Patent Number: 052727432
Section: summary

BACKGROUND ART The present invention pertains to a key member for use in inserting fuel rods into a nuclear fuel assembly grid or in removing the fuel rods from the nuclear fuel assembly grid, and a method for insertion or removal of fuel rods using the same key member. The invention further relates to a method for disassembling a nuclear fuel assembly using the same key member. In a known nuclear fuel assembly, a pair of top and bottom nozzles are arranged in a spaced relation to each other, and a plurality of grids each formed of straps are disposed between the nozzles so as to be spaced from each other. Control-rod guide pipes and measuring instrument pipes are inserted through the grid cells of the grid, fixedly secured to the grid, and connected to the nozzles. A plurality of fuel rods are inserted through the grid cells of the grids and resiliently supported by springs formed on the grids. In a conventional assembly method of the nuclear fuel assembly as constructed above, if the fuel rods are simply inserted into the grid cells, the fuel rods are constricted between with the springs and dimples on the grids, so that the outer peripheral surface of each fuel rod may be scratched. Various proposals have hitherto been made in order to circumvent this problem. For example, as disclosed in U.S. Pat. No. 5,068,081, a method utilizing a deflecting jig of an expander type capable of being enlarged in diameter has been proposed. In this method, an opening for inserting a key member is formed in each grid, and the springs are deflected by inserting the jig into the grid cells of the grid and enlarging the same. Then, the key member is inserted through the opening to maintain the springs deflected, and fuel rods are smoothly inserted through the grid cells. However, in the above method, the deflecting jig must be used since the key member is not provided with the function to deflect the springs, and hence, in addition to the necessity of preparing the deflecting jig, an extra step of applying the deflecting jig is required. Furthermore, when it is necessary to dismantle the nuclear fuel assembly, if the fuel rods can be removed from the grid by, for example, inserting the key member through the opening and operating the same to release the engagement of the fuel rods with the grids, the fuel rods thus removed can be put into repeated use for a new nuclear fuel assembly. However, once the assembling of the nuclear fuel assembly is completed, the opening for the insertion of the key member is obstructed by the control-rod guide pipes or measuring instrument pipes, so that the key member cannot be inserted any longer. Therefore, it has been desired to devise an arrangement to provide access to the space for the insertion of the key member. SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide a novel key member which can release the constrictive engagement of the fuel rods with the springs and dimples, without using a special device such as the aforementioned spring-deflecting jig, and ensuring sufficient cell sizes required for the assembling and/or disassembling of the nuclear fuel assembly. Another object of the present invention is to provide a method for insertion and removal of fuel rods which can be effected very efficiently using the aforesaid key member, and by which the fuel rods can be easily inserted into and removed from the grid without being damaged. A further object of the invention is to provide a method for disassembling a nuclear fuel assembly which is particularly suitably practiced using the aforesaid key member. According to a primary aspect of the present invention, there is provided a key member for use in insertion and removal of a fuel rod into and from a nuclear fuel assembly grid which includes a plurality of elongated straps intersecting generally perpendicularly 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 such that each pair of dimple and spring are disposed in facing relation to each other, and which includes key member-accommodating openings formed at the intersections of the straps so as to define peripheral ends therearound, the key member comprising an elongated key body having a pair of opposite surfaces, the key body including a plurality of first projections adapted to be held in engagement with the peripheral end of the opening of the strap and a plurality of second projections adapted to be held in engagement with the spring to deflect the spring away from the dimple facing thereto, the first projections and the second projections being formed on the opposite surfaces alternately in a staggered relation to one another. According to another aspect of the present invention, there is provided a method for insertion and removal of a fuel rod into and from a nuclear fuel assembly grid which includes a plurality of elongated straps intersecting generally perpendicularly 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 such that each pair of dimple and spring are disposed in facing relation to each other, and which includes key member-accommodating openings formed at the intersections of the straps so as to define peripheral ends therearound, the method comprising the steps of: (a) preparing a plurality of key members each including an elongated key body having a longitudinal axis and a pair of opposite surfaces, the key body including a plurality of first projections and a plurality of second projections formed on the opposite surfaces alternately in a staggered relation to one another; (b) inserting each of the key members in the grid while causing the key member to pass through the openings of the straps disposed parallel to each other in such a manner as to extend along a respective strap intersecting the aforesaid parallel straps; (c) turning each of the key members about the longitudinal axis to bring the plurality of first projections into engagement with the peripheral ends of the openings of the aforesaid parallel straps and to bring the plurality of second projections into engagement with the springs on the aforesaid strap intersecting the parallel straps to deflect the springs away from the dimples opposing thereto; and (d) inserting the fuel rods into the grid cells of the grid or removing the same therefrom. According to a further aspect of the invention, there is provided a method for disassembling a nuclear fuel assembly which includes top and bottom nozzles disposed in spaced relation to each other, a plurality of grids disposed between the top nozzle and the bottom nozzle so as to be spaced from each other and to define grid cells, a plurality of pipes connected to the top and bottom nozzles for accommodating control-rods and measuring instrument, a plurality of pairs of dimples and springs formed on the straps for supporting a plurality of fuel rods such that each pair of dimple and spring are disposed in facing relation to each other, the method comprising the steps of: (a) preparing a plurality of key members each including an elongated key body having a longitudinal axis and a pair of opposite surfaces, the key body including a plurality of first projections and a plurality of second projections formed on the opposite surfaces alternately in a staggered relation to one another; (b) working the plurality of pipes to ensure spacings in the grid for inserting the key members; (c) inserting each of the key members into the grid while causing the key member to pass through the spacings formed by the working of the pipes; (d) turning each of the key members about the longitudinal axis to bring the plurality of first projections into engagement with the peripheral ends of the openings of the straps disposed parallel to each other and to bring the plurality of second projections into engagement with the springs on a respective strap intersecting the aforesaid parallel straps to deflect the springs away from the dimples opposing thereto; and (e) subsequently removing the fuel rods from the grid cells of the grid. In the foregoing, the working step (b) may include cutting the pipes transversely thereof at positions adjacent to the openings at the intersections of the straps, and enlarging the pipes at the cut portions to reduce the longitudinal lengths of the pipes to thereby ensure the aforesaid spacings for inserting the key members. Furthermore, the same working step may include forming slots at bulged portions of the pipes, and removing the pipes to thereby ensure spacings for inserting the key members.