Patent Number: 052895157
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an assembly method for a nuclear fuel assembly, capable of preventing scratches from occurring on outer peripheral surfaces of nuclear fuel rods when the nuclear fuel assembly is assembled, and to a grid in the nuclear fuel assembly for supporting the fuel rods. 2. Prior Art FIG. 10 of the attached drawings depicts an example of a nuclear fuel assembly which is mounted on a nuclear reactor such as a pressurized water reactor or the like. In this assembly, a pair of top and bottom nozzles 1 and 2 are arranged in facing relation to each other and in vertically spaced relation to each other. A plurality of control-rod guide thimbles 3 extend between and are securely fixed to the top and bottom nozzles 1 and 2. A plurality of grids 4 are secured to intermediate portions of the respective control-rod guide thimbles 3 in vertically spaced relation to each other. As shown in FIGS. 11 through 13, each of the grids 4 is formed as follows. That is, a plurality of straps 7, each in the form of a thin strip sheet, are assembled perpendicularly to each other into a grid by mutual fitting of slits 8 which are formed in the straps 7 in longitudinally equidistantly spaced relation to each other. A plurality of grid cells 5 are defined in each of the grids 4. A pair of dimples 9 and 9 and a pair of springs 10 and 10 for supporting a fuel rod 6 are formed on the wall surface of each of the grid cells 5 in opposed relation to each other. The fuel rod 6 inserted in the grid cell 5 is supported in urging relation to the dimples 9 by the springs 10. An assembly method of the nuclear fuel assembly constructed above will next be described. First, the grids 4 are arranged in vertically spaced relation to each other at a predetermined spacing. The control-rod guide thimbles 3 are then inserted into and fixed to predetermined grid cells 5 of each of the grids 4, respectively. Subsequently, the fuel rods 6 are inserted into corresponding grid cells 5 in each of the grids 4 which are supported by the control-rod guide thimbles 3, with the fuel rod 6 in sliding contact with the dimples 9 and the springs 10. In this manner, the fuel rods 6 are fixedly arranged in corresponding grid cells 5 through the dimples 9 and the springs 10. After insertion of all the fuel rods 6, the pair of top and bottom nozzles 1 and 2 are fixedly mounted respectively to the opposite ends of the control-rod guide thimbles 3. In the aforesaid assembly method of the nuclear fuel assembly, when the fuel rods 6 are inserted respectively into the grid cells 5 in each of the grids 4, the outer peripheral surface of each of the fuel rods 6 is clamped between the dimples 9 and the springs 10, and the fuel rod 6 must be inserted into the corresponding grid cells 5, while the fuel rod 6 resists the resilient force of the springs 10. Thus, there is a problem that the outer peripheral surface of the fuel rod 6 is scratched by the springs 10 and the dimples along its longitudinal direction. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an assembly method for a nuclear fuel assembly, capable of preventing scratches from occurring on an outer peripheral surface of each fuel rod upon assembling of the nuclear fuel assembly. Another object of the invention is to provide a combination of a grid in the above-mentioned nuclear fuel assembly and a plurality of elongated key members. According to a first aspect of the invention, there is provided an assembly method of a nuclear fuel assembly, comprising the steps of: preparing a plurality of grids each of which comprises a plurality of elongated straps intersected with each other to define a plurality of grid cells therein, and a plurality of pairs of dimples and springs formed on the straps for supporting a plurality of fuel rods, each pair of dimples and springs being disposed in facing relation to each other, on wall sections of the straps, which cooperate with each other to define one of the grid cells, the pair of dimples and springs projecting into the grid cell; subsequently inserting a deflecting jig into one of the grid cells defined in each of the grids, the deflecting jig being in the form of a rod having a diameter capable of being enlarged; subsequently enlarging the diameter of the deflecting jig to urge the spring associated therewith against resilient force of the spring to deflect the spring away from the dimples associated therewith; subsequently inserting a plurality of elongated key members, respectively along a longitudinal direction of the straps forming the grid, into the grid cells through a plurality of openings which are defined at intersections between the straps, each of the key members being formed with a plurality of hooks which are spaced a predetermined spacing from each other along the longitudinal direction of the key member; subsequently rotating each of the key members about its axis to cause the hooks of the key member to project from a wall surface of the strap associated with the key member, through the openings, in a direction opposite to the projecting direction of the springs formed on the strap; subsequently moving the key member forwardly in the longitudinal direction of the strap to engage the hooks of the key member with the wall surface of the strap, thereby fixedly mounting the key member to the strap to maintain the springs deflected; subsequently releasing the urging of the spring due to the deflecting jig to withdraw the same from the grid cell and, subsequently, inserting the fuel rods respectively into the grid cells; subsequently moving the key member rearwardly to release retention of the springs due to the hooks of the ke member thereby bringing the springs into pressure contact with the fuel rods, respectively; and withdrawing the key members from the grid cells. According to a second aspect of the invention, there is provided a combination of a grid in a nuclear fuel assembly and a plurality of elongated key members, the grid comprising a plurality of elongated straps intersected with each other to define a plurality of grid cells therein, a plurality of pairs of dimples and springs formed on the straps for supporting a plurality of fuel rods, and a plurality of openings which are defined at intersections among the straps, each pair of dimples and springs being disposed in facing relation to each other, on wall sections of the straps, which cooperate with each other to define one of the grid cells, the pair of dimples and springs projecting into the grid cell, and each of the elongated key members being provided for maintaining the springs deflected respectively away from the dimples, the key member being capable of being inserted into the grid cells through the openings along a longitudinal direction of a corresponding one of the straps. According to the assembly method of the nuclear fuel assembly and the combination of the grid and the elongated key members, when the fuel rods are inserted respectively into the grid cells of the grid, the deflecting jig is inserted into the grid cells and is enlarged in diameter to deflect the springs away from the dimples facing thereto, the key member is inserted into the grid cells through the openings formed at intersections among the straps, and the key member is rotated and moved forwardly whereby the hooks of the key member are engaged with the wall surface of the strap, to maintain the springs deflected. Under such condition, the distance between the spring and the dimples facing thereto becomes larger than the diameter of the fuel rod. Therefore, the fuel rods do not slide in contact with the springs and dimples, and the outer peripheral surfaces of the respective fuel rods are not clamped between the spring and the dimples. Accordingly, no scratches occur on the outer peripheral surfaces of the fuel rods. In addition, since, when the fuel rods are inserted respectively into the grid cells, the fuel rods are not clamped between the springs and the dimples, no unreasonable tension force is applied to the fuel rods, and it is possible to reduce a power source for inserting the fuel rods respectively into the grid cells. Thus, it is possible to made an inserting unit and so on compact.