Patent Number: 051924960
Section: claims

1. A fuel assembly comprising: a plurality of fuel rods arranged in a square 10.times.10 lattice other than at a central region thereof corresponding to a rectangular space for accommodating an arrangement of 4.times.4 fuel rods;  four fuel rods arranged so that a respective one of the four fuel rods is disposed at each of four corner portions of the central region; and  water rod means including one of a plurality of water rods and a plurality of spectral shift water rods being disposed in the central region, one of the plurality of water rods and the plurality of spectral shift water rods being arranged adjacent to and spaced form one another along a circular path so as to delimit intervals therebetween in the central region in a space of the central region other than the four corner portions for accommodating 12 fuel rods. 2. A fuel assembly according to claim 1, wherein a plurality of water rods are provided, each of the water rods having a large diameter. 3. A fuel assembly according to claim 1, wherein a plurality of spectral shift water rods are provided, each of the spectral shift water rods having an internal liquid level adjustment by control of coolant flow rate in a reactor core. 4. A fuel assembly according to claim 1, wherein one of the plurality of water rods and the plurality of spectral shift rods are arranged to surround a central portion of the central region and enable formation of a first coolant flow path which communicates with second coolant flow paths formed at fuel rods surrounding the central region. 5. A fuel assembly according to claim 4, wherein the first coolant flow path communicates with the second coolant flow paths through the intervals delimited between the water rod means. 6. A fuel assembly according to claim 1, wherein the plurality of fuel rods arranged in the square lattice includes at least first fuel rods and the four fuel rods arranged at the four corners of the central region are second fuel rods. 7. A fuel assembly according to claim 6, wherein the second fuel rods have a shorter axial length than an axial length of the first fuel rods. 8. A fuel assembly according to claim 2 wherein the plurality of fuel rods arranged in the square lattice includes at least first fuel rods and the four fuel rods arranged at the four corners of the central region are second fuel rods. 9. A fuel assembly according to claim 8, wherein each of the water rods has a large cross-section and a horizontal cross-sectional area corresponding to a cross-sectional area of three first fuel rods, the plurality of water rods including four water rods arranged along the circular path. 10. A fuel assembly according to claim 8, wherein the second fuel rods have a shorter axial length than an axial length of the first fuel rods. 11. A fuel assembly according to claim 8, further comprising an upper tie plate for supporting upper end portions of at least the first fuel rod, the upper tie plate delimiting an opening corresponding to a central portion of the central region delimited by the plurality of water rods arranged along the circular path. 12. A fuel assembly according to claim 8, wherein the fuel rods arranged in the square lattice further include third fuel rods, the third fuel rods having an axial length shorter than an axial length of the first fuel rods and being arranged in the outermost portion of the square 10.times.10 lattice. 13. A fuel assembly according to claim 12, wherein the fuel rods arranged in the square lattice include third fuel rods, the third fuel rods having an axial length longer than an axial length of the second fuel rods.