Abstract:
A fast reactor has: a reactor vessel containing a coolant; a reactor core housed in the reactor vessel; a core supporting plate; a reflector; a partition arranged to surround the reflector on the side of the reactor vessel, for forming a passage of the coolant; a thermal shield arranged to cover at least one of the core side and the reactor vessel side of the partition; and a neutron shield. The thermal shield is mounted on the partition. The thermal shield includes a metallic thermal shield plate and a heat insulator mounted in the thermal shield plate, and has its inside filled with an inert gas. By the thermal shield, the thermal insulation of the partition can be improved to suppress the heat exchange between primary coolants on the core side and on the side of the reactor vessel of the partition.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part (CIP) application based upon the International Application PCT/JP2008/002578, the International Filing Date of which is Sep. 18, 2008, the entire content of which is incorporated herein by reference, and claims the benefit of priority from the prior Japanese Patent Application No. 2007-244257, filed in the Japanese Patent Office on Sep. 20, 2007, the entire content of which is incorporated herein by reference. 
     
    
     BACKGROUND ART 
       [0002]    The present invention relates to a fast reactor and, more particularly, to a fast reactor provided with a partition with high thermal insulation between the high and low temperature regions of primary coolant so as to improve reliability. 
         [0003]    A conventional small-type fast reactor has a configuration in which a reactor core is surrounded by a plurality of vessels and a reflector is provided outside the vessels. Neutrons emitted from the reactor core to the outside are reflected by the reflector to thereby promote burnup of the reactor core. 
         [0004]    However, in the fast reactor having such a configuration, the reflector is provided outside the reactor vessel, so that a vast amount of heat is dissipated in a structure housing the fast reactor by the reactor vessel and the reflector. Further, there is provided no neutron shield inside the reactor vessels, so that a large amount of neutrons are emitted outside the reactor vessel to activate argon or nitrogen retained in the upper portion of a shield structure. As a result, there arise problems that large scale cooling facilities or high-security containment facilities are required and that a stainless steel cannot be used as a structural material due to increased emission of neutrons and the use amount of a relatively expensive chrome steel is increased. 
         [0005]    As an apparatus for eliminating such problems, there is known a fast reactor having a configuration disclosed in, e.g., Japanese Patent No. 3,126,524, the entire content of which is incorporated herein by reference. This fast reactor will be described with reference to  FIG. 11 . 
         [0006]      FIG. 11  is a vertical cross-sectional view showing the outline of a fast reactor disclosed in Japanese Patent No. 3,126,524. A fast reactor  101  has a reactor core  102  including fuel assemblies and is formed into substantially a circular cylinder shape. The outer periphery of the reactor core  102  is surrounded by a core tank  103 . Outside the core tank  103 , an annular reflector  104  is provided so as to surround the core tank  103 . The reflector  104  is connected to a reflector driver  112  through reflector drive shafts  111  and is driven upward and downward by the reflector driver  112 . Outside the reflector  104 , a partition  106  constituting the inner wall of a passage for primary coolant is provided so as to surround the reflector  104 . Outside the partition  106 , a reactor vessel  107  constituting the outer wall of the coolant passage is provided at a predetermined distance from the partition  106 . The reactor vessel  107  is protected by a guard vessel  109  provided thereoutside. Further, a neutron shield  108  is provided outside the partition  106  and inside the coolant passage so as to surround the reactor core  102 . Above the neutron shield  108 , an electromagnetic pump  114 , an intermediate heat exchanger  115 , and a decay heat removal coil  116  are provided in order from the bottom. The intermediate heat exchanger  115  exchanges heat between secondary coolant flowing from a secondary coolant inlet nozzle  118  and primary coolant in the reactor vessel  107 , and discharges the secondary coolant to a secondary coolant outlet nozzle  119 . The reactor core  102 , the core tank  103 , the partition  106 , and the neutron shield  108  are supported by a core support plate  113 . An upper plug  110  is provided above the reactor vessel  107  so as to support the reflector driver  112 . 
         [0007]    According to the fast reactor  101  having the above configuration, the following effects can be obtained: the neutrons are effectively reflected by the reflector  104  disposed closely to the outer periphery of the reactor core  102  and the burnup and the breeding of the nuclear fuel can be hence effectively performed; the heat generated by the reflector  104  is utilized as a power of the fast reactor, thus improving the heat efficiency of the reactor; and the amount of neutrons emitted to the reactor vessel  107  or outside the reactor vessel  107  is decreased. 
         [0008]    In the case where sodium is used as the coolant in the above fast reactor, the temperature of the coolant is assumed to be about 350° C. to 500° C. More specifically, a range (hereinafter, referred to as “high temperature region”) from the outlet of the reactor core to the inlet of the intermediate heat exchanger has a temperature of about 500° C., and a range (hereinafter, referred to as “low temperature region”) from the outlet of the intermediate heat exchanger to the inlet of the reactor core has a temperature of about 350° C. That is, the partition is operated in an environment where a temperature difference between the inner and outer peripheral sides thereof is excessive. 
         [0009]    When heat is exchanged between the coolant in the high temperature region and the coolant in the low temperature region, a temperature drop between the inlet and the outlet of the intermediate heat exchanger is decreased to decrease power generation efficiency. Further, an increase in the temperature of the coolant in the low temperature region causes the temperature of the electromagnetic pump provided below the intermediate heat exchanger to rise, which is unfavorable in terms of the safety and efficiency of the electromagnetic pump. Further, there exist the reactor vessel, neutron shield, and core support plate in the low temperature region continued to the reactor core inlet, and the temperature rise of the coolant in the low temperature region may exert unfavorable influence in terms of the strength of these structures. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention has been made in view of the above situation, and an object thereof is to provide a fast reactor with higher reliability than conventional ones, which is capable of enhancing thermal insulation performance of the partition so as to prevent decrease in power generation efficiency. 
         [0011]    According to the present invention, there is presented a fast reactor comprising: a reactor vessel in which coolant is housed; a reactor core which is housed in the reactor vessel and which includes a fuel assembly; a core support plate which is fitted in the reactor vessel so as to support the reactor core; a reflector which surrounds the outer periphery of the reactor core and which can be moved in vertical direction; a partition which surrounds the reflector from the reactor vessel side thereof so as to form a flow channel of the coolant; a thermal shield which surrounds the partition from the reactor core side of the partition and/or the reactor vessel side thereof; a neutron shield which is provided in the flow channel of the coolant so as to surround the partition from the reactor vessel side thereof; an upper support plate which is fitted to the reactor vessel so as to support the reactor core, the partition, and the neutron shield; an intermediate heat exchanger which is set above the upper support plate; a pump which is provided in the flow channel of the coolant so as to drive the coolant; and an upper plug which is set in an upper part or above the reactor vessel and which includes a neutron shield layer and a thermal shield layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The above and other features and advantages of the present invention will become apparent from the discussion hereinbelow of specific, illustrative embodiments thereof presented in conjunction with the accompanying drawings, in which: 
           [0013]      FIG. 1  is a vertical cross-sectional view showing the outline of a fast reactor according to a first embodiment of the present invention; 
           [0014]      FIG. 2A  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the upper end of a thermal shield  40  according to the first embodiment; 
           [0015]      FIG. 2B  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the lower end of the thermal shield  40  according to the first embodiment; 
           [0016]      FIG. 3A  is an enlarged vertical cross-sectional view showing an example of a heat expansion absorbing means according to the first embodiment; 
           [0017]      FIG. 3B  is an enlarged vertical cross-sectional view showing another example of the heat expansion absorbing means according to the first embodiment; 
           [0018]      FIG. 4A  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the upper end of the thermal shield  40  according to a second embodiment of the present invention; 
           [0019]      FIG. 4B  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the lower end of the thermal shield  40  according to the second embodiment; 
           [0020]      FIG. 5  is an enlarged vertical cross-sectional view showing the area between the upper plug and thermal shield of the fast reactor according to a third embodiment of the present invention; 
           [0021]      FIG. 6  is a vertical cross-sectional view showing the outline of the fast reactor according to a fourth embodiment; 
           [0022]      FIG. 7A  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the upper end of the thermal shield  40  according to the fourth embodiment; 
           [0023]      FIG. 7B  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the intermediate portion of the thermal shield  40  according to the fourth embodiment; 
           [0024]      FIG. 7C  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the lower end of the thermal shield  40  according to the fourth embodiment; 
           [0025]      FIG. 8  is a vertical cross-sectional view showing the outline of the fast reactor according to a fifth embodiment of the present invention; 
           [0026]      FIG. 9A  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the upper end of the thermal shield  40  according to the fifth embodiment; 
           [0027]      FIG. 9B  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the intermediate portion of the thermal shield  40  according to the fifth embodiment; 
           [0028]      FIG. 9C  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the lower end of the thermal shield  40  according to the fifth embodiment; 
           [0029]      FIG. 10A  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the upper end of the thermal shield  40  according to a sixth embodiment; 
           [0030]      FIG. 10B  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the intermediate portion of the thermal shield  40  according to the sixth embodiment; 
           [0031]      FIG. 10C  is an enlarged vertical cross-sectional view showing a portion in the vicinity of the lower end of the thermal shield  40  according to the sixth embodiment; and 
           [0032]      FIG. 11  is a vertical cross-sectional view showing the outline of a conventional reactor. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    Embodiments of the present invention will be described below with reference to the accompanying drawings. 
       First Embodiment 
       [0034]    A first embodiment of the present invention will be described below with reference to  FIGS. 1 ,  2 A,  2 B,  3 A and  3 B.  FIG. 1  is a vertical cross-sectional view showing a configuration of a fast reactor according to the first embodiment of the present invention.  FIG. 2A  is an enlarged view of a portion in the vicinity of the upper end of a thermal shield  40  of the fast reactor  1  of  FIG. 1 , and  FIG. 2B  is an enlarged view of a portion in the vicinity of the lower end of the thermal shield  40 .  FIGS. 3A and 3B  are each a cross-sectional view showing an example of a heat expansion absorbing means provided in the thermal shield  40 . 
         [0035]    A structure of the fast reactor  1  will be described below using  FIG. 1 . A core support plate  13  is fitted to the lower portion of a reactor vessel  7  surrounded by a guard vessel  9 , and a core support base  39  is set on the core support plate  13 . An entrance module  38  is set on the core support base  39 . A plurality of fuel assemblies  37  are fitted to the entrance module  38  and constitute a reactor core  2 . A safety rod  26  is inserted through the inside of the reactor core  2  and is connected to a safety rod driver  27  provided at the upper portion of the fast reactor  1 . The outer periphery of the reactor core  2  is surrounded by a core tank  3  fitted to the upper surface of the core support plate  13 . A reflector  4  is provided so as to surround the core tank  3 . The reflector  4  is constituted by a neutron reflecting section  4   a  and a hollow cavity section  4   b  and is moved upward and downward by a reflector driver  12  provided at the upper portion of the fast reactor  1 . Inactive gas or metal having a lower neutron reflecting capability than the coolant is encapsulated in the hollow cavity section  4   b.    
         [0036]    A partition  6  is fitted to the upper surface of the core support plate  13  so as to surround the reflector  4 . The intermediate portion of the partition  6  is supported by an upper support plate  29  attached to the reactor vessel  7 . The partition  6  is not fixed to the upper support plate  29  but is freely slid with respect to the upper support plate  29  when being extended or retracted in the vertical direction due to heat expansion. Further, a thermal shield  40  is provided on the reactor vessel  7  side of the partition  6 . A neutron shield  8  is provided on the core support plate  13  so as to surround the outside of the partition  6 . 
         [0037]    An intermediate heat exchanger  15  is provided above the upper support plate  29  in the reactor vessel  7 . The intermediate heat exchanger  15  has a secondary coolant inlet nozzle  18  and a secondary coolant outlet nozzle  19  and exchanges, inside the reactor vessel  7 , heat between primary coolant in the reactor vessel  7  and secondary coolant. An electromagnetic pump  14  is attached to the lower portion of the intermediate heat exchanger  15  and feeds downward the primary coolant that has been subjected to heat exchange in the intermediate heat exchanger  15 . 
         [0038]    An upper plug  10  having a neutron shield layer and a thermal shield layer is provided in the upper part of the reactor vessel  7 . The upper plug  10  supports the reflector driver  12  and the safety rod driver  27 , and a containment dome  28  is provided so as to contain the reflector driver  12  and the safety rod driver  27 . Further, primary coolant  21  is injected into the reactor vessel  7 . The direction in which the primary coolant  21  flows is denoted by arrows in  FIG. 1 . 
         [0039]    The flow of the primary coolant  21  will be described in detail. The primary coolant  21  is heated in the reactor core  2  and rises up. The primary coolant  21  then passes above the partition  6  and the thermal shield  40  and flows into the intermediate heat exchanger  15 . In the intermediate heat exchanger  15 , the primary coolant  21  is subjected to heat exchange with the secondary coolant to be cooled and discharged below the electromagnetic pump  14  by means of the electromagnetic pump  14  provided under the intermediate heat exchanger  15 . The primary coolant  21  discharged from the electromagnetic pump  14  further flows down, passes through the upper support plate  29  and core support plate  13 , and reaches the bottom of the reactor vessel  7 . After that, the primary coolant  21  passes through the core support plate  13 , core support base  39 , and entrance module  38  and flows into the reactor core  2  once again. The primary coolant  21  is circulated repeatedly as described above. In the case where sodium is used as the primary coolant, the temperature of the primary coolant  21  is about 500 ° C. after passage through the reactor core  2  and is about 350° C. after passage through the intermediate heat exchanger  15 . 
         [0040]    A structure of the thermal shield  40  will be described in detail using  FIGS. 2A ,  2 B and  2 C. 
         [0041]    As shown in  FIG. 2A , the thermal shield  40  is mounted so as to be suspended from the upper end of the partition  6 . The thermal shield  40  is constituted by a hollow metal thermal shield plate  40   b,  partitions  40   g  partitioning the internal space of the thermal shield plate  40   b,  heat insulators  40   a  provided in the space partitioned by the partitions  40   g  and internally encapsulated inactive gas  40   c.  The partitions  40   g  are not formed integrally with the inner wall on the partition  6  side in the thermal shield plate  40   b  but can be slid in accordance with a heat expansion difference between the upper and lower portions of the partitions  40   g  which is caused by a temperature difference between inner and outer peripheral sides. Examples of the heat insulator  40   a  materials include, e.g., zirconia ceramics, silicon carbide ceramics, silicon nitride ceramics, alumina ceramics, fiberglass, ceramic fiber, glass wool, rock wool and ceramic wool. Examples of the inactive gas  40   c  include helium, argon and neon. 
         [0042]    A joint  40   e  is formed at the upper end portion of the thermal shield  40 . When the partition  6  and the thermal shield  40  need to be inspected or repaired, it is possible to take out the thermal shield  40  by removing upper structures such as the upper plug  10 , lowering a crane or jig from above of the fast reactor  1 , and connecting the crane or jig to the joint  40   e  by remote control. Further, in the case of the joint  40   e  having a shape as shown in the drawing, it is possible to press down the thermal shield  40  from above, allowing the joint  40   e  to be utilized when the thermal shield  40  is installed in the reactor vessel  7 . 
         [0043]    As shown in  FIG. 2B , bellows  40   d  are provided at the lower portion of the thermal shield plate  40   b.  The internal space of the bellows  40   d  communicates with a space where the heat insulator  40   a  is provided through a clearance between the thermal shield plate  40   b  and the partitions  40   g.  The inactive gas  40   c  is also encapsulated in the bellows  40   d.  The bellows  40   d  function as heat expansion absorbing means for absorbing an extension difference of the thermal shield plate  40   b  in the vertical direction due to heat expansion caused by a temperature difference between the reactor core side and the reactor vessel side thereof. 
         [0044]    In place of the bellows  40   d,  a sliding structure shown in  FIG. 3A  or an omega seal  40   f  shown in  FIG. 3B  may be provided as the heat expansion absorbing means. 
         [0045]    In the sliding structure shown in  FIG. 3A , the lower end of the thermal shield plate  40   b  is formed in a pocket-like shape. A reactor vessel  7  side lower end portion  40   h  of the thermal shield plate  40   b  is slid in the vertical direction in the pocket, whereby the extension difference due to heat expansion is absorbed. In this structure, an internal space  40   i  of the thermal shield plate  40   b  is not sealed, allowing the primary coolant  21  to enter the inside of the thermal shield plate  40   b.    
         [0046]    In the structure shown in  FIG. 3B , the omega seal  40   f  is expanded and contracted in accordance with the extension and retraction of the thermal shield plate  40   b,  whereby the extension difference due to heat expansion is absorbed. Although the heat expansion absorbing means is provided at the lower portion of the thermal shield  40  in  FIG. 1 , the heat expansion absorbing means may be provided at the upper portion or intermediate portion of the thermal shield  40 . 
         [0047]    Although the thermal shield  40  is provided on the reactor vessel  7  side of the partition  6  in the present embodiment, the thermal shield  40  may be alternatively provided on the reactor core  2  side of the partition  6 . 
         [0048]    According to the fast reactor  1  of the present embodiment, it is possible to enhance thermal insulation performance by fitting the thermal shield  40  to the partition  6  and prevent heat exchange between the primary coolant  21  that has been heated by the reactor core  2  and the primary coolant  21  discharged from the electromagnetic pump  14  through the partition  6 , thereby preventing decrease in power generation efficiency. Further, the use of the joint  40   e  formed at the upper portion of the thermal shield  40  makes it easy to take out the thermal shield  40  from the reactor, thereby obtaining excellent maintainability and repairability. 
         [0049]    The partition  6  may be divided into two in the vertical direction by the upper support plate  29  and, in this case, the upper side of the partition  6  can be fixed on the upper support plate  29 . Thus, by dividing the partition  6  into upper and lower portions by the upper support plate  29 , the size of the partition  6  having an elongated structure is reduced to improve manufacturability. The thermal shield  40  extends from the upper end of the partition  6  to the upper support plate  29 , so that even when the partition  6  is divided in two in the vertical direction, the same effect can be obtained. 
       Second Embodiment 
       [0050]    A second embodiment of the present invention will be described below with reference to  FIGS. 4A and 4B .  FIG. 4A  is an enlarged view of the upper portion of the thermal shield  40  according to the present embodiment, and  FIG. 4B  is an enlarged view of the lower portion of the thermal shield  40  according to the present embodiment. The same reference numerals are given to the same or similar configurations as those in the first embodiment, and a duplicate description thereof is omitted. 
         [0051]    As shown in  FIG. 4B , in the fast reactor  1  according to the present embodiment, the thermal shield  40  is not suspended from the partition  6 , but is configured to stand alone on the upper surface of the upper support plate  29 . 
         [0052]    According to the present embodiment, the thermal shield  40  is arranged independently, so that a load of the thermal shield  40  is not applied to the partition  6 , thereby reducing a load on the partition  6 . Since the thermal shield  40  is fixed only to the upper support plate  29 , heat expansion thereof in the vertical direction is not constrained. 
         [0053]    As described above, according to the present embodiment, a load on the partition  6  can be reduced. 
       Third Embodiment 
       [0054]    A third embodiment of the present embodiment will be described with reference to  FIG. 5 .  FIG. 5  is an enlarged cross-sectional view of an area from the upper portions of the partition  6  and thermal shield  40  of the fast reactor  1  to the upper plug  10 . The same reference numerals are given to the same or similar configurations as those in the first and second embodiments, and a duplicate description thereof is omitted. 
         [0055]    As shown in  FIG. 5 , thermal shield support rods  42  fitted to the upper plug  10  support the upper end of the thermal shield  40 . By adopting the configuration in which the thermal shield support rods  42  support the thermal shield  40  by pressing the upper end thereof from above as described above, the positional stability of the thermal shield  40  can be enhanced. Further, the thermal shield support rods  42  function as heat expansion absorbing means and can absorb a heat expansion difference generated between the lower end of the upper plug and the upper end of the partition  6 , preventing an excessive load from being applied to the thermal shield  40  and the partition  6  during operation. 
         [0056]    The above support structure may be applied not only to the configuration as shown in  FIG. 5  in which the thermal shield  40  is suspended from the upper end of the partition  6 , but also to a configuration as shown in the second embodiment in which the partition  6  and the thermal shield  40  are separated from each other. Further, the use of the thermal shield support rods  42  makes it easy to take out the thermal shield  40  from the reactor. 
       Fourth Embodiment 
       [0057]    A fourth embodiment of the present invention will be described below with reference to  FIGS. 6 ,  7 A,  7 B and  7 C.  FIG. 6  is a vertical cross-sectional view showing a configuration of the fast reactor according to the fourth embodiment.  FIG. 7A  is an enlarged view of a portion in the vicinity of the upper end of the thermal shield  40  of the fast reactor  1  of  FIG. 6 ,  FIG. 7B  is an enlarged view of a portion in the vicinity of the intermediate portion of the thermal shield  40 , and  FIG. 7C  is an enlarged view of a portion in the vicinity of the lower end of the thermal shield  40 . The same reference numerals are given to the same or similar configurations as those in the first embodiment, and a duplicate description thereof is omitted. 
         [0058]    As shown in  FIG. 7A , in the present embodiment, the thermal shield  40  is suspended from the upper end of the partition  6 . Further, as shown in  FIG. 7B , the thermal shield  40  includes the hollow thermal shield plate  40   b  and a pad  40   p  for retaining the internal space of the thermal shield plate  40   b . The inactive gas  40   c  is encapsulated in the internal space surrounded by the thermal shield plate  40   b.  A gap between the inside (reactor core side) of the thermal shield  40  and outside (pressure vessel wall side) thereof is retained by the pad  40   p , thereby preventing the inside and outside from contacting each other. The pad  40   p  is not formed integrally with the inner wall on the opposite side to the partition  6  in the thermal shield plate  40   b  but can be slid in accordance with a heat expansion difference between the upper and lower portions of the pad  40   p  which is caused by a temperature difference between inner and outer peripheral sides. Preferable examples of the inactive gas  40   c  include helium, argon and neon. 
         [0059]    Further, a fastening portion  40   k  is formed at the upper end of the thermal shield  40 , and the thermal shield  40  is fixed to the upper end of the partition  6  by a fastening member  40   j.  The fastening member  40   j  can retain the thermal shield  40  against vertical acceleration induced by an earthquake. When the partition  6  and the thermal shield  40  need to be inspected or repaired, it is possible to take out the thermal shield  40  by removing upper structures such as the upper plug  10 , lowering a crane or jig from above of the fast reactor  1 , removing the fastening member  40   j  by remote control, and connecting the jig or the like to the fastening portion  40   k.    
         [0060]    As shown in  FIG. 7C , the bellows  40   d  are provided at the lower portion of the thermal shield plate  40   b.  The inactive gas  40   c  is encapsulated in the internal space of the bellows  40   d . The bellows  40   d  function as heat expansion absorbing means for absorbing an extension difference of the thermal shield plate  40   b  in the vertical direction due to heat expansion caused by a temperature difference between the reactor core side and reactor vessel side thereof. 
         [0061]    Although the thermal shield  40  is provided on the reactor vessel  7  side of the partition  6  in the present embodiment, the thermal shield  40  may be alternatively provided on the reactor core  2  side of the partition  6 . 
         [0062]    According to the fast reactor  1  of the present embodiment, it is possible to obtain the same effect as that obtained in the first embodiment and to stably retain the thermal shield  40  even when vertical acceleration is applied thereto by an earthquake. Further, by dividing the partition  6  into upper and lower portions by the upper support plate  29 , the size of the partition  6  having an elongated structure is reduced to improve manufacturability. The thermal shield  40  extends from the upper end of the partition  6  to the upper support plate  29 , so that even when the partition  6  is divided in two in the vertical direction, the same effect can be obtained. 
         [0063]    As in the case of the first embodiment, the joint  40   e  (see  FIG. 2A ) may be fitted. Further, the configuration in which the thermal shield according to the fourth embodiment is fixed to the partition may be applied to the thermal shield of the first embodiment. 
         [0064]    Although not described above, there are slight differences between the configurations of  FIGS. 1 and 6 . For example, in the configuration of  FIG. 6 , a left/right positional relationship between the secondary coolant inlet nozzle  18  and secondary coolant outlet nozzle  19  is reversed with respect to that in  FIG. 1  and, further, a positional relationship between the neutron reflecting section  4   a  and the cavity section  4   b  is reversed between the left and right sides. However, the above differences are not related to the essence of the present invention and therefore it can be said that the configurations of  FIGS. 1 and 6  are substantially the same. 
       Fifth Embodiment 
       [0065]    A fifth embodiment of the present invention will be described with reference to  FIGS. 8 ,  9 A,  9 B and  9 C.  FIG. 8  is a vertical cross-sectional view showing a configuration of the fast reactor according to the fifth embodiment of the present invention.  FIG. 9A  is an enlarged view of the upper portion of the thermal shield  40  according to the present embodiment,  FIG. 9B  is an enlarged view of the intermediate portion of the thermal shield  40  according to the present embodiment, and  FIG. 9C  is an enlarged view of the lower portion of the thermal shield  40  according to the present embodiment. The same reference numerals are given to the same or similar configurations as those in the fourth embodiment, and a duplicate description thereof is omitted. 
         [0066]    In the fast reactor  1  according to the present embodiment, as shown in  FIG. 9A , the thermal shield  40  is suspended not from the partition  6  but from the upper portion of the intermediate heat exchanger  15 . 
         [0067]    According to the present embodiment, the configuration in which the thermal shield  40  is suspended from the intermediate heat exchanger  15  prevents a load of the thermal shield  40  from being applied to the partition  6  existing in a high temperature environment, reducing a load on the partition  6 . Further, the thermal shield  40  need not be suspended from the upper end of the partition  6 , so that the length of the partition  6  can be shortened so as to improve manufacturability of the partition  6 . 
         [0068]    Further, both side surfaces of the thermal shield  40  are exposed in this configuration, making it much easier to perform repair and maintenance of the thermal shield. In the case where the length of the partition  6  is shortened as described above, the thermal shield  40  constitutes a part of the flow channel of the primary coolant. 
         [0069]    Further, the configuration of the fifth embodiment in which the thermal shield is fixed to the intermediate heat exchanger may be applied to the thermal shield of the first embodiment. 
       Sixth Embodiment 
       [0070]    A sixth embodiment of the present invention will be described below with reference to  FIGS. 10A ,  10 B and  10 C.  FIGS. 10A ,  10 B and  10 C are vertical cross-sectional views each showing a configuration of the fast reactor according to the sixth embodiment of the present invention.  FIG. 10A  is an enlarged view of the upper portion of the thermal shield  40  according to the present embodiment,  FIG. 10B  is an enlarged view of the intermediate portion of the thermal shield  40  according to the present embodiment, and  FIG. 10C  is an enlarged view of the lower portion of the thermal shield  40  according to the present embodiment. The same reference numerals are given to the same or similar configurations as those in the fourth or fifth embodiments, and a duplicate description thereof is omitted. 
         [0071]    In the fast reactor  1  according to the present embodiment, as shown in  FIG. 10A , the thermal shield  40  is suspended not from the partition  6  but from the upper portion of the intermediate heat exchanger  15  as in the fifth embodiment. Further, in this embodiment, the thermal shield  40  is constituted by the thermal shield plate  40   b  having a multilayer structure comprising. e.g., three layers in the radial direction of the fast reactor  1 . An opening portion  40   q  that opens downward is formed in the vicinity of the lower end portion  40   h.    
         [0072]    The inside of the thermal shield  40  is filled with the inactive gas  40   c.  For example, the inside of the reactor vessel  7  is drawn vacuum and displaced with the inactive gas  40   c  before filling with the primary coolant  21 , followed by the filling of the reactor vessel  7  with the primary coolant  21 . Thus, the inside of the thermal shield  40  is filled with the inactive gas  40   c.  At this time, a coolant liquid level  40   n  is formed in the opening portion  40   q.  Although the coolant liquid level  40   n  varies depending on the differential pressure between the inactive gas  40   c  and the primary coolant  21  at the lower end portion  40   h  generated in accordance with the operating condition of the fast reactor  1 , the coolant liquid level  40   n  is positioned almost in the vicinity of the lower end portion  40   h.  The pad  40   p  is attached to each of the thermal shield plates  40   b  as shown in  FIG. 10B  and is configured to be able to be slid in accordance with a heat expansion difference between the upper and lower portions of the pad  40   p  which is caused by a temperature difference between inner and outer peripheral sides. 
         [0073]    According to the present embodiment, by constituting the thermal shield  40  by the thermal shield plate  40   b  having a multilayer structure, it is possible to form a plurality of separate gas spaces. Thus, even if any of the thermal shield plates  40   b  is damaged, the inactive gas  40   c  filled in the inside of the thermal shield  40  is not discharged into the reactor vessel  7  at a time. This ensures multiple safety to improve reliability of the thermal shield  40 . Further, the structure can be made simple, thereby improving manufacturability and reducing cost. 
         [0074]    Although the thermal shield  40  is suspended from the upper portion of the intermediate heat exchanger  15  in the configuration shown in  FIGS. 10A ,  10 B and  10 C as in the cases of the fifth embodiment, it is possible to adopt the configuration of the fourth embodiment in which the thermal shield  40  is suspended from the upper portion of the partition  6 . Further, as shown in  FIGS. 7C and 9C , it is possible to set the thermal shield plate  40   b  having a multilayer structure in the thermal shield  40  even in the thermal shield  40  in which the inactive gas  40   c  is encapsulated in the internal space of the bellows  40   d.  In this case, even if the bellows  40   d  have been broken, the coolant liquid level  40   n  is formed in the lower end portion  40   h  of the thermal shield  40  and performs the same thermal insulation function as in the present embodiment. 
         [0075]    As described above, according to the present embodiment, it is possible to improve reliability and manufacturability of the thermal shield  40 . 
         [0076]    The configuration in which the heat expansion absorbing means of the sixth embodiment is made opened may be applied to the thermal shields according to the first to fourth embodiments. 
         [0077]    Although the embodiments of the present invention have been described with reference to the accompanying drawings, it should be understood that the present invention is not limited to the above representative examples, but various modifications may be adopted, for example, by combining the first to sixth embodiments without departing from the scope of the present invention. Thus, various modifications and changes may be made to the concrete embodiments by those skilled in the art without departing from the technical concept and technical scope of the invention.