Patent Number: 
Section: description

This application is a Continuation of U.S. application Ser. No. 12/764,163 filed Apr. 21, 2010, which is a Division of U.S. application Ser. No. 12/128,524, filed May 28, 2008, which is a Continuation of and claims the benefit and priority under 35 USC §120 from U.S. Ser. No. 11/348,333, filed Feb. 7, 2006, the entire contents of each of which are incorporated herein by reference. U.S. Ser. No. 11/348,333 is a Division of U.S. Pat. No. 7,139,352, U.S. Ser. No. 09/749,547, issued Nov. 21, 2006, and claims the benefit of priority under 35 U.S.C. §119 from Japanese Patent Application No. 2000-049031, filed Feb. 25, 2000, and Japanese Patent Application No. 11-375240, filed Dec. 28, 1999. 1. Field of the Invention The present invention relates to a reactivity control rod for a core, a core of a nuclear, a nuclear reactor and a nuclear power plant. More particularly, the present invention relates to a reactivity control rod for a core, which can elongate the lifetime of the core, a core of a nuclear reactor composed of the reactivity control rod, which can have a long lifetime, a nuclear reactor which is cooled by a liquid metal and is able to reduce scattering of the liquid metal so as to be made into a small size thereof and a nuclear power plant which comprises the nuclear reactor. 2. Description of the Related Art A conventional liquid metal cooled nuclear reactor with a small size, that is, a fast reactor is disclosed in U.S. Pat. No. 5,420,897. Moreover, a conventional fast reactor has a structure for moving a neutron reflector in a vertical direction so as to adjust (control) a leakage of neutron from the core thereof, thus to compensate a change of reactivity of the core due to a burn-up (combustion) thereof. In the aforesaid conventional liquid metal cooled nuclear reactors, an intermediate heat exchanger is arranged in a reactor vessel. A primary coolant performs the heat exchanging operation with a secondary coolant in the intermediate heat exchanger, and the exchanged secondary coolant is circulated to a steam generator arranged outside the reactor vessel so as to generate a steam. Namely, the conventional liquid metal cooled nuclear reactor has a structure of requiring a steam generator for generating a steam, an electromagnetic pump for circulating a secondary coolant between the reactor vessel and the steam generator, and piping equipments connecting them. An activated liquid metal such as sodium is used as each of the coolants. For this reason, the reactor vessel and a facility using the liquid metal arranged around the reactor vessel have complicated structures, so that there is the possibility that an auxiliary facility is required in preparation for a leakage of the activated liquid metal, fire caused thereby or the like. Moreover, in the conventional liquid metal cooled nuclear reactor, the liquid metal which is easily activated, such as sodium is used as the coolant. That is, in the steam generator, the liquid metal which is easily activated reacts to water to generate a steam. For this reason, in cases where a water leakage occurs in a heating tube of the steam generator, it is difficult to avoid an occurrence of an accident caused by the reaction between the sodium and the leaked water. The reaction between the sodium and the leaked water causes a reaction product, so that, in order to prevent the reaction product from directly being radiated, a secondary cooling system facility must be required. In addition, a facility for housing the reaction product must be required so that there is the possibility that the reactor system, as a whole, is made into a large size thereof, and that the cost of manufacturing the reactor system is made to be increased. Furthermore, the electromagnetic pump is arranged in a liquid metal; however, it is coaxially arranged in series on a downstream side (lower side) of the intermediate heat exchanger in view of a heat resistant characteristic of a large-sized conductive coil of the electromagnetic pump or the like. On the other hand, each of tube plates arranged above and below the intermediate heat exchanger has a structure which is easy to receive a thermal stress, and an enlargement of its diameter causes an increase of the thermal stress so that it is taken into consideration to prevent each of the tube plates from being made into a large size thereof. As described above, in the conventional liquid metal cooled reactor, the intermediate heat exchanger and the electromagnetic pump are vertically arranged in series; for this reason, the reactor is made into a large size thereof in its height direction (in its axial direction). The reactor with a large size in its axial direction has a structure which is easily oscillated, thereby making it unstable. On the other hand, in a conventional neutron reflector migration type of fast reactor, when elongating the lifetime of the core thereof, it must be necessary to make long the length of fuel assembly in the core. That is, according to the progress of combustion of the fuel assembly, a reactivity of the fuel assembly becomes negative. Therefore, in order to offset the negative reactivity, a neutron reflector is left up from a lower portion of the core to cover the height thereof so as to improve the ability of reflecting neutron, thereby increasing a positive reactivity of the neutron reflector, so that a reactivity of the whole core of the reactor needs to be set to 0; that is, it is necessary to make the reactor operate so as to keep a combustion in a critical state. Thus, in order to elongate an operating period of the reactor, a fuel length of the fuel assembly must be made long. Furthermore, in cases where the fuel length of the fuel assembly is made long, the reactor vessel of the reactor becomes long as a whole; as a result, there is the possibility of deteriorating the economics of the reactor. Furthermore, there are problems of causing a change of reactivity by deformation of the core in the lifetime thereof the core, an increase of pullout force of the fuel assembly. The present invention is made in view of the aforesaid problems in the related art. Accordingly, it is an object of the present invention to provide a nuclear reactor, which is capable of limiting a space for housing a liquid metal used as a coolant into an inside of a reactor vessel thereof so as to prevent scattering of the coolant to the outside thereof, whereby it is possible to make simple the whole structure of the nuclear reactor with a cooling facility, and to make compact the whole structure thereof, and to provide a nuclear power plant comprising the nuclear reactor. In order to achieve such object, according to one aspect of the present invention, there is provided a nuclear reactor in which a primary coolant is contained, including: a core composed of nuclear fuel, the coolant moving upwardly from the core by an operation thereof; an annular steam generator arranged in an upper side of the core into which the upwardly moving coolant flows and adapted to transfer heat in the coolant into water therein to generate a steam; a passage structure that defines a coolant passage for the coolant to an outside of the core, the heat-transferred coolant in the annular steam generator flowing downwardly in the coolant passage so as to flow into the core, thereby moving upwardly; and a reactor vessel arranged to surround the coolant passage so as to contain the core, the annular steam generator and the coolant passage therein. In order to achieve such object, according to another aspect of the present invention, there is provided a nuclear power plant comprising a nuclear reactor in which a coolant is contained, the nuclear reactor including: a core composed of nuclear fuel, the coolant moving upwardly from the core by an operation thereof; an annular steam generator having a plurality of heat transfer tubes and arranged in an upper side of the core into which the upwardly moving coolant flows, the annular steam generator transferring heat in the coolant with water in the heat transfer tubes to generate a steam; a passage structure that defines a coolant passage for the coolant to an outside of the core, the heat-transferred coolant in the annular steam generator flowing downwardly in the coolant passage so as to flow into the core, thereby moving upwardly; and a reactor vessel arranged to surround the coolant passage so as to contain the core, the annular steam generator and the passage means therein; a feed water branch pipe connecting to corresponding to heat transfer tubes; a steam branch pipe connecting to corresponding to heat transfer tubes, the feed water branch pipe and the steam branch pipe independently penetrating through a reactor container facility; a first feed water pipe; a steam pipe, the feed water branch pipe and the steam branch pipe being connected to the first feed water pipe and the steam pipe outside the reactor container facility, respectively; a steam bypass pipe branching from the steam branch pipe and provided with a steam separator having a bottom portion; an air conditioner provided for the steam separator via a steam facility pipe thereof; and a second feed water pipe with a feed-water pump, the bottom portion of the steam separator being connected through the second feed water pipe to the feed water branch pipe. In order to achieve such object, according to further aspect of the present invention, there is provided a reactivity control rod for use in a reactor core and for controlling a reactivity therein, comprising: a tube portion; and a mixture filled in the tube portion, the mixture being made by mixing a neutron absorber that absorbs a neutron and a neutron moderator that moderates a neutron. In order to achieve such object, according to still further aspect of the present invention, there is provided a reactor core in a core barrel of a nuclear reactor, comprising: a plurality of fuel assemblies contained in the core barrel; and a mixture contained in the core barrel, the mixture being made of a neutron absorber that absorbs a neutron in the core and a neutron moderator that moderates a neutron therein so that a reactivity of the core is controlled. According to the present invention, it is possible to reduce a heat value dispersed to the outside, thereby improving a heat efficiency thereof, and to make the reactor vessel compact into a small size as a whole, thereby securely preventing a leakage of the liquid metal. Furthermore, according to the present invention, because the whole of the reactor vessel is kept at a suitable temperature, and is protected from a rapid heat transit, it is possible to secure a structural safety of the reactor, and to make an operation of the reactor for a long period. In addition, after a shutdown of the reactor, because a natural circulating force generated by heating of the core and radiation from the reactor vessel is effectively used, it is possible to stably carry out a decay heat removal operation of the reactor. Still furthermore, in particular, the shape of the reactor is miniaturized in its longitudinal direction, and therefore, it is possible to prevent a contact of the liquid metal with the water so as to make an operation of the reactor for a long period. The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. In these embodiments, as one example of a nuclear reactor according to the present invention, a liquid metal cooled reactor is described  Next, with reference to FIG. 23, a fast reactor 1G according to a sixth embodiment of the present invention will be described below. FIG. 23 shows principal parts of the fast reactor 1G in this sixth embodiment, and corresponds to FIG. 19A. In FIG. 23, for simplification of explanation, like reference numerals are used to designate the same parts as FIG. 19A. The fast reactor 1G of the sixth embodiment is different from the fast reactor 1F of the above fifth embodiment in that a neutron absorber 124 with a neutron moderator is provided above the neutron reflector 4. The neutron absorber 124 with the neutron moderator includes a material produced by mixing a neutron moderator and a neutron absorber. Conventionally, the upper portion of the neutron reflector 4 is formed into a cavity in order to improve its value. In this sixth embodiment, the neutron absorber 124 with the neutron moderator is mounted into the cavity. According to the structure, in addition to the effect of the fifth embodiment, because the neutrons irradiated from the core 2A is moderated to be absorbed in the neutron absorber 124, it is possible to give a neutron shielding function to the reactor 1G, and to simplify the upper structure of the reactor 1G.   FIGS. 15A, 15B and FIG. 16 show a third embodiment of the present invention. These FIGS. 15A, 15B and FIG. 16 are correspondent to FIG. 3 and FIG. 4 as described before, respectively, and show a liquid surface state and a flow of primary coolant in an operation of reactor. In FIG. 15A and FIG. 16, arrows “a” show flowing directions of the primary coolant. A liquid metal cooled nuclear reactor 1B of this third embodiment basically has the same structure as that of the above first embodiment, and therefore, overlapping explanation is omitted with reference to FIG. 1 and FIG. 2. The liquid metal cooled nuclear reactor 1B of this third embodiment differs from the above first embodiment in that the steam generator 14 is provided with an opening portion 44 of the inner shell 23 of the steam generator 14, which communicates with a cover gas space 45 of the reactor vessel 9, and is located at the upper portion from the liquid surface of the reactor vessel 9. Moreover, in this third embodiment, each of the heating tubes 16 of the steam generator 14 has a double pipe structure provided with an inner tube 16S and an outer tube 16T surrounding an outer periphery of the inner tube 16S, as shown in FIG. 15B. In addition, the reactor 1B comprises a continuous leakage monitoring unit 46 that detects a leakage in both outer and inner tubes 16T and 16S. If a large-scale water leakage occurs in a liquid metal by simultaneous breakdown of the double tubes, a water vapor or bubble of the reaction product caused by contacting the liquid metal with the water is transferred to the surroundings from the leakage portion. In this case, in the heat exchange portion, a gas transferred upwardly from the leakage portion flows to a cover gas space of the steam generator 14. On the other hand, a gas transferred downwardly from there flows through each liquid surface of the space between the intermediate shell 25 and the outer shell 24 and the space between the outer shell 24 and the reactor vessel 9 to the cover gas space of the steam generator 14. At that time, the opening portion 44 of the inner shell 23 operates so that the cover gas space 45 of the reactor vessel 9 communicates with the cover gas space of the steam generator 14. Therefore, the water vapor or bubble of the reaction product by the large-scale water leakage generated in the liquid metal is all guided to the cover gas space 45 of the reactor vessel 9 through the opening portion 44. In this third embodiment, even if a large-scale water leakage occurs in the heating tube 16 of the steam generator 14, it is possible to maintain a safety of the reactor 1B without mixing the bubble into the core 2. Incidentally, in the third embodiment, partial modification may be made. For example, as shown in FIG. 16, the lower end portion 23b of the inner shell 23 of the steam generator 14 in the reactor 1C may be arranged at a position lower than the lower end portion 24a of the outer shell 24 thereof and the lower end portion 25a of the intermediate shell 25 in the primary coolant outlet portion of the steam generator 14. According to the above construction of the modification, if a large-scale water leakage occurs, because the lower end portion 23b of the steam generator inner shell 23 in the primary coolant outlet portion of the steam generator 14 is arranged at the position lower than the lower end portion 25a of the intermediate shell 25 and the lower end portion 24a of the outer shell 24, a gas transferred downwardly of water vapor or reaction product generated by the leakage selectively flows to the upper cover gas space of the steam generator 14 via each liquid surface of the space between the intermediate shell 25 and the outer shell 24 and the space between the outer shell 24 and the reactor vessel 9. Moreover, the opening portion 44 of the steam generator 23 operates so that the cover has space 45 of the reactor vessel 9 communicates with the cover gas space of the steam generator 14, whereby the water vapor or bubble of the reaction product by the large-scale water leakage generated in the liquid metal is all guided to the cover gas space 45 of the reactor vessel 9. In this modification of the third embodiment, even if a large-scale water leakage occurs in the heating tube 16 of the steam generator 14, it is possible to maintain a safety of the reactor 1C without mixing bubble into the core 2. Furthermore, in this third embodiment, another modification with a construction may be made. More specifically, as shown in FIG. 17, the reactor 1D comprises a detecting unit 47 that detects a peculiar change in flow rate generated due to a pressure rise of the shell side of the steam generator 14 by using a change in a current of the electromagnetic pump 13. In addition, the reactor comprises an operation control unit 48 that performs a control for stopping the operation of the electromagnetic pump 13 by a detected signal outputted from the detecting unit 47. In addition, the lower end portion 23b of the steam generator inner shell 23 is arranged at a position lower than the lower end portion 24a of the outer shell 24 and the lower end portion 25a of the intermediate shell 25. According to the above construction of the reactor 1D in the another modification, the following operation is carried out. That is, if a water vapor or reaction product gas is generated in the steam generator 14 by a large-scale water leakage, the pressure rise brings about a change in a flow rate of the primary coolant in the steam generator 14. The change in the flow rate of the primary coolant in the electromagnetic pump 13 is detected by the detecting unit 47 via the outlet portion of the steam generator 14 and the coolant passage 5, and then, the electromagnetic pump 13 stopped by the control of the operation unit 47 and, after that, the electromagnetic pump 13 is again operated. In this case, a gas transferred downwardly in the steam generator 14 is transferred selectively to the upward cover gas space thereof via each liquid surface of the space between the intermediate shell 25 and the outer shell 24 and the space between the outer shell 24 and the reactor vessel 9. Because the lower end portion 23b of the steam generator inner shell 23 is arranged at the position lower than the lower end portion 25a of the intermediate shell 25 and the lower end portion 24a of the outer shell 24. Moreover, the opening portion 44 of the steam generator 23 operates so that the reactor vessel 9 communicates with the cover gas space 45 of the steam generator 14. Therefore, the water vapor or bubble of the reaction product by the large-scale water leakage generated in the liquid metal is all guided to the cover gas space 45 of the reactor vessel 9 so that, even if a large-scale water leakage occurs in the heating tube 16 of the steam generator 14, it is possible to maintain a safety of the reactor 1D without mixing a bubble into the core 2. Moreover, another construction of a further modification according to the third embodiment may be made according to the present invention. For example, the outer tube 16T is arranged at a gap to the outer periphery of the inner tube 16S so that an inert gas such as helium or the like is sealed in the gap. Furthermore, in order to detect a leakage in both inner and outer tubes 16S and 16T, a continuous leakage monitoring unit such as a helium pressure gage, a moisture content concentration monitor or the like, is provided for the reactor according to the modification. According to the above construction of the reactor in the further modification, the heating tube 16 has a double tube structure, and the continuous leakage monitoring unit detects a leakage in both inner and outer tubes 16S and 16T by the inert gas such as helium or the like sealed in the gap between the inner and outer tubes 16S and 16T so that it is possible to securely prevent a contact of the water in the tubes 16S and 16T with the liquid metal of the shell side of the steam generator 14. Accordingly, with the above construction, because of preventing the water from contacting the liquid metal, it is possible to make a stable operation of the reactor for a long period.    Next, a fast reactor according to a seventh embodiment of the present invention will be described below. According to this seventh embodiment, the reactivity control assembly has the structure in that the distribution of the neutron moderator in the diametrical direction of the cladding tube 121 is gradually dense toward an inside of the cladding tube 121. The fast reactor of the seventh embodiment has almost the same effects as the fifth embodiment. Besides, according to the fast reactor of this seventh embodiment, it is possible to prevent a reduction of the initial neutron absorption effect, and to provide a linear reduction of the reactivity. Therefore, according to this seventh embodiment, the reactivity is linear, and the excess reactivity change by the burn-up is linear in appearance. Therefore, it is possible to linearly carry out the burn-up control by the operation of the neutron reflector 4, and thus, to carry out the operation of the neutron reflector 4 at an approximately constant speed, thereby readily performing the burn-up control. Next, a fast reactor according to an eighth embodiment of the present invention will be described below. According to this eighth embodiment, the mixture 122 in the cladding tube 121 of the reactivity control assembly 119 is formed so that the neutron moderator and the neutron absorber are mixed to be filed in the cladding tube 121, and, in this embodiment, as the neutron moderator, graphite is used. The eighth embodiment has almost the same effects as the fifth embodiment. Besides, because of using the graphite as the neutron moderator, it is possible to improve the safety of the fast reactor under the condition of high temperature, to increase the flexibility of designing the fast reactor and to correspond to the fast reactor wherein a coolant outlet temperature thereof is made high. Next, a fast reactor according to a ninth embodiment of the present invention will be described below. In this ninth embodiment, as shown in FIG. 20 in the fifth embodiment, the neutron absorber rod 123 is produced by mounting, as the mixture 122, the neutron moderator and the neutron absorber into the cladding tube 121 by a vibration compaction process. More specifically, in the case of mixing zirconium hydride and gadolinium as the mixture 122 of the neutron moderator and the neutron absorber, both zirconium hydride and gadolinium are weighted by a predetermined amount, and thereafter, are molded like granules. These granules are gradually put from a top opening portion of the cladding tube 121 whose bottom end is sealed, to be filled therein, while vibration is applied to the cladding tube 121 by a vibrator. After vibration filling, an upper plug is attached onto the top opening portion of the cladding tube 121 to be sealed thereto, and thus, the neutron absorber rod 123 is completed. In this case, the cladding tube 121 is attached on a vibration base of the vibrator, and then, a predetermined vibration is applied the cladding tube 121 thereby. According to this eighth embodiment, it is possible to simplify a process for forming the neutron absorber rod 123 containing the neutron moderator, and to carry out a remote control in forming of the neutron absorber rod 123. Furthermore, even in the case where the neutron moderator or the neutron absorber is a dangerous material such as a radioactive material, the neutron absorber rod 123 can be readily formed. Next, a fast reactor according to a tenth embodiment of the present invention will be described below. In this tenth embodiment, the cladding tube 121 or the wrapper tube 120 shown in FIG. 20 in the fifth embodiment is provided at its inner surface with an inside coat for preventing hydrogen from being transmitted, for example, a chromium coating layer. The chromium coating layer contacts with the mixture 122 of the neutron moderator and the neutron absorber, for example, the mixture of zirconium hydride and gadolinium. According to this tenth embodiment, the reactivity control assembly 119 is provided at its inner surface with the inside coat for preventing hydrogen from being transmitted, and then, the reactivity control assembly 119 is mounted into the center portion of the core 2A as shown in FIG. 19A and FIG. 19B. According to the structure, it is possible to prevent hydrogen generated by the burn-up in the core 2A from leaking outside the reactivity control assembly 119. Other effects are the same as the above fifth embodiment. Next, a fast reactor according to an eleventh embodiment of the present invention will be described below. In this eleventh embodiment, in order to improve a neutron absorptive power of the reactivity control assembly 119, the neutron absorber rod 123 is formed with the mixture 122 made by mixing a fission product (FP) as a neutron absorber and a zirconium hydride as a neutron moderator, and the neutron absorber rod 123 is mounted in the core 2A. According to this eleventh embodiment, the fission product (FP) is used as the neutron absorber, and thereby, it is possible to effectively use a radioactive material generated by another reactor, and thus, to contribute for a reduction of fission products. Other effects are the same as the fifth embodiment. Next, a fast reactor according to a twelfth embodiment of the present invention will be described below. In this twelfth embodiment, a mixture 122 of the neutron moderator and a thermal neutron absorber, for example, zirconium hydride and gadolinium in the fifth embodiment, is filled in the fuel assembly 116 at the vicinity of the central portion of the core, and thereby improving a neutron absorptive power. According to this twelfth embodiment, the fuel assembly 116 is provided with the mixture of the neutron moderator and a thermal neutron absorber, and thereby, there is no need of mounting the reactivity control assembly 119 in the central portion of the core. Further, this serves to readily make a design of the neutron absorber rod mounted in the center of the core or a neutron absorptive channel. Incidentally, in this embodiment, the mixture 122 is filled in the fuel assembly 116 in the vicinity of the central portion of the core. However, the present invention is not limited to the structure. That is, the neutron absorber may be filled in one of the fuel assemblies 116 in the vicinity of the central portion of the core, and the neutron moderator may be filled in another one of the fuel assembles 116 which is also in the vicinity of the central portion thereof. Next, a fast reactor according to a thirteenth embodiment of the present invention will be described below. In this thirteenth embodiment, in each of the aforesaid fast reactors, a mixture of a neutron moderator and a neutron absorber, for example, zirconium hydride and gadolinium, is provided in a burnable poison assembly at the central portion of the core, and thereby, a void reactivity of the final burn-up is transferred to a positive side. The reflector control type of fast reactor of this embodiment has the same function as the fifth embodiment. In general, in the fast reactor, with the burn-up of the core, a void reactivity rises to a positive side. This means that in the final burn-up, the positive reactivity is increased by spectral hardening in the case where void is generated. However, as this embodiment, in the case of the fast reactor, which is provided with the neutron absorber rod with the neutron moderator, in the final burn-up, an absorptive effect is reduced in a small neutron energy range. For this reason, in the final burn-up, the burn-up to fission is great in a low neutron energy range as compared with a general fast reactor. As a result, in the final burn-up, no transfer to a positive reactivity is made with respect to spectral hardening by coolant void generation. Therefore, in the final burn-up, the void reactivity is hard to be transferred to the positive side, and therefore, it is possible to improve safety of the fast reactor. Next, a fast reactor according to a fourteenth embodiment of the present invention will be described below. In this fourteenth embodiment, lead or lead-bismuth alloy is used in place of sodium used as the liquid metal coolant in the fifth embodiment. Other construction is the same as the fifth embodiment. According to this fourteenth embodiment, a fast neutron is moderated so as to be absorbed in the neutron absorber, and thereby, it is possible to improve a neutron absorptive power, and to provide a fast reactor which has a high neutron breeding ratio, thereby elongating a lifetime of the core. In this embodiment, the volume percent ratios of the neutron moderator and the neutron absorber in the neutron absorber rod 123 in the reactivity control assembly 119 mounted in the core 2A are not uniformed but different according to different positions in the axial direction of the core 2A. That is, the volume percent ratio of a predetermined portion of the mixture 122 in the neutron absorber rod 123 of the reactivity control assembly 119, which has a height in the axial direction thereof corresponding to the height H1 of the core 2A, is X1 to Y1, wherein the X1 percent is bigger than the Y1 percent, and the volume percent ratio of another predetermined portion of the mixture 122 in the neutron absorber rod 123 of the reactivity control assembly 119, which has a height in the axial direction thereof corresponding to the height H2 of the plenum is X2 to Y2, wherein the X2 percent is bigger than the Y2 percent, and the X1 percent and the Y1 percent are bigger than the X2 percent and the Y2 percent, respectively. Incidentally, in the above embodiments, the primary coolant, such as the liquid metal is circulated by means of the electromagnetic pump, but the present invention is not limited to the structure. That is, the electromagnetic pump is omitted in each reactor in each embodiment of the present invention, and the primary coolant is circulated by a natural circulating force generated by, for example, the heating of the core, the radiation from the reactor vessel and the like. In this modification, it is further possible to reduce the cost of manufacturing the reactor, and because of no use of the electromagnetic pump, it is possible to improve the safety of each reactor in the present invention. Furthermore, in the fifth embodiment to the fifteenth embodiment of the present invention, as a nuclear reactor, the liquid metal cooled type of fast reactor is applied, but the present invention is not limited to the structure. That is, in the fifth embodiment to the fifteenth embodiment, as a nuclear reactor, a light water reactor is able to be applied to the present invention, which has the described system for cooling the core, and furthermore, other nuclear reactors can be applied to the present invention. While there has been described what is at present considered to be the preferred embodiments and modifications of the present invention, it will be understood that various modifications which are not described yet may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.