Patent Number: 053965264
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for assembling a fuel assembly and more specifically to an apparatus for mechanically or automatically removing the keys after loading the fuel rods. The keys are inserted into support grids of a fuel assembly for deflecting the springs away from the dimples prior to loading the fuel rods into the supporting grids. 2. Background Technology In general, pressurized light water nuclear reactors utilize a type of fuel assembly comprising: an upper nozzle and a lower nozzle; a plurality of supporting grids for supporting the fuel rods (shortened to grids henceforth) disposed between the nozzles with a certain spacing; instrumentation pipes and fuel rod guide pipes inserted into and attached to the grid cells of the grids; and a plurality of fuel rods inserted into and held elastically in the grid cells with the springs formed inside the grid cells. When assembling such a fuel assembly, there is a danger of introducing scratches on the surface of the fuel rods if the rods are inserted into the grid cells with the springs and dimples as they are, because the springs and dimples are formed on the inside walls of the grid cells. To prevent damaging the surface of the fuel rods, there have been various methods suggested to deal with the prevention of surface damage problem. For example, a method proposed in a U.S. Pat. No. 5,068,081 proposed the following fuel assembly. In this fuel assembly, key insertion opening are formed in the grids, and the spring deflection jigs are inserted into the grid cells to deflect the springs away from the dimples formed in each wall of the grid cell for elastically holding the fuel rod. The keys are inserted through the key openings to keep the springs in the deflected position, so as to allow the fuel rods to be inserted into the grid cells smoothly without the danger of causing surface damages. However, such key insertion and removal operations are designed to be carried out manually, thus causing an operational problem because the tasks are cumbersome, the operations is lengthy and the efficiency is low. In particular, the workers involved in the key removal operation were required to work in the proximity of the fuel rods, and were particularly vulnerable to radiation exposure. From these viewpoints, there has been a need to automate the key removal operation. SUMMARY OF THE INVENTION The present invention was made in view of the background described above, and the objective is to present a key removal apparatus which can carry out the task of removing the keys from a loaded assembly smoothly and reliably, and offer opportunities for mechanization and automation. The above objective is achieved in an apparatus for removing keys from a grid of a fuel assembly having fuel rods inserted in a plurality of grid cells, formed by a plurality of straps of a thin longitudinal strip form intersecting at right angles to each other, having dimples formed on one adjacent pair of walls of said grid cells and springs formed on opposing pair of walls of said grid cells, wherein a plurality of keys which had been inserted in said grid in the longitudinal direction of said strip form through opening sections formed near the intersections of said plurality of straps are removed by said apparatus comprising; at least one key rotation means, which rotates one set of said plurality of keys in one rotational direction about the key axis, and the other set of said plurality of keys in the opposite rotational direction; and at least one key moving means, for moving said plurality of keys in the direction of the key axis. Accordingly, when the keys are used to deflect the springs away from the dimples in a grid having the springs and the dimples disposed rotationally symmetrically in four quadrants of the grid cells, the plurality of keys in any one line of grid cells are disposed so that the keys are oriented symmetrically with respect to each of the two diametrical lines passing through the center O of the grid. Therefore, the apparatus of the present invention enables a set of keys to be rotated in the relative opposite directions depending on the location of the keys in the line of keys. By so doing, all the keys in one line of keys can be disengaged from the grid, and the line of keys are removed from the cells by pulling them out of the grid cells by the key moving devices. The apparatus thus enables the keys to be removed completely from the grid, and allows mechanization and automation of the key removal operation.