Patent Number: 052951693
Section: summary

BACKGROUND OF THE INVENTION This invention relates to reactor containment facilities and, in particular, to reactor containment facilities improved in terms of the heat dissipation characteristic of a reactor containment vessel. As disclosed in JP.A.63-75594 and JP.A.63-191096, a reactor containment vessel includes a dry well, which defines a space where a reactor pressure vessel containing a core is arranged, and a suppression chamber. The suppression chamber holds suppression-pool water and defines a wet well in the space above it, with the dry well communicating with the suppression-pool water through vent pipes. The outer periphery of this suppression chamber is surrounded by a steel wall, which constitutes the containment vessel, with the steel wall being surrounded by an outer peripheral pool containing a cooling water that is in contact therewith. In this reactor containment vessel, the coolant in the reactor pressure vessel, turned into steam that is at high temperature and pressure by being heated by the core, is conveyed from the reactor pressure vessel to the exterior of the reactor containment vessel through pipes. Any rupture in the pipes will cause some of the coolant in the reactor pressure vessel to leak into the dry well as steam at high temperature and pressure to occupy the same (a loss-of-coolant accident); then, the coolant steam will be discharged therefrom, along with the nitrogen with which the dry well has been filled, through the vent pipes into the suppression-pool water, where the steam condenses, with the nitrogen being accumulated in the wet well as noncondensing gas. The transfer of the noncondensing gas from the dry well to the wet well is completed in several minutes after the occurrence of the accident; afterwards, it is only the steam discharged from the reactor pressure vessel that flows into the suppression-pool water. The condensation of this steam causes the temperature of the suppression-pool water to be increased, generating a difference in temperature between the suppression-pool water and the outer-peripheral-pool water. Since the containment-vessel wall separating the suppression chamber from the outer peripheral pool is made of steel, which is a good conductor of heat, the above-mentioned difference in temperature causes the heat held by the suppression-pool water to be transferred to the outer-peripheral-pool water through the wall of the reactor containment vessel. Due to this arrangement, the heat in the reactor containment vessel can be discharged to the exterior thereof over a long period of time after the occurrence of the accident, without using any dynamic apparatus, with the result that a rise in pressure in the reactor containment vessel is suppressed, thereby ensuring the soundness of the reactor containment vessel. Furthermore, since it promotes the heat dissipation from the reactor containment vessel in a natural manner, without using any dynamic apparatus, the above-described containment vessel is referred to as a natural-heat-dissipation-type or natural-cooling-type containment vessel, which provides a high level of reliability since it employs no dynamic apparatus. Thus, of those reactor containment vessels endowed with a pressure-rise suppressing function to cope with a loss-of-coolant accident, which is to be taken into account from the viewpoint of safety when designing a nuclear reactor, the natural-heat-dissipation-type containment vessel, which is equipped with a cooling water pool in the outer periphery thereof, can be cooled by transferring heat from the suppression chamber to the outer-peripheral-pool water through the containment-vessel wall, thereby suppressing pressure rise in the containment vessel. When applied to a plant of a relatively large output power, this natural-heat-dissipation-type containment vessel entails, at the time of an accident, an increase in decay heat, which is discharged from the reactor core into the space in the containment vessel; this increase in decay heat is in proportion to the output power, so that it is necessary to proportionately increase the quantity of heat that can be dissipated to the exterior of the containment vessel. One method of increasing the heat dissipation from the natural-heat-dissipation-type containment vessel is to enlarge the area of the heat transfer surface through which heat is transferred from the suppression chamber to the outer-peripheral-pool water. In the case where the wall of the reactor containment vessel is used as the heat transfer surface, the heat transfer area can be increased by enlarging the diameter of the containment vessel, or increasing the water depth of the vent pipes so as to attain an enlargement in the height direction of the region which is effective in transferring heat to the outer peripheral pool. Enlarging the diameter of the reactor containment vessel, however, is not desirable since it would entail deterioration in the pressure withstanding capacity of the containment vessel, which would lead to a decrease in the allowable temperature of the suppression chamber, resulting in a degeneration in heat dissipation characteristic. Increasing the water depth of the vent pipes, on the other hand, involves an excessive swell of the suppression-pool water when a great amount of steam rapidly enters the suppression chamber at the initial stage of an accident, so that it is necessary to increase the height of the space above the pool water or augment the strength of the structures inside the suppression chamber. Thus, this method is not desirable, either. Prior-art techniques for enlarging the heat transfer area without enlarging the diameter of the containment vessel or increasing the water depth of the vent pipes, are disclosed in JP.A.64-91089 and JP.A.2-181696, according to which the outer-peripheral-pool water is circulated through pipes running through the interior of the suppression chamber, thus utilizing the heat dissipation from the pipes running through the suppression chamber as well as the natural heat dissipation through the containment-vessel wall. Another prior-art technique in this regard was presented in the "Fall Meeting of Atomic Energy Society of Japan in the Year 1989". According to the technique presented, a convection promoting plate is provided in the suppression pool to promote the pool water circulation in the lower region of the suppression pool, thereby mitigating the temperature stratification in the suppression pool; due to this arrangement, that region of the suppression pool which is effective in absorbing the heat from the nuclear reactor and the heat transfer area for heat dissipation can be enlarged in the vertical direction. According to still another prior-art technique, not only the suppression-pool water but also the wet well is cooled by utilizing the containment-vessel wall; in this prior-art technique, which is shown in JP.A.2-227699, the entire containment vessel is surrounded by a flow passage, through which air is circulated to effect cooling. The prior-art techniques mentioned above, however, have the following problems: In the prior-art techniques described in JP.A.64-91089 and JP.A.2-181696, the outer-peripheral-pool water which has been heated to high temperature by the heat released from the suppression-pool water, is allowed to circulate, so that, in the region below the vent-pipe outlets, it is always the temperature on the side of the outer peripheral pool that rises first. As a result, heat transfer takes place in that region from the outer peripheral pool toward the suppression chamber, so that the heat which has been released to the outer peripheral pool in the region above the vent-pipe outlets is again absorbed in the lower region by the suppression chamber. Thus, while an increase in heat reserve can be expected in the region below the vent-pipe outlets, the heat dissipation area for releasing heat to the outer peripheral pool is not increased; on the contrary, it rather decreases. Further, in this prior-art technique, no consideration is given to the continuity of the water circulation in the heat transfer pipes, which circulation is based on the difference in density due to the difference in temperature between the heat transfer pipes and the outer peripheral pool. The water in the heat transfer pipes and that in the outer peripheral pool are heated by the heat released from the suppression chamber and are reduced in density to be accumulated in the upper section of the pool. Since this accumulation takes place both in the heat transfer pipes and in the outer peripheral pool, and the outer peripheral pool is open to the atmospheric air, these two regions are eventually filled with water at the saturation temperature thereof (100.degree. C.), so that the requisite temperature difference cannot be secured between the two regions, resulting in the water circulation being stopped. With the prior-art technique for increasing heat dissipation presented in the Atomic Energy Society of Japan, the water in the suppression pool is circulated by the convection promoting plate installed in the suppression pool; due to this arrangement, that problem to which no consideration was given in the above prior-art technique can be solved, making it possible to utilize the region below the vent-pipe outlets and attain continuity in circulation. Since, however, only the containment-vessel wall is used as the heat transfer surface for releasing heat from the suppression chamber to the outer peripheral pool, the transfer area can only be enlarged in proportion to enlargement of the high-temperature region of the suppression pool, which means the suppression pool has to be enlarged if a further increase in heat dissipation is desired. That would entail an increase in the size of the reactor containment vessel. In the prior-art technique described in JP.A.2-227699, in which air cooling is effected, the rate of the heat transfer by the air circulation is lower than that of the convection heat transfer in the pool water, so that a large heat transfer area is needed to attain the requisite heat dissipation characteristic. Further, in this prior-art technique, no consideration is given to the above-mentioned necessity of raising the allowable temperature for the suppression pool. To attain a further improvement in heat dissipation characteristic in these prior-art reactor containment vessels so that they may be adapted to a nuclear plant of a larger output power, the heat dissipation area might be increased by enlarging the size of the reactor containment vessel. However, such an increase in the size of the reactor containment vessel would be a problem. In the reactor containment vessels described in JP.A.63-75594 (exclusive of FIG. 4) and JP.A.63-191096, which have been mentioned above, any rupture occurring, for example, in the main steam piping, will, as described above, cause the coolant in the reactor pressure vessel to enter the dry well as steam at high temperature and pressure, which steam will further flow through the vent pipes into the suppression-pool water to condense therein. In this process, part of the coolant in the reactor containment vessel will be drawn down in the dry well. It should be noted here that reactor containment facilities are generally equipped with emergency core cooling systems; when the pressure in the reactor pressure vessel has become lower than a predetermined value, the emergency core cooling system operates to cause water to be fed into the reactor pressure vessel for the purpose of submerging the core. This water further overflows from the rupture opening to be discharged into the dry well, with the result that the water level in the dry well is raised by the drawdown water and this overflow water. When the water level in the dry well has been raised up to the dry-well-side openings of the vent pipes, these hot waters flow into the suppression pool through the vent pipes. In the above prior-art techniques, however, the positions of the dry-well-side openings of the vent pipes are at high level, so that a large amount of water accumulates in the dry well at the time of an a loss-of-coolant accident as mentioned above; accordingly, the water level in the suppression pool only rises to a small degree, with the result that the contact area with the outer peripheral pool of the containment vessel cannot be greater than a fixed value. Further, since a large amount of hot water accumulates in the dry well, the temperature rise of the suppression-pool water is correspondingly dull and the difference in the temperature thereof and that of the outer-peripheral-pool water is small, with the result that the heat transfer from the suppression pool to the outer peripheral pool only occurs to a small degree. In other words, the cooling capacity of the containment vessel has to remain rather poor. By lowering the level of the dry-well-side openings of the vent pipes, the amount of water accumulating in the dry well is reduced and the amount of suppression-pool water is augmented, with the heat transfer to the outer pool of the containment vessel increasing. However, depending on the degree to which their level is lowered, it may happen that the dry-well-side openings of the vent pipes are immersed in water. In that case, it is difficult for the water level in the dry well to be smoothly lowered even if the pressure in the dry well is raised by the hot water accumulated therein (Pascal's principle), so that the pressure in the dry well rises to an excessive degree, resulting in the containment vessel being deteriorated in terms of safety. Thus, it is necessary to ascertain the degree to which the level of the dry-well-side openings of the vent pipes can be lowered without involving any problems. A prior-art technique for reducing the amount of water accumulated in the dry well is described in JP.A.63-229390, according to which a return line is provided, which extends through the wall in which the vent pipes are formed, i.e., the vent wall, and the opening on the dry well side of this return line is situated higher than the water surface of the suppression pool in the normal condition, whereby the suppression-pool water is prevented from flowing backwards to the dry well. Apart from this, shown in FIG. 4 of JP.A.63-75594, which has been mentioned above, is a structure which includes a core submerging hole allowing the dry well to communicate with the vent-pipes; this core submerging hole is provided in that portion of the vent wall which is on the dry well side, and at a height which is above the normal water level of the suppression pool and which allows the reactor core to be submerged. When, in these prior-art techniques, core coolant is drawn down into the dry well through any rupture opening, the water level in the dry well rises; when the water level has reached the height of the return line or that of the core submerging hole, the drawdown water flows into the suppression-pool water through the return line or the core submerging hole, thereby preventing the water level in the dry well from being raised. Further, since the drawdown water enters the suppression chamber, the water level of the suppression-pool water rises, thereby increasing the area of the heat transfer surface through which heat is transferred from the suppression chamber to the outer peripheral pool and improving the rate of heat dissipation from the containment vessel, which is required for a medium or long period of time after accident. In these prior-art techniques, however, the return line or the submerging hole is provided in the vent wall, so that the diameter of the return line or the submerging hole cannot be made large because of the necessity of retaining the requisite level of strength of the vent wall. Therefore, the amount of flow from the dry well to the suppression pool through the return line or the submerging hole is limited, so that, while the water amount in the dry well is increasing at high rate, it is impossible to completely prevent the water-level in the dry well from rising. Thus, the uppermost and hottest portion of the water in the dry well is not transferred to the suppression pool, so that, for a short period after the occurrence of an accident, the containment vessel suffers deterioration in its ability to transfer heat from the dry well to the suppression pool, resulting in the vessel being deteriorated in safety. Further, there are prior-art techniques in which the reactor core is cooled at the occurrence of a loss-of-coolant accident by supplying water into the core by a static means, as disclosed in "Simplicity; the key improved safety, performance and economics", Nuc. Eng. November 1989, and JP.A.63-229390 mentioned above. According to the prior-art technique described in Nuc. Eng. November 1989, the cooling of the reactor core for a short period after the occurrence of any loss-of-coolant accident is effected by means of a gravity-driven water pool in an emergency core cooling system, and the cooling of the reactor core for a long period after the occurrence of the same is achieved by returning the pool water in the suppression pool to the pressure vessel through an equalizing system. For this purpose, the equalizing system comprises an equalizing line which connects the suppression-pool water with the pressure vessel, a blasting valve provided in this equalizing line such as to remain closed during normal operation and as to be opened only at the time of an accident, and a check valve for preventing the coolant in the pressure vessel from flowing into the suppression pool. For a long period after the occurrence of an accident, the water in the containment vessel fills the lower dry well to the full by the gravity-driven water pool, and further fills dry well to the height of the inlets of the vent pipes (or the height of the return line leading to the suppression pool), with drawdown water flowing into the suppression-pool water to raise the water level thereof. As a result, the area of the heat transfer surface through which heat is transferred from the suppression chamber to the outer peripheral pool is augmented, thereby improving the rate of heat dissipation from the containment vessel as required for a medium or long period of time. In this case, it is necessary for the water in the containment vessel to fill the same up to the height of the inlets of the vent pipes (or the height of the return line leading to the suppression pool), with the result that a large amount of gravity-driven-pool water is required. In the prior-art technique described in JP.A.63-229390, the core cooling for a short period after the occurrence of a loss-of-coolant accident is effected by means of an accumulator water tank provided in the emergency core cooling system, and the core cooling for a long period after the occurrence of the accident is attained by the equalizing system connecting the suppression pool with the pressure vessel, as in the prior-art technique described in Nuc. Eng. November 1989. Also in this case, the water in the containment vessel for a long period in the containment vessel has to fill the dry well up to the height of the return line leading to the suppression pool, so that an accumulator water tank of a large capacity is required. Thus, in both of the prior-art techniques described in Nuc. Eng. November 1989 and JP.A.63-229390, it is necessary to previously set the water amount of the gravity-driven water pool or the accumulator water tank at a high level, with the result that the wall of the building structure supporting the same must be made thick. In addition, there is the problem that a more strict requirement is imposed on the structure in terms of earthquake-proof property. SUMMARY OF THE INVENTION It is a first object of this invention to provide a reactor containment facility in which an improvement is attained in terms of the heat dissipation characteristic of the reactor containment vessel for a long period of time after the occurrence of a loss-of-coolant accident while avoiding, as far as possible, augmentation in the size of the reactor containment vessel, so as to be suitable for use in a nuclear plant of a larger output power. A second object of this invention is to provide a reactor containment facility which realizes an improvement in the heat dissipation characteristic of the reactor containment vessel by increasing the allowable temperature for the suppression chamber. A third object of this invention is to provide a reactor containment facility in which an improvement is attained in terms of safety for a short period of time after the occurrence of a loss-of-coolant accident as well as in terms of the heat dissipation characteristic of the reactor containment vessel. A fourth object of this invention is to provide a reactor containment facility in which a reduction can be attained in the volume of the water source for the emergency core cooling system as well as an improvement in the heat dissipation characteristic of the reactor containment vessel. To achieve the above first and second objects, there is provided, according to a first aspect of this invention, a reactor containment facility comprising: a reactor pressure vessel containing a core; a dry well in which the reactor pressure vessel is arranged; a containment vessel housing the dry well; a suppression chamber holding a suppression-pool water and forming above it a first wet well; and passage means allowing the dry well to communicate with the pool water; wherein the facility further comprises: (a) means for defining a second wet well communicating with the first wet well; and (b) cooling means for keeping the second wet well at a temperature lower than that of the first wet well at the time of a loss-of-coolant accident. The reactor containment facility in accordance with the first aspect of this invention operates as follows: At the time of a loss-of-coolant accident, the high-temperature/pressure steam leaked into the dry well is transferred in a pressurized condition to the pool water in the suppression chamber along with the non-condensing gas in the dry well, and the steam is condensed in the pool water, with the noncondensing gas accumulating in the first wet well. The pool water is vaporized by the heat transferred thereto as a result of the condensation of the steam, the first wet well being filled with a mixture fluid of the steam and the noncondensing gas. The second wet well is at a relatively low temperature, so that when the mixture fluid is introduced into the second wet well from the first wet well, the steam in the mixture fluid is condensed into liquid to cause a reduction in pressure, with the second wet well becoming a noncondensing-gas region having practically no steam. The first wet well, in contrast, is brought to a condition in which it is substantially steam only that exists therein. Thus, when considering the pressure-proof property of the containment vessel, it is only necessary to take into account the vapor pressure in the first wet well, which is at a relatively high pressure level. Therefore, it is possible to raise the allowable temperature for the pool water to a saturated-steam temperature corresponding to the withstanding pressure of the containment vessel, which means the difference in temperature between the suppression-pool water and the portion outside thereof can be made so much the larger, thus attaining an improvement in heat dissipation capacity. By thus improving the heat dissipation capacity, the pressure-rise suppression effect of the reactor containment vessel is improved, thereby making it possible to provide a reactor containment vessel suitable for use in a nuclear plant of a larger output power. The second wet well may be provided separately from the conventional wet well, and the suppression chamber may be divided into a first chamber containing the pool water and a second chamber, the second wet well being defined by said second chamber. In the latter case, the second wet well is formed in the suppression chamber, so that the reactor containment facility of the present invention can be realized in a compact structure. The reactor containment facility preferably further comprises: (c) a steel wall which is in contact with the suppression-pool water and which surrounds at least the pool water so as to form the above-mentioned containment vessel; and (d) an outer peripheral pool containing a cooling water in contact with the outer peripheral surface of the steel wall. With this construction, the pool water of the suppression pool is in contact with the outer-peripheral-pool water through the intermediation of a wall that is made of steel, which is a good conductor of heat, so that a satisfactory level of heat transfer efficiency can be obtained, thus attaining a further improvement in heat dissipation capacity. To achieve the above first and second objects, there is provided, according to a second aspect of this invention, a reactor containment facility comprising: a reactor pressure vessel containing a core; a dry well in which the reactor pressure vessel is arranged; a containment vessel housing the dry well; a suppression chamber holding a suppression-pool water and forming above it a first wet well; and a passage means allowing the dry well to communicate with the pool water; wherein the facility further comprises: (a) means for defining a second wet well communicating with the first wet well; and (b) means which separates a mixture fluid consisting of the noncondensing gas in the suppression chamber and the steam from the pool water into the noncondensing gas and the steam and which causes the steam after the separation to remain in the first wet well and the noncondensing gas to be collected in the second wet well. The operation of the reactor containment facility in accordance with the second aspect of this invention is substantially the same as that of the reactor containment facility in accordance with the first aspect thereof. Also in this case, the reactor containment facility preferably further comprises: (c) a steel wall which is in contact with the suppression-pool water and which surrounds at least the pool water so as to form the above-mentioned containment vessel; and (d) cooling means for cooling the outer peripheral surface of the steel wall. Further, to achieve the above first and second object of this invention, there is provided, according to a third aspect of this invention, a reactor containment facility comprising: a reactor pressure vessel containing a core; a dry well in which the reactor pressure vessel is arranged; a suppression chamber holding a suppression-pool water and forming, in the space above the same, a wet well; a plurality of vent pipes allowing the dry well to communicate with the pool water; a steel wall which is in contact with the pool water of the suppression chamber and which surrounds at least the pool water so as to form a containment vessel which houses the dry well and the suppression chamber; and an outer peripheral pool containing a cooling water in contact with the outer peripheral surface of the steel wall; wherein the facility further comprises: (a) dividing means for dividing the wet well of the suppression chamber into a first space which is in contact with the water surface of the pool water and a second space which is not in contact therewith; (b) first passage means which allows the first space to communicate with the second space and which has an area smaller than that of the dividing means; and (c) cooling means for keeping the second space at a temperature lower than that of the first space. The reactor containment facility in accordance with the third aspect of this invention operates as follows: At the time of a loss-of-coolant accident, there exists in the first space a mixture fluid consisting of the steam generated from the pool water and noncondensing gas; in the second space, the steam is condensed and the noncondensing gas accumulates; as a result, the allowable temperature of the suppression chamber is raised, as stated above, thereby improving the heat dissipating characteristic of the reactor containment vessel. Here, the first and second spaces communicate with each other through a narrow passage means, so that the intrusion of the mixture fluid from the first into the second space takes place gradually, whereby the condensation of the steam in the mixture fluid can take place steadily in the second space, leaving no steam to remain uncondensed for a long time. Thus, fractional collection of noncondensing gas and steam can be effected reliably. Further, since the suppression-pool water is water-cooled by the outer peripheral pool, the suppression effect of the reactor containment vessel is improved. Preferably, the above reactor containment facility further comprises: (d) second passage means allowing the lower section of the second space to communicate with the suppression-pool water. Due to this construction, the water condensed in the second space is returned to the suppression pool through the second passage means, thereby further enhancing the degree of repletion of the noncondensing gas in the second space. Further, the above-mentioned steel wall preferably further surrounds the first and second spaces, the above-mentioned cooling means including an air passage formed outside the steel wall. The cooling means may include a recess region formed by extending downwards the outer peripheral section of the second space to be in thermal contact with the cooling water of the outer peripheral pool. The reactor containment facility preferably further comprises: (e) at least one convection promoting pipe which is arranged in the outer peripheral pool and which has at least one upper opening situated below the water surface of the suppression-pool water at a position above the outlets of the vent pipes and at least one lower opening situated in the pool water at a position below the outlets of the vent pipes, with the upper and lower openings communicating with each other to allow the pool water to pass therethrough. With this construction, a further improvement in heat dissipation characteristic can be attained due to the action of the convection promoting pipes described below. Further, the reactor containment facility may further comprise: (f) a convection promoting plate which is arranged in the suppression-pool water along the stee wall, the upper end of the plate being positioned higher than the outlets of the vent pipes, the lower end of the plate being positioned lower than the outlets of the vent pipes, with the difference in height between the upper end and the outlets of the vent pipes being larger than the difference in height between the outlets of the vent pipes and the lower end. With this construction, the convection promoting plate helps to enlarge the convection region of the pool water to enhance the heat dissipation through the steel wall, thereby attaining a further improvement in terms of heat dissipation characteristic. Further, to achieve the above first and second objects, there is provided, according to a fourth aspect of this invention, a reactor containment facility comprising: a reactor pressure vessel containing a core; a dry well in which the reactor pressure vessel is arranged; a containment vessel housing the dry well; a suppression chamber holding a suppression-pool water and forming, in the space above the same, a wet well; and passage means allowing the dry well to communicate with the pool water; wherein the facility further comprises: (a) means arranged on the water surface of the pool water of the suppression chamber for serving to restrain the evaporation of the pool water. The operation of the reactor containment facility in accordance with the fourth aspect of this invention is as follows: At the time of a loss-of-coolant accident, some of the steam at high temperature and pressure is leaked into the dry well and transferred in a pressurized condition to the pool water of the suppression chamber along with the noncondensing gas in the dry well; the steam is condensed, and the noncondensing gas accumulates in the wet well. As a result of this condensation, the temperature of the suppression-pool water is raised; since, however, the evaporation is restrained by the evaporation restraining means, the evaporation of the pool water, which, in the prior art, would start at the saturation temperature corresponding to the vapor partial pressure in the wet well, starts at the saturation temperature corresponding to the total pressure in the wet well. As a result, the temperature of the suppression-pool water can be kept at a higher level under the same wet-well pressure, so that the difference in temperature between the suppression-pool water and the outside of the reactor containment vessel is augmented, thereby attaining an improvement in heat dissipation effect. The reactor containment facility preferably further comprises: (b) a steel wall in contact with the suppression-pool water and surrounding at least the pool water so as to form the above-mentioned containment vessel; and (c) cooling means for cooling the outer peripheral surface of the steel wall. Further, to achieve the above first and second objects, there is provided, according to a fifth aspect of this invention, a reactor containment facility comprising: a reactor pressure vessel containing a core; a dry well in which the reactor pressure vessel is arranged; a suppression chamber holding a suppression-pool water and forming, in the space above the same, a wet well; a plurality of vent pipes allowing the dry well to communicate with the pool water; a steel wall which is in contact with the pool water of the suppression chamber and which surrounds at least the pool water so as to form a containment vessel which houses the dry well and the suppression chamber; and an outer peripheral pool containing a cooling water in contact with the outer peripheral surface of the steel wall; wherein the facility further comprises: (a) a hydrophobic-material layer which is formed on the water surface of the suppression-pool water and which has a saturation vapor pressure and a density that are lower than those of the pool water. The operation of the reactor containment facility in accordance with the fifth aspect of this invention is as follows: When, at the time of a loss-of-coolant accident, the temperature of the suppression-pool water rises as a result of steam condensation, the evaporation of the pool water is restrained since the saturation vapor pressure of the hydrophobic-material layer floating on the pool-water surface is lower than that of water. That is, this hydrophobic-material layer functions as a means of restraining the evaporation of the pool water. Accordingly, the temperature of the suppression-pool water can be kept at a higher level, as stated above, thereby attaining an improvement in heat dissipation effect. Further, since the suppression-pool water is water-cooled by the outer peripheral pool, the suppression effect of the reactor containment vessel is improved. Further, to achieve the above first and second objects, there is provided, according to a sixth aspect of this invention, a reactor containment facility comprising: a reactor pressure vessel containing a core; a dry well in which the reactor pressure vessel is arranged; a containment vessel housing the dry well; a suppression chamber holding a suppression-pool water and forming, in the space above the same, a wet well; and passage means allowing the dry well to communicate with the pool water; wherein the facility further comprises: (a) circulation passage means which has an intake opening situated in the pool water at a position higher than the outlet of the passage means leading to the pool water and a discharge opening situated in the pool water at a position lower than the same, with at least a part of the circulation passage means being situated outside the suppression chamber. The reactor containment facility in accordance with the sixth aspect of this invention operates as follows: At the time of a loss-of-coolant accident, the steam at high temperature and pressure is leaked into the dry well and transferred in a pressurized state to the pool water through the passage means along with the noncondensing gas in the dry well, the steam being condensed and the noncondensing gas accumulating in the wet well. The pool water in that portion of the passage means which is higher than the above-mentioned outlet leading to the pool water is at a higher temperature as compared with the pool water in that portion of the passage means which is lower than that. That portion of the pool water which is at a relatively high temperature is taken up by the circulation passage means. Since the circulation passage means is situated outside, the pool water thus taken up is cooled to be increased in density, and descends of its own accord to be returned to the suppression-pool water through the discharge opening of the circulation passage means. This causes a circulation flow to be generated, by means of which the suppression-pool water is moved to promote heat dissipation. The above reactor containment facility preferably further comprises: (b) cooling means provided in that portion of the circulation passage means which is situated outside the suppression chamber. By cooling the circulation passage means by the cooling means, the heat dissipation capacity of the facility is further enhanced. Further, to achieve the above first and second objects, there is provided, according to a seventh aspect of this invention, a reactor containment facility comprising: a reactor pressure vessel containing a core; a dry well in which the reactor pressure vessel is arranged; a containment vessel housing the dry well; a suppression chamber holding a suppression-pool water and forming, in the space above the same, a wet well; and passage means allowing the dry well to communicate with the pool water; wherein the facility further comprises: (a) circulation passage means at least a part of which is situated outside the suppression chamber for causing the pool water to be circulated from a position higher than the pool-water side outlet of the passage means to a position lower than the same. The operation of the reactor containment facility in accordance with the seventh aspect of this invention is substantially the same as that of the facility in accordance with the sixth aspect of this invention. Also in this case, the reactor containment facility preferably further comprises: (b) cooling means provided in that portion of the circulation passage means which is situated outside the suppression chamber. Further, to achieve the above first and second objects, there is provided, according to an eighth aspect of this invention, a reactor containment facility comprising: a reactor pressure vessel containing a core; a dry well in which the reactor pressure vessel is arranged; a suppression chamber holding a suppression-pool water and forming, in the space above the same, a wet well; a plurality of vent pipes allowing the dry well to communicate with the pool water; a steel wall which is in contact with the pool water of the suppression chamber and which surrounds at least the pool water so as to form a containment vessel which houses the dry well and the suppression chamber; and an outer peripheral pool containing a cooling water in contact with the outer peripheral surface of the steel wall; wherein the facility further comprises: (a) at least one convection promoting pipe which is arranged in the outer peripheral pool and which has at least one upper opening situated below the water surface of the suppression-pool water at a position above the outlets of the vent pipes and at least one lower opening situated in the pool water at a position below the outlets of the vent pipes, with the upper and lower openings communicating with each other to allow the pool water to pass therethrough. The operation of the reactor containment facility in accordance with the eighth aspect of this invention is substantially the same as that of the facility in accordance with the seventh aspect, except for the fact that the pressure suppression effect of the reactor containment vessel is further improved due to the water-cooling of the suppression-pool water by the outer peripheral pool. In the above containment facility, the difference in height between the upper opening mentioned above and the outlets of the vent pipes is larger than the difference in height between the outlets of the vent pipes and the lower opening mentioned above. Further, the above-mentioned convection promoting pipe preferably includes upper and lower header pipes respectively arranged at upper and lower positions in the outer peripheral pool and a plurality of heat transfer pipes allowing the upper and lower header pipes to communicate with each other. To achieve the above first and third objects, there is provided, according to a ninth aspect of this invention, a reactor containment facility comprising: a reactor pressure vessel containing a core; a dry well in which the reactor pressure vessel is arranged; a first suppression chamber holding a suppression-pool water and forming, in the space above the same, a wet well; a plurality of first vent pipes allowing the dry well to communicate with the pool water; a steel wall which is in contact with the pool water of the suppression chamber and which surrounds at least the pool water so as to form a containment vessel which houses the dry well and the first suppression chamber; an outer peripheral pool containing a cooling water in contact with the outer peripheral surface of the steel wall; and an emergency core cooling system adapted to cool the core by supplying a water into the pressure vessel at the time of a loss-of-coolant accident; wherein the height of the dry-well-side openings of the first vent pipes is so determined that when the water level in the dry well, in which water overflowing from the reactor pressure vessel accumulates at the time of a loss-of-coolant accident, has attained a core submerging level which allows submergence cooling of the core, the water in the dry well starts to flow into the first suppression chamber through the first vent pipes. The operation of the reactor containment facility in accordance with the ninth aspect of this invention is as follows: If an accident should occur in which some of the coolant in the reactor pressure vessel is lost due to a rupture in the piping, etc., the water in the emergency core cooling system, e.g., that in the accumulator tank, flows into the reactor pressure vessel and enters the dry well through the rupture opening, thereby raising the water level in the dry well. Since the pressure in the reactor containment vessel has been sufficiently reduced by the time the water level in the dry well reaches the rupture opening, the water level in the reactor pressure vessel rises from the time onwards at which the water level in the dry well has exceeded the rupture opening as does the water level in the dry well, and attains a level equal to the submergence level of the dry well. Here, the core submerging level of the dry well, which partly depends on the structure of the reactor containment vessel, is set at a value which is obtained by adding a margin height, e.g., of approx. 50 cm, to the height of the upper end of the core in the reactor pressure vessel, taking some fluctuation in water level into account. Thus, it is also possible to cope sufficiently with the evaporation of the cooling water in the reactor pressure vessel due to the core decay heat after the submergence. When the water level in the dry well has reached the core submerging level, the water in the dry well flows, starting with the uppermost hot water portion, which is the hottest water portion in the dry well through the vent pipes to the suppression pool, causing the water level and temperature of the suppression-pool water to rise. When the water in the accumulator tank, etc. has been used up, the water level of the suppression-pool water ceases to rise. As a result, the water level in the suppression pool rises higher than in the normal state and the area of the heat transfer surface through which heat is transferred to the outer peripheral pool is enlarged, thereby attaining an improvement in terms of heat dissipation for a long period of time after accident. Further, when the water level in the dry well has reached the core submerging level, the uppermost hot water portion in the dry well immediately starts to flow through the vent pipes into the suppression pool, so that heat transfer to the suppression chamber is effected in an efficient manner, thereby attaining an improvement in terms of safety for a short period after accident. Further, since the temperature of the suppression-pool water is raised to a maximum, the difference in temperature between the suppression pool and the outer peripheral pool of the reactor containment vessel is augmented, thereby making it also possible to increase the quantity of heat that is transferred. In the above reactor containment facility, the amount of coolant stored in the water source of the emergency core cooling system is preferably set such as to be substantially equal to the sum of the amount of coolant needed for raising the water level in the dry well up to the core submerging level and the amount of coolant required for making the water level of the suppression-pool water equal to the core submerging level. Due to this arrangement, the water level in the pressure suppression pool can be raised up to the core submerging level for the dry well, thereby making it possible to enlarge to a maximum the heat transfer area between the suppression pool and the outer peripheral pool of the containment vessel. Further, a structure for reducing the amount of coolant when the water level in the dry well has been raised to the core submerging level is preferably provided in that portion of the space in the dry well which is below the core submerging level. This helps to reduce the time it takes for the water level in the dry well to reach the core submerging level, thereby attaining an improvement in terms of safety. Moreover, since the amount of coolant needed for the submergence may be small, it is possible to reduce the capacity of the accumulator tank, etc. for supplying water to the core. Further, to achieve the above first and third objects, there is provided, according to a tenth aspect of this invention, a reactor containment facility comprising: a reactor pressure vessel containing a core; a dry well in which the reactor pressure vessel is arranged; a first suppression chamber holding a suppression-pool water and forming, in the space above the same, a wet well; a plurality of first vent pipes allowing the dry well to communicate with the pool water; a steel wall which is in contact with the pool water of the suppression chamber and which surrounds at least the pool water so as to form a containment vessel which houses the dry well and the first suppression chamber; and an outer peripheral pool containing cooling water in contact with the outer peripheral surface of the steel wall; wherein the facility further comprises: (a) a second suppression chamber situated above the first suppression chamber and including a suppression pool and a wet well, the suppression pool communicating with the dry well through a plurality of second vent pipes; and (b) a line equipped with a valve and connecting the second suppression chamber with the reactor pressure vessel to provide an emergency core cooling system. The operation of the reactor containment facility in accordance with the tenth aspect of this invention is as follows: If an accident should occur which causes some of the reactor cooling water in the reactor pressure vessel to be discharged into the dry well due to a rupture in the piping, etc., the high-temperature water discharged into the dry well accumulates in the lower section of the dry well; the steam, however, flows through the first and second (the upper and lower) vent pipes and condenses in the suppression pools, so that there is no excessive rise in the pressure in the containment vessel. The valve provided in the line leading from the pool water of the second suppression chamber (the upper-suppression-pool water) to the reactor pressure vessel is opened when the water level in the reactor pressure vessel has been lowered to reduce the drawdown from the rupture opening, and the pressure in the reactor pressure vessel has decreased, or when the pressure in the reactor pressure vessel has been lowered to a sufficient degree by means of a safety relief valve. By thus opening the above valve provided in the line, the water in the upper suppression pool is fed into the reactor pressure vessel to cool the core. The water thus fed flows out through the rupture opening into the dry well, causing the water level in the dry well to rise. When the dry-well water level has reached the dry-well-side openings of the first vent pipes, water flows into the pool water of the first suppression chamber (the lower suppression pool), causing the water level in the lower suppression pool to rise. Due to this arrangement, the submergence cooling of the reactor core can be performed if there is no drive source such as a pump, and, since the water level in the lower suppression pool rises to a large degree, a large area can be secured for the heat transfer to the outer peripheral pool of the containment vessel. That is, an improvement is attained in terms of heat dissipation for a long period of time after accident. Further, by arranging suppression pools at upper and lower positions, the number of vent pipes can be augmented, so that the overshoot of the pressure in the dry well due to the vent-pipe resistance immediately after accident can be reduced, thereby achieving an improvement in terms of safety. Further, by dividing the requisite coolant amount of the suppression pool into upper and lower portions, the water depth in the suppression pools can be set at the same level as in the prior art, so that it is possible to reduce the size (the area or the outer diameter) of the upper and lower suppression pools, that is, the diameter of the containment vessel. In the above reactor containment facility, the height of the dry-well-side openings of the first vent pipes is preferably set at a level substantially equal to a core submerging level which is that water-level in the dry well at which submergence cooling of the core can be effected with the water in the dry well, which has overflowed from the reactor pressure vessel and accumulated in the dry well at the time of a loss-of-coolant accident. As in the case of the facility in accordance with the ninth aspect, this arrangement is advantageous in that when the water level in the dry well has reached the core submerging level, the water in the dry well flows, starting with the uppermost hot water portion, which is the hottest water portion in the dry well, to the suppression pool through the vent pipes, and the water level and temperature of the suppression-pool water start to rise, whereby heat transfer to the suppression chamber is effected in an efficient manner, thus attaining a further improvement in terms of safety for a short period of time after accident. Further, the amount of coolant of the second suppression chamber is preferably set such as to be substantially equal to the sum of the amount of coolant needed for raising the water level in the dry well up to the core submerging level and the amount of coolant required for making the water level of the suppression-pool water equal to the core submerging level. Due to this arrangement, the water level in the lower suppression pool rises to the level of the dry-well-side openings of the vent pipes, so that the heat transfer area between the suppression pool and the outer peripheral pool of the containment vessel can be enlarged to a maximum. To achieve the above first and fourth objects, there is provided, according to an eleventh aspect of this invention, a reactor containment facility comprising: a reactor pressure vessel containing a core; a dry well in which the reactor pressure vessel is arranged; a suppression chamber holding a suppression-pool water and forming, in the space above the same, a wet well; a plurality of vent pipes allowing the dry well to communicate with the pool water; a steel wall which is in contact with the pool water of the suppression chamber and which surrounds at least the pool water so as to form a containment vessel which houses the dry well and the suppression chamber; an outer peripheral pool containing cooling water in contact with the outer peripheral surface of the steel wall; and an emergency core cooling system adapted to cool the core by supplying water into the pressure vessel at the time of a loss-of-coolant accident; wherein the facility further comprises: (a) equalizing means which, for a long period of time after a loss-of-coolant accident, cools the core by supplying water into the pressure vessel, utilizing the suppression-pool water and the drawdown water accumulated in the dry well as a water source. The reactor containment facility in accordance with the eleventh aspect of this invention operates as follows: Due to the provision of an equalizing means which, for a long period of time after a loss-of-coolant accident, supplies water into the pressure vessel by utilizing the suppression-pool water and the drawdown water in the lower section of the dry well as the water source, the drawdown water in the lower dry well can be directly utilized as a new water source,. so that there is no need to fill the dry well with drawdown water up to the height of the vent pipes and return it to the suppression pool for the purpose of using the same. Therefore, it is only necessary for the water source of the emergency core cooling system to have a capacity large enough to fill the lower dry well up to the core submerging level, thus making it possible to reduce the capacity of the water source. By the "drawdown water" is meant that portion of the cooling water in the pressure vessel which has outflowed through the rupture opening and that portion of the cooling water which has been leaked out through the rupture opening after being fed into the pressure vessel from the emergency core cooling system. Further, since it is also possible to transfer the decay heat from the suppression pool to the outer peripheral pool through the wall of the containment vessel, the cooling of the core and the containment vessel can be effected by a static means for a long period of time after the occurrence of an accident. In the above reactor containment facility, the opening in the suppression-pool water of the equalizing means is preferably at a height near the water surface of the pool water. Due to this arrangement, the water level in the suppression pool can be maintained at a high level when it is so set, so that the area of the heat transfer surface through which heat is transferred to the outer peripheral pool can be enlarged, thereby attaining an improvement in terms of heat dissipation. Further, the above-mentioned equalizing means preferably includes: a first equalizing line connecting the suppression-pool water with the pressure vessel; a second equalizing line branching off from the first equalizing line and opening at a position below the dry well; an isolation valve provided between the point at which the first equalizing line is connected with the pressure vessel and the branching point at which the second equalizing line branches off; and check valves respectively provided in the first and second equalizing lines, said check valves in the first equalizing line being positioned between the branching point and the point at which the first equalizing line is connected with the suppression-pool water and in the second equalizing line. In this case, the opening in the suppression-pool water of the first equalizing line is preferably at a level near the water surface of the pool water. The above reactor containment facility preferably further comprises: (b) first detection means for detecting the pressure in the pressure vessel; (c) second detection means for detecting the water level in the pressure vessel; (d) third detection means for detecting the pressure in the dry well; (e) a pressure reducing valve connected with the pressure vessel and adapted to allow the steam in the pressure vessel to escape to the dry well; and (f) control means which is adapted to open the pressure reducing valve in response to a low-water-level signal indicative of low water level in the pressure vessel and supplied from the second detection means and to a high-pressure signal indicative of high pressure in the dry well and supplied from the third detection means so as to allow the steam in the pressure vessel to escape therefrom, and which, afterwards, opens the isolation valve to operate the equalizing means, in response to a low-pressure signal indicative of low pressure in the pressure vessel. With this construction, the isolation valve of the equalizing means is opened after the pressure reducing valve has been automatically opened in the process during which the pressure in the pressure vessel decreases after the occurrence of an accident. When the isolation valve of the equalizing means has been opened, a reduction to a sufficient degree in the pressure in the pressure vessel after a long time after the occurrence of the accident will cause the pool water in the suppression pool and the drawdown water in the dry well to flow into the pressure vessel due to water head. Since check valves are provided in the first and second equalizing lines of the equalizing means, there is no risk of the water in the pressure vessel flowing backwards to the dry well or the suppression pool, and none of the pool water in the suppression pool will flow into the dry well. The above-mentioned isolation valve may comprise a blasting valve or an electrically operated valve. Further, the first and second equalizing lines are preferably respectively equipped with each two of isolation valves and check valves as mentioned above that are arranged in parallel; thus, if one of the systems becomes out of order, the equalizing means can be reliably operated by the other one.