Patent Number: 048760625
Section: claims

1. A fuel assembly comprising a plurality of fuel rods and a moderating rod containing a moderator and disposed at a generally central portion, of said fuel assembly, the ratio A.sub.M /A.sub.C of the area A.sub.M of a moderator region in said moderating rod in the cross-sectional plane in which said moderator is present to the area A.sub.C of the coolant passages in said fuel assembly being within the range of 0.07 to 0.11, and said area A.sub.M being 75% or more of the total area of the fuel lattice units in which none of said fuel rods is arranged, but said moderating rod is arranged. 2. A fuel assembly according to claim 1, wherein if the average enrichment of the fuel rods of said fuel rods arranged in the periphery of said fuel assembly is E.sub.A, the average enrichment of the fuel rods which are adjacent to said moderating rod and each face the two sides thereof is E.sub.B, and average the enrichment of the other fuel rods is E.sub.C, the inequality E.sub.A &lt;E.sub.B &lt;E.sub.C is established. 3. A fuel assembly comprising a moderating rod containing a moderator and a plurality of fuel rods which are arranged around said moderating rod, said plurality of fuel rods being arranged in a lattice form having 9 rows and 9 columns so as to surround said moderating rod, the ratio A.sub.M /A.sub.C of the area A.sub.M of a moderator region in said moderating rod in the cross-sectional plane in which said moderator is present to the area A.sub.C of the coolant passages in said fuel assembly being within the range of 0.07 to 0.11, and said area A.sub.M being 75% or more of the total area of the fuel lattice units in which none of said fuel rods is arranged, but said moderating rod is arranged. 4. A fuel assembly according to claim 3, wherein if the average enrichment of the fuel rods of said fuel rods arranged in the periphery of said fuel assembly is E.sub.A, the average enrichment of the fuel rods which are adjacent to said moderating rod and each face the two sides thereof is E.sub.B, and average the enrichment of the other fuel rods is E.sub.C, the inequality E.sub.A &lt;E.sub.B &lt;E.sub.C is established. 5. A fuel assembly comprising a moderating rod containing a moderator and having a cruciate cross-sectional form of a region in which said moderator is present, and a plurality of fuel rods arranged around said moderating rod so as to surround it, said fuel rods being arranged between the sides of said moderating rod and sides of the said fuel assembly, the ratio A.sub.M /A.sub.C of the area A.sub.M of a moderator region in said moderating rod in the cross-sectional plane in which said moderator is present to the area A.sub.C of the coolant passages in said fuel assembly being within the range of 0.07 to 0.11, and said area A.sub.M being 75% or more of the total area of the fuel lattice units in which none of said fuel rods is arranged, but said moderating rod is arranged. 6. A fuel assembly according to claim 5, wherein the projecting portions of a cruciate form of said moderating rod are placed at right angles with respect to the sides of said fuel assembly. 7. A fuel assembly according to claim 5, wherein if the average enrichment of the fuel rods of said fuel rods arranged in the periphery of said fuel assembly is E.sub.A, the average enrichment of the fuel rods which are adjacent to said moderating rod and each face the two sides thereof is E.sub.B, and average the enrichment of the other fuel rods is E.sub.C, the inequality E.sub.A &lt;E.sub.B &lt;E.sub.C is established. 8. A fuel assembly according to claim 7, wherein the projecting portions of a cruciate form of said moderating rod are placed at right angles with respect to the sides of said fuel assembly. 9. A fuel assembly according to claim 5, wherein said moderating rod has a passage through which a coolant serving as a moderator for a reactor, a means which is provided at the lower end of said moderating rod and introduces said coolant into said passage from the outside thereof, and a means which is provided at the upper end thereof and discharges said coolant from said passage to the outside. 10. A fuel assembly according to claim 5, wherein said moderating rod has a cylindrical body which is sealed at the upper and lower ends thereof and has a cruciate cross-sectional form, a lower end plug which is engaged with said lower tie plate at the lower end of said mode-rating rod, and a cylindrical member which is provided in said cylindrical body in a state wherein part of said cylindrical member is inserted into the center of said cylindrical body. 11. A fuel assembly according to claim 5, wherein said moderating rod has a shape at its lower portion in which the cross-sectional area thereof upwardly and gradually increases.