Patent Number: 
Section: claims

1. A fuel assembly comprising: a plurality of fuel rods placed in a square lattice array of n-rows/n-columns (n: integer);  at least one water rod placed in a region in which one or more of said fuel rods can be placed;  a lower tie plate for supporting the lower end of a fuel bundle composed of said plurality of fuel rods and said at least one water rod; and  a guide post provided for fixing a channel fastener;  wherein said integer n is set at n=10;  the fuel rod pitch of said plurality of fuel rods is in a range of 12.65 mm to 13.15 mm; and  means for offsetting and holding said fuel bundle is provided in such a manner that the center in a cross section of said fuel bundle is offset from the center in a cross section of said lower tie plate toward said channel fastener side, the center in a cross section of said lower tie plate being equal to the center in a cross section of a channel box for covering said fuel bundle. 2. A fuel assembly comprising: a plurality of fuel rods placed in a square lattice array of n-rows/n-columns (n: integer);  at least one water rod placed in a region in which one or more of said fuel rods can be placed;  a lower tie plate for supporting the lower ends of a fuel bundle composed of said plurality of fuel rods and said at least one water rod; and  a guide post provided for fixing a channel fastener;  wherein said integer n is set at n=10;  the fuel rod pitch of said plurality of fuel rods is in a range of 12.65 mm to 13.15 mm; and  means for offsetting and holding said fuel bundle is provided in such a manner that the center in a cross section of said fuel bundle is offset a value Y greater than 2-3/2 mm from the center in a cross section of said lower tie plate toward said channel fastener side, the center in a cross section of said lower tie plate being equal to the center in a cross section of a channel box for covering said fuel bundle. 3. A fuel assembly according to  claim 2 , wherein said value Y is in a range of 7xc3x972 xe2x88x921/2  mmxe2x89xa7Yxe2x89xa72 xe2x88x923/2  mm. claim 2 4. A fuel assembly comprising: a plurality of fuel rods placed in a square lattice array of n-rows/n-columns (n: integer);  at least one water rod placed in a region in which one or more of said fuel rods can be placed;  a lower tie plate for supporting the lower end of a fuel bundle composed of said plurality of fuel rods and said at least one water rod;  fuel spacers for holding a plurality of axial positions of said fuel bundle; and  a guide post provided for fixing a channel fastener;  wherein said integer n is set at n=10;  the fuel rod pitch of said plurality of fuel rods is in a range of 12.65 mm to 13.15 mm;  each of said fuel spacers includes a plurality of tabs for offsetting and holding said fuel bundle in such a manner that the center in a cross section of said fuel bundle is offset from the center in a cross section of said lower tie plate toward said channel fastener side, the center in a cross section of said lower tie plate being equal to the center in a cross section of a channel box for covering said fuel bundle, said tabs being provided in such a manner as to project outwardly from the outer periphery of said fuel spacer; and  a distance L 1  between the leading end of one of said plurality of tabs positioned on the channel fastener side and one of said fuel rods positioned at the outermost periphery of said square lattice array and a distance L 2  between the leading end of one of said plurality of tabs positioned on the anti-channel fastener side and one of said fuel rods positioned at the outer most periphery of said square lattice array are determined in such a manner as to satisfy a relationship of L 2 xe2x88x92L 1  greater than 0.5 mm. 5. A fuel assembly according to  claim 4 , wherein said distances L 1  and L 2  are determined in such a manner as to satisfy a relationship of 7.0 mmxe2x89xa7L 2 xe2x88x92L 1 xe2x89xa70.5 mm. claim 4 6. A fuel assembly comprising: a plurality of fuel rods placed in a square lattice array if n-rows/n-columns (n: integer);  at least one water rod placed in a region in which one or more of said fuel rods can be placed;  a channel box provided in such a manner as to surround a fuel bundle composed of said plurality of fuel rods and said at least one water rod; and  a guide post provided for fixing a channel fastener;  wherein said integer n is set at n=10;  the fuel rod pitch of said plurality of fuel rods is in a range of 12.65 mm to 13.15 mm;  the inner width of said channel box is in a range of 133.5 mm to 134.5 mm; and  means for offsetting and holding said fuel bundle is provided in such a manner that the center in a cross section of said fuel bundle is offset from the center in a cross section of said channel box toward said channel fastener side, the center in a cross section of said channel box being equal to the center in a cross section of a lower tie plate for supporting the lower end of said fuel bundle.