Patent Number: 056573600
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a reactor container or reactor containment vessel provided with a dry well cooling system for use in a nuclear power plant. A reactor container of a boiling water reactor (hereinafter referred to as BWR) is generally divided into a dry well and a wet well. A reactor pressure vessel (hereinafter referred to as RPV) and a reactor primary system are housed in the dry well, and a suppression pool filled with a suppression pool water is also located in the wet well. In accordance with the nuclear policy in various countries such as Japan, design of a nuclear container is obliged to meet with strict requirements in an anticipation of a loss of coolant accident caused by breakage of reactor primary pipings that is one design reference event. The vapor discharged in the event of breakage of the reactor primary pipings is introduced from the dry well to the suppression pool through a vent pipe for condensation of the vapor so that a pressure increase in the reactor container is suppressed. When the water temperature of the suppression pool is raised due to the condensation of the vapor, a residual heat removal system (hereinafter referred to as an RHR system) is operated to remove the heat from the suppression pool and, hence, the water temperature of the suppression pool can always be kept low. A dry well cooler is also provided for the purpose of removing the heat dissipated from the reactor container and the reactor primary system during a normal operation and for controlling the temperature in the dry well so as to be prevented from rising excessively and be held within a certain range. A first example of the dry well cooling system comprises, as shown in FIG. 18, a heat exchanger 3 mounted in a dry well 2 inside a reactor container 1, a blower 4 and a duct, not shown, for circulating an atmosphere in the dry well 2 through the heat exchanger 3, and a normal cooling system 6 for introducing cooling water to the secondary side of the heat exchanger 3 and finally conveying the heat in the dry well 2 to the sea 5 outside the power plant. With such arrangements, the temperature in the dry well 2 is controlled during normal operation. A wet well is not shown but is arranged bellow the dry well in the reactor container. The cooling water on the secondary side of the heat exchanger 3, disposed in the dry well 2, of the dry well cooling system is conveyed from an equipment cooling pump 7 to the heat exchanger 3 through an equipment cooling heat exchanger 8 and a circulating pipe 9. It is also arranged such that, if necessary, the cooling water can be conveyed to a load 10 for an RHR heat exchanger and a load 11 for equipment cooling as well. The secondary side of the equipment cooling heat exchanger 8 is cooled by conveying seawater 12 to the equipment cooling heat exchanger 8 through the operation of a seawater pump 13. The reason why the seawater is used for the BWR of the described type is that the BWR is usually located near the seaside. Alternatively, the dry well cooling system comprises a plurality of heat exchangers mounted in a dry well, a plurality of blowers and ducts for circulating a gas in the dry well through the heat exchangers, and a cooling system for circulating cooling water on the secondary side of the heat exchanger and finally conveying the heat in the dry well to the sea outside the power plant, thereby controlling the temperature in the dry well during a normal operation of a nuclear reactor. FIG. 19 is a block diagram showing a second example of the dry well cooling system for emergency. An atmosphere in a dry well 2 is taken out by a blower 4a disposed outside a reactor container 1, cooled by a heat exchanger 3a, and then returned to the dry well 2 again for removal of heat from the reactor container 1. The cooling water on the secondary side of the heat exchanger 3a of the dry well cooling system for emergency is conveyed from a reactor equipment cooling pump 7 to the heat exchanger 3a through a reactor equipment cooling heat exchanger 8. It is also designed such that, if necessary, the cooling water of a reactor equipment cooling line 9 can also be conveyed to an RHR heat exchanger load 10 and a load 11 for equipment cooling as well. The secondary side of the reactor equipment cooling heat exchanger 8 is cooled by conveying the seawater 12 to the reactor equipment cooling heat exchanger 8 through the operation of a reactor equipment cooling seawater pump 13. A nuclear power plant is designed in anticipation of an accident that heat cannot completely be removed from a reactor container because of an occurrence of an abnormal event and simultaneous failure of the RHR system, or an accident wherein a re-supply means of cooling water to the RPV fails to work at the same time as the occurrence of an abnormal event (hereinafter referred to as a severe accident), although the probability of such a severe accident is so very small as to be practically improbable. Even if a severe accident should occur, adequate countermeasures are taken to surely keep safety of the nuclear power plant. If a severe accident leading to complete an outage of heat removal from a reactor container occurs and the outage continues for a long time, the interior of the nuclear container experiences a condition of high temperature and high pressure. In other words, vapor and incondensable gas at high temperature are built up in a dry well and a wet well to produce a high-temperature and high-pressure condition in the reactor container. In anticipation of such a severe accident, it has been contemplated to install a nuclear container vent system for discharging vapor and incondensable gas at high temperature fills in the dry well and the wet well to open air from an exhaust tower through a suppression pool. When a nuclear container vent system is installed and operated to discharge the atmosphere in the reactor container to open air, fission products (hereinafter referred to as FP) contained in the atmosphere in the reactor container are held in water of the suppression pool and almost no FP is contained in the gas discharged to the open air. However, the possibility that a small amount of FP may be discharged to the open air is not zero. Meanwhile, it is now under consideration to use a cooler mounted in a dry well (hereinafter referred to as a dry well cooler) as means for removing heat from the reactor container. Heat removal using the dry well cooler has no possibility that the atmosphere in the reactor container is directly discharged to the open air. However, since the dry well cooler is intended to remove heat during the normal operation, sufficient heat removal from the reactor container cannot be expected in a condition under high temperature and high pressure in the event of a severe accident. Further, since the dry well cooler generally shares a cooling system with the RHR system, there is a problem that if the RHR system should be disabled, the dry well cooler also does not work. Moreover, there is also provided a problem that reliability of the dry well cooler is low. SUMMARY OF THE INVENTION A primary object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art described above and to provide a reactor container provided with a dry well cooling system of a reactor container capable of removing heat from the reactor container without discharging a fission product into the open air even in the event of a severe accident, and hence, achieving high reliability. Another object of the present invention is to provide a reactor container provided with a dry well cooling system of capable of removing heat from a dry well independently in the event of an accident so that heat can be removed from the reactor container even when the function of a residual heat removing system is disabled, thereby maintaining the soundness of the reactor container. These and other objects can be achieved according to the present invention by providing, in one aspect, a reactor container composed of a dry well and a wet well and provided with a dry well cooling system, the dry well cooling system comprising: an in-dry-well heat exchanger disposed in the dry well of the reactor container; PA1 an in-dry-well blower connected to a primary side of the in-dry-well heat exchanger; PA1 a circulation pipe connected to a secondary side of the in-dry-well heat exchanger; PA1 a normal cooling system connected to the secondary side of the in-dry-well heat exchanger through the circulation pipe and including an equipment cooling pump, an equipment cooling heat exchanger and a seawater pump which are operatively connected to each other; and PA1 a standby cooling system connected to and branched from the circulation pipe. PA1 an in-dry-well heat exchanger disposed in a dry well of the reactor container; PA1 an in-dry-well blower connected to a primary side of the in-dry-well heat exchanger; and PA1 a dedicated dry well cooling system connected to a secondary side of the in-dry-well heat exchanger, the dedicated dry well cooling system comprising a dedicated cooling pump, a dedicated cooling heat exchanger and a dedicated cooling seawater pump which are operatively connected to each other. PA1 an in-dry-well heat exchanger disposed in a dry well of the reactor container; PA1 an in-dry-well blower connected to a primary side of the in-dry-well heat exchanger; PA1 a circulation pipe connected to a secondary side of the in-dry-well heat exchanger; and PA1 a cooling system connected to the secondary side of the in-dry-well heat exchanger and including an equipment cooling pump, an equipment cooling heat exchanger and a seawater pump, PA1 the in-dry-well heat exchanger and the in-dry-well blower being surrounded by environmental condition resistance maintaining equipment. PA1 an extra-dry-well blower disposed outside the reactor container and connected to the dry well of the reactor container; PA1 an extra-dry-well heat exchanger connected to the extra-dry-well blower and having a primary side connected to the dry well; and PA1 an emergency dry well cooling system equipment cooling system connected to a secondary side of the extra-dry-well heat exchanger, the emergency dry well cooling system equipment cooling system comprising an emergency dry well cooling system equipment cooling pump, an emergency dry well cooling system equipment cooling heat exchanger and an emergency dry well cooling system equipment cooling seawater pump which are operatively connected to each other. PA1 an extra-dry-well blower disposed outside the reactor container and connected to the dry well of the reactor container; PA1 an extra-dry-well heat exchanger connected to the extra-dry-well blower and having a primary side connected to the dry well; and PA1 an emergency dry well cooling system equipment air cooling system connected to a secondary side of the extra-dry-well heat exchanger, the emergency dry well cooling system equipment air cooling system comprising an emergency dry well cooling system equipment air cooling pump and an air cooler. PA1 an extra-dry-well blower disposed outside the reactor container and connected to the dry well of the reactor container; PA1 an extra-dry-well heat exchanger connected to the extra-dry-well blower and having a primary side connected to the dry well; PA1 pipe lines having one ends connected to the dry well and the wet well of the reactor container and other ends connected to the extra-dry-well heat exchanger; PA1 a circulation pipe connected to a secondary side of the extra-dry-well heat exchanger; PA1 a primary cooling system provided with a pump and a heat exchanger connected through the circulation pipe; and PA1 a secondary cooling system provided with a seawater pump. PA1 an extra-dry-well blower disposed outside the reactor container and connected to the dry well of the reactor container; PA1 an extra-dry-well heat exchanger connected to the extra-dry-well blower and having a primary side connected to the dry well; PA1 pipe lines having one end of each thereof respectively connected to the dry well and the wet well of the reactor container and the other end thereof respectively connected to the extra-dry-well heat exchanger; PA1 a circulation pipe connected to a secondary side of the extra-dry-well heat exchanger; and PA1 a cooling system provided with a seawater circulation pipe connected to the extra-dry-well heat exchanger and a seawater pump connected through the seawater circulation pipe. PA1 an extra-dry-well blower disposed outside the reactor container and connected to the dry well of the reactor container; PA1 an extra-dry-well heat exchanger connected to the extra-dry-well blower and having a primary side connected to the dry well; and PA1 pipe lines having one end thereof respectively connected to the dry well and the wet well of the reactor container and the other end thereof respectively connected to the extra-dry-well heat exchanger, PA1 the extra-dry-well heat exchanger being disposed in a cooling pool opened to atmosphere and filled up with water, the cooling pool acting as a heat sink. The standby cooling system comprises a standby cooling pump and a standby cooling heat exchanger connected to each other and comprises a standby seawater pump connected to the standby cooling heat exchanger. A normal power supply means and an emergency power supply means are connected to the in-dry-well blower, the seawater pump, the standby cooling pump and the standby seawater pump. The standby cooling system comprises a seawater circulation line for directly circulating the seawater therethrough and a seawater pump for pumping up the seawater. The standby cooling system may comprise a standby cooling pump and an air cooler connected to each other. In another aspect, there is provided a reactor container composed of a dry well and a wet well and provided with a dry well cooling system, the dry well cooling system comprising: In a further aspect, there is provided a reactor container composed of a dry well and a wet well and provided with a dry well cooling system, the dry well cooing system comprising: In a further aspect, there is provided a reactor container composed of a dry well and a wet well and provided with a dry well cooling system, the dry well cooling system comprising: A normal power supply means and an emergency power supply means are connected to the extra-dry-well blower, the emergency dry well cooling system equipment cooling pump, and the emergency dry well cooling system equipment cooling seawater pump. The dry well cooling system further comprises a pressure sensor and a temperature sensor which are disposed in the reactor container, an emergency dry well cooling system automatic start-up circuit adapted to receive respective output signals from the pressure sensor and the temperature sensor, the emergency dry well cooling system automatic start-up circuit being also adapted to receive a function outage signal output from a residual heat removing system, and signal lines for applying output signals from the emergency dry well cooling system automatic start-up circuit to the extra-dry-well blower, the emergency dry well cooling system equipment cooling pump and the emergency dry well cooling system equipment cooling seawater pump. The extra-dry-well blower is connected to a downstream side of the extra-dry-well heat exchanger. The dry well cooling system further comprises a header disposed in the reactor container and connected to a cooling water delivery port on the primary side of the extra-dry-well heat exchanger. A branch pipe is connected to a line between the reactor container and the extra-dry-well blower, a safety valve is connected to the branch pipe, and a pipe on a discharge side of the safety valve is connected to a vent line. In a further aspect, there is provided a reactor container composed of a dry well and a wet well and provided with a dry well cooling system, the dry well cooling system comprising: In a further aspect, there is provided a reactor container composed of a dry well and a wet well and provided with a dry well cooling system, the dry well cooling system comprising: A blower duct means is disposed in the dry well of the reactor container and connected to the primary side of the extra-dry-well heat exchanger through the extra-dry-well blower. In a further aspect, there is provided a reactor container composed of a dry well and a wet well and provided with a dry well cooling system, the dry well cooling system comprising: In a further aspect, there is provided a reactor container composed of a dry well and a wet well and provided with a dry well cooling system, the dry well cooling system comprising: According to the various aspects and features of the present invention, the following functions and effects will be achieved. First, concerning the first to sixth embodiments, which will be described concretely hereinafter with reference to FIGS. 1 to 6, the cooling system for a dry well cooler includes, in addition to the cooling system used during the normal operation of the reactor, the standby cooling system having a capacity comparable to the normal cooling system. Therefore, even if the normal cooling system should fail to work and the function of removing heat from the reactor container should be totally disabled, the heat removal from the reactor container can be performed through the dry well cooler by switching the cooling system from the normal one to the standby one. The dedicated cooling system or the standby cooling system includes an air cooler and a pump for conveying the cooling water, unlike the normal cooling system using the seawater. Therefore, even during the inspection period of the seawater cooling system, the dry well cooler can be operated. Also, since the dedicated or standby cooling system employs a cooling method different from the normal cooling system, the possibility that at the same time as when the normal cooling system fails, the dedicated or standby cooling system also fails due to the common cause, is avoided. The present invention, the dry well cooler can also be operated by the electric power supplied from an emergency power supply means. Therefore, even if the normal power supply depending on the external power source should fail to work and the function of the dry well cooler should be disabled, it is possible to operate the dry well cooler by starting up the emergency power supply means, i.e., an emergency diesel generator (EDG). The cooling system for the dry well cooler includes the dedicated cooling system. Therefore, even if the function of the RHR system for removing heat from the reactor container should be disabled due to a failure of the associated cooling system, the function of the dry well cooler can be maintained because the cooling system for the dry well cooler is independent of the RHR cooling system. In addition, the blower and the heat exchanger of the dry well cooler both mounted in the dry well are designed to have such a environment-resistant capability that is sufficient to protect the blower and the heat exchanger against the high-temperature, high-pressure, high-humidity and aqueous atmosphere condition produced in the event of a severe accident. Therefore, sufficient heat removal by the dry well cooler can be ensured under various environmental conditions that result in the event of a severe accident. As described above, according to these embodiments the reliability of the dry well cooler in the event of a severe accident can be improved and the heat removal from the reactor container can more surely be performed even if a severe accident should occur. Next, concerning the seventh to thirteenth embodiments of the present invention, which will be described concretely hereinafter with reference to FIGS. 7 to 13, the emergency dry well cooling system can remove heat from the reactor independently. Therefore, even if the function of the RHR system for removing the heat from the reactor container should be also disabled at the same time as the occurrence of an accident, it is possible to prevent breakage of the reactor container and avoid discharge of the FP in a large amount by ensuring the sufficient heat removal from the reactor container. The secondary cooling system of the emergency dry well cooling system is a dedicated cooling system. Therefore, even if the function of the secondary cooling system for the RHR system should be disabled, the emergency dry well cooling system can be operated to remove the heat from the reactor container without suffering from any effects. The secondary cooling system of the emergency dry well cooling system includes an air cooler and a pump for conveying cooling water. Although the usual seawater cooling system cannot be operated during the term for inspection of a seawater intake port and so on, the present emergency dry well cooling system can be operated during such inspection term since the air cooler is used for cooling the heat exchanger. Also, since the present emergency dry well cooling system employs a different cooling method from the normal cooling system, it is possible to prevent both the cooling systems from being failed with the common cause. The emergency dry well cooling system can also be operated by the electric power supplied from the emergency power supply means. Therefore, even if the normal power supply means depending on the external power source should fails to work, the heat removal from the reactor container can be performed by the electric power supplied from the emergency diesel generator. In addition, the automatic start-up logic for the emergency dry well cooling system is provided. Therefore, if the temperature or pressure in the reactor container is excessively raised due to a functional outage of the residual heat removing system in the event of an accident, the emergency dry well cooling system is automatically started up to perform heat removal from the reactor container. As a result, a remarkable improvement in safety operation can be expected. Furthermore, the emergency dry well cooling system is designed to have the blower disposed downstream of the heat exchanger. With this embodiment, the air cooled by the heat exchanger is returned into the reactor container by the blower. Therefore, the thermal load imposed on the blower is reduced and the total reliability of the emergency dry well cooling system can be increased. The emergency dry well cooling system is designed to have a header provided at its delivery, i.e., discharge, port of cooling water into the dry well. Therefore, the efficiency in cooling atmosphere in the dry well can be increased. The emergency dry well cooling system is designed to have a vent line connected to the intermediate portion of the cooling system line. Therefore, even if the pressure in the dry well should abruptly rise, the pressure in the reactor container can be suppressed while preventing breakage of the emergency dry well cooling system. Furthermore, concerning the fourteenth to seventeenth embodiments of the present invention, which will be described concretely hereinafter with reference to FIGS. 14 to 17, in a case where an accident occurs in a normal operation condition, the reactor scrams, and thereafter, vapor in the RPV is flown out in the vent tube or into the suppression pool through a safety relief valve. At this time, the vapor is condensed by the water in the suppression pool and a fission product contained in the vapor is captured in the suppression pool water through a scrubbing effect and transferred to the wet well gas phase. According to these embodiments of the present invention, the atmosphere in the wet well is circulated to the suppression pool liquid phase through the cooling system, the dry well and the vent tube, thus the fission product in the suppression pool gas phase can be scrubbed more effectively. When an accident occurs during the normal operation, the reactor is shut-down and the core is cooled. Thereafter, when the reactor container is cooled, by the RHR system through one of the suppression pool cooling mode and the dry well spray mode. In the prior art technology, the suppression pool cooling means is not provided, but according to the present invention, the safeness of the reactor can be realized with high performance. Further, it will be easily understood that the arrangement of the first to thirteenth embodiments may be selectively applicable to the fourteenth to seventeenth embodiments of the present invention by persons skilled in the art without specifically describing the same herein with reference to the accompanying drawings. The nature and further features of the present invention will be made more clear from the following descriptions made with reference to the accompanying drawings.