Patent Number: 051046091
Section: claims

1. An assembly method for a nuclear fuel assembly, comprising the steps of: preparing at least one first grid member with springs on wall sections thereof, and preparing at least one second grid member with dimples on wall sections thereof;  arranging the first grid member and the second grid member face to face such that a plurality of grid cells of the first grid member and a plurality of grid cells of the second grid member communicate with each other and that said dimples of the second grid member and said springs of said first grid member are disposed on parallel planes which are opposed to each other;  providing a sleeve on one of said first and second grid members for allowing lateral shifting of said first grid member with respect to said second grid member,  retaining the first and second grid members by a grid retainer,  inserting a guide thimble into a thimble cell of the first and second grid members while retaining a clearance between the first and second grid members,  fixing the guide thimble to the second grid member,  inserting fuel rods respectively into the grid cells of the first grip member and the grid cells of the second grid member such that the springs of said grid member and the dimples of the second grid member are shifted relative to each other in such a direction that the springs and the dimples move away from each other;  subsequently moving at least one of the first grid member and the second grid member such that the grid cells in the first grid member and the grid cells in the second grid member are in alignment with each other; and  connecting the first grid member and the second grid member to each other. 2. An assembly method according to claim 1, including preparing a pair of said at least one second grid members, arranging said first grid member and said pair of second grid members such that the three grid members are superimposed on each other with said first grid member interposed between said pair of second grid members, and connecting said superimposed three grid members to each other by an outer frame. 3. An assembly method according to claim 2, wherein each of said three grid members includes a rectangular outer strap and a plurality of inner straps, and which comprises mounting said outer strap to four sides corresponding to ends of said innerstraps which are assembled together into a grid form to define the grid cells, each of said second grid members having a plurality of pairs of dimples, each of said pair of dimples being provided respectively on adjacent two of four wall sections formed by the inner and outer straps of the second grid member, said four wall sections defining one of the grid cells of each of the second grid members, said first grid member having a plurality of pairs of springs, each pair of springs being provided respectively on adjacent two of four wall sections formed by the inner and outer straps of the first grid member with springs, and said four wall sections defining one of the grid cells of the grid member with springs. 4. As assembly method according to claim 3, including, prior to inserting the fuel rods, mounting a part of the outer frame onto a Y-shaped support, mounting an assembly of said three grid members onto the Y-shaped support, and after inserting the fuel rods into the respective grid cells of the pair of second grid members and the grid cells of the first grid member, moving the first grid member such that the grid cells in the pair of second grid members and the grid cells in the first grid member are aligned with each other, and fixing the other part of the outer frame to the assembly of the three grid members and to said one part of the outer frame. 5. An assembly method according to claim 1, including arranging a pair of the firs grid members and a pair of the second grid members such that the four grid members are superimposed on each other with the pair of first grid members interposed between the pair of second grid members, and connecting the superimposed four grid members to each other by an outer frame. 6. An assembly method according to claim 5, wherein each of said four grid members comprises an outer strap and a plurality of inner straps, said outer strap being mounted to four sides corresponding to the ends of said inner straps which are assembled together into a grid form to define the grid cells, each of said pair of second grid members having a plurality of pairs of dimples each pair provided respectively on adjacent two of four wall sections of the inner and outer straps of the second grid members, said four wall sections defining one of the grid cells of the second grid members, each of said pair of first grid members having a plurality of springs each provided on one of four wall sections of the inner and outer straps of the first grid members, said four wall sections defining one of the grid cells of the first grid members, and wherein the springs of the pair of first grid members are arranged in facing relation to the dimples of said pair of second grid members. 7. An assembly method according to claim 6, including, prior to inserting the fuel rods, retaining the four grid members onto a grid retainer, inserting a guide thimble into thimble cells of the respective four grid members, while retaining slight clearances respectively between the four grid members, bulge-fixing the guide thimble to the respective pair of second grid members, mounting one part of the outer frame onto a Y-shaped support, and, after mounting an assembly of said four grid members onto the Y-shaped support, inserting the fuel rods respectively into the grid cells of the pair of second grid members and the grid cells of the pair of first rid members, moving the pair of first grid members such that the grid cells in the pair of second grid members and the grid cells in the pair of first grid members are aligned to each other, and fixing the other part of the outer frame to the assembly of the four grid members and to said one part of the outer frame.