Patent Number: 052951693
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of this invention will now be described with reference to the accompanying drawings. First, the first embodiment of this invention will be described with reference to FIGS. 1 to 3. This embodiment is shown as applied to a facility including a steel reactor containment vessel having a diameter of 34 m. The reason for the containment-vessel diameter of 34 m is that this dimension will allow the reactor vessel, the piping system, and the requisite equipment for operation to be housed in the containment vessel in the case of a plant whose output electrical power ranges from 600 to 1500 MW. In the other embodiments described below, containment vessels of the same diameter are adopted. In FIG. 1, the reactor containment facility of this embodiment comprises: a reactor pressure vessel 2 containing a reactor core 1; a dry well 3 providing a space in which the core 1 is arranged; a suppression chamber 4 consisting of suppression-pool water (hereinafter referred to simply as "suppression pool" or "pool water") 5 and a gaseous-phase-space wet well 6 defined above the suppression pool; a plurality of vent pipes 7 connecting the dry well 3 and the suppression pool 5 to each other; a reactor-containment-vessel wall 8 made of steel; and an outer peripheral pool 9 arranged outside the suppression pool 5. The facility further comprises: an accumulator water tank 25 and a gravity-driven water tank 26, which are situated above the reactor pressure vessel 2 and connected thereto through check valves 28; and a submerging system 27 connecting the suppression pool 5 and the reactor pressure vessel 2 to each other through a check valve. By "reactor containment vessel" is implied here the steel reactor-containment-vessel wall 8 and the structures integrally built therein, i.e., the suppression chamber 4, etc. This reactor containment vessel is formed as a vessel of natural-cooling type, with the outer peripheral pool 9 being arranged around it. The components featuring this embodiment will be described. The wet well 6 is divided by a partition 63 into first and second spaces 61 and 62, with the first space 61 being in contact with the surface of the suppression pool water. The two spaces communicate with each other through a plurality of pipes 64 extending through the partition 63. Further, provided in the partition 63 are a plurality of pipes 65, which allows the bottom section of the second space to communicate with the suppression pool water. Provided outside that portion of the steel reactor-containment-vessel wall 8 which is adjacent to the second space of the wet well, is an air passage 66, which sucks in air through an inlet in the lower section of the building and discharges it through an outlet in the upper section of the same. FIG. 2 is an enlarged view of a part of the portion around the suppression chamber. The allowable temperature for the suppression chamber 4 of the reactor containment vessel is determined as follows: The pressure in the wet well 6 (61), which is in contact with the vessel wall 8 constituting the pressure boundary of the reactor containment vessel, is the sum of the noncondensing-gas partial pressure and the vapor partial pressure, in the wet well. At the time of an accident, all the noncondensing gas that exists in the reactor containment vessel in normal operation is accumulated in the wet well, so that the maximum value of the noncondensing-gas partial pressure in the wet well in this condition is determined from the ratio of the total gaseous-phase volume of the reactor containment vessel to the wet-well volume. Further, the vapor partial pressure in the wet well is determined as the saturation vapor pressure corresponding to the surface temperature of the suppression pool 5. The temperature of the suppression pool must be limited such that, at the time of an accident, the pressure in the wet well, which is the sum of the above two categories of pressure, is not higher than the withstanding pressure of the vessel, i.e., such that the vapor partial pressure is not larger than the difference between the withstanding pressure of the vessel and the noncondensing-gas partial pressure. The temperature limit thus obtained constitutes the allowable temperature for the suppression pool. In this embodiment, the allowable temperature for the suppression chamber is raised by the following principle to increase the difference in temperature between the suppression chamber and the outer periphery thereof, and, due to the large temperature difference thus attained, it is possible to dissipate a large quantity of heat to the exterior. At the time of a loss-of-coolant accident, the noncodensing gas in the dry well 3 is forced out by the steam discharged from the reactor pressure vessel and flows through the vent pipes 7 to the suppression pool 5, accompanied by the steam. At this time, the noncondensing gas is first accumulated in the first space 61, which is in contact with the surface of the suppression-pool water, and, after raising the pressure of that region, flows into the second space 62 due to the pressure difference. Afterwards, as a result of the steam flowing through the vent pipes 7 into the suppression pool to condense therein, the water temperature of the suppression pool rises, and the vapor partial pressure in the first space 61 is raised, with the total pressure also rising. Since gaseous-phase circulation/mixing is not restricted in the first space 61, the noncondensing gas and the steam are evenly mixed with each other. Due to the pressure difference between the first and second spaces 61 and 62, this gaseous phase flows from the first space 61 into the second space 62. Since the second space 62 is cooled and the steam flowing into it accompanied by noncondensing gas due to the above action is partly or entirely condensed therein, the pressure in the second space 62 becomes lower than that in the first space 61. As a result, the steam accompanied by noncondensing gas again flows from the first space 61 to the second space 62. As this operation is repeated, the noncondensing gas in the wet well is entirely accumulated in the second space 62. Since the returning of the gaseous phase from the second space 62 to the first space 61 is restricted, the first space 61 is filled with steam only, so that when considering the pressure in this space, it is only necessary to take into account the vapor pressure. The cooling amount required at this time in the second space 62 is that required for making the temperature of the steam flowing into the second space through the passages 64 connecting the two spaces lower than the temperature when it is in the first space 61, so that it need not be a large one. Further, since the wet well is divided into upper and lower sections, the size of the reactor containment vessel is not influenced. As a result of the above operation, the saturation steam temperature corresponding to the withstanding pressure of the vessel is obtained as the allowable temperature for the suppression pool. Due to this arrangement, it is possible to raise the allowable water temperature for the suppression pool under the condition of the same withstanding pressure of the pressure vessel, without changing the thickness of the reactor-containment-vessel wall 8; furthermore, the difference in temperature between the suppression pool and the outer peripheral pool increases, thus attaining an improvement in heat dissipation characteristic. Accordingly, this reactor containment facility can be applied to a plant of a higher output power with the same containment-vessel configuration. Next, the operation of this embodiment will be explained, partly repeating what has been described above. At the time of a loss-of-coolant accident, which is taken into account from the viewpoint of safety when designing a nuclear reactor, the coolant in the reactor pressure vessel 2 flows out into the dry well 3 as steam at high temperature and pressure. Control rods (not shown) are inserted into the reactor core 1 to stop the nuclear fission; in the reactor core, however, the generation of decay heat continues for a long period after that. As the pressure in the reactor pressure vessel decreases, cooling water is supplied thereto from the accumulator water tank 25, the gravity-driven water tank 26 and the core submerging system 27, due to the difference in pressure and gravitation, thereby maintaining the submergence of the reactor core 1. The decay heat in the reactor core 1 is removed by the evaporation of this cooling water and steam is discharged through the rupture section to the dry well 3, whereby the pressure in the dry well 3 is raised to force the water level in the vent pipes 7 downwards, with the steam flowing into the suppression pool 5 to be condensed in the pool water. In this process, the noncondensing gas which has been in the dry well is forced out by the discharged steam and flows, accompanied by it, into the suppression pool, where it ascends to be accumulated in the first space 61. As a result of this accumulation, the pressure in the first space 61 is raised, so that the noncondensing gas flows through the pipes 64 into the second space 62. The transfer of the noncondensing gas from the dry well to the wet well is completed in several minutes after the occurrence of an assumed accident; afterwards, only the steam discharged from the reactor pressure vessel 2 flows into the suppression pool. Because of the decay heat generated during the steam condensation in the suppression pool 5, that portion of the pool water around the vent-pipe outlets 13 is heated, and, due to convection, the temperature of that portion of the pool water which is above the vent-pipe outlets 13 is raised in a substantially uniform fashion. With this rise in temperature, evaporation takes place at the surface of the pool water, and the vapor partial pressure in the first space 61 also rises to cause the pressure in the space to be raised. And the generated steam flows into the second space 62 along with the noncondensing gas remaining in the first space 61. The steam which has entered the second space 62 releases heat to the outer passage 66 through the steel containment-vessel wall 8 and condenses on the wall surface; afterwards, it returns to the suppression pool 5 through the pipes 65. Though its cooling capacity per unit area is small, the natural air-cooling action for cooling the second space 62 is effective since the area of the containment-vessel wall 8 serving as the heat transfer surface is large and the amount of steam flowing in through the pipes 64 is small; thus it is capable of condensing the steam entering the second space 62 and keeping the space at a temperature lower than that of the first space 61. Here, in the pipes 64, the noncondensing gas which has once entered the second space 62 is prevented from flowing back to the first space 61 by the gas flow from the first space 61, and, in the pipes 65, it is prevented by the suppression-pool water in which the pipes are immersed. By repeating the operations described above, substantially the total noncondensing gas is accumulated in the second space 62, with the first space 61 being filled with steam at a temperature equal to the surface temperature of the suppression-pool water. On the other hand, the heat dissipation from the containment vessel at the time of an assumed accident is basically effected by the heat release from the suppression pool 5, which has attained high temperature, to the outer peripheral pool 9 through the steel containment-vessel wall. The improvement in heat dissipation characteristic attained in this embodiment will be explained with reference to FIG. 3, which is a diagram comparing a case where this embodiment is applied with a case where it is not, in terms of the changes in the pressure in the containment vessel with respect to the time elapsing after the occurrence of an assumed accident. In the case where this embodiment is not applied, which is represented by the broken line, the temperature of the suppression-pool water rises in process of time; as the temperature of the suppression-pool water rises, however, the quantity of heat released to the outer peripheral pool is also augmented, until it exceeds the quantity of decay heat generated in the reactor core, with the result that the pressure in the containment vessel starts to decrease, thus keeping the pressure in the containment vessel below the withstanding pressure of the vessel. This established plant output power will be defined as a standardized output power 1.0, which is represented by the solid line A in the drawing. In the case of this standardized output power of 1.0, where this embodiment is applied, it is not the sum of the noncondensing-gas partial pressure and the vapor partial pressure but the vapor partial pressure only that is to be considered to be the pressure in the containment vessel, as stated in the description of the principle. Accordingly, the maximum pressure at the time of an assumed loss-of-coolant accident is relatively low. That is, in this embodiment, the temperature of the suppression-pool water is allowed to rise in correspondence with this lowered pressure, and the difference in temperature between the suppression pool and the outer peripheral pool, which is open to the atmospheric air and, consequently, whose temperature cannot be higher than 100.degree. C., can be augmented, whereby an improvement is attained in terms of heat dissipation characteristic, making it possible for a containment vessel of the same size to be applied to a plant of a larger output power. In the case of this embodiment, the allowable temperature for the suppression pool can be raised from approx. 122.degree. C. to approx. 144.degree. C. By applying this embodiment, the pressure suppression in the containment vessel is established at a standardized plant output power of 1.6, as indicated by the solid line B of the drawing, thus making it possible to make the applicable plant output power 1.6 times larger. When the pressure in the containment vessel begins to exhibit an inclination to decrease, the pressure in the second space 62 becomes higher than that in the first space 61, and part of the noncondensing gas accumulated in the second space 62 returns to the first space 61 through the pipes 64; at this time, however, the quantity of heat dissipated is in excess of that of decay heat, so that no serious problem is involved in terms of heat dissipation characteristic. If it is desired to prevent this returning of noncondensing gas, it is only necessary to provide check valves in the pipes 64. Further, while in this embodiment return pipes 65 for the condensed water in the second space 62 are provided, such pipes are not absolutely necessary; if they are not provided, water will accumulate in the second space, which, however, will entail no problem in terms of operation. A modification of this embodiment will be described with reference to FIG. 4. This modification differs from the above-described embodiment in that the partition 63 exhibits a stepped section at its end on the side of the containment-vessel wall 8, such that the second space 62 is extended downwards, with the extended region being in thermal contact with the outer-peripheral-pool water situated above the water level in the suppression pool 5; and, further, the pipes 65 for returning condensed water are provided in the bottom section of that region of the second space 62 extended downwards. Cooling which is effected by pool water, as in this embodiment, provides a better heat dissipation as compared with the natural air cooling described above, so that the requisite heat transfer area for cooling the second space 62 can be reduced. Accordingly, an air-cooling means which requires a large heat transfer area and which has to be installed and maintained at a relatively high position above the containment vessel, can be dispensed with. The heat dissipation characteristic which is obtained by this modification as a whole is the same as that described with reference to FIG. 3. A still another modification of the above embodiment will be described with reference to FIG. 5. This modification differs from the above embodiment in that it includes a wet-well-cooling-water pool 41, which is in contact with the outer periphery of the reactor-containment-vessel wall 8 so as to cool the wet well 6 and which is separate from the outer peripheral pool 9, as a means for cooling the second space 62 of the wet well 6, with a circumferential ring-like structure 42 being provided on that portion of the wet well 6 which constitutes the inner periphery of the reactor-containment-vessel wall 8. With this modification, the second space is cooled by pool water, which provides a better heat dissipation as compared with natural air cooling, so that the requisite heat transfer area is reduced, thereby eliminating the need to install a cooling means at a position outside and relatively higher than the reactor containment vessel. This helps to attain an improvement in terms of the ease with which the reactor containment facility is constructed and maintained. Further, by providing, as in this modification, a pool which is separate from the outer peripheral pool 9 situated below and which is intended for that portion of the cooling water which is excessively higher than the water level in the suppression pool 5, the suppression pool 5 can be protected from an excessive external pressure (water head) due to water-level difference during normal operation. Further, as to the external pressure applied to the reactor-containment-vessel wall due to the water level of the wet-well-cooling-water pool 41 that is separately provided, it can be coped with by means of a ring-like structure 42 provided on that portion of the wet well 6 which constitutes the inner periphery of the reactor-containment vessel 8, without changing the thickness of the reactor containment vessel. It is only necessary for this ring-like structure to be installed in the region of the wet well 6, which is a gaseous-phase space; if arranged in water, such a structure would hinder the water convection. As it is, the structure does not hinder the condensation heat transfer at the wall surface, thus avoiding deterioration in heat dissipation characteristic. On the contrary, this ring-like structure helps to prevent the development of a condensate film on the wall surface, so that an improvement can be expected in terms of the heat transfer at the wall surface due to augmentation in liquid film thickness. A second embodiment of this invention will be described with reference to FIG. 6. This embodiment differs from the one shown in FIG. 1 in that, instead of providing a partition structure dividing the wet well 6, there is provided on the water surface of the suppression pool 5 a layer 51 of a hydrophobic material, such as silicone oil or spindle oil, which exhibits a low saturation vapor pressure even at a temperature higher than 100.degree. C. and whose density is smaller than that of water. In this embodiment, the temperature in the suppression chamber is raised by the following principle to increase the difference in temperature between it and the outer periphery thereof, thereby making it possible to dissipate a greater quantity of heat to the outer periphery. By forming on the water surface of the suppression pool 5 a layer 51 of a hydrophobic material, such as silicone oil or spindle oil, which exhibits a low saturation vapor pressure and whose density is smaller than that of water, the pool water 5 is isolated from the wet well 6 by the hydrophobic-material layer 51. The temperature of the hydrophobic material is equal to the temperature of the water surface of the suppression pool; since, however, its saturation vapor pressure is low, the increase in the pressure in the wet well 6 is small. Since the temperature of the suppression pool 5 is raised and the pressure in the wet well 6 is kept at a low level, the water of the suppression pool will presently start boiling. The steam generated as a result of the boiling of the suppression pool passes through the hydrophobic-material layer 51 on the water surface to enter the wet well, thereby raising the pressure therein. Since, however, it is isolated from the pool surface, the wet well has a low humidity and is, consequently, in a superheated-steam condition (in which the temperature is higher than the saturation temperature corresponding to the vapor partial pressure). And when the total pressure in the wet well 6 (the sum of the noncondensing-gas partial pressure and the vapor partial pressure) attains the saturation pressure corresponding to the water temperature of the suppression pool, the suppression pool ceases to boil. This phenomenon repeats itself; that is, due to the formation of the hydrophobic-material layer 51 on the pool surface, 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, can be made to start at the saturation temperature corresponding to the total pressure in the wet well. In other words, the water temperature of the suppression pool can be kept at a higher level under the same wet-well pressure. In the case where the wet well is cooled, the hydrophobic material may be regarded as a substitute for the partition section dividing the wet well in the first embodiment. For the above reason, this embodiment makes it possible to raise the allowable suppression-pool-water temperature under the same allowable steam-partial pressure in the wet well (the same withstanding pressure of the vessel), thus helping to increase the difference in temperature between the suppression pool and the outer peripheral pool and attaining an improvement in terms of heat dissipation. Thus, the embodiment can be applied to a plant of a larger output power with the same reactor-containment-vessel configuration. Next, the operation of this embodiment will be explained, partly repeating what has been described above. The noncondensing gas transferred from the dry well, along with the steam, during the initial period after the occurrence of an accident, is accumulated in the wet well 6 after having passed through the suppression pool 5 and the hydrophobic-material layer 51. On the other hand, the steam coming through the vent pipes 7 is condensed in the pool water, thereby heating the pool water and the hydrophobic-material layer 51. Since, however, the pool surface which is in contact with the wet well 6 consists of a hydrophobic material having a low saturation vapor pressure, the rise in the vapor partial pressure in the wet well 6 is small (practically zero) even if the pool-water temperature is raised, so that it is not necessary to take into account the vapor partial pressure of the hydrophobic material. And, by forming, as in this embodiment, a layer 51 of a hydrophobic material which exhibits a low saturation vapor pressure and whose density is smaller than water, the evaporation of the pool water at the surface of the suppression pool 5 can be restrained, as stated above. Accordingly, it is possible to realize a condition in which the water temperature of the suppression pool 5 is high, with the pressure in the wet well 6 being low. That is, the allowable temperature for the suppression pool can be raised without changing the size of the containment vessel. Further, in the case where the wet well is cooled by natural air cooling as in this embodiment, the hydrophobic material may, as stated above, be regarded as a substitute for the partition 63 and the pipes 64 and 65 of the embodiment shown in FIG. 1, so that the improvement in heat dissipation characteristic attained in this embodiment is the same as that shown in FIG. 3. That is, in this embodiment, the allowable temperature for the suppression pool can be raised from approx. 122.degree. C. to approx. 144.degree. C., and the applicable plant output power can be made 1.6 times larger. A third embodiment of this invention will be described with reference to FIG. 7. This embodiment is featured by upper and lower openings 10 and 11, which are provided in that portion of the steel containment-vessel wall 8 which is in the suppression-pool water, such as to be positioned with the vent-pipe outlets 13 therebetween, the upper and lower openings 10 and 11 being connected with each other through a plurality of convection promoting pipes 12 provided in the outer peripheral pool 9. Apart from this, the main components of this embodiment are the same as those of the embodiment shown in FIG. 1. In this embodiment, the vent-pipe outlets 13 and the upper and lower openings 10 and 11 are positioned in such a manner that the difference in height between the upper openings 10 and the vent-pipe outlets 13 is larger than the difference in height between the vent-pipe outlets 13 and the lower openings 11. In this embodiment, it is possible to release a large quantity of heat from the reactor containment vessel by the following principle: The steam discharged from the pressure vessel 2 into the dry well 3 at the time of a loss-of-coolant accident is introduced into the suppression pool 5 through the vent pipes 7 and condensed in the pool water. As a result, the temperature of that portion of the suppression-pool water which is above the vent-pipe outlets 13 is raised, and, by virtue of the convection in the pool formed by the heating due to the steam condensation at the vent-pipe outlets, that region of the pool water attains a uniform high-temperature condition. On the other hand, the temperature of that portion of the suppression-pool water which is below the vent-pipe outlets 13 is not raised at this time, with the result that a temperature stratification occurs at the height of the vent-pipe outlets. Here, the density condition in that portion of the suppression pool which is between the upper and lower ends of the convection promoting pipes 12 and the density condition in the convection promoting pipes 12, will be considered. That portion of the suppression pool which is above the vent-pipe outlets is at a relatively high temperature and has a small density, whereas that portion thereof which is on the side of the convection promoting pipes is at a relatively low temperature and has a large density. As a result, the water head (production of density and height: .rho.g h) at the lower end of the section being considered is larger on the side of the convection promoting pipes, so that a circulation is formed which flows downwards through the convection promoting pipes 12 to enter the suppression pool. This causes the high-temperature water of the upper portion of the suppression pool to enter the upper section of the convection promoting pipes; since, however, the convection promoting pipes are immersed in the outer peripheral pool 9, which is at low temperature, heat dissipation takes place through the pipe walls, and the temperature of the water is gradually lowered as it flows downwards through the convection promoting pipes. Due to this action, a condition is constantly maintained in the section above the vent-pipe outlets 13 in which the density and, consequently, the water head, are larger on the convection-promoting-pipe side, thereby forming a drive power for the circulation flowing downwards through the convection promoting pipes. In the section below the vent-pipe outlets 13, on the other hand, the portion on the side of the convection promoting pipes first attains high temperature as a result of the circulation, so that the density (water head) condition is such as to cancel the above-mentioned drive power for circulation; however, by appropriately setting the position of the lower ends of the convection promoting pipes (i.e., by setting the position of the lower ends of the convection promoting pipes 12 at such a position as will not completely cancel the downward drive power formed in the upper section), it is possible to cause water at a relatively high temperature to flow into the region below the vent-pipe outlets while maintaining the circulation flowing downwards through the convection promoting pipes. By virtue of this action, the hot water which has flowed into the region below the vent-pipe outlets causes the water temperature of this region to be raised, which helps to eliminate the density (water head) condition which would cancel the drive force for circulation formed in the region below the vent-pipe outlets, thus promoting the circulation. By repeating the above action, the water temperature of the region below the vent-pipe outlets can be continuously raised. At this time, the water temperature of the region below the vent-pipe outlets is substantially the same as the temperature of the water flowing in at the lower ends of the convection promoting pipes, so that it does not become lower than the water temperature of the outer peripheral pool. As a result, pool water at a relatively high temperature is constantly circulated in that region of the suppression pool 5 which is below the vent-pipe outlets 13 and which is at low temperature, thereby increasing the region for absorbing heat from the reactor core 1, and, at the same time, making it possible to utilize not only the reactor-containment-vessel wall 8 corresponding to the suppression-pool region which is at high temperature, but also the walls of the convection promoting pipes 12, as the heat transfer surface through which heat is dissipated to the outer peripheral pool 9. Since it is normal for the convection promoting pipes 12 to have a diameter smaller than that of the reactor containment vessel, the convection promoting pipes can be arbitrarily provided without influencing the withstanding pressure of the reactor containment vessel, so that the heat transfer area can be augmented without changing the size or the withstanding pressure of the reactor containment vessel, thus increasing the quantity of heat that can be dissipated. Accordingly, this embodiment can be applied to a plant of a larger output power with a reactor containment vessel of the same principal dimensions, etc. Next, the operation of this embodiment will be explained, partly repeating what has been described above. The steam discharged from the reactor pressure vessel 2 at the time of an assumed loss-of-coolant accident flows through the vent-pipe outlets 13 into the suppression pool 5 to be condensed in the pool water. The pool water portion which is around the steam openings 13 is heated by the latent heat generated during the steam condensation in the suppression pool 5, and the temperature of the pool water portion which is above the vent-pipe outlets 13 is raised in a substantially uniform fashion. As a result, the density in the section where the convection promoting pipes 12 are arranged is low in the suppression-pool water, which has attained high temperature, and high in the convection promoting pipes 12, which are at low temperature. Due to this difference in density, a flow descending in the convection promoting pipes 12 is generated, and suppression-pool water which is at high temperature passes through the upper openings 10 and enters the upper sections of the convection promoting pipes 12. The high-temperature water which has thus flowed in is cooled inside the convection promoting pipes 12, which are immersed in the outer-peripheral-pool water, and becomes gradually cooler as it descends therein. As a result, a flow descending in the convection promoting pipes 12 is constantly formed, without changing the density condition in the section where the convection promoting pipes 12 are arranged. And, as stated above, water at a temperature higher than that of the water temperature of the outer peripheral pool passes through the lower openings 11 and enters that region of the suppression pool 5 which is below the vent-pipe outlets 13, thereby warming that region. FIGS. 8A and 8B show the temperature distribution and density distribution in the height direction of the suppression pool 5 and the convection promoting pipes 12 at a time after the occurrence of an assumed loss-of-coolant accident. Regarding the temperature, that section of the suppression pool 5 which is above the vent-pipe outlets 13 is at a uniformly high temperature due to the steam from the vent pipes; since the water is cooled in the convection promoting pipes 12, its temperature exhibits a linear reduction toward the lower end; and, as since water at the temperature of this lower end flows in, that region of the suppression pool 5 which is below the vent-pipe outlets 13 attains a temperature substantially equal to that. Here, the reason for the linear reduction in temperature in the convection promoting pipes 12 is that the diameter of the convection promoting pipes is uniform and that, consequently, the cooling in the outer peripheral pool is also uniform in the height direction. Further, the reason for making the water temperature of that region of the suppression pool 5 which is below the vent-pipe outlets 13 substantially equal to the temperature of the water which has flowed in is that the phenomenon in question is a gentle one which covers a long period and, consequently, can be treated as one of a quasi-constant nature. As is known, the density of water is in inverse proportion to the temperature thereof, so that, as shown in FIG. 8B, the density distribution is reverse to the temperature distribution shown in FIG. 8A. As can be seen from the drawing, in the section above the vent-pipe outlets 13, the density on the side of the convection promoting pipes 12 is larger, so that the drive force (water head) in each of the convection promoting pipes 12 works such as to cause a downward flow therein. The sum total of these downward drive forces corresponds to the area of the triangle a formed by the density lines of the two regions in the drawing. In the section below the vent-pipe outlets 13, in contrast, the density on the suppression-pool side is larger, so that the drive forces in this section work such as to cancel the downward drive force generated in the upper section. If the sum total of the drive forces generated in the section below the vent-pipe outlets (i.e., the area of the triangle b formed by the density lines in the drawing) is smaller than the sum total of the drive forces generated in the upper section, the flow in the convection promoting pipes 12 is generally a downward one. That is, if, in the drawing, the area of the triangle a is larger than that of the triangle b, a general flow descending in the convection promoting pipes 12 is constantly formed, thus enabling the present means to operate effectively. It may be concluded from this that, in the case where, as in this embodiment, the configuration of the convection promoting pipes 12 and the cooling condition are both uniform, the condition for the present means to work effectively is that the difference in height between the upper openings 10 and the vent-pipe outlets 13 (L1 in the drawing) be larger than the difference in height between the vent-pipe outlets 13 and the lower openings 11 (L2 in the drawing). Thus, the area of the heat transfer surface through which heat is transferred from the suppression pool 5 to the outer peripheral pool 9 is enlarged, thereby attaining an improvement in heat dissipation characteristic. The improvement in heat dissipation in this embodiment will be explained with reference to FIG. 9. In the drawing, changes in the containment-vessel pressure in process of time after the occurrence of an accident assumed in a case where this embodiment is applied, are, as in the case of FIG. 3, compared with those in a case where it is not. In the case where this embodiment is applied, which is represented by the solid lines in the drawing, it is assumed that approximately 500 convection promoting pipes 12 having a diameter of 50 mm are provided. Since the convection promoting pipes 12 can be arranged over the entire periphery of the containment vessel, no particular problem is involved in terms of practical application. In the case where a standardized plant output power of 1.0 is applied to this embodiment, the heat dissipation area is enlarged, as indicated by the solid line A in the drawing, so that the maximum pressure at the time of an assumed accident is lower than in the case where this embodiment is not applied (indicated by the broken line in the drawing). In correspondence with this reduction in pressure, this embodiment is more suitable for application to a plant of a larger output power. As indicated by the solid line B in the drawing, the applicable standard plant output power is 1.5; which means, this embodiment makes it possible to make the applicable plant output power 1.5 times larger. A modification of the third embodiment will be described with reference to FIG. 10. This modification differs from the embodiment shown in FIG. 7 in that isolation valves 21 are provided on the outer and inner sides of the upper and outer openings 10 and 11 in the reactor-containment-vessel wall 8. FIG. 10 shows a part of this modification where the suppression pool and the outer peripheral pool are arranged. During normal operation, the isolation valves 21 are open; when performing periodical inspection, these isolation valves 21 are closed to isolate the reactor containment vessel from the convection promoting pipes 12, thereby facilitating maintenance operations, such as the replacement of the convection promoting pipes 12. Further, if, for some reason, a leak from the convection promoting pipes 12 should occur, the reactor containment vessel can be isolated more reliably by closing the isolation valves 21. A still another modification of this embodiment will be described with reference to FIGS. 11 and 12. This modification differs from the above embodiments in that the convection promoting pipes are composed of upper header pipes 31 connected with the upper openings 10, lower header pipes 32 connected with the lower openings 11, and heat transfer pipes 33 connecting the upper and lower header pipes 31 and 32 with each other. FIG. 11 is a longitudinal sectional view showing a part of the suppression pool and the outer peripheral pool, and FIG. 12 is a cross-sectional view taken along the line A--A of FIG. 12. The water temperature in the upper header pipes 31 and that in the lower header pipes 32 are respectively made uniform by the water flowing in, so that the operation of this modification is the same as that of the above embodiment. This modification is advantageous in that the number of upper and lower openings 10 and 11 can be reduced, thereby attaining an improvement in terms of machinability and producibility. At the same time, by adjusting the diameter of the header pipes, it is possible to provide as many heat transfer pipes 33 as required, irrespective of the diameter of the reactor containment vessel. Further, by imparting a curved configuration to the heat transfer pipes 33, any expansion or contraction of the heat transfer pipes 33 due to temperature changes can be absorbed. A further modification of this embodiment will be described with reference to FIG. 13. In this modification, the convection promoting pipes are divided into two sections: In the upper section, they are composed of upper header pipes 31 connected with the upper openings 10, lower header pipes 32, and a plurality of heat transfer pipes 33 connecting the upper and lower header pipes with each other; in the lower section, they are composed of the lower header pipes 32 and pipes 34 of a relatively large diameter which are less in number than the heat transfer pipes 33 and which connect the lower header pipes 32 with the lower openings 11. FIGS. 14A and 14B show the temperature distribution and density distribution in the height direction in that section of the suppression pool and the convection promoting pipes which is between the upper and lower openings 10 and 11. In the case of this modification, the area of the contact surface with the outer peripheral pool is different between the upper and lower sections on the side of the convection promoting pipes, so that the cooling is effected to a larger degree in the upper section, which has a larger surface area. As a result, the temperature distribution in the convection promoting pipes is such that the temperature decreasing rate is high in the upper section and low in the lower section, with the density distribution being in correspondence therewith, as shown in the drawing. In this case, the generation of a drive force which would cancel the flow descending in the convection promoting pipes only occurs to a small degree, so that the lower openings 11 can be provided at a lower position as compared with the case of the embodiment shown in FIG. 7. Due to this arrangement, the region which can be effectively utilized can be enlarged in that section of the suppression pool 5 which is below the vent-pipe outlets 13. An embodiment which consists of a combination of the first and third embodiments will be described with reference to FIG. 15. Basically, this embodiment is a combination of the embodiments described with reference to FIGS. 5 and 7. In this embodiment, the allowable temperature for the suppression pool is raised by dividing the wet well into first and second spaces 61 and 62; and the heat dissipation area is enlarged by providing convection promoting pipes 12 in the region corresponding to the suppression pool. Further, for the purpose of cooling the second space 62 of the wet well, there are provided a wet well cooling pool 41 which is separate from the outer peripheral pool 9 and a ring-like structure 42, thereby attaining an improvement in terms of the withstanding pressure of the vessel during normal operation and the ease with which it is built and maintained. As for the improvement in heat dissipation characteristic, the rise in the allowable temperature for the suppression pool and the enlargement of the heat transfer area are combined with each other to make it possible to make the applicable plant output power 2.3 times larger (i.e., to realize an established standard plant output power of 2.3), as shown in FIG. 16. It is known from a presentation in the "Fall Meeting of the Atomic Energy Society of Japan in the Year 1989", mentioned in connection with the prior art, that an enlargement of the high-temperature region of the suppression pool can be realized by providing a convection promoting plate in the suppression pool. An embodiment which consists of a combination of this technique and the first embodiment will be described with reference to FIG. 17. In this embodiment, a convection promoting plate 70 is arranged in the suppression pool 5 of the embodiment shown in FIG. 1, along the reactor-containment-vessel wall 8. As stated in the above-mentioned presentation, this convection promoting plate 70 is arranged such that the vent-pipe outlets 13 are situated between the upper and lower ends of this plate, with the difference in height between the upper end and the vent-pipe outlets 13 being larger than the difference in height between the vent-pipe outlets 13 and the lower end, whereby the high-temperature region of the suppression pool is enlarged. According to this embodiment, the high-temperature region of the suppression pool is enlarged by the action of the convection promoting plate 70, and, though restricted to the section of the reactor-containment-vessel wall 8, the heat dissipation area can be enlarged. At the same time, the noncondensing gas in the wet well 6 can be concentrically collected in the second space 62, so that the allowable temperature for the suppression pool can be raised. Thus, with the enlargement of the heat dissipation area and the increase in temperature difference between the pools, it is possible to attain an enhancement in heat dissipation characteristic. The above-described basic embodiments can also be applied to a plant whose reactor-containment-vessel wall is mainly formed of concrete by forming that region thereof which corresponds to the suppression pool and the wet well as a steel wall 8, which is a good conductor of heat. In this embodiment, the wall outside the outer peripheral pool 9 constitutes the principal structure wall of the building, so that, if the reactor-containment-vessel wall, which has conventionally been formed of concrete, is formed as a steel wall 8, no serious problem is involved in terms of the strength of the building. In this embodiment, the wet well in the suppression chamber, which is partly formed by the steel reactor-containment-vessel wall 8, is divided into first and second spaces 61 and 62, and convection promoting pipes 12 are provided on that portion of the steel reactor-containment-vessel wall 8 which is in the suppression pool. Further, a wet-well-cooling-water pool 41, which is in contact with the outer periphery of the reactor-containment-vessel wall 8, is provided as a cooling means for the second space 62, with a circumferential ring-like structure 42 being provided on the inner periphery of the reactor-containment-vessel wall 8 of the wet well 6. By virtue of this arrangement, the allowable temperature for the suppression pool is raised, and the area of the heat dissipation surface through which heat is dissipated to the outer peripheral pool 9 is enlarged. While typical combinations of the different means have been presented, it is also possible realize other various combinations such as combinations of the embodiment shown in FIG. 6 similar to the examples shown in FIGS. 15, 17 and 18; such combinations do not involve any problems due to mutual interference in terms of practical application or effect. A fourth embodiment of the present invention will be described with reference to FIG. 19. Referring to FIG. 19, a reactor containment vessel of natural cooling type contains a nuclear reactor pressure vessel 102 which accommodates a nuclear reactor core 101 and which is disposed in a dry well 103. The dry well 103 communicates with a CRD chamber 104 which is below the reactor pressure vessel 102 via the internal space of a gamma shield 105. A suppression chamber 107 having a suppression pool 106 is and wet well 107A above the suppression pool 106 disposed outside the dry well 103. The dry well 103 and the suppression pool 106 communicate with each other through a plurality of vent pipes 108. The level of the openings 108A of the vent pipes 108 opening to the dry well 103 is determined in conformity with the core submerging level which is set above the reactor core 101. The discharge opening 108B of the suppression pool 106 is set to a level which is at a suitable depth in water which is determined on the basis of the results of a steam condensation test. The term "core submerging level" is used to mean the level which is reached by the water rushing from the nuclear reactor pressure vessel 102 into the dry well 103 in the event of a loss-of-coolant accident and which is high enough to ensure submerging cooling of the reactor core 101. Therefore, when the water level in the dry well has reached the core submerging level, the water starts to flood from the dry well into the suppression chamber 107 through the vent pipes 108, with the result that the water level rises in the suppression pool 106. The submerging level should be determined in consideration of factors such as the construction of the reactor pressure vessel, power of the nuclear reactor, and so forth. Practically, however, the submerging level is higher than the upper end of the reactor core 101 in the reactor pressure vessel by s suitable safety margin which is determined by possible fluctuation of the water level. For instance, the submerging level is set to be at least 50 cm higher than the upper end of the reactor core 101. The containment vessel wall 109 made of steel serves as the outer wall which defines the radially outer end of the suppression pool 106 and the suppression chamber 107. The wall 109 is surrounded by a outer peripheral pool 110 The outer peripheral pool contains water to a level which high enough to measure itself with the depth of water in the suppression pool 106 after an accident, in order to ensure efficient transfer of heat from the suppression pool 106. A relief pipe 11 with a valve 112 leads from a space above the water surface in the outer peripheral pool 110, in order to relieve steam which is generated due to a temperature rise of the water in the pool 110 in the event of an accident. The steam which is generated in the reactor pressure vessel 102 is transmitted through a main steam pipe 113 into a turbine (not shown) and is liquefied by condensation so as to be finally returned to the reactor pressure vessel 102 via a feedwater pipe 114. In case of an emergency, a main steam isolation valve (MSIV) 115 is closed and a check valve 116 in the feedwater pipe 114 prevents coolant in liquid or liquid/vapor mixed phase from flowing backward out of the reactor pressure vessel 102. However, in the event of a rupture taking place in a portion of the main steam line upstream of the MSIV 115 or in a portion of the feedwater line downstream of the check valve 116, the coolant would undesirably be allowed to flow from the reactor pressure vessel 102 into the dry well 106 so as to expose the reactor core 101, resulting in a serious accident, i.e., a loss-of-coolant accident. In order to obviate this problem, an injection line 119 having a valve 118 is provided for the purpose of injecting water from a pressure accumulator water tank 117 into the reactor pressure vessel 102. Several types of water injection system using such a pressure accumulator water tank 117 are usable. For instance, the water in the pressure accumulator water tank 117 may always be pressurized. In another type, the pressure accumulator water tank 117 is brought into communication with the vapor space in the reactor pressure vessel 102 through a specific line only when the injection is required, so that water can flow into the reactor pressure vessel as a result of the difference in the water head between the pressure accumulator water tank 117 and the reactor pressure vessel 102. In still another type of the injection system, the internal pressure of the reactor pressure vessel 102 is relieved and reduced through a relief safety valve (not shown) and then introduced into the pressure accumulator water tank 117 so as to drive the water therefrom into the reactor pressure vessel 102. All these injection systems do not require any specific power source such as a pump and are operable just through a simple valve actuation. In the drawings, therefore, only the pressure accumulator water tank 117, the valve 118 and the injection line 119 are shown. Needless to say, an injection system employing a pump may be used equally, provided that the required injection rate is ensured. The quantity W of water stored in the pressure accumulator water tank 117 is determined to be substantially equal to the sum of the water quantity W1 in the dry well 103 necessary for building up the water column up to the aforementioned submerging level and the water quantity W2 necessary for accumulating the water in the suppression pool 106 up to the submerging level, i.e., the level of the openings 108A of the vent pipes 108 opening in the dry well 103. That is, the condition of W=W1+W2 is substantially met. Under this condition, the water level in the suppression pool 106 can rise up to the same level as the submerging level in the dry well, thus maximizing the area of heat transfer between the suppression pool 106 and the outer peripheral pool 110. The suppression chamber 107 and the dry well 103 communicate with each other through a communication pipe 121 having a check valve 120 which permits fluid to flow only in a predetermined direction, i.e., from the suppression chamber 107 into the dry well 103. A communication line 125 with valves 123, 124 is provided in order that the submerging water level is maintained both in the reactor pressure vessel 102 and the dry well 103 for a long time after the occurrence of the accident. In this embodiment, a structure 125A of, for example, concrete is placed to fill a vacant space around the outer wall of the dry well 103 up to a level below the level of the openings 108A of the vent pipes 108 in the dry well 103, in such a manner as not to cause impediment to works such as installation and periodical survey. This arrangement enables the water level in the dry well 103 to rise up to the submerging level in a shorter time and also to reduce the internal volume of the pressure accumulator water tank 117, because the dead space which may otherwise be filled with water is filled by the concrete structure. In the event of a rupture of the feedwater line 114, the coolant in liquid/vapor mixed phase is discharged from the interior of the reactor pressure vessel 102 into the dry well 103 through the fracture 122. The liquid phase portion of the hot coolant thus discharged flows into the CRD chamber 104 which is below the dry well 103 to raise the water level in this chamber 104. Meanwhile, the vapor phase portion of the coolant raises the pressure inside the dry well 103, which lowers the water level in the vent pipes 108 to allow the atmosphere in the dry well 103 to be transferred to the suppression chamber 107 through the water in the suppression pool 106. Since the vapor phase portion of the discharged coolant is condensed into liquid phase through the contact with the water in the suppression pool 106, only the air from the dry well 103 reaches the suppression chamber 107, with the result that the pressure rises in the suppression chamber 107. The nuclear reactor automatically stops in response to the rise in the internal pressure of the dry well 103 or the lowering of the water level in the reactor pressure vessel 102. At the same time, the MSIV 115 is closed to terminate the supply of steam. Supply of the condensate feedwater through the feedwater line also is stopped. Then, the valve 118 is opened to allow injection of water into the reactor pressure vessel 102 from the pressure accumulator water tank 117 so as to recover the water level in the reactor pressure vessel, thereby preventing damage to the reactor core which may otherwise be caused by overheat. The water injected into the reactor pressure vessel 102 is heated to a high temperature by the decay heat derived from the reactor core 1 and rushes into the dry well 103 through the fracture 122, so that the water level in the CRD chamber 104 which is below the dry well 103 rises. A further rise of the water level in the CRD chamber 104 causes the water to flood into the dry well 103 to raise the water level therein nearly to the level of the openings 108A of the vent pipes 108. The discharge of hot water from the rupture continues further, so that hot water is introduced into the water in the suppression pool 106 from the dry well 103 via the vent pipes 108, causing both the level and temperature of the water in the suppression pool 106 to be raised. When the water level has been raised to a level near the level of the openings 108A of the vent pipes 108 in the dry well 102, the pressure accumulator water tank 117 becomes almost empty and the discharge of water through the fracture 22 is ceased. As a result, the rise in the water level inside the suppression pool 106 also ceases. The rise of the water level in the suppression pool 106 tends to cause a rise of the pressure inside the suppression chamber 107. This, however, does not hinder the rise of the water level in the suppression pool 106, because the check valve 120 is opened by the pressure differential between the suppression chamber 107 and the dry well 103, so that air returns from the suppression chamber 107 into the dry well 103 to attain a pressure equilibrium therebetween. As a result of the rise of the water level in the suppression pool 106, heat is transferred from the suppression pool 106 to the outer peripheral pool 110 through the steel wall 109 of the containment vessel. As a consequence, the water in the outer peripheral pool 110 is boiled to generate vapor. The vapor is then relieved to the exterior through the relief pipe 111 by forcibly opening the valve 112. When the water injection from the pressure accumulator water tank 117 is ceased, water levels which are almost the same are attained in the reactor pressure vessel 102, dry well 103 and in the suppression pool 106. Generation of decay heat inside the reactor core 101 progressively decreases but is still effective in heating and evaporating water inside the reactor pressure vessel 102. Consequently, the pressure inside the reactor pressure vessel 102 continues to rise as a result of generation of heat. A further rise of the internal pressure of the reactor pressure vessel 102 due to generation of vapor causes the vapor to be relieved through a relief safety valve (not shown) or to be discharged together with hot water into the dry well 103 through the fracture. Consequently, the pressure rises in the dry well 103, which serves to lower the level of water in the vent pipes 108, so that the vapor component is discharged into the suppression pool 106 to be condensed into liquid phase through contact with the water in the pool 106. The heat given to the water as a result of the condensation is transferred to the water in the outer peripheral pool 110 so as to be dissipated therefrom. Meanwhile, the level of the water inside the reactor pressure vessel 102 tends to progressively come down as a result of the evaporation. However, the valve 123 is opened when the reduction of the water level is sensed so that the water flows from the suppression pool 106 into the reactor pressure vessel 102 due to the difference in the water head, whereby the water level is recovered in the reactor pressure vessel 102. By opening the valve 124 simultaneously with the opening of the valve 123, it is possible to maintain the same submerging level in the reactor pressure vessel 102, dry well 103 and the suppression pool 106. As will be understood from the foregoing description, the embodiment shown in FIG. 19 appreciably shortens the time required for the dry well 103 to be filled with water up to the submerging level in the event of an accident. In addition, submerging cooling of the reactor core can be started quickly because the water level in the suppression vessel 106 can be raised to the highest level without causing the openings 108A of the vent pipes 108 to be flooded by the water. Furthermore, the transfer of heat from the dry well 103 to the suppression pool 106 can be promoted by virtue of condensation of vapor component in contact with the water inside the suppression pool 106, partly because the upper hot portion of the water column in the dry well 103 first moves into the suppression pool 106 and partly because the water in the vent pipes 108 can easily be displaced as the time elapses to allow vapor component to flow from the dry well 103 into the suppression pool 106. In addition, transfer of heat from the suppression pool 106 to the outer peripheral pool 110 is promoted, because a large heat transfer area, as well as a large temperature differential, is provided between the water inside the suppression pool 106 and the water inside the outer peripheral pool 110. Thus, the fourth embodiment described in connection with FIG. 19 performs quick and efficient cooling of the reactor core, as well as efficient transfer of heat from the dry well 103 to the suppression pool 106 and further to the outer peripheral pool 110 therefrom, thus offering a remarkable improvement in the safety of the natural cooling type reactor core containment vessel. It is also to be noted that this embodiment makes it possible to set the initial water level in the dry well 103 to a level which is lower than that in existing equipment, which, in combination with the use of the structure 125A filling dead space inside the dry well 103, enables the capacity of the pressure accumulator water tank 117 to be reduced. A fifth embodiment of the present invention will be described with reference to FIGS. 20 and 21. Referring to these Figures, there is shown a natural cooling type reactor containment vessel having upper and lower suppression chambers. More specifically, a reactor pressure vessel 202 accommodating a reactor core 201 is placed in a dry well 203 which is surrounded by a lower suppression chamber 207 having a lower suppression pool 205 and a wet well 207A above the suppression pool 205. The dry well 203 and the lower suppression pool 205 are communicated with each other through a plurality of vent pipes 211. An upper suppression chamber 206 provided on the upper side of the lower suppression chamber 207 has an upper suppression pool 204 which communicates with the dry well 203 through a plurality of vent pipes 208 and a wet well 206A above the suppression pool 204. A water injection line 217 having a valve 216 leads from the upper suppression pool 204 to the reactor pressure vessel 202. The steel wall 214 of the containment vessel serves also as an outer wall which defines the radially outer ends of the lower suppression pool 205 and the lower suppression chamber 207. This outer wall is surrounded by a containment vessel outer peripheral pool 215 which is filled with water up to a level high enough to cover the core submerging level of water inside the dry well. The term "submerged level" is used in the same sense as that explained in the description of the preceding embodiment. Namely, the core submerging level is a level which is higher than the top of the reactor pressure vessel 201 by a height, e.g., 50 cm, which provides a margin for fluctuation of the water level. The quantity W of water in the upper suppression pool 205 is determined to be substantially equal to the sum of the water quantity W1 in the dry well 203 necessary for raising the water level therein to the aforementioned submerging level and the water quantity W2 which is required for raising the water level inside the lower suppression pool 205 to a level substantially equal to the aforementioned submerging level. That is, the condition of W=W1+W2 is substantially met. As a consequence, the water level in the lower suppression pool 205 rises almost to the same level as the openings 212 of the vent pipes 211 opening to the dry well 212, so that the area for the transfer of heat from the lower suppression pool 205 to the outer peripheral pool 215 can be maximized. Referring to FIG. 20, in the event of a rupture in a pipe which is directly connected to the reactor pressure vessel 202, a coolant in the form of a liquid/vapor mixed phase is discharged from the reactor pressure vessel 202 into the dry well 203. The liquid phase, i.e, water component, of the discharged hot coolant is accumulated on the bottom of the dry well 203, while the steam component of the same raises the pressure inside the dry well 203 so as to lower the water levels in the vent pipes 208, 211 Consequently, the atmosphere inside the dry well 203, composed of air and steam, is moved into the suppression chambers 206, 207 via the water in the respective suppression pools 204, 205. As the atmosphere passes through the water, the steam component of the same is condensed into liquid phase through contact with the water in the suppression pools 204, 205, so that only the air component of the atmosphere reaches the suppression chambers 206, 207 so as to contribute to the rise of the pressure in the suppression chambers 206, 207 and the dry well 203. The nuclear reactor is automatically stopped and isolated when the rise of the pressure inside the dry well 203 or the lowering of water level inside the reactor pressure vessel 202 is sensed in the event of a rupture. Only vapor phase is discharged from the fracture 218 after the water level inside the reactor pressure vessel has comedown below the level of the fracture 218. The pressure inside the reactor pressure vessel decreases accordingly. If the pressure does not decrease, a safety valve (not shown) operates to relief the pressure. When the pressure in the reactor pressure vessel has been lowered sufficiently, the valve 216 in the water injection line 217 leading from the upper suppression pool 204 is opened to allow injection of water into the reactor pressure vessel 202 thereby to cool the reactor core 201 by submerging. The water injected into the reactor pressure vessel 202 is heated to a high temperature by the decay heat derived from the reactor core 201 so that hot water is discharged from the fracture 218 into the dry well 202. As a consequence, the water level inside the dry well 202 is raised up to the level of the openings 212 of the vent pipes. As the discharge of hot water from the fracture 218 continues, hot water flows from the dry well 202 into the water in the lower suppression pool 205 via the vent pipes 211, causing rises of the water level and water temperature inside the lower suppression pool 205. The upper pressure suppression pool 204 becomes almost empty so that the discharge of hot water from the fracture 218 is materially ceased when the water level in the lower suppression chamber has been raised to the level of the openings 212 of the vent pipes. The rise of the water level in the lower suppression pool also is ceased accordingly. The rise of the water level inside the lower suppression pool 205 tends to cause a rise in the pressure inside the lower suppression chamber 207. This tendency, however, is canceled as the check valve 219 opens when the internal pressure of the lower suppression chamber 207 has become higher than that of the dry well 203, so as to attain a pressure equilibrium between the lower suppression chamber 207 and the dry well 203. The water level in the lower suppression chamber 205, therefore, can be raised without impediment. As a result of the rise of the water temperature in the lower suppression pool 205, heat is transferred from the water in the lower suppression pool 205 to the water in the outer peripheral pool 215 through the steel wall 214 of the containment vessel, so that the water in the outer peripheral pool 215 is heated and evaporated to generate steam which is relieved to the exterior, so that the steel wall 214 of the containment vessel is cooled to and maintained at a certain temperature. The water inside the reactor pressure vessel 202 progressively decreases as the time elapses. It is, however, possible to maintain a predetermined water level in the reactor pressure vessel, by providing a line (not shown) which interconnects the lower suppression pool 205 and the interior of the reactor pressure vessel 202 or a line (not shown) which interconnects a power portion of the dry well 203 and the interior of the reactor pressure vessel 202. In this regard, a reference be made to the communication line 125 with valves 123, 124 (see FIG. 19) or to a pressure equalizing system 313 which will be described later in connection with FIG. 23. As will be seen from the foregoing description, the fifth embodiment offers the following advantages. In the event of an accident such as a rupture causing discharge of coolant into the dry well, the water component of the discharged coolant is accumulated on the bottom of the dry well but the steam component of the same is introduced via the upper and lower vent pipes 208, 211 into the upper and lower suppression pools 204, 205 so as to be condensed into liquid phase through contact wit water contained in these pools. This prevents overshoot of pressure rise in the dry well which may otherwise be caused due to flow resistance along the vent pipes immediately after the occurrence of the accident. Furthermore, the time required for the water level inside the dry well 203 to reach the submerging level is shortened and the water level in the suppression pool 205 can be raised to the highest level without causing flooding of the openings 212 of the vent pipes 211 in the dry well 203. For these reasons, the submerge cooling of the reactor core can be commenced quickly. Furthermore, the transfer of heat from the dry well 203 to the suppression pool 205 can be promoted by virtue of condensation of vapor component in contact with the water inside the suppression pool 205, partly because the upper hot portion of the water column in the dry well 203 first moves into the suppression pool 205 and partly because for a long period of time the water in the vent pipes 211 can easily be displaced to allow vapor component to flow from the dry well 203 into the suppression pool 205. In addition, transfer of heat from the suppression pool 205 to the outer peripheral pool 215 is promoted, because a large heat transfer area, as well as a large temperature differential, is provided between the water inside the suppression pool 205 and the water inside the outer peripheral pool 215. Thus, the fifth embodiment performs quick and efficient cooling of the reactor core, as well as efficient transfer of heat from the dry well 203 to the suppression pool 205 and further to the outer peripheral pool 215 therefrom, thus offering a remarkable improvement in the safety of the natural cooling type reactor core containment vessel. The fifth embodiment offers an additional advantage in that, since the water required for suppression of pressure rise is stored partly in the upper suppression pool and partly in the lower suppression pool, the size of the suppression pools in terms of area or diameter can be reduced as compared with arrangements having only one suppression pool, provided that the depth of water in the pool is the same, which makes it possible to reduce the diameter of the containment vessel. The described fifth embodiment may be modified such that water is initially charged only in the upper suppression pool 204 while the lower suppression pool 205 is kept empty. In such a modification, the water in the upper suppression pool 204 condenses and liquefied the vapor portion of the coolant discharged as a result of a rupture, while the lower suppression pool 207 directly receives the atmosphere of the dry well. Then, water is injected from the upper suppression pool 204 into the reactor pressure vessel 202, so that hot water is discharged from the fracture into the dry well 203 and then into the lower suppression chamber 207 so as to be accumulated in the lower suppression pool 205. Once the water level in the lower suppression pool 205 is raised above the level of the outlets 213 of the vent pipes 213, vapor component is steadily condensed by the water in the suppression pool 205 for a long time. The merit of the fifth embodiment in regard to the reduction in the size of the containment vessel cannot be fully enjoyed with this modification because the area of the suppression pools cannot be reduced sufficiently. Reduction in the diameter of the containment vessel, however, is possible to a certain extent because the upper suppression pool 204 can be used as a water injection source which does not necessitate any additional equipment such as a pump and because the lower suppression pool 205 may be designed to have a smaller area than the upper suppression pool 204. In addition, the effect to cool the containment vessel is enhanced after the water level is raised in the lower suppression pool 205. A sixth embodiment will be described with reference to FIGS. 22 to 27. Referring to FIG. 22, a boiling water reactor has a pressure vessel 302 which accommodates a reactor core 301 and a containment vessel 303 which contains the pressure vessel 302. The containment vessel 303 has a dry well 302 and a suppression chamber 306. The dry well 302 receives the pressure vessel 302 and a primary line through which a fluid of high pressure and high temperature is circulated. The bottom portion of the suppression chamber 306 defines a suppression pool 305 with a wet well 306A being provided above the suppression pool 305. The dry well 304 and the suppressing pool 305 communicate with each other through a plurality of vent tubes 307 the lower ends of which are immersed in water. The containment vessel 303 is made of a steel. In the event of a loss-of-coolant accident, decay heat is transferred to water in the suppression pool 305 and further to an outer peripheral pool 308 through the steel wall of the containment vessel 303. Referring to FIG. 23, the pressure vessel 302 is provided with a reducing valve 309. A gravity pool 310, which serves as the water supply source of an emergency core cooling system (ECCS) 311, is disposed in an upper portion of the space inside the containment vessel 303. In a short period immediately after occurrence of a loss-of-coolant accident, the reducing valve 309 is opened to relief vapor from the pressure vessel 302 so as to quickly reduce the pressure inside the pressure vessel 302, and the water in the gravity pool 310 is supplied by the force of gravity, i.e., by the difference in the water head, into the pressure vessel 302, thereby cooling the reactor core 301. As an alternative, the ECCS 311 may employ, in place of the gravity pool 310, a pressure accumulator tank 312 installed outside the containment vessel 303 as shown in FIG. 24. In case of a emergency, water is driven by the pressure accumulated in the pressure accumulator tank 312 so as to be supplied into the pressure vessel 302 to cool the reactor core 301. Thus, water is supplied into the pressure vessel 302 from the gravity pool 311 or from the pressure accumulator tank 312 for a short period immediately after the occurrence of the accident. The water thus supplied effectively cools the reactor core 301 and is discharged into the dry well 304 through the fracture (not shown) so as to fill the lower portion of the space inside the dry well 304 as denoted by 317 in FIG. 25. As a consequence, lower half part of the pressure vessel 302 is immersed. The pressure vessel 302 also is equipped with an equalizing system 313 which utilizes, as water sources, the water pooled in the suppression pool 305 and the draw-down water accumulated on the bottom portion of the dry well 304. The equalizing system 313 serves to supply water from the suppressing pool 305 and/or the draw-down water 317 from the dry well 304 into the pressure vessel for a long period after the occurrence of the loss-of-coolant accident. More specifically, the equalizing system 313 includes a first equalizer line 314a having one end connected to the pressure vessel 302 and the other end opening in the suppression pool 305, a second equalizer line 314b shunting from the first equalizing line 314a and opening in a lower portion of the dry well 304, a blasting valve 315 disposed in the first equalizing line 314a at a portion between the pressure vessel 302 and the point where the second equalizer line 314b shunts from the first equalizing line 314a, and a check valve 316 provided in each of the equalizer lines 314a and 314b. The height of the connection between the first equalizer line 314a and the pressure vessel 302 (referred to as "height of equalizer line connection") is so determined as to provide a sufficient margin for enabling submerging cooling of the reactor core for a long time after occurrence of a loss-of-coolant accident. Practically, the height of the equalizer line connection is determined to be 50 to 150 cm, preferably 100 cm or so, above the top of the reactor core. As will be described later, the force with which the water is driven from the suppression pool 305 is derived from the head difference between the outlets of the vent pipes 307 and the height of equalizer line connection. In order to obtain a sufficient force for driving the water, the outlets of the vent pipes 307 are set at a level which is 50 to 150 cm, preferably 70 cm or more, above the height of equalizer line connection and which is 100 to 200 cm above the top of the reactor core 301. The term "level of outlets of the vent pipes" means the level of the lower end openings of the vent pipes opening into the suppression pool. When vent pipes 307 have outlets at different levels, the highest one of these levels is determined as the level of outlets of the vent pipes. The force for driving the draw-down water 317 from the dry well 304 is derived from the head difference between the level of the draw-down water 317 and the height of equalizer line connection at which the first equalizer line 314a is connected to the pressure vessel The opening of the second equalizer line 314b opening to the lower portion of the dry well is determined to be substantially the same as the level of the connection to the pressure vessel 302. In addition, the second equalizer line 314b is designed and laid to have minimum length including horizontal portion, and the check valve 316 is provided in the horizontal portion, in order to reduce loss of the water driving power due to stagnation of air in the second equalizer line 314b. In FIG. 23, the portion of the equalizer system inside the one-dot-and-dash line is shown in plan, for the purpose of easier understanding. This applies also to other Figures of the drawings. The quantity of water charged in the gravity pool 310 or the pressure accumulator tank 311 of the ECCS is so determined that the level of the draw-down water in the dry well is maintained above the level of the height of equalizer connection to the pressure vessel, i.e., above the level of the second equalizer line 314b, for a long time after occurrence of the loss-of-coolant accident. In this embodiment, the wall of the containment vessel 303 functions as a heat-transmission wall through which heat is transferred from the water in the suppression pool 305 to the water in the outer peripheral pool 308. In order to obtain a large area of heat transfer through the containment vessel wall, the water level in the suppression pool 35 is set comparatively high. The operation of the equalizer system 313 should not draw water from the suppression pool 305 at such a large rate as to cause a reduction in the water level inside the suppression pool 305. In order to meet this requirement, the opening 314c of the first equalizer line 314a opening in the suppression pool 305 is set to a level near the initial water level in the suppression pool 305, e.g., 50 cm or so below the initial level, so that the water level in the suppression pool 305 does not come down below this level even when water is drawn through the equalizing system 313. To explain in more detail, in this embodiment of the present invention, the equalizer system 303 starts to operate when the pressure inside the pressure vessel 302, which progressively decreases in a long time after occurrence of the accident, is lowered to a level substantially equal to that in the dry well 304, so that water in the suppression pool 305 and the draw-down water 317 in the bottom portion of the dry well 304 are supplied into the pressure vessel 302, as shown in FIG. 25. The power with which water is driven from the suppression pool 305 and the power with which draw-down water 317 is driven from the dry well 304 are respectively given by the following formulae: (1) Power .DELTA.P.sub.1 for driving water from suppression pool 305 ##EQU1## since, EQU P.sub.WW +H.sub.V .multidot..gamma.=P.sub.R (2) (2) Power .DELTA.P.sub.2 for driving draw-down water from dry well ##EQU2## where, P.sub.R : pressure in pressure vessel P.sub.DW : pressure in dry well PA1 P.sub.WW : pressure in suppression chamber PA1 H.sub.V : depth of immersion of vent pipe PA1 H.sub.NV : height difference between vent pipe outlet and equalizer line connection to pressure vessel PA1 H.sub.D : height difference between drawn-down water level and equalizer line connection to pressure vessel PA1 .gamma.: density of water As will be seen from formula (1) above, the driving power .DELTA.P.sub.1 for driving water from the suppressing pool 305 is derived from the water head difference between the level of the outlet of the vent pipe 307 and the level at which the equalizer line 314a is connected to the pressure vessel 302. It will also be seen from formula (3) that the driving power .DELTA.P.sub.2 for driving the draw-down water 317 in the bottom of the dry well 304 is derived from the difference in the water head between the level of the draw-down water and the level at which the equalizer line 314b is connected to the pressure vessel 302. A control system for controlling the above-described equalizer system will be described with reference to FIGS. 26 and 27. Referring first to FIG. 26, the control system includes a water level sensor 319 for sensing the water level LR in the pressure vessel 302, a pressure sensor 320 for sensing the internal pressure PD in the dry well, and a pressure sensor 321 for sensing the pressure PR inside the pressure vessel 302. Signals from the water level sensor 319 and pressure sensors 320, 321 are sent to a controller 322 which operates to activate the reducing valve 309 and the equalizer system 303 in accordance with an operation logic as shown in FIG. 27. When a loss-of-coolant has taken place, the water level L.sub.R in the pressure vessel 302 is lowered while the pressure P.sub.D in the dry well is increased, whereby low-L.sub.R signal and high P.sub.D signal are transmitted from the respective sensors 319 and 320. Upon receipt of these signals, the controller 322 operates to open the reducing valve 309 so that the internal pressure P.sub.R of the pressure vessel 302 is drastically lowered and progressively approaches the level of the pressure inside the dry well 304. The controller 322, upon receipt of a signal indicative of the reduction in the internal pressure of the pressure vessel, operates to open the blast valve 315 in the equalizing system 313. After the blast valve 315 is opened, the coolant is automatically injected when the conditions of the formulae (1) and (3) are met. When either one of these conditions is not met, the check valve 316 in the equalizer system 313 prevents water from being discharged through the equalizer system 313. As will be understood from the foregoing description, in the described embodiment, the blasting valve 315 is opened in the course of the reduction in the internal pressure of the pressure vessel after occurrence of the loss-of-coolant accident, so that water in the suppression pool 305, as well as the draw-down water 317 in the dry well 304, is injected into the pressure vessel 302, thereby cooling the reactor core 301 for a long period after occurrence of a loss-of-coolant accident. Furthermore, in this embodiment which utilizes both the water in the suppression pool 305 and the draw-down water 317 on the bottom of the dry well 304, injection of water into the pressure vessel 302 is performed when the dry well is filled to a level which is slightly higher, e.g., 100 cm or less above, the level at which the equalizer lines 314a and 314b are connected to the pressure vessel 302, without requiring that the lower portion of the dry well 304 is completely filled. It is therefore possible to effectively cool the reactor core 301 by submersion for a long time after occurrence of a loss-of-coolant accident. This makes it possible to reduce the quantity of water initially held in the gravity pool 310 or the pressure accumulator tank 312 of the ECCS. Consequently, the mass which is to be held by upper portion of the nuclear reactor building is reduced to facilitate anti-earthquake designing of the nuclear reactor system. The wall of the containment vessel 303 provides surfaces through which heat is transmitted and transferred from the water in the suppression pool 305 to the water in the suppression pool 308. In the illustrated embodiment, the opening of the equalizer line 314a opening in the suppression pool 305 is set to a level which is only slightly below the initial water level so that the water level in the suppression pool 305 does not come down substantially below the initial level. As a consequence, high water level is maintained in the suppression pool 305 even during operation of the equalizer system 313 so that a large area is preserved for the heat transfer from the water in the suppression pool 305 and the water in the outer peripheral pool 308, thus attaining an improvement in the heat dissipation characteristic. In the event that the quantity of water in the suppression pool 305 has come down to such a level that the opening 314c of the first equalizer line 314a is exposed, the inlet of the second equalizer line 314b is flooded with draw-down water which is necessarily charged into the dry well 304 due to the head balance of water in the containment vessel 303, so that cooling of the reactor core 301 is continued without fail. Consequently, a remarkable improvement in the safety is achieved by a combination of the static cooling of the containment vessel offered by the above-mentioned heat dissipation characteristic and the static cooling of the reactor core performed by the equalizer system 313. A modification of the sixth embodiment will be described with reference to FIG. 28. This modification employs, in place of the blasting valve 315 used as an isolation valve in the equalizer line 314a of the fifth embodiment shown in FIG. 23, a normally-closed electrically driven valve 323 disposed in an equalizer system 313A. The electrically driven valve 323 also is driven and controlled in accordance with a logic similar to that shown in FIG. 27. This modification offers an advantage in that reliability of the isolation valve, which is an electrically driven valve, can easily be confirmed through a periodical check. In addition, operation and administration of the safety system are facilitated by virtue of the use of the electrically driven valve. FIG. 29 shows another modification which is different from the sixth embodiment shown in FIG. 23 in that a parallel connection of a pair of blasting valves 315 and a parallel connection of a pair of check valve 316 are used in each line. In general, it is necessary to assume that a normally-closed valve which is expected to open in the event of a loss-of-coolant accident may fail to open due to an unexpected reason. With the dual valve arrangement employed in this modification, it is possible to supply emergency cooling water to the pressure vessel even when one of the dynamic components, e.g., valve, has failed to operate.