Abstract:
A spacer grid for dual-cooling nuclear fuel rods arranged at a narrow interval. The spacer grid solves the problem in which, since the dual-cooling nuclear fuel rods are used to improve the cooling performance and stability of nuclear fuel and obtain high burnup and output, the outer diameter of each dual-cooling nuclear fuel rod is increased, and thus the gap between each dual-cooling nuclear fuel rod and the grid strap is decreased. The spacer grid includes first grid straps and second grid straps, which are crossed and arranged in transverse and longitudinal directions at regular intervals and have the shape of a flat strip, and support structures, which are fitted into the first and second grid straps around intersections of the first and second grid straps so as to support the dual-cooling nuclear fuel rods.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates, in general, to a spacer grid for dual-cooling nuclear fuel rods, which has been developed in order to reduce the core temperature of a nuclear fuel rod, thereby ensuring the safety of the nuclear fuel rod even at super high burnup, and increasing output to obtain economical effects and, more particularly, to a spacer grid for dual-cooling nuclear fuel rods using intersectional support structures, in which the intersectional support structures are fitted into grid straps around intersections of the grid straps in order to compensate for reduction in a gap between the nuclear fuel rod and the support structure due to an increase in diameter compared to an existing nuclear fuel rod, thereby supporting each dual-cooling nuclear fuel rod in a diagonal direction. 
     2. Description of the Related Art 
       FIG. 1  is a schematic perspective view illustrating a conventional nuclear fuel assembly.  FIG. 2  is a schematic horizontal cross-sectional view illustrating a conventional nuclear fuel assembly.  FIG. 3  is a schematic top plan view illustrating a spacer grid, which is applied to a conventional nuclear fuel assembly.  FIG. 4  is a schematic perspective view illustrating a spacer grid, which is applied to a conventional nuclear fuel assembly.  FIG. 5  is a schematic perspective view illustrating a unit grid strap for a spacer grid supporting a conventional nuclear fuel assembly. 
     As illustrated in the figures, the nuclear fuel assembly  100  comprises nuclear fuel rods  110 , guide pipes  140 , spacer grids  150 , a top end piece  120 , and a bottom end piece  130 . 
     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 produce high-temperature heat. 
     Meanwhile, each guide pipe  140  is used as a passage for a control rod, which moves up and down in order to control the output of a reactor core and to stop a fission reaction. Each spacer grid  150  is one of the components constituting the nuclear fuel assembly in a nuclear reactor, and is designed so that a spring  118  and dimples  119  of each unit grid strap support the nuclear fuel rods  110  such that the nuclear fuel rods  110  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 supported at a designated position, and thus may not be soundly supported. 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 thereof due to excessive frictional resistance 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. When this longitudinal growth is not properly accommodated, the nuclear fuel rods  110  undergo bowing. 
     In this manner, when the nuclear fuel rods  110  undergo bowing, they come nearer to or contact neighboring nuclear fuel rods  110 . Thus, a coolant channel, i.e. a sub-channel  115 , 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. 
     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 filter (not shown) for filtering foreign materials floating in the reactor core. 
     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 pipe cells, into which the guide pipes  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 of the nuclear fuel rod cell, 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. 
     A cylindrical uranium dioxide compact is charged into each nuclear fuel rod  110 , and the coolant rapidly flows from the bottom to the top of the reactor core in the 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 pipe  140 . 
     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. 
     Meanwhile, as illustrated in  FIGS. 6 and 7 , a dual-cooling 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. 
     Here, the dual-cooling 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-cooling nuclear fuel rod  10 , so that heat transfer is doubled. As a result, the dual-cooling nuclear fuel rod  10  maintains a low surface temperature. 
     In this manner, in the case in which the dual-cooling nuclear fuel rod  10  is maintained at a low core temperature, the possibility of damaging the fuel due to an increase in the core temperature of the nuclear fuel is reduced, so that the safety margin of the dual-cooling nuclear fuel rod  10  can be increased, and the dual-cooling nuclear fuel rod  10  provides high burnup and high output. 
     However, in order to make the dual-cooling 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 pipes  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 design draft for the dual-cooling nuclear fuel rods having a 12×12 array, the gap between the nuclear fuel rod and the unit grid strap is reduced from 1.45 mm, which is the size of the existing gap, to about 0.39 mm. 
     Thus, due to the narrow gap between the nuclear fuel rod and the unit grid strap, it is impossible to use the existing method of forming fuel rod support structures to realize contact at the portions where the fuel rod support structures intersect the nuclear fuel rods in order to support the nuclear fuel rods. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an object of the present invention is directed to a spacer grid for the stable support of dual-cooling nuclear fuel rods, which are used for improving the cooling performance and stability of nuclear fuel and obtaining high burnup and output, despite a narrow gap by fitting separate support structures, which are capable of supporting the dual-cooling nuclear fuel rods, around intersections of grid straps for a spacer grid instead of springs and dimples formed for each existing grid strap, such that the dual-cooling nuclear fuel rods are soundly supported for their entire lifespan. 
     According to one aspect of the present invention, there is provided a spacer grid for dual-cooling nuclear fuel rods using intersectional support structures. The spacer grid comprises: first grid straps and second grid straps, which are crossed and arranged in transverse and longitudinal directions at regular intervals and have a shape of a flat strip; and support structures, which are fitted into the first and second grid straps around intersections of the first and second grid straps so as to support the dual-cooling nuclear fuel rods. 
     Here, each of the first grid straps may include first grid slits spaced apart from each other at regular intervals at one of upper and lower portions thereof, and first fastening slits formed on opposite sides of the respective first grid slits. Each of the second grid straps may include second grid slits spaced apart from each other at regular intervals at one of upper and lower portions thereof, and second fastening slits formed on opposite sides of the respective second grid slits at a remaining portion thereof. 
     Further, each support structure may include slits at corners of a prism having a quadrilateral cross section so as to be coupled with the first fastening slits and the second fastening slits, and supports supporting the dual-cooling nuclear fuel rods on faces thereof. 
     Alternatively, each support structure may include slits in corners of a prism having a quadrilateral cross section so as to be coupled with the first fastening slits and the second fastening slits, and supports having one of concave, convex and planar shapes in a longitudinal direction on faces thereof so as to be in contact with outer circumferences of the dual-cooling nuclear fuel rods. 
     At this time, each support structure may be formed by bending a rectangular sheet at a right angle. 
     Meanwhile, each support structure may include four slits in a circumference of a cylinder having a circular cross section at right angles so as to be coupled with the first fastening slits and the second fastening slits, and supports supporting the dual-cooling nuclear fuel rods between the four slits. 
     Further, each support may include a contact portion, which comes into contact with an outer circumference of each dual-cooling nuclear fuel rod after each dual-cooling nuclear fuel rod is charged, and connector portions, which extend from upper and lower ends of the contact portion in a curved shape. 
     Here, the contact portion may have a concave shape so as to have a curvature equal to that of the outer circumference of each dual-cooling nuclear fuel rod after each dual-cooling nuclear fuel rod is charged. Alternatively, the contact portion may have a convex shape or a simple planar shape protruding toward each dual-cooling nuclear fuel rod in consideration of the rigidity of the supports. 
     Further, each support structure may additionally include dimples at upper and lower portions of each support. 
     In addition, the first grid straps, the second grid straps, and the support structures may be welded to each other. 
     As apparent from the foregoing description, the support structures are fitted into the first and second grid straps around the intersections of the first and second grid straps so as to allow the dual cooling nuclear fuel rods, each of which undergoes an increase in diameter and has very high burnup, to be soundly supported during an entire lifespan, despite a narrow gap between each dual cooling nuclear fuel rod and each grid strap, so that the spacer grid can stably support the dual cooling nuclear fuel rods. 
     Further, it is not necessary to change the positions of the guide pipes within the nuclear fuel assembly, and it is possible to use the top and bottom end pieces as they stand, so that the spacer grid can maintain compatibility with constituent parts of an existing nuclear fuel assembly without changing or modifying an existing reactor core structure. 
     In addition, the support structures are fitted into the grid straps around the intersections of the grid straps without performing plastic working on each grid strap to form the springs and dimples, so that the spacer grid can promote impact strength against lateral load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic perspective view illustrating a conventional nuclear fuel assembly; 
         FIG. 2  is a schematic horizontal cross-sectional view illustrating a conventional nuclear fuel assembly; 
         FIG. 3  is a schematic top plan view illustrating a spacer grid, which is applied to a conventional nuclear fuel assembly; 
         FIG. 4  is a schematic perspective view illustrating a spacer grid, which is applied to a conventional nuclear fuel assembly; 
         FIG. 5  is a schematic perspective view illustrating a unit grid strap for a spacer grid supporting a conventional nuclear fuel assembly; 
         FIG. 6  is a schematic horizontal cross-sectional view illustrating a dual-cooling nuclear fuel rod; 
         FIG. 7  is a schematic horizontal cross-sectional view illustrating a dual-cooling nuclear fuel assembly; 
         FIG. 8  is a perspective view illustrating a first grid strap according to an exemplary embodiment of the present invention; 
         FIG. 9  is a perspective view illustrating a second grid strap according to an exemplary embodiment of the present invention; 
         FIG. 10  is a perspective view illustrating a support structure applied to a first embodiment of the present invention. 
         FIG. 11  is an assembled view illustrating the sequence of assembling a first grid strap, a second grid strap and a support structure; 
         FIG. 12  is a top plan view illustrating a support structure applied to a first embodiment of the present invention; 
         FIG. 13  is a perspective view illustrating a spacer grid according to a first embodiment of the present invention; 
         FIG. 14  is a perspective view illustrating a support structure, which is applied to a first embodiment of the present invention and to which dimples are additionally provided; 
         FIG. 15  is a perspective view illustrating a support structure applied to a second embodiment of the present invention; 
         FIG. 16  is a top plan view illustrating a spacer grid according to a second embodiment of the present invention; 
         FIG. 17  is a perspective view illustrating a spacer grid according to a second embodiment of the present invention; 
         FIG. 18  is a perspective view illustrating a support structure, which is applied to a second embodiment of the present invention and has convex supports; 
         FIG. 19  is a perspective view illustrating a support structure applied to a third embodiment of the present invention; 
         FIG. 20  is a top plan view illustrating a spacer grid according to a third embodiment of the present invention; 
         FIG. 21  is a perspective view illustrating a spacer grid according to a third embodiment of the present invention; 
         FIG. 22  is a perspective view illustrating a support structure, which is applied to a third embodiment of the present invention and to which dimples are additionally provided; 
         FIG. 23  is a perspective view illustrating a support structure applied to a fourth embodiment of the present invention; 
         FIG. 24  is a top plan view illustrating a spacer grid according to a fourth embodiment of the present invention; and 
         FIG. 25  is a perspective view illustrating a spacer grid according to a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in greater detail to exemplary embodiments of the invention, an example of which is 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. 
       FIG. 8  is a perspective view illustrating a first grid strap according to an exemplary embodiment of the present invention.  FIG. 9  is a perspective view illustrating a second grid strap according to an exemplary embodiment of the present invention.  FIG. 10  is a perspective view illustrating a support structure applied to a first embodiment of the present invention.  FIG. 11  is an assembled view illustrating the sequence of assembling a first grid strap, a second grid strap and a support structure.  FIG. 12  is a top plan view illustrating a support structure applied to a first embodiment of the present invention.  FIG. 13  is a perspective view illustrating a spacer grid according to a first embodiment of the present invention.  FIG. 14  is a perspective view illustrating a support structure, which is applied to a first embodiment of the present invention and to which dimples are additionally provided. 
     The spacer grid  1  of the present invention comprises first grid straps  20  and second grid straps  30 , which are crossed and arranged in transverse and longitudinal directions, respectively, at regular intervals and have the shape of flat strips, and support structures  40 , which are fitted into the first and second grid straps  20  and  30  around intersections of the first and second grid straps  20  and  30  so as to support dual-cooling nuclear fuel rods  10 . 
     In detail, the plurality of first grid straps  20  and the plurality of second grid straps  30  are crossed and arranged in the transverse and longitudinal directions to thereby define spaces in which the dual-cooling nuclear fuel rods  10  are accommodated. In order to stably support the dual-cooling nuclear fuel rods  10 , the support structures  40  are fitted into the first and second grid straps  20  and  30  around the intersections of the first and second grid straps  20  and  30 . 
     As illustrated in  FIG. 8 , each of the first grid straps  20  has the shape of a flat strip, and is provided with two types of slits, first grid slits  21  and first fastening slits  22 , at one of upper and lower portions thereof. 
     First, the first grid slits  21  are spaced apart from each other at regular intervals at the upper portion of each of the first grid straps  20  so as to be able to be coupled with second grid slits  31  of the second grid straps  30 , which will be described below. The first fastening slits  22  are formed on opposite sides, that is, left-hand and right-hand sides, of the respective first grid slits  21  such that the support structures  40  for supporting the dual-cooling nuclear fuel rods  10  can be inserted. 
     In other words, the plurality of first grid slits  21  and the plurality of first grid slits  22 , which are formed in each of the first grid straps  20 , are formed on the upper portion of each of the first grid straps  20  at regular intervals. The first grid slits  21  are coupled with second grid slits  31  of each of the second grid straps  30 , and the first fastening slits  22  are coupled with slits  41  formed in each support structure  40 , which will be described below. 
     Here, the first grid slits  21  and the first fastening slits  22  are formed at the upper portion of each of the first grid straps  20 . However, the first grid slits  21  and the first fastening slits  22  may be formed in the lower portion of each of the first grid straps  20 . 
     As illustrated in  FIG. 9 , each of the second grid straps  30  has the shape of a flat strip, and is provided with two types of slits, second grid slits  31  and second fastening slits  32 , at lower and upper portions thereof, like the first grid straps  20 . 
     The second grid slits  31  are spaced apart from each other at regular intervals at the lower portion of each of the second grid straps  30  so as to be able to be coupled with the first grid slits  21  of the first grid straps  20 , which will be described below. The second fastening slits  32  have the same intervals therebetween as the first fastening slits  22 , and are formed on opposite sides, that is, left-hand and right-hand sides, of the respective second grid slits  31  at the upper portion of each of the second grid straps  30 . 
     If the first grid slits  21  and the first grid slits  22  are provided at the lower portion of each of the first grid straps  20 , the second grid slits  31  must be provided at the upper portion of each of the second grid straps  30 , and the second fastening slits  32  must be provided at the lower portion of each of the second grid straps  30 . 
     In this case, the support structures  40  for supporting the dual-cooling nuclear fuel rods  10 , which will be described below, are coupled to the first and second grid straps  20  and  30  from the bottom to the top. 
     As illustrated in  FIG. 10 , each support structure  40 , applied to a first embodiment of the present invention, is provided with slits  41  at corners of a prism having a quadrilateral cross section so as to be coupled with the first fastening slits  22  and the second fastening slits  32 . 
     At this time, the diagonal length of each support structure  40  must be equal to the interval between the first fastening slits  22  of each of the first grid straps  20  and the interval between the second fastening slits  32  of each of the second grid straps  30 . 
     Specifically, in order to mutually couple the first grid straps  20  and the second grid straps  20  in a crossed state and then fit the support structures  40  into the first and second fastening slits  22  and  32  formed in the first and second grid straps, the diagonal length between the slits  41  formed in the corners of each support structure  40  in a diagonal direction must be equal to the transverse or longitudinal interval between the first fastening slits  22  and the transverse or longitudinal interval between the second fastening slits  32 . 
     In this embodiment, since the slits  41  are provided with the respective lower corners of each support structure  40 , the support structures  40  are adapted to be assembled with the first and second grid straps  20  and  30  from the bottom to the top. Alternatively, the slits  41  may be provided with respective upper corners of each support structure  40 , and the support structures  40  may be assembled with the first and second grid straps  20  and  30  from the top to the bottom. 
     To this end, the first fastening slits  22  and the second fastening slits  32  must be provided at the lower portion of each of the first grid straps  20  and at the lower portion of each of the second grid straps  30 , respectively. 
     Each support structure  40  having the quadrilateral cross section is provided with supports  42 , which support the dual-cooling nuclear fuel rods  10 , on faces thereof. Each support  42  includes a contact portion  43 , which comes into contact with the outer circumference of each dual-cooling nuclear fuel rod  10  after each dual-cooling nuclear fuel rod  10  is charged, and connector portions  44 , which extend from upper and lower ends of the contact portion  43  in a curved shape. 
     Here, the contact portion  43  of the support  42 , which is formed on each face of the support structure  40 , protrudes toward each dual-cooling nuclear fuel rod  10  around the intersections of the first and second grid straps  20  and  30  so as to be in contact with the outer circumference of each dual-cooling nuclear fuel rod  10 . 
     Further, the connector portions  44  extend from the upper and lower ends of the contact portion  43  in a curved shape in order to provide constant elastic force to the contact  43 . 
     Thus, the connector portions  44  elastically support the contact portion  43  protruding from each face of the support structure  40 , and thus elastically support each dual-cooling nuclear fuel rod  10  through the contact portion  43 . 
     In  FIG. 10 , the contact portion  43  has a concave shape, the curvature of which is equal to that of the outer circumference of each dual-cooling nuclear fuel rod  10  so as to be in surface contact with each dual-cooling nuclear fuel rod  10  after each dual-cooling nuclear fuel rod  10  is charged. Alternatively, in consideration of the rigidity of each support  42 , the contact portion  43  may protrude toward each dual-cooling nuclear fuel rod  10  in a convex shape or in a simple plane shape. 
     The support structure  40  is obtained by performing punching and plastic working on a rectangular sheet using a press machine, etc. so as to form the slits  41  and the supports  42 , having the contact portion  43  and the connector portions  44 , and then by bending the rectangular sheet into a quadrilateral prism. 
       FIG. 11  illustrates an example in which the first grid strap  20 , the second grid strap  30 , and the support structure  40  are assembled into the spacer grid  1  of the present invention. 
     As illustrated in  FIG. 11 , the second grid strap  30  is assembled to the first grid strap  20  from the top to the bottom such that the second grid slits  31  of the second grid strap  30  are coupled to the first grid slits  21  of the first grid strap  20 . Then, the first and second grid straps  20  and  30  are attached to each other around the intersections thereof by, for instance, welding. 
     Then, the support structure  40  is fitted into the first and second grid straps  20  and  30  from the top to the bottom such that the slits  41  in the corners of the support structure  40  are coupled with the first and second fastening slits  22  and  32  in the grid straps. Subsequently, the support structure  40  and the grid straps are attached to each other around the intersections thereof by, for instance, welding. 
     As in  FIGS. 12 and 13 , the support structures  40  are fitted into the first and second grid straps  20  and  30  around the intersections of the first and second grid straps  20  and  30 , and then are coupled with the first and second grid straps  20  and  30  by, for instance, welding. Thereby, the dual-cooling nuclear fuel rods  10  can be accommodated into square spaces surrounded by the first and second grid straps, and can then be stably supported by the supports  42  of the support structures  40  fitted into the intersections of the first and second grid straps. 
     As illustrated in  FIG. 14 , each face of the support structure  40  illustrated in  FIG. 10  is additionally provided with dimples  46  in upper and lower portions thereof in order to maintain sound supporting performance of each dual-cooling nuclear fuel rod  10 . 
     The dimples  46  protrude from the respective faces of the support structure  40  toward the respective dual-cooling nuclear fuel rods  10  so as to be in surface contact with the outer circumferences of the dual-cooling nuclear fuel rods  10 , and have a concave contact face, the curvature of which is equal to that of the outer circumference of each dual-cooling nuclear fuel rod  10  after each dual-cooling nuclear fuel rod  10  is charged. 
     In  FIG. 14 , although each dimple  46  has the concave contact face, the curvature of which is equal to that of the outer circumference of each dual-cooling nuclear fuel rod  10 , so as to be in surface contact with the outer circumference of each dual-cooling nuclear fuel rod  10 , each dimple may have a contact face having a convex shape or a simple planar shape which protrudes toward each dual-cooling nuclear fuel rod  10  in order to ensure reliable supporting performance of each dual-cooling nuclear fuel rod  10 . 
       FIG. 15  illustrates a support structure applied to a second embodiment of the present invention. The support structure  40  is provided with slits  41  in corners of a prism having a quadrilateral cross section so as to be coupled with the first fastening slits  22  and the second fastening slits  32 , and concave supports  42  in faces thereof in the longitudinal direction so as to be in surface contact with the outer circumference of each dual-cooling nuclear fuel rod  10 . 
     Further, each support  42  may be formed after cutting parts of upper and lower portions of each face of the support structure  40  in consideration of the rigidity of the support structure  40  or the fretting (or wear) characteristics of each dual-cooling nuclear fuel rod  10  resulting from the support structure  40 . 
     In the case in which each support  42  has a concave shape, it is preferably adapted to have the same curvature as the outer circumference of each dual-cooling nuclear fuel rod  10  so as to be in surface contact with the outer circumference of each dual-cooling nuclear fuel rod  10  after each dual-cooling nuclear fuel rod  10  is charged. 
     Further, the diagonal length of the support structure  40  must be equal to the interval between the first fastening slits  22  of each of the first grid straps  20  and the interval between the second fastening slits  32  of each of the second grid straps  30 . 
     Specifically, in order to mutually couple the first grid straps  20  and the second grid straps  20  in a crossed state and then insert the support structures  40  into the first and second fastening slits  22  and  32  formed in the first and second grid straps, the diagonal length between the slits  41  formed in the corners of each support structure  40  in a diagonal direction must be equal to the transverse or longitudinal interval between the first fastening slits  22  and the transverse or longitudinal interval between the second fastening slits  32 . 
     The support structure  40  is obtained by performing punching and plastic working on a rectangular sheet using a press machine or the like so as to form the slits  41  and the supports  42 , and then by bending the rectangular sheet into a quadrilateral prism. 
     The method of manufacturing the spacer grid according to a second embodiment of the present invention is performed in the same way as in the first embodiment, and thus will not be described below. 
     As in  FIGS. 16 and 17 , the support structures  40  are fitted into the first and second grid straps  20  and  30  around the intersections of the first and second grid straps  20  and  30 , and then are coupled with the first and second grid straps  20  and  30  by, for instance, welding. Thereby, the dual-cooling nuclear fuel rods  10  can be accommodated into square spaces surrounded by the first and second grid straps, and can then be stably supported by the supports  42  of the support structures  40  fitted into the intersections of the first and second grid straps. 
       FIG. 18  is a perspective view illustrating a support structure which is applied to the second embodiment of the present invention and has convex supports. Each support  42  protrudes toward each dual-cooling nuclear fuel rod  10  in a convex shape, and thus contacts the outer circumference of each dual-cooling nuclear fuel rod  10  so as to be able to support each dual-cooling nuclear fuel rod  10 . 
     Although not illustrated in  FIGS. 17 and 18 , each support  42  may have the shape of a flat plate. 
     Further, each support  42  may be formed after cutting parts of upper and lower portions of each face of the support structure  40  in consideration of the rigidity of the support structure  40  or the fretting (or wear) characteristics of each dual-cooling nuclear fuel rod  10 , attributable to the support structure  40 . 
       FIG. 19  illustrates a support structure applied to a third embodiment of the present invention. The support structure  40  is provided with four slits  41  in the circumference of a cylinder having a circular cross section at right angles so as to be coupled with the first fastening slits  22  and the second fastening slits  32 , and supports  42 , supporting the dual-cooling nuclear fuel rods  10  between the four slits  41 . 
     At this time, the interval between the first slit  41  in the circumference of the support structure  40  and the third slit  41  facing the first slit in a diagonal direction must be equal to the interval between the first fastening slits  22  in each of the first grid straps  20  and the interval between the second fastening slits  32  in each of the second grid straps  30 . 
     The supports  42  supporting the dual-cooling nuclear fuel rods  10  are arranged at four places on the circumference of the support structure  40  having the circular cross section at the same interval. Each support  42  includes a contact portion  43 , which comes into contact with the outer circumference of each dual-cooling nuclear fuel rod  10  after each dual-cooling nuclear fuel rod  10  is charged, and connector portions  44 , which extend from upper and lower ends of the contact portion  43  in a curved shape. 
     Here, the contact portion  43  of the support  42  of the support structure  40  protrudes toward each dual-cooling nuclear fuel rod  10  around the intersections of the first and second grid straps  20  and  30  so as to be in contact with the outer circumference of each dual-cooling nuclear fuel rod  10 . 
     Further, the connector portions  44  extend from the upper and lower ends of the contact portion  43  in a curved shape in order to provide a constant elastic force to the contact  43 . 
     Thus, the connector portions  44  elastically support the contact portion  43  protruding from one of the four places of the circumference of the support structure  40 , and thus elastically support each dual-cooling nuclear fuel rod  10  through the contact portion  43 . 
     In  FIG. 19 , the contact portion  43  has a concave shape, the curvature of which is equal to that of the outer circumference of each dual-cooling nuclear fuel rod  10  so as to be in surface contact with each dual-cooling nuclear fuel rod  10  after each dual-cooling nuclear fuel rod  10  is charged. Alternatively, in consideration of the rigidity of each support  42 , the contact portion  43  may protrude toward each dual-cooling nuclear fuel rod  10  in a convex shape or in a simple planar shape. 
     The support structure  40  is obtained by performing punching and plastic working on a rectangular sheet using a press machine or the like so as to form the slits  41  and the supports  42  having the contact portion  43  and the connector portions  44 , and then by rolling the rectangular sheet in a cylindrical shape. 
     As in  FIGS. 20 and 21 , the support structures  40  are fitted into the first and second grid straps  20  and  30  around the intersections of the first and second grid straps  20  and  30 , and then are coupled with the first and second grid straps  20  and  30  by, for instance, welding. Thereby, the dual-cooling nuclear fuel rods  10  can be accommodated into square spaces surrounded by the first and second grid straps, and can then be stably supported by the supports  42  of the support structures  40  fitted into the intersections of the first and second grid straps. 
     As illustrated in  FIG. 22 , the circumference of the support structure  40  illustrated in  FIG. 19  is additionally provided with dimples  46  at upper and lower portions thereof in order to maintain sound supporting performance of each dual-cooling nuclear fuel rod  10 . 
     The dimples  46  protrude from the circumference of the support structure  40  toward the respective dual-cooling nuclear fuel rods  10  so as to be in surface contact with the outer circumferences of the dual-cooling nuclear fuel rods  10 , and each have a concave contact face, the curvature of which is equal to that of the outer circumference of each dual-cooling nuclear fuel rod  10  after each dual-cooling nuclear fuel rod  10  is charged. 
     In  FIG. 22 , although each dimple  46  has a concave contact face, the curvature of which is equal to that of the outer circumference of each dual-cooling nuclear fuel rod  10 , so as to be in surface contact with the outer circumference of each dual-cooling nuclear fuel rod  10 , each dimple may have a contact face having a convex shape or a simple planar shape which protrudes toward each dual-cooling nuclear fuel rod  10  in order to ensure reliable supporting performance of each dual-cooling nuclear fuel rod  10 . 
       FIG. 23  illustrates a support structure applied to a fourth embodiment of the present invention. The support structure  40  is provided with four slits  41  in the circumference of a cylindrical tube at right angles so as to be coupled with the first fastening slits  22  and the second fastening slits  32 . Thus, each dual-cooling nuclear fuel rod  10  is supported by directly contacting the outer circumference of the support structure  40 . 
     At this time, the interval between the first slit  41  in the circumference of the support structure  40  and the third slit  41  facing the first slit in a diagonal direction must be equal to the interval between the first fastening slits  22  of each of the first grid straps  20  and the interval between the second fastening slits  32  of each of the second grid straps  30 . 
     In the first, second and third embodiments, the support structure  40  is formed by bending the rectangular sheet at a right angle, or by rolling the rectangular sheet in a cylindrical shape. However, in the fourth embodiment, the support structure  40  is provided using the cylindrical tube itself, having a circular cross section, and performs punching on the slits  41  arranged at each right angle using a press machine or the like. 
     Thus, as in  FIGS. 24 and 25 , the support structures  40  are fitted into the first and second grid straps  20  and  30  around the intersections of the first and second grid straps  20  and  30 , and then are coupled with the first and second grid straps  20  and  30  by, for instance, welding. For example, as shown in  FIG. 25 , welds  50  are formed to connect upper portions of the support structures  40  to the first and second grid straps  20  and  30 . Similar welds (not shown) may be formed to connect lower portions of the support structures  40  to the first and second grid straps  20  and  30 . Thereby, the dual-cooling nuclear fuel rods  10  are accommodated into square spaces surrounded by the first and second grid straps, and thus the outer circumferences thereof are in direct contact with the outer circumferences of the support structures  40 , which have the circular cross section and are fitted into the first and second grid straps around the intersections of the first and second grid straps. As a result, the dual-cooling nuclear fuel rods  10  can be stably supported using the intrinsic rigidity of each support structure  40 . 
     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.