Patent Number: 053596331
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION Referring first to FIG. 1, there is shown, in an enlarged fragmentary perspective view, one of a plurality of grids 20 in a nuclear fuel assembly which is mounted on a nuclear reactor such as a pressurized water reactor or the like. In this connection, the nuclear fuel assembly is similar in construction to that described previously with reference to FIG. 10, and the description of the nuclear fuel assembly will therefore be omitted to avoid duplication. The grid 20 is composed of a plurality of elongated straps 21 each in the form of a strip sheet, which are assembled into a lattice. The straps 21 cooperate with each other to define a plurality of grid cells 23 each of which is substantially rectangular in shape. Each of the elongated straps 21 is formed with a plurality of cut-outs 22 in a widthwise central portion of the strap 21. The cut-outs 22 in the strap 21 are each in the form of a rectangle and are spaced a predetermined distance from each other in a longitudinal direction of the strap 21. A spring 24 is formed between each pair of adjacent cut-outs 22 and 22 in the strap 21, and is projected into the grid cell 23. The spring 24 is formed such that a wall section of the strap 21 is cut at two locations, and a portion between the two cuts is projected into the grid cell 23. The spring 24 is formed at its center with a projection 24a which is in pressure contact with a fuel rod 36 (refer to FIGS. 8 and 9). A pair of wall sections of the strap 21 in the longitudinal direction thereof, between which the spring 24 is located, are formed into a pair of ribs 25 and 25, one of which being engaged with a corresponding one of a plurality of hooks of a key member subsequently to be described. A pair of dimples 26 and 26 are formed at respective positions in the widthwise direction of the strap 21, between which the spring 24 is located. The dimples 26 and 26 are projected in a direction opposite to the spring 24. A plurality of straps 21 each constructed as described above are assembled into a grid thereby forming the grid 20 having a plurality of grid cells 23 into which the fuel rods 16 are inserted respectively. In each of the grid cells 23, the two pairs of springs 24 and dimples 26, which are projected into the grid cell 23, are arranged on the wall sections of the respective straps 21 in respectively facing relation to each other. Further, the cut-outs 22 in one of the straps 21 and the cut-outs 22 in the other strap 21 are combined with each other respectively at intersections between the straps 21 to form a plurality of openings 27. The openings 27 formed respectively at the intersections are arranged straight along the longitudinal direction of the strap 21. Each of a plurality of elongated key members 30 is capable of being inserted into these openings 27 which are arranged straight along the strap 21. The elongated key member 30 is in the form of a rod having a rectangular cross-section, and being longer than one side of the grid 20. The key member 30 has a plurality of hooks 31 formed on one side. The hooks 31 are oriented in the same direction, and are spaced a predetermined distance from each other in the longitudinal direction of the key member 30. In other words, the hooks 31 are spaced from each other by a distance identical with that by which the cut-outs 22 formed in the strap 21 are spaced from each other. The hooks 31 are each in the form of an L-shape, and are engaged respectively with the ribs 25 to clamp the ribs 25 and the springs 24 between the hooks 31 and the body of the key member 30. Thus, the hooks 31 hold the springs 24 such that the springs are deflected so as to be flush with the ribs 25. A method of assembling the nuclear fuel assembly, which uses the grid 20 constructed above, will next be described with reference to FIGS. 2 through 9. As shown in FIG. 2, each of a plurality of deflecting jigs 32 in the form of a rod having a circular cross-section is inserted into a corresponding one of the plurality of grid cells 23 in the grid 20. Here, as shown in FIGS. 2 and 3, the deflecting jig 32 is such that a tapered pin 34 is inserted for axial sliding movement in a sleeve 33 divided into four sleeve pieces 33a which are arranged circumferentially. Withdrawal or retraction of the tapered pin 34 causes the four sleeve pieces 33a of the sleeve 33 to be urged or pushed outwardly so that the sleeve 33 is opened, that is, its diameter is expanded or enlarged. Subsequently, the tapered pin 34 of the deflecting jig 32 inserted in each of the grid cells 23 is retracted. Whereupon, as shown in FIG. 4, the sleeve 33 is enlarged to urge the projection 24a on the spring 24 against the resilient or elastic force thereof. Thus, the spring 24 is deflected away from the dimples 26 facing thereto, so that the spring 24 is made substantially flush with the ribs 25. Under the above condition, as shown in FIG. 5, the key member 30 is inserted into the openings 27 from the side face of the grid 20, to position the hooks 31 of the key member 30 respectively at the openings 27. At this time, the hooks 31 are oriented upwardly in FIG. 1, that is, are located in a vertical plane. Subsequently, the key member 30 is rotated about its axis through 90 degrees to position the hooks 31 within the openings 27, that is, to orient the hooks 31 horizontally. Thus, the hooks 31 project from the wall surface of the strap 21 in a direction opposite to the projecting direction of the spring 24. Subsequently, as shown in FIG. 6, the key member 30 is moved forwardly or advanced along the longitudinal direction of the strap 21 as indicated by an arrow. The hooks 31 are engaged respectively with the ribs 25 to fixedly mount the key member 30 to the strap 21. Thus, the spring 24 is maintained deflected. In this connection, a plurality of key members 30 are inserted into the openings 27 in the straps 21 from both directions, that is, from the longitudinal and lateral directions, to hold each pair of springs 24 and 24 projecting into the same grid cell 23 under such a condition that the springs 24 are deflected. Subsequently, as shown in FIG. 7, the tapered pin 34 of the deflecting Jig 32 is pushed or returned to release the urging of the springs 24 due to the sleeve 33, thereby enabling the deflecting Jig 32 to be removed from the grid cell 23. In this manner, even though the urging of the springs 24 is released, the springs 24 are held under such a condition that the springs 24 are deflected by the key members 30. A predetermined number of grids 20, in which the key members are disposed as described, are arranged in spaced relation to each other by a predetermined distance. Subsequently, as shown in FIG. 8, each of the fuel rods 36 is gripped by a pull-in rod (not shown) to draw the fuel rods 36 into the respective corresponding grid cells 23 in a state of non-contact with the springs 24 and the dimples 26. Thus, the fuel rod 16 rests against the dimples 26. Subsequently, the key members 30 are moved rearwardly, that is, are retracted to disengage the hooks 31 respectively from the ribs 25, thereby releasing retention of the springs 24. Whereupon, as shown in FIG. 9, the springs 24 come into pressure contact with the fuel rod 36. Thus, the fuel rod 36 is clamped between the two pairs of springs 24 and dimples 26, and is fixedly mounted to the grid 20. In addition, the key member 30 is rotated through 90 degrees in a direction opposite to that described above, to return the hooks 31 upwardly within the openings 27. Thus, the key member 30 is withdrawn or removed from the grid 20. Subsequently, a plurality of control-rod guide thimbles similar to those illustrated in FIG. 10 are inserted into the predetermined grid cells 23, and are fixedly mounted thereto. Lastly, a pair of top and bottom nozzles, which are similar to the pair of top and bottom nozzles 1 and 2 illustrated in FIG. 10, are mounted to the nuclear fuel assembly to complete assembling thereof. According to the assembly method of the nuclear fuel assembly, which uses the grids 20, a plurality of fuel rods 36 are inserted respectively into the grid cells 23 in each of the grids 20 such that each of the springs 24 is moved away from corresponding two of the dimples 26 facing thereto by the key member 30, and the spring 24 is maintained deflected by the key member 30. Under such condition, the distance between the spring 24 and the dimples 26 facing thereto increases more than the diameter of the fuel rod 36. Accordingly, the fuel rod 36 is not in sliding contact with the springs 24 and the dimples 26 and the outer peripheral surface of the fuel rod 36 is not clamped between the springs 24 and the dimples 26. Thus, no scratches occur on the outer peripheral surface of the fuel rod 36. Further, when the fuel rod 36 is drawn into the grid cell 23, the fuel rod 36 is not clamped between the springs 24 and the dimples 26. Accordingly, no unreasonable tension is applied to the fuel rod 36, and it is possible to reduce a power source for drawing the fuel rod 36 into the grid cell 23. Thus, it is possible to make a drawing unit or the like compact. Further, since the pair of ribs 25 and 25 are provided at respective positions between which the spring 24 is located, it is possible to obtain stabilized spring characteristics by the ribs 25 and 25. In addition, the deflecting jig 32 is composed of the sleeve 33 divided circumferentially into the four sleeve pieces 33a, and the tapered pin 34 inserted into the sleeve for axial sliding movement. Drawing of the tapered pin 34 into the sleeve 33 causes the four divided sleeve pieces 33a of the sleeve 33 to be urged and moved outwardly, whereby the springs 24 are deflected. Thus, when the springs 24 are deflected, no torsion stress is applied to the springs 24, so that strain, cracks and so on that could be caused by the torsion stresses do not occur on the springs 24. Further, each of the hooks 31 of the key member 30 is engaged with one of the ribs 25 and 25 provided at locations between which the spring 24 is located. Thus, with the ribs 25 and 25 subjected to a reaction force of the spring 24, the spring 24 is maintained deflected. Thus, no undesirable influence such as torsion or the like is exerted upon the spring 24.