Patent Number: 
Section: description

FIG. 5 is a perspective view of a duct-type spacer grid for nuclear fuel assemblies in accordance with the primary embodiment of this invention. As shown in the drawing, the duct-type spacer grid 2 of this invention comprises a plurality of duct-shaped grid elements 11, individually provided with both a fuel rod support spring 12 of FIG. 8 and a swirl flow vane of FIG. 9. The above spacer grid 2 is fabricated by horizontally and arranging in parallel the grid elements 11, each of which has an octagonal cross-section. In such a case, the grid elements 11 are welded together at the upper and lower area of the wall thereof. In the present invention, each of the grid elements 11 may be produced using a tube-having an octagonal cross-section. Alternatively, each of the grid elements 11 may be made of a thin and narrow strip by forming the strip into a hollow single structure having an octagonal cross-section. When the grid element 11 is made of an octagonal tube, the tube is machined through a pressing process so as to form a plurality of spring windows 13, 14, line contact springs 12, and swirl flow vanes 30 on the tube. On the other hand, when the grid element 11 is produced using a thin and narrow strip, the strip is primarily formed into a tube structure having an octagonal cross-section, thus forming a tube having a desired size. The tube is, thereafter, machined through a pressing process wherein a plurality of spring windows 13, 14, surface contact springs 12, and swirl flow vanes 30 are formed on the tube in the same manner as that described for the case of using an octagonal tube. After the pressing process, the tube is subjected to a welding process wherein the edges are welded and seamed together. A desired grid element 11 is thus completely produced. The duct-type spacer grid 2, having a plurality of independent octagonal cells 8 within the grid elements 11, has an agreeable structure capable of more effectively resisting against a lateral impact in comparison with a conventional grid structure formed using the inner strips that intersect each other at right angles at the center of a subchannel 107. The reason why the duct-type grid 2 has such a structural advantage is as follows. That is, when the spacer grid 2 is geometrically designed to have a plurality of independent octagonal cells 8 as described above, the grid 2 more quickly and effectively transfers the lateral impact in every direction than in the case of a conventional strip-type spacer grid. Therefore, when the same lateral impact is applied to both types of spacer grids, the allowable impact load of the duct-type grid of this invention is remarkably greater than that of the conventional strip-type grid. As shown in FIGS. 6 and 7, a plurality of longitudinal spring windows, or left- and right-side windows 13 and 14 are formed on the sidewall of each of the grid elements 11 through a pressing process, with a strip-shaped line contact spring 12 being left within each of the windows 13 and 14 while extending at the center of the window. The central portion of each spring 12 is bent toward the center of the grid element 11. The spring 12 thus elastically supports an elongated fuel rod 6 at the bulged portion when the fuel rod 6 is inserted into the cell 8 of the grid element 11. Within each grid element 11, four line contact springs 12 are formed on diametrically opposite four of eight sidewalls. Therefore, the four springs 12 uniformly apply the same spring force to the external surface of a fuel rod 6, inserted into the cell 8, while accomplishing a balance. The spring windows 13 and 14 are used as openings for allowing coolant to pass through so as to more effectively cool the fuel rods 6 within the spacer grid 2. A collateral objective of the windows 13 and 14 is to give additional flexibility to the springs 12. FIG. 8 is a view, showing the operation of the springs 12 when they elastically support a fuel rod 6 within a grid element 11 of the spacer grid 2. When the springs 12 support the fuel rod 6 within the grid element 11, the springs 12 are brought into line contact with the external surface of the fuel rod 6. Therefore, the spring 12 is so-called a line contact spring. Since the springs 12 come into line contact with the fuel rod 6 as described above, the surface contact area of each spring 6 is increased, while contact pressure is applied from the spring 12 to the fuel rod 6. Therefore, it is possible for the spacer grid 2 of this invention to minimize surface damage of the fuel rods 6 due to fretting wear. FIG. 9 is a perspective view, showing the top portion of an octagonal grid element 11 included in the spacer grid of this invention, with two integral type swirl flow vanes 30 being provided at the top of the grid element 11. As shown in the drawing, each of the two vanes 30 comprises two blade parts: a main blade 31 and a sub-blade 32. Within each of the grid elements 11, the two vanes 30 are positioned to have different heights. In order to form each swirl flow vane 30 within a grid element 11, an extension part, integrally and axially extending from one sidewall of a grid element 11, is primarily bent toward the center of the main flow path 7, thus forming a sub-blade 32. Thereafter, the extension part is secondarily bent at the top of the sub-blade 32 toward the center of the main flow path 7, thus forming a main blade 31. In the swirl flow vanes 30, each sub-blade 32 provides an inclined surface, at which the main blade 31 starts to extend. The sub-blade 32 maximizes the size of the main blade 31. The different heights of the flow vanes 30 within each grid element 11 are accomplished by the different heights of the sub-blades 32. As the sub-blades 32 have such different heights, the cross-sectioned area of the flow path gradually varies, thus reducing the pressure loss caused by the swirl flow vanes 30. FIG. 10 is a plan view, showing an arrangement of integral type swirl flow vanes provided at the top of the duct-type spacer grid 2 of this invention. As shown in the drawing, two swirl flow vanes 30 are provided within each main flow path 7 of the spacer grid 2. Since each of the vanes 30 is bent outwardly, the vanes 30 are almost completely free from being undesirably brought into contact with the fuel rods 6. In addition, the swirling directions of the vanes 30 provided at the main flow paths 7 of the grid 2 are designed as follows. That is, the swirl flow vanes 30, provided at the main flow paths 7 on a perpendicular arrangement, are designed in that their swirling directions are opposite to each other. However, the vanes 30, provided at the main flow paths 7 on a diagonal arrangement, are designed to have the same swirling direction. FIG. 11 is a perspective view, showing the two swirl flow vanes 30 before they are bent to a desired configuration. As shown in the drawing, each of the vanes 30 extends from a unit grid element It while forming a triangular plate shape having a specifically curved profile and/or a specifically bent linear profile at both edges. Of course, it should be understood that each of the vanes 30 may have another shape in place of the above-mentioned triangular shape and/or another edge profile in place of the above-mentioned profiles in accordance with a desired swirl flow. The above duct-type spacer grid 2 has the following operational effect. That is, the grid element 11 of the spacer grid 2 comprises a duct having an octagonal cross-section, and so the grid element 11 does not pass across the center of the subchannel 107, through which coolant flows at a high speed. Therefore, the spacer grid 2 reduces pressure loss caused by the grid elements 11. Each of the grid elements 11 is formed as an independent cell 8 for placing and supporting an elongated fuel rod 6, thus having an improved resistance against a lateral impact applied to the sidewall of the grid 2. Within each of the main flow paths 7 of the spacer grid 2, four swirl flow vanes 30 are axially positioned to have different heights, thus reducing pressure loss at the main blades 32 of the vanes 30. Since each of the main blades 32 of the swirl flow vanes 30 is bent outwardly from the cells 8, the main blades 32 are almost completely free from being undesirably brought into contact with the fuel rods 6 when the fuel rods 6 are inserted into the cells 8. FIG. 12 is a perspective view of a duct-type spacer grid 2a for nuclear fuel assemblies in accordance with the second embodiment of this invention. In the spacer grid 2a of the second embodiment, the construction of both the duct-shaped grid elements 11 and the swirl flow vanes 30 remains the same as that described for the primary embodiment. But, the line contact springs 12a of the spacer grid 2a are positioned on the sidewalls around the main flow paths 7 different from the springs 12 of the primary embodiment. Therefore, the spring windows 13 and 14 are positioned on said sidewalls around the main flow paths 7 in the second embodiment. This structure finally increases the amount of coolant flowing through the windows 13 and 14 since a large amount of coolant passes through the main flow paths 7. Therefore, the spacer grid 2a of this embodiment improves the cooling effect for the fuel rods 6 within the grid elements 11. FIG. 13 is a perspective view of a duct-type spacer grid 2b for nuclear fuel assemblies in accordance with the third embodiment of this invention. In the spacer grid 2b of this embodiment, the construction of both the duct-shaped grid elements 11 and the swirl flow vanes 30 remains the same as that described for the primary embodiment. However, the arrangement of the line contact springs 12b of this embodiment is altered as follows. That is, the arrangement of the springs 12b of the neighboring grid elements 11 is rotated at an angle of 45xc2x0 one by one. In other words, the arrangement of the springs 12b in the third embodiment is accomplished by alternately using the arrangements of the springs 12 and 12a of the primary and second embodiments. FIG. 14 is a perspective view of a duct-type spacer grid 2c for nuclear fuel assemblies in accordance with the fourth embodiment of this invention. In the spacer grid 2c of this embodiment, the construction of the duct-shaped grid elements 11, the swirl flow vanes 30, the line contact springs 12 and the spring windows 13 and 14 remains the same as that described for the primary embodiment. However, the spacer grid 2c of this embodiment further comprises a plurality of additional coolant flow windows 15. The additional windows 15 are formed on the sidewalls between the spring-provided sidewalls of each grid element 11. This structure increases the amount of coolant flow between the cells 8, thus improving the cooling effect for the fuel rods 6 within the grid elements 11. As described above, the present invention provides a duct-type spacer grid for nuclear fuel assemblies. The spacer grid of this invention consists of a plurality of duct-shaped grid elements individually having an octagonal cross-section. The grid elements are closely arranged in parallel into a matrix structure prior to being welded together. In the spacer grid, the duct-shaped grid elements do not pass across the center of the subchannel 107, through which coolant flows at a high speed. Therefore, the spacer grid of this invention effectively reduces pressure loss caused by the grid elements. Each of the grid elements is formed as an independent cell effectively resisting against a lateral impact applied to the sidewall of the grid. In the duct-type spacer grid of this invention, two swirl flow vanes are axially positioned to have different heights within each subchannel 107. The swirl flow vanes thus reduce pressure loss at their main blades. In addition, since each of the main blades of the swirl flow vanes is bent outwardly from the cells, the main blades are almost completely free from being undesirably brought into contact with fuel rods when the fuel rods are inserted into the cells. Another advantage of this invention resides in that each elongated fuel rod is supported within a cell by line contact springs without using any dimple, with the surface contact springs being positioned at the same height. The spacer grid of this invention thus uniformly distributes its spring force on the spring contact area of each fuel rod, and so it almost completely prevents damage of the fuel rod due to fretting wear. Although the preferred embodiments of the present invention have been disclosed 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.