Abstract:
A fast reactor performing reflector control to control reactivity of the core by moving a neutron reflector in the vertical direction, including: a core fuel assembly; a neutron absorption assembly in the middle of the core fuel assembly; a reflector assembly at the circumference of the core fuel assembly; plural inner neutron shields at the circumference of the reflector assembly; a cylindrical core barrel surrounding entirety of the plural neutron shields; and a drive mechanism controlling the reflector. The reflector assembly includes: a reflector element that reflects neutrons from the core fuel assembly towards the core; a cavity section, arranged thereabove, that permits leakage of neutrons to outside the core; a linkage mechanism that links the reflector element and the cavity section; a guide tube that defines a space for removal/insertion of these; and a connecting section that connects the drive mechanism and the cavity section.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims benefit of priority from Japanese Application No. JP2012-90813 filed Apr. 12, 2012, the entire contents of which are incorporated by reference herein. 
       FIELD  
       [0002]    Embodiments described herein relate generally to a reflector-controlled type fast reactor, and to a reflector assembly used therein. 
       BACKGROUND  
       [0003]    In a fast reactor in which liquid metal is employed as the coolant, the reactor vessel has the important function of containing the coolant in the event of an accident; consequently, the reactivity of the core is controlled by insertion or removal of control rods into the core from above. 
         [0004]    The drive device for the control rods is therefore positioned above the core: there is therefore the problem that this necessitates the provision of a core top mechanism or the like above the core, which makes the construction of the reactor more complicated and tends to increase its weight and cost. 
         [0005]    Accordingly, in for example Japanese Laid-open Patent Application Number Tokkai 2005-233751 (hereinafter referred to as Patent Reference 1), reactor technology of the reflector-controlled type has been disclosed, in which the reactivity of the core is controlled by adjustment of neutron leakage from the core by moving a neutron reflector provided outside the core in the vertical direction. 
         [0006]      FIG. 1  is a horizontal cross-sectional view showing the reactor peripheral layout in a reactor vessel according to an example of a prior art fast reactor. Also,  FIG. 2  is a vertical cross-sectional view showing the reactor peripheral layout in a reactor vessel according to an example of a prior art fast reactor. 
         [0007]    In the fast reactor of this example, a single neutron absorption assembly  12  is arranged in the middle and 18 core fuel assemblies  11  are arranged at the periphery, being provided in two layers in the radial direction. A circular core barrel  16  is arranged outside these core fuel assemblies  11  so as to surround these in the radial direction, and a reflector movement zone  100  constituted by an annular space of the neutron reflector is provided between the outside of the core barrel  16  and a shroud  17 , and a plurality of sector-shaped reflector devices or fan-shaped reflector devices, not shown, are arranged therein. The reflector devices are moved in the perpendicular direction through the interior of the reflector movement zone  100  and are provided with reflector sections  110  and a cavity section  120  at the top thereof. A neutron shield  15  is provided outside this reflector movement zone  100 . 
         [0008]    The weight of the core fuel assemblies  11  constituting the core is supported from below by a core support plate, not shown. Furthermore, if the fuel assemblies  11  are subjected to load in the horizontal direction for example by an earthquake or the like, the load acting on the top of these fuel assemblies  11  is transmitted to the core barrel  16  and is thence transmitted to the reactor vessel  1  through a load transmission path such as a linkage construction at the top of the reflector movement zone  100  outside the core barrel  16 . 
         [0009]    With such a core construction, with increase in the life of the power plant, the amount of neutron irradiation is increased, so a ferrite material that can withstand high levels of irradiation is employed as the material of the core barrel  16 . However, if further prolongation of the life of the power plant is envisioned, it is thought that replacement of the reactor barrel may be necessitated by irradiation-induced degradation of the barrel material. 
         [0010]    For example, albeit a fast reactor has the advantage that fuel replacement during the reactor life is unnecessary or need only be performed a very small number of times, reducing the risk of diffusion of the nuclear material or raising the nonproliferation of the nuclear material, such irradiation-induced degradation of the barrel material has the effect of reducing this advantage, because it makes it necessary to open up the reactor vessel in order to replace the core barrel. Also, it is necessary to adopt a construction of the reactor vessel interior and its periphery such as will enable reactor barrel replacement during the period of operation of the power plant: this results in increased costs. 
         [0011]    According to an aspect of the present technology, an object of the present invention is therefore to provide a reflector construction whereby replacement of the core barrel due to irradiation-induced degradation is unnecessary, even in cases where the life of the power plant is further moved. 
         [0012]    In order to achieve the above object, the present invention is constructed as follows. Specifically,
       a fast reactor in which reflector control is performed to control the core reactivity, by adjusting neutron leakage from the core by moving in the vertical direction a neutron reflector that is arranged radially outside the core, which is bathed in liquid metal coolant, comprising:   a plurality of core fuel assemblies that move in mutually parallel fashion in the vertical direction;   a neutron absorption assembly that moves in the vertical direction, provided in the middle in the horizontal direction of said plurality of core fuel assemblies;   a plurality of reflector assemblies moving in the vertical direction, arranged in the horizontal peripheral region of said core fuel assemblies;   a plurality of neutron shields arranged in the horizontal peripheral region of said core fuel assemblies;   a cylindrical core barrel provided surrounding the entirety of said plurality of neutron shields in the horizontal direction; and   a drive mechanism for said reflector control,   wherein said reflector assemblies comprise:
           a reflector element that reflects neutrons flowing out from said core fuel assembly towards the core;   a hollow cavity section arranged perpendicularly above said reflector element for permitting leakage of the neutrons flowing out from said core fuel assembly towards outside the core;   a linkage mechanism that links said reflector element and said cavity section;   a guide tube defining a space for insertion/removal of said reflector element, said cavity section and said linkage mechanism; and   a connecting section that connects said drive mechanism and said cavity section,   
           wherein said reflector element and said cavity section are moved vertically through the interior of said guide tube by said drive mechanism.       
 
         [0027]    Also, further according to the present invention, the following construction is provided. Specifically,
       a reflector-controlled type fast reactor reflector assembly, in which the core reactivity is controlled by adjusting neutron leakage from the core by moving in the vertical direction a neutron reflector that is arranged radially outside the core, which is bathed in liquid metal coolant, comprising:   a reflector element that reflects neutrons flowing out from said core fuel assembly towards the core; a hollow cavity section arranged perpendicularly above said reflector element for permitting leakage of the neutrons flowing out from said core fuel assembly towards outside the core;   a linkage mechanism that links said reflector element and said cavity section;   a guide tube defining a space for insertion/removal of said reflector element, said cavity section and said linkage mechanism; and   a connecting section that connects said drive mechanism and said cavity section,   wherein said reflector element and said cavity section are moved vertically through the interior of said guide tube by said drive mechanism.       
 
         [0034]    According to the present invention, replacement of the core barrel due to high irradiation-induced degradation is unnecessary, even in cases where the life of the power plant is further moved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0035]      FIG. 1  is a horizontal cross-sectional view showing the construction of the core periphery in a reactor vessel according to an example of a conventional fast reactor; 
           [0036]      FIG. 2  is a vertical cross-sectional view showing the construction of the core periphery in a reactor vessel according to an example of a conventional fast reactor; 
           [0037]      FIG. 3  is a horizontal cross-sectional view showing the construction of the core periphery in a reactor vessel according to a first embodiment of a fast reactor according to the present invention; 
           [0038]      FIG. 4  is a vertical cross-sectional view showing the construction of the core periphery in a reactor vessel according to a first embodiment of a fast reactor according to the present invention; 
           [0039]      FIG. 5  is a bird&#39;s eye view showing a reflector assembly constituting a first embodiment of a fast reactor according to the present invention; 
           [0040]      FIG. 6  is a bird&#39;s eye view showing a reflector assembly constituting a second embodiment of a fast reactor according to the present invention; 
           [0041]      FIG. 7  is a vertical cross-sectional view showing a reflector assembly constituting a third embodiment of a fast reactor according to the present invention; 
           [0042]      FIG. 8  is a bird&#39;s eye view showing a reflector assembly constituting a fourth embodiment of a fast reactor according to the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0043]    Embodiments of a fast reactor according to the present invention and a reflector assembly for a fast reactor are described below with reference to the drawings. Identical or similar portions are given the same reference symbols, to avoid repeated description. 
       First Embodiment  
       [0044]      FIG. 3  is a horizontal cross-sectional view showing the construction of the core periphery in a reactor vessel according to a first embodiment of a fast reactor according to the present invention. 
         [0045]      FIG. 4  is a vertical cross-sectional view showing the construction of the core periphery in a reactor vessel according to a first embodiment of a fast reactor according to the present invention; 
         [0046]    The present invention relates to a fast reactor that is cooled by liquid metal and in which reflector control is performed to control the reactivity of the core, by adjusting leakage of neutrons from the core, by moving a neutron reflector arranged radially outside the core in the vertical direction. 
         [0047]    The core constituent elements that are arranged around the core of a reactor  10  in a reactor vessel  1 , as shown, comprise: core fuel assemblies  11 , neutron absorption assemblies  12 , reflector assemblies  20 , and neutron shields  15  comprising inner neutron shields  15   a  and outer neutron shields  15   b;  these are arranged in mutually parallel fashion moving vertically in the perpendicular direction. 
         [0048]    27 core fuel assemblies  11  are provided. In the middle of the core fuel assemblies  11 , there is provided a single neutron absorption assembly  12  and three neutron absorption assemblies  12  are provided outside the core fuel assemblies  11 . These neutron absorption assemblies  12  are inserted when the reactor  10  is to be shut down. 
         [0049]    The region radially outside the core fuel assemblies  11  and neutron absorption assemblies  12  is surrounded by reflector assemblies  20 . There are provided  60  reflector assemblies  20 , arranged in two layers in the radial direction. 
         [0050]    The outside in the radial direction of the reflector assemblies  20  is surrounded by inner neutron shields  15   a.  There are 120 inner neutron shields  15   a,  arranged in two layers in the radial direction. 
         [0051]    A cylindrical core barrel  16  is provided surrounding in the horizontal direction the entirety of the inner neutron shields  15   a,  on the radial outside of the inner neutron shields  15   a.    
         [0052]    The core constituent elements inside the core barrel  16 , specifically, the core fuel assemblies  11 , neutron absorption assemblies  12 , reflector assemblies  20  and inner neutron shields  15   a  are all of the same external hexagonal shape, with their adjacent faces opposite each other and these opposite faces being mutually parallel. 
         [0053]    A plurality of outer neutron shields  15   b  are provided outside the core barrel  16 , forming two layers in the radial direction. 
         [0054]    The outside of the outer neutron shields  15   b  is surrounded by the reactor vessel  1 . 
         [0055]      FIG. 5  is a bird&#39;s eye view showing a reflector assembly constituting an embodiment of a fast reactor according to the present invention. 
         [0056]    A reflector assembly is provided with a guide tube  26  made of stainless steel at its radially outermost section. The external shape of the guide tube  26  in horizontal section is a regular hexagon. However, its bottom portion is of cylindrical shape of smaller diameter, for the insertion of a core support plate, not shown, and is provided with a plurality of orifices  28  for inflow of coolant such as for example liquid metal such as liquid metallic sodium. 
         [0057]    The material of the guide tube  26  is not restricted to stainless steel but should be a material of small neutron absorption cross-section, such as aluminum or zirconium. 
         [0058]    Also, convex-shaped pads  27  are provided at the entire circumference of the outside of the guide tube  26  in the radially outwards direction at the same height in the perpendicular direction. The pads  27  are provided at a plurality of heights in the perpendicular direction. 
         [0059]    A reflector element  21  and a cavity section  23  are accommodated in the interior of the guide tube  26 . The reflector element  21  is suspended through a plurality of connecting rods  25  by means of a suspension disc  41 . A cavity section  23  and a spring  24  at the top thereof are provided between the reflector element  21  and the suspension disc  41 , so that the cavity section  23  is pressed towards the reflector element  21  by the spring  24 . 
         [0060]    The plurality of connecting rods  25  are subjected to reaction when the spring  24  presses the reflector element  21 , and restrict sideways movement thereof in the radial direction of the cavity section  23 . 
         [0061]    The suspension disc  41  is connected with a drive mechanism  60  through a connecting section  29  and is moved by the drive mechanism  60  in the perpendicular direction through the space within the guide tube  29  integrally with the suspension disc  41 , connecting rods  25 , reflector element  21 , spring  24  and cavity section  23 . 
         [0062]    The elements that move through the interior of the guide tube  26  are of circular external shape in horizontal cross-section, and thus cannot interfere with the inside of the guide tube  26  with regard to the direction of rotation and so do not need to be fixed in the direction of rotation in order to avoid buffering and hence can be simplified in construction. 
         [0063]    The reflector element  21  reflects neutrons from the core fuel assemblies  11  towards the core and is of a laminated construction in which discs of material, such as for example stainless steel, having a reflective effect for neutrons are laminated in unitary fashion. Also, the radial periphery of the laminated structure of the reflector elements  21  is covered with a covering, not shown. 
         [0064]    The cavity section  23  is a vessel that defines a space to allow leakage of neutrons from the core fuel assemblies  11  directly, without reflection, to outside the core, and has sealed in its interior an inert gas such as for example argon. 
         [0065]    With this embodiment constructed as described above, the core barrel  16  is provided outside the region where the inner neutron shields  15   a  are arranged, whereas, in the prior art example, the core barrel  16  is arranged immediately outside the region of the core fuel assemblies  11 : thus there is a considerable difference in regard to the positional relationship thereof with respect to the core fuel assemblies  11 . 
         [0066]    Specifically, in this embodiment, the reflector assemblies  20  and inner neutron shields  15   a  are interposed between the core fuel assemblies  11  and the core barrel  16 , so the neutron irradiation flux received by the core barrel  16  is greatly reduced. 
         [0067]    Consequently, in this embodiment, replacement of the core barrel caused by the high level of irradiation can be rendered unnecessary, even when the power plant life is further moved. 
       Second Embodiment  
       [0068]      FIG. 6  is a bird&#39;s eye view showing a reflector assembly constituting a second embodiment of a fast reactor according to the present invention. 
         [0069]    This embodiment is a modification of the first embodiment. Whereas, in the case of the first embodiment, the cross-sectional shape of the guide tube  26  is externally a regular hexagonal shape, the guide tube of this embodiment is a guide tube  31  furnished with apertures having apertures  31   a  such as to permit passage of neutrons at each side face thereof. 
         [0070]    The apertures  31   a  consist in apertures  31   a  constituting the major portion of the side face, excluding a sufficient portion for the pads  27  and edges of the regular hexagonal prism that is necessary in order to guarantee structural strength of the guide tube  31  furnished with apertures. 
         [0071]    With the reflector assemblies  20  according to the present embodiment, owing to the provision of the apertures  31   a  at the side faces of the guide tube  31  furnished with apertures, neutrons from the core fuel assemblies  11  arrive directly at the reflector elements  21  by passing through the apertures  31   a:  reflection efficiency is thereby improved. Control of the reactivity by the reflector assemblies  20  is thereby made more reliable. 
         [0072]    With the construction of the present embodiment as described above, even in cases where the power plant life is further moved, replacement of the core barrel caused by high irradiation levels can be made unnecessary and more reliable reflector control can be achieved. 
       Third Embodiment  
       [0073]      FIG. 7  is a vertical cross-sectional view showing a reflector assembly constituting a third embodiment of a fast reactor according to the present invention. 
         [0074]    In this embodiment, the linkage of the reflector elements  21  and the cavity sections  23  is different from that of the first embodiment etc. Specifically, in this embodiment, the reflector element  21  and the cavity section  23  are fitted together in telescopic fashion, and this fitting together is secured by means of bolts  44 . 
         [0075]    As shown in  FIG. 7 , the reflector element  21  has a convex section  42  at the top thereof. Also, the cavity section  23  has a concave section  43  at the bottom thereof. The convex section  42  of the reflector element  21  and the concave section  43  of the cavity section  23  are mutually fitted together. 
         [0076]    At the height of this fitting-together section, bolt holes are formed from the side face of the portion of the cavity section  23  provided with the concave section  43  towards the center, passing through as far as part of the convex section  42  of the reflector element  21 , so that the fitting-together is secured by the bolts  44 . 
         [0077]    The top of the cavity section  23  is directly connected with the suspension disc  41 . 
         [0078]    With this construction of the present invention, the cavity section  23  is reliably fixed to the reflector element  21 , so there is no possibility of minute displacements of the cavity section  23  being caused by for example mechanical vibration or fluid oscillation etc., and precise control can thus be achieved. 
         [0079]    Also, since the suspension disc  41 , cavity section  23  and reflector element  21  are reliably coupled, there is no need for connecting rods  25 , so the radius of the cavity section  23  can be increased, further simplifying the construction and reducing causes of failure. Increasing the radius of the cavity section  23  increases the neutron leakage rate. 
         [0080]    Reflection of neutrons from portions outside the range in which the reactivity is controlled by the reflectors reduces the efficiency of reactivity control by the reflectors, so increasing the volume of the cavity section  23  can improve the effectiveness of control of the reactor rate. Also, in cases where the reactivity must be reduced, a large volume of the cavity section  23  increases the neutron leakage effect, so the lowering of the reactivity can be made more positive, improving stability. 
         [0081]    With the construction of the present embodiment as described above, even in cases where the power plant life is further moved, replacement of the core barrel caused by high irradiation levels can be made unnecessary and even more effective reflector control with excellent precision can be achieved by a simplified construction. 
       Fourth Embodiment  
       [0082]      FIG. 8  is a bird&#39;s eye view showing the elements in a guide tube of a reflector assembly constituting a fourth embodiment of a fast reactor according to the present invention. In the Figure, the guide tube  26  is not shown, only the elements within the tube being shown. 
         [0083]    The present embodiment is a modification of the first embodiment. Whereas, in the case of the first embodiment, the cross-sectional shape of the reflector element  21  and the cavity section  23  is circular, in contrast, a hexagonal reflector assembly  50  according to this embodiment is provided with a hexagonal reflector element  51  whose horizontal cross-section is of regular hexagonal shape, a hexagonal cavity section  53  whose horizontal cross-section is of regular hexagonal shape, and a hexagonal suspension plate  54  within a guide tube  26 , which is of regular hexagonal shape, just as in the first embodiment. The circumference of the hexagonal reflector element  51  is covered by a hexagonal cover member  52 . 
         [0084]    In this way, by combining a guide tube  26  whose external shape is a regular hexagon in horizontal cross-section with a structure whose external shape is a regular hexagon disposed in the interior thereof, the gap between the guide tube  26  and the reflector element  21  can be minimized. Consequently, in conditions in which reflective function by the reflector elements  21  is required, the ratio of leakage of neutrons from the gap between the reflector element  21  and the guide tube  26  to outside the core can be minimized and the effectiveness of control of reactivity in the reactor assemblies  20  can thus be raised. 
         [0085]    Also, by making the external shape of the reflector elements  21  a regular hexagonal shape, the reflector elements  21  can be made large compared with when a cylindrical shape is employed, improving the neutron reflection performance. 
         [0086]    With the construction of the present embodiment as described above, even in cases where the power plant life is further moved, replacement of the core barrel caused by high irradiation levels can be made unnecessary and even more effective reflector control can be achieved. 
       Other Embodiments  
       [0087]    While various embodiments of the present invention have been described above, these embodiments are presented merely by way of example and are not intended to restrict the scope of the invention. 
         [0088]    Also, the characteristic features of various embodiments may be combined. For example, an embodiment could be adopted that is provided with a guide tube  31  furnished with apertures as in the second embodiment and also provided with linkage between the reflector element  21  and the cavity section  23  as in the third embodiment. Also, a hexagonal reflector element  51 , hexagonal cavity section  53  and hexagonal suspension plate  54  that move through the interior of the guide tube  26  as in the fourth embodiment could be employed in these embodiments. 
         [0089]    In addition, these embodiments could be put into practice in various other modified forms and various deletions, substitutions or alterations could be made without departing from the gist of the invention. 
         [0090]    Just as these embodiments and modifications thereof are included in the scope and gist of the invention, they are included in the invention as set out in the patent claims, and equivalents thereof. 
       EXPLANATION OF THE REFERENCE SYMBOLS  
       [0091]      1  . . . reactor vessel 
         [0092]      2  . . . guard vessel 
         [0093]      10  . . . reactor 
         [0094]      11  . . . core fuel assembly 
         [0095]      12  . . . neutron absorption assembly 
         [0096]      13  . . . core fuel region 
         [0097]      14  . . . gas plenum region 
         [0098]      15  . . . neutron shield 
         [0099]      15   a  . . . inner neutron shield 
         [0100]      15   b  . . . outer neutron shield 
         [0101]      16  . . . core barrel 
         [0102]      17  . . . shroud 
         [0103]      20  . . . reflector assembly 
         [0104]      21  . . . reflector element 
         [0105]      23  . . . cavity section 
         [0106]      24  . . . spring 
         [0107]      25  . . . connecting rod 
         [0108]      26  . . . guide tube 
         [0109]      27  . . . pad 
         [0110]      28  . . . orifice 
         [0111]      29  . . . connecting section 
         [0112]      31  . . . guide tube furnished with apertures 
         [0113]      31   a  . . . apertures 
         [0114]      41  . . . suspension disc 
         [0115]      42  . . . convex section 
         [0116]      43  . . . concave section 
         [0117]      44  . . . bolt 
         [0118]      50  . . . hexagonal reflector assembly 
         [0119]      51  . . . hexagonal reflector element 
         [0120]      52  . . . hexagonal covering member 
         [0121]      53  . . . hexagonal cavity section 
         [0122]      54  . . . hexagonal suspension plate 
         [0123]      60  . . . drive mechanism 
         [0124]      100  . . . reflector movement region 
         [0125]      110  . . . reflector section 
         [0126]      120  . . . cavity section