Patent Number: 054328292
Section: claims

1. A fuel assembly comprising a number of fuel rods arrayed in a square lattice pattern, and at least one neutron moderating rod having a cross-sectional area of a moderator larger than a cross-sectional area of a unit lattice of the fuel rod array, wherein: (a) said fuel rods include a plurality of first fuel rods and one or more second fuel rods having a shorter fuel effective length than said first fuel rods;  (b) said second fuel rods are arranged in an outermost layer of said fuel rod array in the square lattice pattern at positions other than corners of the outermost layer; and  (c) among the fuel rods inside said outermost layer of said fuel rod array in the square lattice pattern and arranged in a layer adjacent to said outmost layer, all of the fuel rods adjacent to said second fuel rods in said outermost layer are said first fuel rods.  (a) said fuel rods include a plurality of first fuel rods and one or more second fuel rods having a shorter fuel effective length than said first fuel rods;  (b) said second fuel rods are arranged in an outermost layer of said fuel rod array in the square lattice pattern at positions other than corners of the outermost layer; and  (c) all of the fuel rods inside said outermost layer of said fuel rod array in the square lattice pattern and arranged in a layer adjacent to said outermost layer are said first fuel rods.  (a) said core includes a plurality of first fuel assemblies and a plurality of second fuel assemblies, said second fuel assemblies each comprise a number of fuel rods;  (b) said first fuel assemblies each comprise a number of fuel rods arrayed in a square lattice pattern and at least one neutron moderating rod having a cross-sectional area of a moderator larger than a cross-sectional area of a unit lattice of the fuel rod array, said fuel rods including a plurality of first fuel rods and one or more second than said first fuel rods, said second fuel rods being arranged in an outermost layer of said fuel rod array in the square lattice pattern at positions other than corners of the outermost layer, among the fuel rods inside said outermost layer of said fuel rod array in the square lattice pattern and arranged in a layer adjacent to said outermost layer, all of the fuel rods adjacent to said second fuel rods in said outermost layer being said first fuel rods; and  (c) said first fuel assemblies and said second fuel assemblies are loaded in a core central portion and a core circumferential portion, said first fuel assemblies having a smaller loading ratio in the core central portion than in the core circumferential portion. 2. A fuel assembly according to claim 1, wherein said second fuel rods in said outermost layer are arranged at positions other than the corners and positions adjacent to the corners. 3. A fuel assembly according to claim 1, wherein when said neutron moderating rod is projected in two directions orthogonal to each other in said fuel rod array in the square lattice pattern, said second fuel rods arranged in said outermost layer are located inside a projected range of said neutron moderating rod including the outermost opposite regions of the projected range. 4. A fuel assembly according to claim 1, wherein the cross-sectional area of the moderator in said neutron moderating rod is from 7 to 14 cm.sup.2. 5. A fuel assembly according to claim 1, wherein said neutron moderating rod is arranged in a region able to accommodate 7 to 12 said fuel rods. 6. A fuel assembly according to claim 1, wherein said fuel rods further comprise one or more third fuel rods having a shorter fuel effective length than said first fuel rods, said third fuel rods being arranged adjacent to said neutron moderating rod. 7. A fuel assembly according to claim 1, wherein the number of said second fuel rods arranged in said outer layer is larger than the number of said third fuel rods arranged adjacent to said neutron moderating rod. 8. A fuel assembly according to claim 1, wherein said fuel rods further comprise one or more third fuel rods having a shorter fuel effective length than said first fuel rods, said third fuel rods being arranged in a fuel rod layer adjacent to said outermost layer inside thereof at positions of corners of said fuel rod layer. 9. A fuel assembly according to claim 1, wherein said second fuel rods are arranged two adjacent to each other in said outermost layer. 10. A fuel assembly according to claim 1, wherein said second fuel rods have a fuel effective length in a range of 1/2 to 3/4 of the fuel effective length of said first fuel rods. 11. A fuel assembly according to claim 1, wherein said neutron moderating rod is circular in cross-section and arranged three in a lattice region of 4.times.4 at the center of said fuel assembly along a diagonal line of said lattice region. 12. A fuel assembly according to claim 1, wherein said neutron moderating rod is circular in cross-section and arranged two in a lattice region of 3.times.3 at the center of said fuel assembly along a diagonal line of said lattice region. 13. A fuel assembly according to claim 1, wherein said neutron moderating rod includes a spectral shift rod in which an axial water level is changed depending on a core flow rate. 14. A fuel assembly according to claim 1, wherein the fuel rods arranged inside said outermost layer of said fuel rod array in the square lattice pattern in one layer adjacent to said outermost layer and another one layer adjacent to said one layer are said first fuel rods. 15. A fuel assembly comprising a number of fuel rods arrayed in a square lattice pattern, and at least one neutron moderating rod having a cross-sectional area of a moderator larger than a cross-sectional area of a unit lattice of the fuel rod array, wherein: 16. A fuel assembly according to claim 15, wherein said second fuel rods in said outermost layer are arranged at positions other than the corners and positions adjacent to the corners. 17. A fuel assembly according to claim 15, wherein when said neutron moderating rod is projected in two directions orthogonal to each other in said fuel rod array in the square lattice pattern, said second fuel rods arranged in said outermost layer are located inside a projected range of said neutron moderating rod including the outermost opposite regions of the projected range. 18. A fuel assembly according to claim 15, wherein said neutron moderating rod is arranged in a region able to accommodate 7 to 12 said fuel rods. 19. A fuel assembly according to claim 15, wherein said fuel rods further comprise one or more third fuel rods having a shorter fuel effective length than said first fuel rods, said third fuel rods being arranged adjacent to said neutron moderating rod. 20. A fuel assembly according to claim 15, wherein the number of said second fuel rods arranged in said outer layer is larger than the number of said third fuel rods arranged adjacent to said neutron moderating rod. 21. A fuel assembly according to claim 15, wherein said second fuel rods are arranged two adjacent to each other in said outermost layer. 22. A fuel assembly according to claim 15, wherein said second fuel rods have a fuel effective length in a range of 1/2 to 3/4 of the fuel effective length of said first fuel rods. 23. A fuel assembly according to claim 15, wherein said neutron moderating rod is circular in cross-section and arranged three in a lattice region of 4.times.4 at the center of said fuel assembly along a diagonal line of said lattice region. 24. A fuel assembly according to claim 15, wherein said neutron moderating rod is circular in cross-section and arranged two in a lattice region of 3.times.3 at the center of said fuel assembly along a diagonal line of said lattice region. 25. A fuel assembly according to claim 15, wherein said neutron moderating rod includes a spectral shift rod in which an axial water level is changed depending on a core flow rate. 26. A fuel assembly according to claim 15, wherein the fuel rods arranged inside said outermost layer of said fuel rod array in the square lattice pattern in one layer adjacent to said outermost layer and another one layer adjacent to said one layer are said first fuel rods. 27. A fuel assembly according to claim 15, wherein the lattice pattern of said fuel rod array comprises 10 rows and 10 columns. 28. A reactor core using light water as a coolant, wherein: 29. A reactor core according to claim 28, wherein in said first fuel assemblies, said second fuel rods in said outermost layer are arranged at positions other than the corners and positions adjacent to the corners. 30. A reactor core according to claim 28, wherein said second fuel assemblies include no fuel rods having a shorter fuel effective length than said first fuel rods. 31. A reactor core according to claim 28, wherein said second fuel assemblies include one or more third fuel rods having a shorter fuel effective length than said first fuel rods, the number of said third fuel rods being smaller than the number of said second fuel rods.