Abstract:
[Problem] To build a secure and high-efficiency power generation system while achieving a further reduction in the size of the entire system. [Solution] A compact nuclear power generation system is provided with a reactor ( 3 ) comprising a core ( 2 ) which uses metal fuel containing either or both of uranium-235/238 and plutonium-239, a reactor vessel ( 1 ) which houses the core ( 2 ), metal sodium which is filled into the reactor vessel ( 1 ) and heated by the core ( 2 ), and a neutron reflector ( 9 ) which maintains the effective multiplication factor of neutrons emitted from the core ( 2 ) at approximately one or more to bring the core into a critical state. A main heat exchanger ( 15 ) is installed outside the reactor ( 3 ). The metal sodium heated by the reactor ( 3 ) is supplied to the main heat exchanger ( 15 ), and supercritical carbon dioxide that is subjected to heat exchange with the heated metal sodium circulates therethrough. The supercritical carbon dioxide heated by the main heat exchanger ( 15 ) drives a turbine ( 20 ), and a power generator ( 21 ) is operated by the drive of the turbine ( 20 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a National Stage of International Application No. PCT/JP2012/008124 filed Dec. 19, 2012 and which claims priority to Japanese Patent Application No. JP2011-278363 filed Dec. 20, 2011, the entire contents of which are being incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    This invention relates to a nuclear power generation system and, more particularly, to a compact nuclear power generation system in which a cooling system is divided at least into a primary cooling system and a secondary one. 
       BACKGROUND ART 
       [0003]    Heretofore, as a nuclear reactor used for a nuclear power generation system, there is known an indirect cycle nuclear reactor in which a turbine is run in rotation to generate electrical power by steam not contaminated by radiations. The nuclear reactor of this sort includes a steam generator and a heat exchanger between the first and second cooling systems. 
         [0004]    It is observed that, in a loop type fast breeder reactor, aimed to construct a large size electrical power generation system, the heat in a primary sodium system (primary cooling system), heated on cooling the reactor core, is transmitted by an intermediate heat exchanger to a secondary sodium system (secondary cooling system). The heat of the secondary sodium system is further transmitted by an evaporator and a superheater to the water/steam system. In a tank type fast breeder reactor in which the reactor vessel is enlarged in size and an intermediate heat exchanger as well as a pump of a primary sodium system is housed in the reactor vessel, the heat in the primary sodium system is transmitted by the intermediate heat exchanger to a secondary sodium system and further the heat in the secondary sodium system is transmitted by a steam generator to a water/steam system. 
         [0005]    The nuclear reactor used in the large size power generation system of this sort is provided with a reactor core including larger numbers of fuel rods assembled together. Each fuel rod is formed of fuel obtained on molding metal oxide, containing uranium-235 or plutonium-239 of a low heat transfer characteristic, in the form of a bar or a pellet, and larger numbers of these pellets are then stacked in a sheathing tube. In a reactor used in a large size nuclear reactor, scores of fuel rods are bundled together to form a fuel assembly of the fuel rods. About 200 of the fuel assemblies are aggregated together, and each of a plurality of control rods, used for controlling the rate of the fuel reaction, is arranged in-between two adjoining fuel assemblies. If, in a large size nuclear reactor, employing the control rods, the fuel rods become unable to operate due to, for example, failures of a control mechanism that controls the fuel rod positions, there is a risk of runaway of the nuclear reaction in the reactor core. 
         [0006]    Furthermore in a nuclear reactor different than the fast breeder reactor, such as a pressurized light water reactor, the heat of the primary cooling water, heated on cooling the reactor core, is transferred by a steam generator to a water/steam system. In the nuclear reactor of this sort, control rods likewise are arranged in-between adjoining fuel assemblies within the reactor in order to control the reaction rate in the reactor core. 
         [0007]    In an indirect cycle nuclear reactor, such as a pressurized light water reactor or a loop type fast breeder reactor, aimed to construct the above mentioned large size electrical power generation system, heat transfer between respective cooling systems is by steam generators or heat exchangers that are respectively different entities independent of one another or that are housed within respective different chambers and interconnected by piping. Hence, the cooling system in its entirety is complicated and bulky in size. In particular, in a fast breeder reactor, aimed at power generation, the primary coolant system, exploiting metal sodium as a coolant, is formed by larger numbers of loops, each connected to a plurality of loops of the secondary coolant system. Hence, the numbers of tubes, pumps, heat exchangers or steam generators are increased, with the result that the cooling system becomes complicated or bulky in size. 
         [0008]    Furthermore, in a large size nuclear reactor, constituting a large size power generation system, in which the rate of the nuclear reaction in the reactor core is controlled by control rods arranged between neighbored fuel assemblies, the monitor system for control rods is required, thus complicating the structure of the reactor itself. Hence, the manufacturing cost of the nuclear reactor is drastically increased, while many crews as well as larger numbers of monitor equipment are needed for maintenance and management. 
         [0009]    It is observed that a tank fast breeder reactor has been proposed to simplify and reduce the size of the cooling system. In the fast breeder reactor of this sort, intermediate heat exchangers or steam generators are still needed to circumvent the risk proper to sodium used for cooling the reactor core, while simplifying the structure or reducing the size of the cooling system may not be said to be sufficient. 
         [0010]    There is thus a demand for further simplifying and reducing the size of the cooling system. To accomplish the objective, such a nuclear reactor disclosed in Patent Document 1 has been proposed. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL1: WO03/007310 
       
     
       SUMMARY 
     Technical Problem 
       [0012]    It is an object of the present invention to provide a compact nuclear power generation system which will enable the system including the nuclear reactor and the power generation system to be further reduced in size in its entirety. 
         [0013]    It is another object of the present invention to provide a compact nuclear power generation system which is of the load follower type, safe and easy to control. 
         [0014]    It is yet another object of the present invention to provide a compact nuclear power generation system which will enable the manufacturing cost as well as the cost for maintenance and management to be reduced. 
       Solution to the Problem 
       [0015]    The compact nuclear power generation system according to the present invention has been proposed in order to accomplish the above objects, and includes a nuclear reactor, which in turn includes a reactor core, a reactor vessel containing the reactor core therein, a primary coolant that is formed of metal sodium charged into the reactor vessel and that is heated by the reactor core, and a neutron reflector mounted surrounding the reactor core. The neutron reflector operates to maintain the effective multiplication factor for neutrons radiated from the reactor core at about unity or higher to set the reactor core to the critical state. The reactor core includes a plurality of fuel rods each formed of a metal fuel containing one or both of uranium-235/238 and plutonium-239. A certain number of the fuel rods are sealed within each of a plurality of sheathing tubes. It is observed that uranium-238 contained in the uranium fuel absorbs neutrons and generates plutonium-238 as the nuclear reactor is maintained in operation. 
         [0016]    In this compact nuclear power generation system, the main heat exchanger is installed outside the nuclear reactor. The primary coolant, heated by the primary coolant, is supplied to the main heat exchanger via a conduit. The secondary coolant, heated by heat exchange with the primary coolant, is circulated in the main heat exchanger. According to the present invention, supercritical carbon dioxide is used as the secondary coolant. The compact nuclear power generation system includes a turbine, driven by the secondary coolant, heated by the main heat exchanger, and a generator run in operation by the driving of the turbine. 
         [0017]    The neutron reflector, mounted surrounding the reactor core, is of a height smaller than the height of the reactor core, and is moved by a movement mechanism in a direction from a lower side towards an upper side of the reactor core. 
         [0018]    The reactor core of the nuclear reactor is formed by a collection of a plurality of fuel rods that are formed by a metal fuel and that are sealed in a sheathing tube formed of ferrite-based stainless steel or chromium molybdenum steel. The metal fuel is formed of an alloy composed of zirconium, uranium-235/238 and plutonium-239 or an alloy composed of zirconium and one of uranium-235/238 and plutonium-239. 
         [0019]    The reactor vessel is formed as a cylinder having a diameter not larger than 2 m and a height not larger than 12 m, and the reactor core housed within the reactor vessel is formed by a collection of fuel rods each having a diameter of 5 to 15 mm and a collective length not larger than 2 m. 
         [0020]    The present system further includes a pump that circulates the primary coolant, charged into the reactor vessel, in a direction from the reactor vessel to the main heat exchanger. 
         [0021]    The present system further includes a cooler that drives the turbine to cool the secondary coolant discharged from the turbine, a compressor that compresses the secondary coolant from the turbine, and a recycling heat exchanger. The recycling heat exchanger is supplied with the secondary coolant discharged from the turbine and with the secondary coolant compressed by the compressor. The recycling heat exchanger heats the secondary coolant compressed by the compressor by the secondary coolant supplied from the turbine to return the heated secondary coolant to the main heat exchanger. The compressor, compressing the secondary coolant, is driven by the turbine. 
       Advantageous Effect of the Invention 
       [0022]    According to the present invention, the nuclear reactor is constituted without using the control rods that control the rate of the nuclear reaction, such as to reduce the size of the nuclear reactor. In addition, supercritical carbon dioxide is used as the secondary coolant heated by the nuclear reactor to drive a turbine, thereby reducing the size of the driving system of the power generation system including the turbine and also reducing the size of the nuclear power generation system in its entirety. 
         [0023]    Furthermore, in the nuclear reactor, having the compact nuclear power generation system, according to the present invention, metal sodium is used as the primary coolant. Thus, in a case that the nuclear reactor is performing load follower driving in which a power generation output is varied to follow variations in the power consumption by the load connected to the power generation system, it becomes possible to automatically control the reactivity of the nuclear fuel so as to follow variations in the power consumption by the load, thus enabling an automatic operation of the power generation system. 
         [0024]    Additionally, according to the present invention, the neutron reflector is formed to a height smaller than the height of the fuel assembly, and is carried for movement from the lower to the upper side of the fuel assembly. The neutron reflector is thus moved from the portion of the fuel assembly where the nuclear fuel has been consumed to its portion where the nuclear fuel has not been consumed. Hence, the nuclear reaction may be sustained for a prolonged time as control is managed of the reactivity of the nuclear fuel. 
         [0025]    According to the present invention, the reactor vessel has a diameter not larger than 2 m and a height not larger than 12 m, while the reactor core, housed in the reactor vessel, is formed by an assembly of fuel rods each having a diameter of 5 to 15 mm and a collective length not larger than 2.5 m. Hence, the nuclear reactor may be reduced in size. 
         [0026]    According to the present invention, the primary coolant, charged into the reactor vessel, is circulated using a pump. Hence, metal sodium, constituting the primary coolant, may positively be circulated. 
         [0027]    According to the present invention, the primary coolant, heated within the nuclear reactor, is supplied to a heat exchanger provided outside the nuclear reactor so as to exchange heat with the secondary coolant formed of supercritical carbon dioxide. It is thus possible to provide the circulation system, comprised of the heat exchanger and the turbine, outside the nuclear reactor, thus assuring facilitated maintenance and inspection of the power generation system. 
         [0028]    Since the circulation channel, in which the secondary coolant, driving the turbine, is circulated, is formed as a closed loop, the power generation system may further be reduced in size, while loss of the secondary coolant may be reduced. 
         [0029]    By using supercritical carbon dioxide as the secondary coolant, the turbine, driving the generator, may further be reduced in size. Viz., the turbine may be driven at a high efficiency because supercritical carbon dioxide has a density sufficiently higher than e.g., water. 
         [0030]    By using supercritical carbon dioxide as the secondary coolant, it is possible to prevent accidents, such as explosions, which might be caused by reaction of sodium with water, even if the secondary coolant contacts metal sodium of the primary coolant, thus assuring improved system safety. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0031]      FIG. 1  is a schematic view showing a structure of a compact nuclear power generation system according to an embodiment of the present invention. 
           [0032]      FIG. 2  is a side view showing fuel rods used in the nuclear reactor according to the present invention. 
           [0033]      FIG. 3  is a transverse cross-sectional view showing an inner part of the nuclear reactor. 
           [0034]      FIG. 4  is a schematic view showing an example installation of the compact nuclear power generation system according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Referring to  FIG. 1 , a compact nuclear power generation system according to the present invention includes a nuclear reactor  3  comprised of a reactor core  2  loaded within a reactor vessel  1 . 
         [0036]    In an embodiment illustrated, the reactor vessel  1  is cylindrically-shaped, with a diameter R1 and a height H1 of the cylinder being not larger than 2 m and not larger than 12 m. More specifically, the reactor vessel  1  is cylindrically-shaped, with the diameter R1 of the cylinder being 1.5 m and the height H1 being 3 m. 
         [0037]    Referring to  FIG. 2 , a reactor core  2 , loaded within the reactor vessel  1 , is constituted by a fuel assembly  5 , and includes a plurality of fuel rods  4  bundled together, as shown in  FIG. 2 . Each fuel rod is comprised of metal fuel containing uranium-235/238 and plutonium-239, with the metal fuel being enclosed within a sheathing tube. 
         [0038]    The metal fuel used in the present embodiment is formed of an alloy of zirconium, uranium-235/238 and plutonium-239, and is in the form of a bar with a diameter of 5 to 15 mm and a height of not lesser than 10 cm. This metal fuel is sealed within a sheathing tube of ferrite-based stainless steel to constitute the fuel rod  4 . 
         [0039]    The sheathing tube, in which the metal fuel is sealed, is formed as an elongated cylinder having a diameter of 5 to 15 mm and a length of not larger than 2.5 m. Hence, the fuel rod  4 , formed using the sheathing tube, is in the form of a column with a diameter about equal to 9 mm and a length about equal to 1.5 m. 
         [0040]    About 50 of the fuel rods  4 , each comprised of a metal fuel sheathed within the sheathing tube, are bundled together by a fastener band  4   a , as shown in  FIG. 2 , to form the sole fuel assembly  5 . About 40 of the fuel assemblies  5  are unified together to form a fuel assembly structure  7 . These about 40 of the fuel assemblies  5  are unified together by a core barrel  6 , provided around the outer periphery of the fuel assemblies, as shown in  FIG. 3 . The fuel assembly structure  7  is loaded within the reactor vessel  11  to constitute the reactor core  2 . 
         [0041]    The fuel assembly structure  7  is formed by the fuel rods  4 , each about 1.5 m long, and hence is about 1.5 m long approximately equal to the length of the fuel rod  4 . Since the fuel assembly structure  7 , thus approximately 1.5 m in height, is loaded on the bottom side of the reactor vessel  1 , having the height H1 of 3 m, there is left a space  1   a  of 1 to 1.2 m on an upper side of the reactor vessel  1 . 
         [0042]    It is noted that metal fuel may be formed of an alloy of zirconium with one of uranium-235/238 and plutonium-239. 
         [0043]    The sheathing tube, within which the metal fuel is sheathed to form the fuel rod  4 , may be formed of chromium or molybdenum steel to a tube shape. 
         [0044]    A primary coolant  8 , heated by heat generated by the reactor core  2  being set to a critical state, is charged within the reactor vessel  1 . In the present embodiment, metal sodium is used as the primary coolant  8 . 
         [0045]    A neutron reflector  9  is arranged on an outer peripheral side of the fuel assembly structure  7  formed by a plurality of the fuel assemblies  5  unified together. The neutron reflector  9  is mounted surrounding the fuel assembly structure  7  as a constant spacing is maintained between the neutron reflector and the fuel assembly structure  7 . The neutron reflector  9  reflects neutrons radiated from the fuel rods  4  constituting the reactor core  2  to control the neutron density within the reactor vessel  1  such as to maintain the effective multiplication factor for neutrons radiated from the fuel rods  4  at approximately unity or higher in order to set the reactor core to the critical state. 
         [0046]    In the nuclear reactor  3  according to the present invention, the neutron reflector  9  reflects the neutrons, radiated from the fuel rods  4 , towards the reactor core  2  to control the neutron density in the reactor vessel  1  to set the critical state of the reactor vessel  1 . Therefore, only a minor quantity of fuel suffices, thus reducing the size of the nuclear reactor  3 . 
         [0047]    In the present embodiment, the neutron reflector  9  is formed e.g., of stainless steel capable of reflecting the neutrons radiated from the fuel rods  4 . The neutron reflector  9  is formed as a double-walled annular body made up of an inner tube and an outer tube arranged at a preset interval in-between. Within the inner side of the neutron reflector  9 , there are charged fine particles of graphite  9   a  having a neutron reflecting capability. The neutron reflector  9 , thus charged in its inner side with the fine particulate graphite  9   a , reflects neutrons, radiated from the fuel rods  4 , surrounded by the neutron reflector  9 , back towards the fuel rods  4 . The neutron reflector also causes neutrons, transmitted through its reflective inner wall member, so as to impinge on the fine graphite particles so that the neutrons are thereby reflected and returned to the inside of the reactor core. 
         [0048]    It is observed that a carbonaceous material, such as graphite, may absorb only a minor amount of neutrons, while having a relatively large cross-sectional area as a target for neutron impingement. For this reason, the material has a high probability of reflection and hence is used as a neutron reflection material in order to take advantage of the characteristic of high reflection probability proper to the fine particulate graphite  9   a.    
         [0049]    It is observed that the neutron reflector  9  is formed as a cylinder of a dimension such that, when the neutron reflector is placed around the fuel assembly structure  7  within the reactor vessel  1 , an interval or a gap of 1 to 10 cm, preferably 1 to 2 cm, will be created between the outer peripheral surface of the fuel assembly structure  7  and the inner peripheral surface of the neutron reflector. 
         [0050]    It is observed that, should there be not provided the neutron reflector  9  around the fuel assembly structure  7 , the neutrons radiated from the metal fuel is lowered in density, and hence the critical state may not be maintained. On the other hand, as the reactor core  2  is brought to the critical state to undergo the reaction, uranium-235 or plutonium-239, contained in the metal fuel, is consumed. 
         [0051]    Thus, the neutron reflector  9  is of a height H2, which is one-half or less of the length of the fuel rods  4  loaded within the reactor vessel  1 , and is carried for movement in the up-and-down direction of the fuel assembly structure  7 . The neutron reflector  9  is moved gradually from the portions of the fuel rods  4  where the metal fuel of the fuel rods has been consumed to those of the fuel rods where the metal fuel has not been consumed, thereby maintaining the critical state of the reactor core  2  for prolonged time. 
         [0052]    In the present embodiment, the neutron reflector  9  is of a height H2 about equal to 40 cm. The neutron reflector  9  is arranged parallel to the reactor core  2 , as shown in  FIG. 1 , and carried by a driving shaft  11  which is run in rotation by a rotary driving mechanism  10 . The neutron reflector is driven into movement along the height of the reactor core  2  by the driving shaft  11  being run in rotation. 
         [0053]    The driving shaft  11  is passed through a partitioning wall member  12  configured for delimiting and hermetically sealing an upper part of the reactor vessel  1 . The driving shaft  11  is connected to the rotary driving mechanism  10 , arranged on top of the partitioning wall member  12 , and is driven to perform revolutions by the operation of the rotary driving mechanism  10 , whereby the neutron reflector  9  may be moved in the up-and-down direction along the reactor core  2 . 
         [0054]    The compact nuclear power generation system of the present invention includes a main heat exchanger  15 , which is supplied with the primary coolant  8 , heated within the nuclear reactor  3 , and within which is circulated a secondary coolant heated by heat exchange with the primary coolant  8 . The main heat exchanger  15 , provided outside the nuclear reactor  3 , is connected to the nuclear reactor  3  via conduits  16 ,  17 , constituting a flow channel for the primary coolant  8  charged into the reactor vessel  1 . 
         [0055]    The primary coolant  8 , heated within the reactor vessel  1 , is delivered to the main heat exchanger  15  via the supply side conduit  16 . The primary coolant  8 , delivered to the main heat exchanger  15 , flows through the inside of the main heat exchanger  15  so as to be then returned into the reactor vessel  1  via the return side conduit  17 . That is, the primary coolant  8 , charged into the reactor vessel  1 , flows through a closed loop circulation channel interconnecting the reactor vessel  1  and the main heat exchanger  15 . 
         [0056]    Within the circulation channel for the primary coolant  8 , there is provided a pump  18  configured for accelerating the convection of the primary coolant  8  of the primary coolant  8  heated within the reactor vessel  1 . The pump  18  is provided halfway on the circulation channel within which the primary coolant  8  exiting the main heat exchanger  15  flows towards the reactor vessel  1 . By providing the pump  18 , configured for accelerating the convection of the primary coolant  8 , it is possible to permit smooth circulation of the primary coolant within the circulation channel extending from the reactor vessel  1  to the main heat exchanger  15 , even though metal sodium of higher viscosity is used as the primary coolant  8 . 
         [0057]    Since electrically conductive metal sodium is used as the primary coolant  8 , an electromagnetic pump is used as the pump  18 . The electromagnetic pump is configured so that, when metal sodium flows through a circulation channel, the electric current is sent to metal sodium, with the current then co-acting with the magnetic field from the electromagnetic pump  18  to produce a force in the flow direction of metal sodium to promote its circulation. 
         [0058]    A heat transfer tube  19 , within which circulates the secondary coolant, is provided within the main heat exchanger  15 . When flowing through the inside of the heat transfer tube  19 , the secondary coolant is heated by thermal contact with the heated primary coolant  8  delivered into the inside of the main heat exchanger  15 . In the subject embodiment, supercritical carbon dioxide is used as the secondary coolant. 
         [0059]    The compact nuclear power generation system according to the present invention includes a turbine  20 , driven by supercritical carbon dioxide, as the secondary coolant, heated by the main heat exchanger  15 , and a generator  21  which is run in rotation by the turbine  20  being run in operation. 
         [0060]    The present system further includes a cooler  22 , used for cooling supercritical carbon dioxide, which has driven the turbine  20 , a compressor  23 , used for compressing the supercritical carbon dioxide, cooled by the cooler  22 , and a recycling heat exchanger  24 . The recycling heat exchanger is supplied with supercritical carbon dioxide, discharged from the turbine  20 , and with supercritical carbon dioxide compressed by the compressor  23 . The supercritical carbon dioxide, compressed by the compressor  23 , is heated by the supercritical carbon dioxide supplied from the turbine  20  so as to be then returned to the main heat exchanger  15 . 
         [0061]    There is also provided a pump for circulation  25  by means of which the supercritical carbon dioxide, used for driving the turbine  20  and then supplied to the recycling heat exchanger  24 , is delivered to the cooler  22 . 
         [0062]    The supercritical carbon dioxide, delivered from the recycling heat exchanger  24  to the cooler  22 , flows through the inside of a heat transfer tube  26  provided within the cooler  22 . The supercritical carbon dioxide, flowing through the inside of the heat transfer tube  26 , is delivered to the cooler  22  so as to be cooled by having thermal contact with cooling water (H 2 O) supplied to the cooler  22  via a cooling water conduit  27 . 
         [0063]    The supercritical carbon dioxide, cooled by the cooler  22 , is delivered to and compressed by the compressor  23 . It is observed that the compressor  23  is connected via a connecting shaft  28  to the turbine  20  and is run in operation by the driving of the turbine  20 . 
         [0064]    The supercritical carbon dioxide, compressed by the compressor  23 , is delivered to a heat transfer tube  29  provided within the recycling heat exchanger  24 . The supercritical carbon dioxide, delivered to the heat transfer tube  29  provided within the recycling heat exchanger  24 , is heated by having thermal contact with the high-temperature supercritical carbon dioxide delivered from the turbine  20  to the inside of the recycling heat exchanger  24 . The compressed supercritical carbon dioxide, heated by the recycling heat exchanger  24 , is returned via a return channel  30  to the heat transfer tube  19  provided within the main heat exchanger  15 . 
         [0065]    The compressed supercritical carbon dioxide, heated by the main heat exchanger  15 , drives the turbine  20 , and is then circulated to the cooler  22 , thence to the compressor  23 , thence to the recycling heat exchanger  24  and thence to the main heat exchanger  15 . During this time, the supercritical carbon dioxide is subject to heating, expansion and compression in a repetitive manner to drive the turbine  20  to actuate the generator  21  for power generation. 
         [0066]    The operation of the compact nuclear power generation system will now be detailed further. In an initial state of the compact nuclear power generation system, prior to start of an operation, the neutron reflector  9  is positioned in a space  1   a  in an upper part of the reactor vessel  1 . 
         [0067]    To start the operation, the rotary driving mechanism  10  is driven to move the neutron reflector  9  to a lower part of the reactor vessel  1 , so that the neutron reflector will face the lower part of the reactor core  2 , as shown in  FIG. 1 . When the neutron reflector  9  faces the reactor core  2 , the neutrons radiated from the fuel rods  4  of the fuel assembly structure  7  are reflected by the neutron reflector  9  to raise the neutron density within the reactor core  2 . With the effective multiplication factor for neutrons, radiated from the fuel rods  4 , equal to about unity or higher, the reactor core  2  is in a critical state. 
         [0068]    With the reactor core  2  in the critical state, the fuel in the fuel rods  4  is reacted and heated, thus heating the primary coolant  8  charged into the reactor vessel  1 . The heated primary coolant  8  convects within the reactor vessel  1  so as to be supplied to the main heat exchanger  15  via supply side conduit  16 . The primary coolant delivered to the main heat exchanger  15  comes into thermal contact with the heat transfer tube  19  provided in the main heat exchanger  15  to heat the secondary coolant circulated within the heat transfer tube  19 . At this time, the primary coolant  8  that heated the secondary coolant is circulated from the main heat exchanger  15  through the inside of the reactor vessel  1  as it is accelerated in its flow movement by the pump  18  provided halfway in the return side conduit  17 . 
         [0069]    According to the present invention, the supercritical carbon dioxide is used as the secondary coolant, while metal sodium is used as the primary coolant  8  for heat exchange in the main heat exchanger  15 . It is thus possible to positively circumvent the risk of, for example, explosions, even though the heat transfer tube  19 , for example, is damaged such that the high-temperature primary coolant  8  comes into direct contact with the secondary coolant. 
         [0070]    If heated to an order of ca. 600° C. and contacted in this state with CO 2 , metal sodium undergoes a chemical reaction. However, if such chemical reaction should take place, simply sodium carbonate and carbon, as such, are generated, as indicated by the following formula: 
         [0000]      4Na+3CO 2 →2NaCO 3 +C
 
         [0000]    while there is no risk of accidents, such as explosions, which might take place on reaction with water. 
         [0071]    It is observed that, when in a critical state, the reactor core  2  of the nuclear reactor  3  is at a temperature of 600 to 800° C., thus heating the primary coolant  8  within the reactor vessel  1 . When heated to a temperature of ca. 500 to 750° C., due to heating in the reactor core  2 , the primary coolant  8  is delivered to the main heat exchanger  15  as it is circulated by the pump  18 . The primary coolant then undergoes heat exchange with the supercritical carbon dioxide, flowing through the heat transfer tube  19 , provided within the main heat exchanger  15 , thus heating the supercritical carbon dioxide to ca. 450 to 700° C. 
         [0072]    It is observed that the supercritical carbon dioxide, flowing through the inside of the main heat exchanger  15 , has been compressed by the compressor  23  to 12 to 20 MPa. 
         [0073]    If injected to the turbine  20  provided within an ambient temperature normal pressure air, the supercritical carbon dioxide, heated by the main heat exchanger  15 , is precipitously expanded in volume, as it is decompressed and cooled, so as to flow within the turbine  20 , thus causing its revolutions. With the turbine  20  run into rotation, the generator  21  is run into operation to generate electric power. 
         [0074]    It is observed that, when injected into the turbine  20 , provided in atmospheric air, the supercritical carbon dioxide, heated to 450 to 700° C., is cooled to ca. 350 to 500° C. as it is decompressed to ca. 8.0 to 10 MPa. As a result, the supercritical carbon dioxide has its volume expanded by a factor of ca. 2.5 to drive the turbine  20 , after which it is discharged from the turbine  20 . 
         [0075]    Thus, the supercritical carbon dioxide, driving the turbine  20 , has an expansion ratio of ca. 2.5 between the volume at the turbine inlet side and that at the turbine outlet side. It is thus unnecessary to provide a large-sized outlet for the supercritical carbon dioxide, as would be necessary in the case of using water as the secondary coolant, thus enabling the size of the turbine  20  to be reduced. 
         [0076]    The supercritical carbon dioxide, injected into the inside of the turbine  20  to drive it and decompressed/cooled, is delivered to the inside of the recycling heat exchanger  24  so as to be then delivered via the recycling heat exchanger  24  to the cooler  22 . It is observed that the supercritical carbon dioxide is delivered from the recycling heat exchanger  24  to the cooler  22  by the pump for circulation  25 . 
         [0077]    The supercritical carbon dioxide, delivered to the cooler  22 , flows through the inside of a heat transfer tube  26  in the cooler  22  so as to be cooled to near the critical point by heat exchange with the coolant water flowing through the cooler  22 . It is observed that the supercritical carbon dioxide, delivered to the cooler  22 , is maintained at a pressure of 8.5 to 10 MPa. The cooler  22  cools the supercritical carbon dioxide, maintained at the pressure of 8.5 to 10 MPa, to ca. 35° C. which is near the critical point. 
         [0078]    The supercritical carbon dioxide, cooled to near the critical point by the cooler  22 , is delivered to the compressor  23  so as to be compressed to 12 to 20 MPa. 
         [0079]    It is observed that, with the supercritical carbon dioxide at near the critical point, as compared to the carbon dioxide, which is at a temperature or pressure not higher than the critical points, the amount of the work needed to achieve the same compression ratio may be drastically reduced. It is thus possible to reduce the amount of the work of the compressor  23  in compressing the supercritical carbon dioxide. According to the present invention, the compressor  23  is driven by the turbine  20  that actuates the generator  21 , so that it is possible to relatively increase the energy used in driving the generator  21 , as a result of which the ratio of the energy of the turbine  20  in driving the generator  21  may be increased to improve the power generation efficiency. 
         [0080]    On the other hand, the supercritical carbon dioxide, compressed by the compressor  23 , is delivered to the heat transfer tube  29  within the recycling heat exchanger  24  and heat-exchanged with the supercritical carbon dioxide. This supercritical carbon dioxide has been ejected from the turbine  20  so as to be delivered into the inside of the recycling heat exchanger  24  at the temperature of ca. 350 to 500° C. Hence, the supercritical carbon dioxide is preliminarily heated to ca. 250 to 350° C. by such heat exchange. The supercritical carbon dioxide, thus preliminarily heated, is again returned to the main heat exchanger  15  and heated by heat exchange with metal sodium which is the primary coolant heated by the nuclear reactor  3 . 
         [0081]    Thus, in the compact nuclear power generation system, according to the present invention, the supercritical carbon dioxide, used for driving the turbine  20 , is cooled, compressed, pre-heated and ultimately heated to drive the turbine  20  to actuate the generator  21 , so that it becomes possible to construct a high efficiency power generation system. 
         [0082]    Furthermore the nuclear reactor, driving the power generation system, is constructed using a reactor vessel with a diameter not larger than 2 m and a height not larger than 12 m. Hence, the nuclear reactor may be constructed as a compact apparatus, so that the nuclear power generation system, including the nuclear reactor, may be reduced in size. 
         [0083]    It is observed that, in the subject embodiment, there is provided an isolation valve  31  halfway on a flow channel through which supercritical carbon dioxide flows from the main heat exchanger  15  to the turbine  20 , while there is provided another isolation valve  32  halfway on a flow channel through which supercritical carbon dioxide flows from the turbine  20  to the recycling heat exchanger  24 . By providing the isolation valves  31 ,  32 , it is possible to shut off the flow of supercritical carbon dioxide towards the nuclear system side to isolate the turbine  20  from the nuclear reactor  3 , so that maintenance as well as inspection of the power generation system inclusive of the turbine  20  may be accomplished safely and easily. 
         [0084]    Moreover, the present invention may usefully be applied to a nuclear power generation system which performs the load follower operation in which the power generation system is driven as the reactivity of the fuel in the nuclear reactor  3  is automatically controlled so as to follow variations in power consumption by the load connected to the system. 
         [0085]    According to the present invention, metal sodium is used as the primary coolant for the nuclear reactor. If the fuel reactivity is raised such as to increase the power generation output so as to follow the increasing power consumption by the load connected to the power generation system, the density of metal sodium is lower, while its temperature becomes higher. The degree of leakage of the neutron number generated by the fuel reaction will increase to automatically lower the fuel reactivity. If conversely the power generation output of the power generation system is lowered, the density of metal sodium becomes higher, while its temperature becomes lower. The degree of leakage of the neutron number generated by the fuel reaction becomes lower, thus automatically elevating the fuel reactivity. Thus, with the use of metal sodium as the primary coolant, it becomes possible to automatically control the nuclear fuel reactivity so as to follow variations in the power consumption by the load connected to the power generation system. Hence, the present invention may be applied to the nuclear power generation system performing the load follower operation without using control rods. 
         [0086]    Furthermore, in the compact nuclear power generation system, according to the present invention, a system A and a power generation system B are simply correlated to each other by a conduit system through which flows the supercritical carbon dioxide. The system A includes the nuclear reactor  3  and the main heat exchanger  15  as well as the equipment controlling the operation of the nuclear reactor  3 , the system B including the turbine  20  and the generator  21  run in operation by the turbine  20 . The turbine is driven by the supercritical carbon dioxide heated by heat exchange with the main heat exchanger  15 . The two systems may thus be installed as systems isolated and independent from each other. 
         [0087]    Hence, the system A, comprised of the nuclear reactor and neighbored equipment, is installed in, for example an underground concrete building  32 , while the power generation system B is installed in a building  33  on the ground. By providing the systems A and B in isolation from each other, in this manner, it is possible to reduce radiation exposure to assure safety as well as maintenance and inspection of the power generation system B inclusive of the turbine  20 . 
       SYMBOLS 
       [0000]    
       
           1  reactor vessel 
           2  reactor core 
           3  nuclear reactor 
           8  primary coolant 
           9  neutron reflector 
           10  rotary driving mechanism 
           11  driving shaft 
           15  main heat exchanger 
           16 ,  17  conduits 
           20  turbine 
           21  generator 
           22  cooler 
           23  compressor 
           24  recycling heat exchanger 
           25  pump for circulation