Patent Number: 050874080
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a primary containment vessel for enveloping the core of a nuclear reactor constituted by a light water reactor of the boiling water type, and, more particularly, to a primary containment vessel which permits improvement of its inherent safety through a static cooling system replacing a pressure suppression pool water cooling system and of the economic efficiency by streamlining facilities and equipment. In addition, the present invention relates to a natural heat-radiating type primary containment vessel which is suitable for cooling the primary containment vessel and/or reducing the emission of radioactive substances at the time of the loss of a coolant. Furthermore, the present invention relates to a natural heat-radiating type primary containment vessel which is suitable for removing to outside the system by a natural force over an extended period of time thermal energy which is produced by core decay heat released to the primary containment vessel at the time of an emergency when the loss of a coolant has occurred. The present invention also relates to a nuclear power plant provided with a condensate storage pool in a reactor building. The present invention also relates to a primary containment vessel reinforcing ring which is suitable for cooling the inside of a primary containment vessel at the time of the occurrence of damage to the piping in the primary containment vessel. Moreover, the present invention relates to a natural circulation-type nuclear reactor, and, more particularly, to a natural circulation-type nuclear reactor provided with an emergency reactor cooling system which is suitable for use in a boiling water reactor and capable of assuring the cooling of a core by such as maintaining the core immersing water over an extended period of time at the time of the occurrence of a loss-of-coolant accident and/or at the time of an emergency when control rods cannot be inserted. 2. Description of the Related Art As an example of the prior art, there is a primary containment vessel having a pressure suppression of boiling water reactor facilities, as shown in FIG. 20. A primary containment vessel 201 envelops a reactor pressure vessel 202, and the upper space therein surrounding the reactor pressure vessel 202 is called a dry well 203, while a container disposed at a lower portion thereof and filled with pool water 204 is called a pressure suppression 205. The dry well 203 and the pressure suppression 205 are constructed such as to communicate with each other by means of a vent pipe 206. An open end of the vent pipe 206 is immersed in the water of a pressure suppression pool 204 stored in the pressure suppression 205. In the dry well 203 are disposed the piping containing a high-temperature and high-pressure coolant, machines and instruments of a primary system of the reactor, in addition to the reactor pressure vessel 202. Furthermore, containment spray headers 207 for spray the cooling water are provided in the container 201. In addition, a residual heat removal pump 208, a residual heat removal system heat exchanger 209 for removing residual heat, and piping from the pressure suppression pool to the spray header 207 via these machines are provided to supply the cooling water to the spray head 207. Furthermore, piping for returning the cooling water from the heat exchanger 209 for removing residual heat to the pressure suppression pool 204 is also provided. Incidentally, reference numeral 210 denotes a building constituting a biological shield. If an emergency is assumed to have occurred in which the piping of the primary system of the reactor is fractured, the high-temperature, high-pressure coolant of the primary system of the reactor is released into the dry well 203, and a mixture of released steam and water is led to the pressure suppression pools 204 via the vent pipes 206. The released steam is cooled and condensed in the pressure suppression pools 204, thereby suppressing an internal pressure rise of the dry well 203. When the efflux of the coolant from a fracture is completed, the high-temperature and high-pressure steam inside the primary containment vessel 201 is condensed by operating the spray headers 207, which causes the internal pressure of the primary containment vessel 201 to decrease rapidly. When the water temperature of the pressure suppression pools 204 rises by the blow-down of steam, the pressure suppression pool water is cooled by the heat exchangers 209 for removing residual heat. As described above, should the piping of the primary system of the reactor be fractured, when the accident takes place over a short period, the conventional primary containment vessel 201 attains the suppression of pressure by condensation of steam in the water of the pressure suppression pools 204. Meanwhile, when the accident takes place over a long period, the primary containment vessel 201 attains the suppression of pressure by condensation of steam by sprinkling from the spray headers 207 and inhibits a temperature rise of the water of the pressure suppression pool. Since the pressure suppression function in the pressure suppression pool 204 in the former case is constituted by the guiding function of the vent pipes 206 alone, this pressure suppression function is sufficient in ensuring inherent safety as well. On the other hand, to cool the primary containment vessel 201 over a long period of time and cool the pressure suppression pools 204, such dynamic machines as the residual heat removal system pumps 208, the heat exchangers 209, electrically-operated valves, etc., become necessary. In the above-described conventional example, it has been necessary to retain in the pressure suppression pools a large quantity of water for cooling and condensing steam released at the time of a loss-of-coolant accident, and the heat exchangers for removing residual heat have been necessary for cooling the pressure suppression pools over a long period of time. In addition, in the primary containment vessel of a boiling water reactor of the above-described conventional art, a residual heat removal system is provided to cope with the removal of core decay heat over a long period of time after the ECCS is operated subsequent to the accident of loss of the coolant and after the core is submerged with water. As a result, there have been drawbacks in that the costs become high, that the pool water containing fission products is led outside the primary containment vessel, and that it is troublesome to carry out, somewhat periodically, the operation test of dynamic machines such as pumps and heat exchangers to check the operation of the machines installed. In contrast to the pressure suppression pool water-cooling system employing the configuration of dynamic machines and facilities such as the one described above, if as static a heat removing system as possible can be devised as a system having a similar cooling function in place of the facilities which dynamically function, such as rotary equipment, including pumps, large heat exchangers, and large piping loops, it is considered that substantial improvement will be made in the safety and reliability of the system per se through a reduction in the functional requirements for dynamic structural parts, and that the economic efficiency of the plant will be enhanced in conjunction with the streamlining of the facilities per se. As a prior art concerning a cooling system for a primary containment vessel employing such a static system, it is possible to cite a primary containment vessel cooling system based on a heat pipe system disclosed in, for instance, Japanese Unexamined Patent Publication No. (Japanese Patent Application Laid-Open (Kokai) No.) 125483/1980. The arrangement of this system is such that a multiplicity of cylindrical heat pipes with a low-boiling-point liquid sealed therein are installed on the outer surface of a dry well steel plate of the primary containment vessel. This is a heat removing system in which heat retained in a gas inside the container dry well is statically allowed to escape to outside the primary containment vessel via these heat pipes. It is technically feasible to apply the heat pipes of this idea to the above-described water cooling system of the container pressure suppression pools, and a static safety cooling system can be arranged. However, since the heat pipes with the low-boiling-point liquid incorporated therein are installed as a large-scale structure, the heat pipes substantially affect as obstructions the layout of facilities installed on the outer biological shield wall of the primary containment vessel and an external space thereof. In addition, the structure of this system is bound to become large in size in view of the requirements of antiseismic design are an important facility relating to a safety system. Thus, this system has numerous problems in terms of its facilities, its structure does not have an economic advantage, with the result the system is not very realistic. As another example of the prior art relating to a similar primary containment vessel cooling system, it is possible to cite a system disclosed in Japanese Unexamined Utility Model Publication No. 11689/1984 in which the primary containment vessel is filled with a liquid by filling the space between a primary containment vessel concrete wall and a liner with the liquid. In this system, however, the gap between the concrete wall and the liner is 2 mm or thereabout, and since the gap is too small to statically cool the pressure suppression pool water with the liquid of this pertinent portion, a circulating flow of the cooling external liquid does not occur. Consequently, this system is so unrealistic that a high static cooling efficiency cannot be obtained, and that it is impossible to expect its effect. A conventional boiling water reactor plant disclosed in Japanese Unexamined Patent No. 137596/1979 has a condensate storage tank installed outdoors of a reactor building such as to be adjacent to the reactor building and a turbine building. This condensate storage tank is used as a water source for a fuel pool replenishing water system, and a control rod driving hydraulic system, as well as for adjustment of the holding water quantity. In addition, the condensate storage tank is also used as a water source for a cooling system at the time of isolation of the reactor as well as a high-pressure core spraying system, both of which are safety systems. In a conventional boiling water reactor plant, the condensate storage tank is installed on an antiseismic foundation mat (concrete mat). For this reason, a large amount of concrete is required in structuring a special antiseismic mat described above, so that a long period of time has been necessary in constructing the entire foundation mat of a boiling water reactor plant. As a structure for injecting cooling water into a primary containment vessel by making use of gravity at the time of a loss-of-coolant accident, a nuclear reactor disclosed in Japanese Unexamined Patent Publication No. 69289/1982 is proposed. As shown in FIG. 21, this reactor is arranged as follows: A core 211 is disposed in such a manner that a cooling water level 214 formed in a cooling water tank 213 is located above a cooling water level 212 formed in a reactor pressure vessel 202 incorporating the core 211. At the time when a loss-of-coolant accident has occurred, separation valves 215, 216 in a piping 214 communicating with a vapor phase portion of the reactor pressure vessel 202 and a cooling water tank 213 are opened to set the pressure of the two spaces at the same level. A pressure relief valve 218 is provided between a main steam pipe 217 and a pressure suppression 205. At the time of an accident when the coolant has been lost, steam inside the reactor pressure vessel 202 is released to a pressure suppression pool 204 via the pressure relief valve 218 so as to decrease the pressure. When the internal pressure of the reactor pressure vessel 202 has been decreased and dropped below that of the cooling water tank 213, separation valves 220, 221 of a piping 219 communicating with the bottom of the cooling water tank 213 and the reactor pressure vessel 202 are opened so as to supply the cooling water contained in the cooling water tank 213 into the reactor pressure vessel 202 by virtue of the operation of gravity. In addition, the pressure suppression 205 is located above the cooling water level 212 formed inside the reactor pressure vessel 202 so as to be able to constitute an emergency core cooling system of the gravity dropping type. At the time of a loss-of-coolant accident, with the opening of a valve 222, the pool water 204 in the pressure suppression 205 is led into the reactor pressure vessel 202 by means of gravity. A similar structure of a nuclear reactor is disclosed at page 13 of Nuclear Engineering Vol. 31, No. 383 (June, 1986) as well. This example of the prior art is provided with the pressure suppression 205 above the liquid level 212 inside the reactor pressure vessel 202. At the time of a loss-of-coolent accident, when the supply of the pool water 204 to the reactor pressure vessel 202 by the action of gravity is completed, there is a possibility that the pool water 204 inside the pressure suppression disappears. For this reason, the capabilities of condensing steam introduced by a vent pipe 223 disadvantageously decline. SUMMARY OF THE INVENTION Accordingly, a primary object of the present invention is to provide a natural heat-radiating type primary containment vessel which eliminates the use of a heat exchanger for removing residual heat by allowing the heat of the primary containment vessel to escape into the atmosphere through primary containment vessel wall surfaces, and which is capable of effecting cooling for a long period of time after the occurrence of a loss-of-coolant accident. The aforementioned object can be obtained by expanding an annulus portion between the primary containment vessel and a reactor building, by providing a container-outer-periphery pool with water accommodated therein, by providing a vent pipe communicating with an upper gaseous phase portion of the annulus portion to the outside of the reactor building, and by transferring the heat in the primary containment vessel to the outer-periphery pool through a container wall surface and allowing the heat to escape to the atmosphere. The characteristic feature of the present invention lies in that heat is transferred from a pressure suppression pool to the outer-periphery pool through the primary containment vessel wall surface without using any driving force, and is ultimately released into the atmosphere. In addition, since the heat inside the primary containment vessel can be allowed to escape to the atmosphere, a residual heat removal system which has hitherto been employed to allow the heat to escape to a sea becomes unnecessary, so that malfunctioning can be overcome and reliability can be improved. Another object of the present invention is to provide a primary containment vessel which has, as a cooling system for container pressure suppression pool water, a heat removal system which is capable of effecting static cooling for a long period of time with a high heat removal efficiency after the occurrence of an accident, in place of dynamic facilities such as rotary equipment such as a pump, a large heat exchanger, and a large piping loop. The aforementioned object can be attained by providing a pool area around the outer periphery of a container in a space portion formed between a wet well of the primary containment vessel and a biological shield wall surrounding the same, said pool area being arranged such that a ratio between the depth L of a suppression pool inside the wet well and a gap distance d of a cylindrical portion between the primary containment vessel and a biological shield wall is set to such a value that will facilitate the occurrence of natural circulating flow for enhancing the efficiency of statically cooling the suppression pool water; facilities for injecting noncontaminated water from a water injection tank for injecting the cooling water for the outer-periphery pool through a water injection line; and a vent line provided between a gaseous phase portion of the upper space of the outer-periphery pool and an outside atmosphere portion of reactor facilities and designed to radiate the heat of the heat-sinking outer-periphery pool. Still another object of the present invention is to provide a natural heat-radiation type container which is capable of effecting cooling for a long period of time after the occurrence of a loss-of-coolant accident without needing to provide a heat exchanger for removing residual heat. Specifically, the foregoing object can be attained by providing primary containment vessel facilities comprising: a pipeline led from a dry well to the side of a reactor building by penetrating a dry well wall; a partition plate disposed below a penetrating portion of the pipeline and above the water level of the outer-periphery pool and adapted to divide the outer-periphery pool into upper and lower spaces; a discharge channel having an inlet in a gaseous layer in the lower space and led to the outside; and an emergency gas treatment system having an inlet on the inside of the upper space. The characteristic of the present invention in accordance with this aspect lies in that heat is transferred from a pressure suppression pool to the outer-periphery pool through the container wall surface without needing to use a dynamic driving force, generated steam is ultimately allowed to escape to the atmosphere, and radioactive substances leaking from the container are released after being treated with an emergency gas treatment system. In the present invention, at the time when the heat inside the primary containment vessel is allowed to escape to the atmosphere, heat is transferred from the wall surface of the reactor pressure vessel made of steel to the outer-periphery pool, and the heat is further transferred to the atmosphere through the steam of the outer-periphery pool, thereby attaining the removal of residual heat without any dynamic equipment. With respect to the removal of radioactive substances, since these radioactive substances can be treated by the emergency gas treatment system by differentiating the atmosphere leaking from the reactor pressure vessel from steam generated in the clean pool, the capacity of the facilities can be made smaller. A further object of the invention is to provide a nuclear power plant which is capable of reducing a construction period. The above object can be attained by providing a nuclear power plant in which an outer-periphery pool disposed between a container and a tubular biological shield surrounding the outer periphery thereof is used as a condensate storage pool. The outer periphery pool (condensate storage pool) is disposed around the outer periphery of the container, and a water injection pump, a pump driving turbine, a main steam pipe, and a water injection line are arranged in the primary containment vessel and the reactor building. Consequently, routes of highly important piping in the reactor building, including safety systems such as a reactor isolation cooling system and an emergency core cooling sytem (ECCS), a drive water system such as a control rod driving apparatus, and a fuel pool replenishing water system can be made very short, since the connection of these facilities can be made adjacent to the outer-peripheral pool. In addition, since the outer-peripheral pool is disposed in an excess space portion formed between the primary containment vessel and the biological shield wall (concrete wall) of the reactor building, the pool is installed on an antiseismic concrete mat in the reactor building. Accordingly, since an antiseismic concrete mat used exclusively for a condensate storage tank required in a conventional reactor plant becomes unnecessary, the amount of placed concrete necessary for the antiseismic foundation mat in accordance with this invention can be reduced substantially, with the result that a period of construction of the foundation mat and, hence, the period of construction of the reactor plant can be reduced. When an outer-periphery pool having the function of a conventional condensate storage tank is installed in a reactor building, approximately 2000 m.sup.3 of more of water is retained in the reactor building. Accordingly, it is necessary to pay consideration to the detection and prevention of water leaking to other facilities, such as an emergency core cooling facilities at the time of leakage of the pool water. The outer-periphery pool has a lining pool structure. In the case where this lining pool structure is adopted, it is readily possible to detect leakage from lining welds, and the like by installing the same facilities as conventional facilities for detecting the leakage of a spent fuel pool. In addition, even when a large amount of water held has leaked due to the large damage to the outer-periphery pool, the biological shield wall functions as a flooding water preventing wall. Therefore, no situation occurs where other equipment that is disposed on portions other than the biological shield wall and is important in terms of safety is subjected to flooding water. Furthermore, in order to use the outer-periphery pool as substituent facilities for a condensate storage tank, it is necessary to maintain the water quality at a predetermined value. Unlike the suppression pool in the primary containment vessel, the outer-periphery pool is free from the exhausting of a main steam relief safety valve and the inflow of flashing water and the like from a residual heat removal system, with the result that there is no factor deteriorating the water quality. Accordingly, with respect to the water quality of the outer-periphery pool, its cleanness can be maintained sufficiently throughout the life of the plant, so that the outer-periphery pool is capable of sufficiently attaining the function as a condensate storage tank. A still further object of the present invention is to provide a container reinforcement ring whose mechanical strength is high and which is capable of maintaining the heat radiating characteristics of the outer-periphery pool at a high level. The aforementioned object can be attained by providing a disk type reinforcement ring which is provided on a container and whose thickness is small at opposite ends thereof and large at a central portion thereof. In the case of this invention, by providing a reinforcement ring having a small thickness at the opposite ends thereof and a large thickness at the central portion thereof, the strength of a reinforcement ring having an identical sectional area increases against a tensile force caused by the deformation of the container at the time the occurrence of a container-fracture accident, the thickness of the container can be made thinner by that margin. Consequently, the reinforcement rings are formed into such a configuration as to promote the natural convection. Hence, the relative flow rate of a fluid in the vicinity of the container surface can be increased, which in turn increases the coefficient of heat transfer from the container surface to the water wall, thereby obtaining a high effect of heat removal. A further object of the invention is to provide a primary containment vessel which is capable of substantially increasing natural heat radiation from the wall of a conventional primary containment vessel and of enhancing the economic efficiency and inherent safety. The above object can be attained by a means in which a plurality of projections (fins) are disposed on an inner wall of the primary containment vessel. This means is capable of increasing the area of heat transfer from the projections and of alleviating the hindrance to heat transfer caused by the formation of a film layer on a wall surface by a noncondensible gas (air), with the result that the rate of steam condensation and transfer on the wall surface can be increased. Hence, natural heat radiation from the primary containment vessel wall is promoted. A further object of the present invention is to provide a nuclear reactor which, at the time of a loss-of-coolant accident, is capable of effecting the condensation of steam released into the container and of maintaining submergence of the core in water. An additional object of the present invention is to provide a nuclear reactor which is capable of efficiently effecting submergence of a core. The former object of the present invention can be attained by providing a nuclear reactor comprising: a pressure suppression disposed in the container in such a manner as to surround the outer periphery of the reactor vessel and filled with a coolant in such a manner that a liquid level is formed above an upper end of a core inside the reactor vessel; a submergence line provided with a valve and adapted to introduce the coolant contained in the pressure suppression into the reactor vessel; and a return channel having an opening in a space formed between the reactor vessel and the pressure suppression above the liquid level and communicating with the pressure suppression. The latter object of the present invention can be attained by adding to the foregoing features a gas discharge pipe communicating with a gaseous phase portion above the liquid level in the pressure suppression and having a valve. In the case of a loss-of-coolant accident, the coolant inside the pressure supporession is supplied to the primary containment vessel through the submergence line. In addition, the coolant discharged into the container from the fracture at the time of the loss-of-coolant accident and accumulated in a lower portion thereof is led to the pressure suppression by a return channel. Thus, at the time of occurrence of a loss-of-coolant accident, since the coolant circulates from the pressure suppression back to the same via the submergence line, the reactor vessel, the fracture, the container, and the return channel, the steam condensing capabilities by the coolant in the pressure suppression does not decrease, and the submergence of the core by the supply of the coolant into the primary containment vessel can be effected efficiently. In addition, according to the feature of the latter, since the gas in the gaseous phase in the pressure suppression is released to the outside by means of the gas discharge pipe, the coolant accumulating in the lower portion of the container can be introduced efficiently into the pressure suppression via the return channel. Consequently, since the coolant level of the pressure supporession is increased, the action for supplying the coolant into the primary containment vessel increases. Another object of the present invention is to provide an emergency core cooling system which, by combining a pressure relief valve in the reactor pressure vessel and a plurality of tanks having pressurized water or boric acid water, is capable of reducing the number of dynamic machines and equipment and of thereby improving the reliability, and which has an effective combination for safely shutting down the core and cooling the same at the time of a loss-of-coolant accident and should control rods be incapable of being inserted. The foregoing object can be attained by the following two points; 1) The number of dynamic machines and equipment is reduced and the reliability is enhanced by combining pressure relief valves and pressurized tanks, or tanks disposed above the pressure vessel in of gravity drop. 2) A measure is taken to cope with a loss-of-coolant accident or a case where control rods are incapable of being inserted by adopting a suitable combination of pressure tanks and gravity-drop water tanks and by filling some tanks with boric acid water. By adopting a suitable combination of pressure relief valves and pressurized tanks or gravity-drop water tanks, the pressure relief valve is opened at the time of the occurrence of a loss-of-coolant accident to allow the steam in the pressure container to be released to decrease the internal pressure of the reactor pressure vessel, and by lowering the pressures of the respective tanks to their working pressures, the water in the pressurized tank and then the water in the gravity-drop tank can be consecutively injected into the reactor pressure vessel. In this injection, since the natural force of the pressure of the tank or gravity drop is employed, reliability can be improved as compared with the injection of water using a pump. In addition, since boric acid water is filled in some tanks among the plurality of tanks, boric acid water can be injected into the pressure vessel in the case where the control rod(s) cannot be inserted, thereby making it possible to safely shut down the reactor.