Patent Number: 053012143
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an apparatus for loading fuel rods into a fuel assembly for use in a nuclear reactor, such as a pressurized water reactor or the like. 2. Technical Background of the Invention A conventional fuel assembly such as the one disclosed in U.S. Pat. No. 5,068,081, shown in FIG. 10, is known. In this figure, the numerals 1 and 2 refer to top and bottom nozzles, respectively, which are disposed vertically and oppositely spaced apart, and have a plurality of rigidly fixed control-rod guide pipes 3 (hereinbelow referred to as guide pipes 3) between the top nozzle 1 and the bottom nozzle 2. In the mid section of the guide pipes 3 are a plurality of grids 4 disposed vertically and spaced apart from each other. The grids 4 are constructed of a plurality of straps 7 made of thin metal strips having slits 8 formed in the longitudinal direction, as shown in FIG. 4. The slits 8 are interlocked to form a latticed structure as shown in FIG. 5, and each space bounded by the straps is known as a grid cell 5. Each grid cell 5 is provided with dimples 9 and springs 10 formed on the opposing walls of the grid cells 5. A fuel rod 6 inserted into a grid cell 5, shown in FIG. 6, is pressed against the dimple 9 by the spring 10, thereby holding the fuel rod 6 firmly in the grid cell 5. There are different approaches to the steps involved in assembling such an assembly. For example, in the first method, as shown in FIG. 1, the grids 4 are disposed at a predetermined spacing, and the guide pipes 3 and an instrumentation tube is inserted into and fixed to designated grid cells 5 of each of the grids 4, respectively, followed by insertion of the fuel rods 6 into the other grids cells 5 of each of the grids 4 which are supported by the guide pipes 3. In the second method, the fuel rods 6 are inserted into and firmly held in the grid cells 5 first, followed by insertion of the control-rod guide pipes 3 and an instrumentation tube into the other grid cells 5 of the grids 4, followed by a step of rigidly fixing the guide pipes 3 to the grids. A method of inserting fuel rods 6 in grid cells 5 with the use of a pull-in loader is known, for example, in Japanese Patent Application, Laid open publication, H2-181,699, which involves the use of key means to deactivate the springs 10, followed by extending the pull-in rods of a pull-in device through the grids 5, gripping the tips of the fuel rods 6 housed in a fuel rod magazine with the gripping device attached to the pull-in rods, and loading the fuel assembly with fuel rods 6 by pulling the fuel rods 6 into the grid cells 5 of the grids 4. There are problems in loading the fuel rods 6 with the use of pull-in rods after the guide pipes 3 (and an instrumentation tube) are fixed to the grids 4. One of the problems is the mechanical interference which occurs between the pull-in rods and the installed guide pipes 3 when trying to load the fuel rods 6 into the grids 4. The mechanical interference is due to the fact that certain grid cells 5 are already occupied by the cylindrical insert parts of the guide pipes 3 disposed at the entrance to the grids. In such a system, those pull-in rods corresponding with the grid cells 5 occupied by the guide pipes 3 must not be activated, and only those pull-in rods which correspond with the fuel rods 6 to be pulled into the grid cells 5 should be activated. The result is that the loading mechanism becomes complex because of the need for a special control device to select the correct pull-in rods. Furthermore, because the arrangement of the guide pipes in the grids are different for each tier of grid cells, the operation of the pull-in rods must be adjusted for each tier of grid cells, leading to a lengthy and cumbersome fuel rod installation operation. SUMMARY OF THE PRESENT INVENTION The present invention was made in view of the problems in the conventional fuel rod loading devices of the pull-in rod type, and an objective of the invention is to present a loading apparatus of the pull-in rod type having a simple mechanical construction and operates efficiently and quickly to load the fuel rods into the fuel assembly, without the necessity for having a special selection device. The assembling apparatus of the present invention comprises, from the fuel rod entry-side of the assembling apparatus: (a) a fuel rod magazine aligned about a central axis and disposed on the entry-side end of the apparatus, for housing fuel rods extending longitudinally in parallel alignment; (b) support frames aligned about the central axis, for supporting the grids so that the grid cells face in the direction of longitudinally extending fuel rods; and (c) a pull-in loader for loading fuel rods, aligned about the central axis and freely rotatably disposed on the exit-side end of the apparatus opposite to the fuel rod magazine, the pull-in loader having: a plurality of pull-in rods provided with gripping means attached to the end thereof for gripping and loading the fuel rods in the fuel assembly, wherein (d) the pipe-cells, for inserting the control rod guide pipes, are situated in the lattice structure of the grid, so that the locations of pipe-cells in one quadrant of the grids correspond with the locations of pipe-cells in the remaining three quadrants when the one quadrant is rotated in steps of ninety degrees about the central axis of the grids; and (e) the pull-in loader is provided with: a plurality of pull-in rods, of a length sufficient to reach the entry-side end of the fuel rods by translating longitudinally along the central axis, aligned and corresponded with the locations of the fuel-rod-cells for firmly holding the fuel rods therein. According to the assembling apparatus of the present invention, it is possible to load one quarter of all the fuel rods required for the fuel assembly in one loading operation, simply by inserting the pull-in rods into the fuel rod magazine and after gripping the ends of the fuel rods, loading the fuel rods into the grid cells by retracting the pull-in rods back into the loader. In the above sequence of steps, there is no need to select the pull-in rods to be activated, every time the loader is rotated 90 degrees about the central axis for loading, so as to avoid those grid cells which are occupied by the guide pipes because the pull-in rods are already disposed to correspond with those grid cells which are intended for the fuel rods. 90 After one quarter of the required rods are loaded into the fuel assembly, the pull-in loader is rotated through 90 degrees, and the loading sequence of the pull-in rods is repeated to load the fuel rods into the next quadrant of the grids. Repeating the above steps for the remaining two quadrants of the grid, all the fuel rods are loaded into the fuel assembly. Such a simplified loading operation is made possible by duplicating the assigned locations of the pipe-cells in one quadrant of a grid exactly for the remaining three quadrants, so that for every 90 degree rotation of the pull-in loader, the pull-in rods are arranged to correspond with the grid cells assigned for the fuel rods in the grid, thus inserting the fuel rods into only the grid cells assigned for the fuel rods. The invented loading apparatus thus simplifies the loading process by not requiring the use of a pull-in rod selector which is needed in the conventional pull-in rod type fuel rod loaders.