Patent Number: 052746844
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for assembling a nuclear fuel assembly, and an apparatus for performing such a task. 2. Background Art Fuel assemblies such as the one disclosed in U.S. Pat. NO. 5,068,081 shown in FIG. 7 have been known. In this figure, the numerals 1 and 2 refer to a top and a bottom nozzles, respectively, which are disposed vertically and oppositely spaced apart, and between the top nozzle 1 and the bottom nozzle 2 are a plurality of rigidly fixed control-rod guide pipes 3 (hereinbelow referred to as guide pipes 3), and in the midway section of the control-rod guide pipes 3 are a plurality of grids 4 disposed vertically and spaced apart from each other. The grids 4 are, as shown in FIGS. 8, 9 and 10, constructed of a plurality of straps 7 made of thin metal strips which have slits 8 provided at regular intervals in the longitudinal direction. By inserting the straps 7 into the slits 8 in each other's slits, a plurality of grid cell spaces 5 (hereinbelow referred to as grid cells 5) are formed. The fuel rod 6 is held firmly in the grid 5, by means of a pair of holding means consisting of a dimple 9 and a spring 10 disposed on opposing walls. The fuel rod 6 inserted into a grid cell 5 is pressed against the dimples 9 by the springs 10 as illustrated in FIG. 10, thereby supporting the fuel rod 6 firmly by the springs 10. The conventional method of assembling such a fuel assembly will be described with reference to FIG. 11. First, the grids 4 are placed apart with a certain distance (positioning of grids). The guide pipes 3 are inserted into the predetermined and corresponding grid cells 5 of each of the grids 4, and are then rigidly attached to the grids 4 by bulge forming. Subsequently, the fuel rods 6 are inserted into the predetermined and corresponding grid cells 5 of each of the grids 4, by sliding the rods 6 against the springs 10 and dimples 9. The rods 6 are thereby secured in the grid cells 5 via the pressing force exerted by the springs 10 and dimples 9. When all of the rods 6 are inserted into the grid cells 5, the top and bottom nozzles 1 and 2 are firmly mounted to the opposite ends of the respective guide pipes 3. In the conventional assembling method described above, the rods 6 are inserted, by sliding over the holding device (composed of dimples 9 and springs 10 ), into the spaces of the grid cells 5 after the guide pipes 3 have been rigidly attached to the grids 4. Because of the procedure, the guide pipes 3 are placed under slight non-uniform compressive stresses transmitted from the fuel rods 6 through to the grids 4. Further detailed explanations are provided in the preferred embodiments of the invention, but it suffices to mention that one of the contributions of the present invention is that the completed assembly shows very little distortion, as a result of the invented process of assembling the fuel assembly. SUMMARY OF THE PRESENT INVENTION The present invention was made in consideration of such problems associated with the conventional assembling method of the fuel assembly, and a purpose is to present a method of assembling the fuel assembly so as to prevent the generation of the distortion of the assembly as described above., and to present an apparatus for performing such a task. According to a method of assembling a fuel assembly presented in the present invention, the springs formed on the wall of the grid cell and protruding out into the cell space are deactivated by means of key means for deactivating the holding means and then a fuel rod is inserted into the grid. Then the key means is removed from the grid cell to activate the springs so as to provide a firm holding of the fuel rod in the grid. The next step is to insert control rod guide pipes into the designated grid cells, and rigidly fixing the control rod guide pipe to the grid. By following the procedure described, it becomes possible to avoid introducing non-uniform stresses on the guide pipes, thereby preventing the distortion of the entire fuel assembly. In the following, the explanations on the apparatus for assembling a fuel assembling are based on the fuel rods, shown in FIGS. 1 and 4, travelling from left to right in the illustration. The entry-side (left) is defined as the side from which the fuel rods enter some object, and the exit-side (right) is defined as the side from which the fuel rods exit. The apparatus for assembling the fuel assembly in the present invention comprises: (a) a fuel rod magazine which holds a plurality of parallel fuel rods in a horizontal position before insertion into the grids; PA1 (b) a driving means disposed on the entry-side of the fuel rod magazine for driving the fuel rods from the fuel rod magazine and inserting the fuel rods into the grids; PA1 (c) a plurality of regularly spaced support frames disposed outward of the exit-side of the fuel rod magazine, said frames supporting grids so that the opening of the grid faces the fuel rod magazine; PA1 (d) regularly spaced guiding means disposed on the entry-side of each of the plurality of regularly spaced support frames providing support to the underside of the fuel rods exiting the fuel rod magazine to prevent sagging of the fuel rods; PA1 (e) an expander jig for bulging the control rod guide pipes. According to the assembling apparatus presented above, after the springs which are protruding into the spaces of the grid cell are deactivated manually or automatically with the use of key means, the fuel rods are inserted into the grids, then the springs are reactivated to hold the fuel rods firmly therein. The control rod guide pipes are then inserted into the designated grid cells and are rigidly fixed to the grids. The top and the bottom nozzles are installed to complete the steps of manufacturing a fuel assembly. According to the assembling apparatus presented, guiding means are effective in preventing damages being introduced to the fuel rods, by preventing sagging of the fuel rods while entering the grid cells and causing possible damage to the rod surface or to the grids 4.