Patent Number: 051046091
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an assembly method for a nuclear fuel assembly which simplifies insertion of a plurality of fuel rods into a plurality of grid cells during the assembly of the nuclear fuel assembly. 2. Description of the Prior Art A construction as shown in FIG. 24 of the attached drawings is known as an example of a nuclear fuel assembly which is mounted in a pressurized water reactor. In FIG. 24, top and bottom nozzles 1 and 2 are vertically spaced and arranged in facing relation to each other. A plurality of control-rod guide thimbles 3 are fixed to and extend between the top and bottom nozzles 1 and 2. A plurality of grids 4 are mounted at intermediate portions of the respective control-rod guide thimbles 3 in a vertically spaced relation to each other. As shown in FIG. 25, each of the grids 4 is formed such that a plurality of straps 5, each in the form of a thin plate, are assembled perpendicularly to each other into a grid by a mutual fitting of slits which are formed in the straps 5 in longitudinally equidistantly spaced relation to each other. A plurality of grid cells 6 are defined in each of the grids 4. A pair of dimples 8 and a pair of springs 9 for supporting a fuel rod 7 are mounted on the wall surface of each of the grid cells 6 in facing relation to each other. The fuel rod 7 inserted in the grid cell 6 is supported by being pushed against the dimples 8 by the springs 9. An assembly method for the nuclear fuel assembly constructed above will next be described. First, the grids 4 are arranged in a spaced relation to each other with a predetermined spacing. Then the control-rod guide thimbles 3 are inserted into and fixed to a part of the corresponding grid cells 6 of each of the grids 4. Subsequently, the fuel rods 7 are inserted into corresponding grid cells 6 in each of the grids 4 which are supported by the control-rod guide thimbles 3, with the fuel rod 7 in sliding contact with the dimples 8 and the springs 9. In this manner, the fuel rods 7 are held in fixed arrangement in the grid cells 6 by the dimples 8 and the springs 9. After insertion of all of the fuel rods 7, the top and bottom nozzles 1 and 2 are fixed to respective opposite ends of the control-rod guide thimbles 3. A problem with the aforesaid assembling method of the nuclear fuel assembly is that when the fuel rods 7 are inserted into the respective grid cells 6 in the grids 4, the outer periphery of each of the fuel rods 7 is clamped between the dimples 8 and the springs 9, and insertion of the fuel rod 7 into the corresponding grid cells 6 is restricted by the resilient force of the springs 9. Thus, difficulties arise in the insertion operation detracting from the working or operating efficiency. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide an assembling method for a nuclear fuel assembly, in which, when each of a plurality of fuel rods is inserted into a corresponding one of a plurality of grid cells, this can be done smoothly and easily, and after insertion of the fuel rod, the fuel rod is reliably supported. It is another object of the invention to provide a grid for the above-mentioned nuclear fuel assembly. To achieve these objects, according to the invention, there is provided an assembly method for a nuclear fuel assembly, comprising the steps of: preparing at least a first grid member with springs, wherein the springs are provided on wall sections of the first grid member with springs, and at least a second grid member with dimples, in which the dimples are provided on wall sections of the second grid member; arranging the first grid member with springs and the second grid member with dimples face to face such that a plurality of grid cells of the first grid member with springs and a plurality of grid cells of the second grid member with dimples communicate with each other and that said dimples and said springs are disposed on planes which are opposed to each other; inserting fuel rods respectively into the grid cells of the first grid member with springs and the second grid member with dimples, under such a condition that the springs and the dimples are shifted relative to each other in such a direction that the springs and the dimples move away from each other; subsequently, moving at least one of said first grid member with springs and said second grid member with dimples such that the grid cells in the first grid member with springs and the grid cells in the second grid member with dimples are in alignment with each other; and connecting the first grid member and the second grid member with dimples to each other. According to the invention, there is also provided a grid for a nuclear fuel assembly, comprising: a first grid member with springs having a plurality of straps each in the form of a thin plate, the straps intersecting each other to form a plurality of grid cells; and a second grid member with dimples having a plurality of straps each in the form of a thin plate, the straps of the first grid member with dimples intersecting each other to form a plurality of grid cells, that plane on which the dimples of the second grid member with dimples are disposed being opposed to that plane on which the springs of said grid member with springs are disposed; the first grid member with springs and the second grid member with dimples being connected to each other such that the grid cells in the first grid member with springs and the grid cells in the second grid member with dimples are in alignment with each other. In the assembling method of the nuclear fuel assembly and the grid for the nuclear fuel assembly, the fuel rods are inserted respectively into the grid cells in such a condition that the grid member with springs and the grid members with dimples are shifted relative to each other. With such an arrangement, the difficulty of insertion of the fuel rods with the fuel rods in contact with the springs and the dimples is removed. Thus, it is possible to easily insert the fuel rods into the grid cells. Furthermore, after insertion of the fuel rods, the grid cells in the grid member with springs and the grid cells in the grid member with dimples are aligned with each other. Thus, it is possible to reliably support the fuel rods by the springs and the dimples. A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: