Abstract:
A spacer grid can be applied to close-spaced nuclear fuel rods. The spacer grid is directed to solve the problem in which, as the outer diameter of each nuclear fuel rod increases due to the use of dual-cooled nuclear fuel rods for improving cooling performance and obtaining high combustion and high output power, the gap between the neighboring nuclear fuel rods is narrowed to thus make it impossible to use an existing spacer grid. The spacer grid is a combination of unit grid straps, each of which has supports for supporting each of the nuclear fuel rods set in a narrow array and has a sheet shape, which are combined with each other. The supports are located at positions shifted from the longitudinal central line of each unit grid strap toward sub-channels.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a spacer grid for supporting nuclear fuel rods of a nuclear fuel assembly charged into a nuclear reactor and, more particularly, to a spacer grid for more close-spaced nuclear fuel rods than conventional ones, in which the supports of each grid strap are located at positions shifted from a central line of each grid strap toward a center of a sub-channel in order to accommodate a reduced gap between the nuclear fuel rods due to the use of dual-cooled nuclear fuel rods, which have excellent cooling performance. 
         [0003]    2. Description of the Prior Art 
         [0004]      FIG. 1  is a perspective view schematically illustrating a conventional nuclear fuel assembly.  FIG. 2  is a cross-sectional view schematically illustrating a conventional nuclear fuel assembly.  FIG. 3  is a top plane view schematically illustrating a spacer grid applied to a conventional nuclear fuel assembly.  FIG. 4  is a perspective view schematically illustrating a spacer grid applied to a conventional nuclear fuel assembly.  FIG. 5  is a perspective view schematically illustrating a unit grid strap for a spacer grid supporting a conventional nuclear fuel assembly. 
         [0005]    As illustrated in the figures, the conventional nuclear fuel assembly  100  comprises nuclear fuel rods  110 , guide tubes  140 , spacer grids  150 , a top end piece  120 , and a bottom end piece  130 . 
         [0006]    Here, each nuclear fuel rod  110  has a cylindrical uranium sintered compact in a clad pipe of zircaloy (zirconium alloy). This uranium sintered compact is fissioned to generate high temperature heat. 
         [0007]    Meanwhile, each guide tube  140  is used as a passage for a control rod, which moves up and down in order to control the output power of a reactor core and to stop the fission reaction. The spacer grid  150  is one of the components constituting the nuclear fuel assembly in a nuclear reactor, and includes a plurality of unit grid straps, each of which has a spring  118  and dimples  119  and functions to support and protect the nuclear fuel rods  110  so that they are arranged at designated positions. When the spring force of the spring  118  and the dimples  119  is too weak, each nuclear fuel rod  110  cannot be arranged at a designated position, and thus has a possibility of losing sound supporting performance. In contrast, when the spring force of the spring  118  and the dimples  119  is too strong, each nuclear fuel rod  110  undergoes defects such as scratching on the surface of the clad tube due to excessive frictional gripping force when it is inserted into the spacer grid. Further, during the operation of the nuclear reactor, the nuclear fuel rods  110  experience longitudinal growth by means of the irradiation of neutrons. This longitudinal growth is not properly accommodated, and thus the nuclear fuel rods  110  are bent. In this manner, when the nuclear fuel rods  110  are bent, they come nearer to or contact neighboring nuclear fuel rods  110 . Thus, the coolant channel between the nuclear fuel rods becomes narrow or is blocked. As a result, the heat generated from the nuclear fuel rods is not effectively transmitted to the coolant, thereby increasing the temperature of the nuclear fuel rods. As such, the possibility of generating Departure from Nucleate Boiling (DNB) is increased, which is mainly responsible for the reduction of nuclear fuel output power. 
         [0008]    The top end piece  120  and the bottom end piece  130  fixedly support the nuclear fuel assembly  100  on upper and lower structures of the reactor core. The bottom end piece  130  includes a screening device (not shown) for filtering foreign materials floating in the reactor core. 
         [0009]    Meanwhile, each spacer grid  150  is usually made of zircaloy, and includes nuclear fuel rod cells, which support the nuclear fuel rods  110 , and guide tube cells, into which the guide tubes  140  are inserted. Each nuclear fuel rod cell is designed to support each nuclear fuel rod  110  at a total of six supporting points using a total of two grid springs  118 , which are located on two respective faces, and a total of four dimples  119 , which are located in pairs above and below the two grid springs  118  and on the other two respective faces. 
         [0010]    A cylindrical uranium dioxide compact is inserted into each nuclear fuel rod  110 , and the coolant rapidly flows from the bottom to the top of the reactor core in an axial direction through sub-channels  115 , each of which is enclosed by four nuclear fuel rods  110  or by three nuclear fuel rods  110  and one guide tube  140 . 
         [0011]    Here, each sub-channel  115  refers to a space enclosed by the nuclear fuel rods  110 , and particularly a passage through which a fluid can freely flow to the neighboring sub-channel because it has an open side. 
         [0012]    Meanwhile, as illustrated in  FIGS. 6 and 7 , a dual-cooled nuclear fuel rod  10  having an annular structure instead of the cylindrical nuclear fuel rod  110  is disclosed in U.S. Pat. Nos. 3,928,132 and 6,909,765. 
         [0013]    Here, the dual-cooled nuclear fuel rod  10  having an annular structure includes a sintered compact  11  having an annular shape, an inner clad tube  12  enclosing the inner circumference of the sintered compact  11 , and an outer clad tube  13  enclosing the outer circumference of the sintered compact  11 . Thus, the coolant flows outside and inside the dual-cooled nuclear fuel rod  10 , so that heat transfer is doubled. As a result, the dual-cooled nuclear fuel rod  10  can maintain a low fuel&#39;s centerline temperature, and provide high combustion and high output power. 
         [0014]    In this manner, in the case in which the centerline temperature of the dual-cooled nuclear fuel rod  10  is maintained low, the possibility of damaging the fuel due to an increase in the core temperature of the nuclear fuel is lowered, so that the safety allowance of the dual-cooled nuclear fuel rod  10  can be increased. 
         [0015]    However, in order to make the dual-cooled nuclear fuel rods  10  structurally compatible with an existing pressurized water reactor (PWR) core, the gap between the nuclear fuel rods becomes considerably narrower compared to that between existing nuclear fuel rods because the positions of the guide tubes  140  cannot be changed in the nuclear fuel assembly  100 , and because the outer diameter of each nuclear fuel rod is increased. For example, in the case in which the nuclear fuel assembly is formed according to a candidate design draft for the dual-cooled nuclear fuel rods having a 12×12 array, the gap between the nuclear fuel rods is reduced from 3.35 mm, which is the size of the existing gap, to about 1.24 mm. 
         [0016]    Thus, due to the narrow gap between the nuclear fuel rods, the spacer grid that has been developed to date cannot be used as that for the dual-cooled nuclear fuel rods  10 . 
         [0017]    In other words, after the thickness of the unit grid strap of the existing spacer grid, which is 0.475 mm, is subtracted from the gap of 1.24 mm between the nuclear fuel rods, the obtained result is again divided by two. Consequently, the gap between the unit grid strap and the nuclear fuel rod is no more than about 0.383 mm. Thus, it is impossible to apply such a shape and a supporting position as in an existing leaf spring within this narrow gap to design a spring having spring rigidity and hydraulic characteristic (mainly pressure loss), which an existing supporting structure has. 
       SUMMARY OF THE INVENTION 
       [0018]    Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a spacer grid, capable of sound supporting nuclear fuel rods set in a narrow array, unlike an existing spacer grid, which has a problem in which, as the outer diameter of each nuclear fuel rod increases due to the use of dual-cooled nuclear fuel rods for improving cooling performance and obtaining high combustion and high output power, the gap between neighboring nuclear fuel rods is narrowed to thus make it impossible to use the existing spacer grid. 
         [0019]    In order to achieve the above object, according to one aspect of the present invention, there is provided a spacer grid for close-spaced nuclear fuel rods, in which a plurality of unit grid straps, each of which has supports for supporting each of the nuclear fuel rods set in a narrow array and has a sheet shape, are combined with each other. The supports are located at positions shifted from the longitudinal central line of each unit grid strap toward sub-channels. 
         [0020]    Here, each support may have a cantilever leaf spring shape, a semi-spherical or elliptical shape, or a semi-cylindrical shape in the longitudinal direction of the unit grid strap. 
         [0021]    Further, the supports may be formed at the front and rear of each unit grid strap in symmetry based on the longitudinal central line of each unit grid strap. 
         [0022]    Also, the cantilever leaf spring may include a protrusion, which protrudes from the unit grid strap toward the dual-cooled nuclear fuel rod, and an extension, which extends from the protrusion so as to enclose the outer circumference of the dual-cooled nuclear fuel rod. 
         [0023]    Further, the extension may have the same curvature as the outer circumference of the dual-cooled nuclear fuel rod, and the protrusion may have a radius of curvature smaller than that of the extension. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
           [0025]      FIG. 1  is a perspective view schematically illustrating a conventional nuclear fuel assembly; 
           [0026]      FIG. 2  is a cross-sectional view schematically illustrating a conventional nuclear fuel assembly; 
           [0027]      FIG. 3  is a top plane view schematically illustrating a spacer grid applied to a conventional nuclear fuel assembly; 
           [0028]      FIG. 4  is a perspective view schematically illustrating a spacer grid applied to a conventional nuclear fuel assembly; 
           [0029]      FIG. 5  is a perspective view schematically illustrating a unit grid strap for a spacer grid supporting a conventional nuclear fuel assembly; 
           [0030]      FIG. 6  is a top plane view schematically illustrating a dual-cooled nuclear fuel rod applied to the present invention; 
           [0031]      FIG. 7  is a cross-sectional view schematically illustrating a nuclear fuel assembly for dual-cooled nuclear fuel rods applied to the present invention; 
           [0032]      FIG. 8  is a top plane view illustrating a spacer grid having a cantilever leaf spring support according to the present invention; 
           [0033]      FIG. 9  is a perspective view illustrating a spacer grid having a cantilever leaf spring support according to the present invention; 
           [0034]      FIG. 10  is a perspective view illustrating a unit grid strap having a cantilever leaf spring support according to the present invention; 
           [0035]      FIG. 11  is a top plane view illustrating a spacer grid having semi-spherical supports according to the present invention; 
           [0036]      FIG. 12  is a perspective view illustrating a spacer grid having semi-spherical supports according to the present invention; 
           [0037]      FIG. 13  is a perspective view illustrating a unit grid strap having semi-spherical supports according to the present invention; 
           [0038]      FIG. 14  is a top plan view illustrating a spacer grid having semi-cylindrical supports according to the present invention; 
           [0039]      FIG. 15  is a perspective view illustrating a spacer grid having semi-cylindrical supports according to the present invention; and 
           [0040]      FIG. 16  is a perspective view illustrating a unit grid strap having semi-cylindrical supports according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    Reference will now be made in greater detail to exemplary embodiments of the invention, which are illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
         [0042]    According to an exemplary embodiment of the present invention, a spacer grid  20  supports dual-cooled nuclear fuel rods  10  having a 3×3 array, which is taken as an example. 
         [0043]      FIGS. 8 through 10  illustrate a spacer grid according to a first embodiment of the present invention.  FIG. 8  is a top plan view illustrating a spacer grid having a cantilever leaf spring support,  FIG. 9  is a perspective view illustrating a spacer grid having a cantilever leaf spring support, and  FIG. 10  is a perspective view illustrating a unit grid strap having a cantilever leaf spring support. 
         [0044]    The spacer grid  20  of the present invention is a combination of unit grid straps  21 , each of which has supports  23  for supporting a dual-cooled nuclear fuel rod  10  by coming into contact with the outer circumference of the dual-cooled nuclear fuel rod  10 , and has a sheet shape. The dual-cooled nuclear fuel rod  10  includes a sintered compact  11  having an annular shape, an inner clad tube  12  enclosing the inner circumference of the sintered compact  11 , and an outer clad tube  13  enclosing the outer circumference of the sintered compact  11 . 
         [0045]    In particular, each support  23  is located at a position shifted from the longitudinal central line  22  of each unit grid strap  21  toward a sub-channel  115  so as to support the outer circumference of the dual-cooled nuclear fuel rod  10 . 
         [0046]    Here, the sub-channel  115  refers to a space enclosed by four dual-cooled nuclear fuel rods  10  or by three dual-cooled nuclear fuel rods  10  and one guide tube  140 . Coolant flows in the axial direction through the sub-channel  115 . 
         [0047]    Preferably, each support  23  has the shape of a cantilever leaf spring  25 , and has a predetermined radius of curvature along the outer circumference of the dual-cooled nuclear fuel rod  10  so as to be able to come into surface contact with the dual-cooled nuclear fuel rod  10 . 
         [0048]    To this end, each support  23  includes a protrusion  26 , which protrudes from the unit grid strap  21  toward the dual-cooled nuclear fuel rod  10 , and an extension  27 , which extends from the protrusion  26  so as to enclose the outer circumference of the dual-cooled nuclear fuel rod  10 . 
         [0049]    In other words, the protrusion  26  and the extension  27  are integrally formed with each other by cutting the unit grid strap  21  in a “C” shape at a position shifted from the longitudinal central line  22  of each unit grid strap  21  toward the center of the sub-channel  115 , and by bending the cut piece. 
         [0050]    At this time, the extension  27  is preferably formed so as to have the same radius of curvature as the outer circumference of the dual-cooled nuclear fuel rod  10  for surface contact with the dual-cooled nuclear fuel rod  10 . It is advantageous in view of the rigidity of the spring that the protrusion  26  be formed so as to have an arcuate shape having a predetermined radius of curvature smaller than that of the extension  27 . 
         [0051]    Further, the supports  23  are formed at the front and rear of each unit grid strap  21 , respectively. The front support  23  and the rear support  23  are in axial symmetry based on the longitudinal central line  22  of each unit grid strap  21 . In detail, the front support  23  is adapted to support the dual-cooled nuclear fuel rod  10 , which is located at the front of each unit grid strap  21 , and the rear support  23  is adapted to support the dual-cooled nuclear fuel rod  10 , which is located at the rear of each unit grid strap  21 . Thus, the dual-cooled nuclear fuel rod  10  is supported at a total of four positions by the four unit grid straps  21  enclosing one dual-cooled nuclear fuel rod  10 . 
         [0052]    The supports  23  are formed at the front and rear of each unit grid strap  21  so as to have axial symmetry based on the longitudinal central line  22  of each unit grid strap  21 , so that they can avoid structural interference or loss of the function as a spring. When formed by a pressing process, the supports  23  minimize disorientation (or flexure) caused by residual strain and residual stress, and are complementarily deformed. 
         [0053]    Further, the supports  23 , having the shape of the cantilever leaf spring  25  formed at the front and rear of each unit grid strap  21 , are preferably spaced apart from each other at a predetermined interval so as to be able to maintain proper rigidity when functioning as the spring supporting the dual-cooled nuclear fuel rod  10 . 
         [0054]      FIGS. 11 through 13  illustrate a spacer grid according to a second embodiment of the present invention.  FIG. 11  is a top plan view illustrating a spacer grid having semi-spherical shape supports,  FIG. 12  is a perspective view illustrating a spacer grid having semi-spherical supports, and  FIG. 13  is a perspective view illustrating a unit grid strap having semi-spherical supports. 
         [0055]    The shape of each dual-cooled nuclear fuel rod  10  and the position of each support  30  are the same as in the first embodiment of the present invention. However, the shape of each support  30  is different from that of the first embodiment of the present invention. Thus, only the shape of each support  30  will be described in detail. 
         [0056]    The spacer grid  20  according to a second embodiment of the present invention includes at least one semi-spherical support  30  at a position shifted from the longitudinal central line  22  of each unit grid strap  21  toward the center of each sub-channel  115 . The semi-spherical supports  30  are formed in a semi-spherical or elliptical shape, and protrude from the front and rear of each unit grid strap  21 . 
         [0057]    At this time, preferably, among the semi-spherical supports  30  having the semi-spherical or elliptical shape, two are formed at the front of the unit grid strap  21  in a vertical direction, and one is formed at the rear of the unit grid strap  21 . 
         [0058]    Alternatively, the semi-spherical supports  30  protruding from the front and rear of the unit grid strap  21  may be formed such that one thereof is located at each of the front and rear of the unit grid strap  21  so as to be in axial symmetry based on the longitudinal central line  22  of the unit grid strap  21 . Thereby, when formed by a pressing process, the semi-spherical supports  30  minimize disorientation caused by residual strain and residual stress, and are complementarily deformed. 
         [0059]    Thus, the dual-cooled nuclear fuel rod  10  can be supported at a total of four positions by the semi-spherical supports  30 , which are formed on the four unit grid straps  21  enclosing one dual-cooled nuclear fuel rod  10 . 
         [0060]      FIGS. 14 through 16  illustrate a spacer grid according to a third embodiment of the present invention.  FIG. 14  is a top plan view illustrating a spacer grid having semi-cylindrical supports,  FIG. 15  is a perspective view illustrating a spacer grid having semi-cylindrical supports, and  FIG. 16  is a perspective view illustrating a unit grid strap having semi-cylindrical supports. 
         [0061]    The shape of each dual-cooled nuclear fuel rod  10  and the position of each support  31  are the same as in the first and second embodiments of the present invention. However, the shape of each support  31  is different from that of the first embodiment of the present invention. Thus, only the shape of each support  31  will be described in detail. 
         [0062]    The spacer grid  20  according to a third embodiment of the present invention includes semi-cylindrical supports  31  at positions shifted from the longitudinal central line  22  of each unit grid strap  21  toward the sub-channels  115 . The semi-cylindrical supports  30  protrude from the front and rear of each unit grid strap  21 . 
         [0063]    Similarly, the semi-cylindrical supports  31  are preferably formed at the front and rear of the unit grid strap  21  so as to be in axial symmetry based on the longitudinal central line  22  of the unit grid strap  21 . Thereby, when formed by a pressing process, the semi-cylindrical supports  31  minimize disorientation caused by residual strain and residual stress, and are complementarily deformed. 
         [0064]    Thus, the dual-cooled nuclear fuel rod  10  is supported at a total of four positions by the semi-cylindrical supports  31 , which protrude from the four unit grid straps  21  enclosing one dual-cooled nuclear fuel rod  10 . 
         [0065]    As described above, the dual-cooled nuclear fuel rods  10 , arrayed at narrow intervals, are supported by the spacer grid  20  having the supports  23 , which are located at the positions shifted from the longitudinal central line  22  of each unit grid strap  21  toward the sub-channels  115 , so that the dual-cooled nuclear fuel rods can be stably supported despite the narrow intervals. 
         [0066]    Moreover, it is not necessary to change the positions of the guide tubes  140  in the nuclear fuel assembly  100 , and the top end piece  120  and the bottom end piece  130  can be used as they are. Thus, the spacer grid can be more compatible with the constituents of an existing nuclear fuel assembly  100 . 
         [0067]    Further, the supports  23  are formed into cantilever leaf springs  25 , so that they can come into surface contact with the dual-cooled nuclear fuel rod  10 , thereby inhibiting fretting attrition from being generated by vibration of the nuclear fuel rods which is caused by flow of the coolant. 
         [0068]    Moreover, the present invention can be applied to a piping system for transporting fluids and its supporting structures, and ordinary industrial machinery using boilers or heat exchangers. For example, when elongate rods or pipes are arranged at narrow intervals, the present invention is used as one of the shapes of the supports for supporting these rods and pipes. 
         [0069]    Although exemplary embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.