the water of the suppression pool is cooled by the heat exchanger, and is caused to flow downward to produce a convection in the water of the suppression pool. As a result, the heat is dissipated into the water of the suppression pool over the entire region thereof, and the cooling function as well as the suppression function is maintained more effectively for a prolonged period of time after the accident. According to the thirteenth aspect, in addition to the effects of the twelfth aspect, the wetwell can also be cooled directly, and therefore the cooling function as well as the suppression function is maintained still more effectively for a long period of time after the accident. According to the fourteenth aspect, the uncondensed steam discharged, from the heat exchanger through the gas vent into the suppression pool, is condensed by the water of the suppression pool, and tends to be stagnant at the surface of the water of the suppression pool such that the water at this water surface is higher in temperature than the remainder of this pool water; however, such high-temperature water is cooled by the cooling means to form a downward flow to produce a convection in the water of the suppression pool. With this convection, even if the outlet of the gas vent extending from the heat exchanger is disposed in the water of the suppression pool, the temperature of the water surface of the suppression pool can be lowered, so that the pressure within the primary containment vessel is decreased. According to the fifteenth aspect, in addition to the effects of the fourteenth aspect, the wetwell can also be cooled directly, and therefore even if the outlet of the gas vent extending from the heat exchanger is extended in the water of the suppression pool, the temperature of the water surface of the suppression pool can be further lowered, so that the pressure within the primary containment vessel is further decreased. According to the sixteenth aspect, in addition to the effects of the fourteenth aspect and the fifteenth aspect, the heat, received in the heat exchanger as a result of cooling the water of the suppression pool, is released to the cooling water pool. This heat is released by the evaporation of the cooling water, and the resulting steam is discharged to the atmosphere through the exhaust pipe. Therefore, a pressure increase of the cooling water pool can also be suppressed. According to the seventeenth aspect, in addition to the effects of the fourteenth aspect, the fifteenth aspect and the sixteenth aspect, the effects of the eighth aspect, the ninth aspect and the tenth aspect are added.