Patent Number: 051868903
Section: claims

1. A fuel assembly for a fast breeder reactor comprising a plurality of nuclear fuel rods, and fuel rod containment means for containing the plurality of nuclear fuels rods therein, the fuel rod containment means having an hourglass shape. 2. A fuel assembly according to claim 1, wherein the fuel rod containment means is an elongated tubular member having inner and outer diameters at a middle portion thereof which are smaller than inner and outer diameters at axial end portions thereof. 3. A fuel assembly according to claim 2, wherein a wall thickness of the fuel rod containment means at a portion adjacent a gas plenum portion of the fuel rod is thinner than a wall thickness at a middle portion of the axial length of the fuel containment means having the smaller inner and outer diameters. 4. A fuel assembly according to claim 2, wherein the fuel containment means has a shape such that a wall thickness at a portion adjacent axial end portions of the fuel containment means is thinner than a wall thickness at a portion adjacent to a middle portion of the axial length of the fuel rod containment means. 5. A fuel assembly according to claim 2, wherein the fuel rod containment means has a shape such that a diameter is larger at a portion adjacent axial ends of the fuel containment means than a diameter at a portion adjacent a middle portion of the axial length of the fuel rod containment means. 6. A fuel assembly for a fast breeder reactor comprising a plurality of nuclear fuel rods, and fuel rod containment means for containing the plurality of fuel rods therein, the fuel rod containment means having a shape so that a portion thereof subjected to a larger neutron irradiation dose than other portions thereof and causing a larger amount of swelling of the fuel rod containment means thereat than at the other portions of the fuel rod containment means has smaller inner and outer diameters at the portion subjected to the larger neutron irradiation dose than inner and outer diameters at the other portions thereof. 7. A fuel assembly according to claim 6, wherein the fuel rod containment means is a tubular member and the portion thereof having the smaller inner and outer diameters is disposed adjacent a driver fuel pellet portion of the fuel rods. 8. A fuel assembly for a first breeder reactor comprising a plurality of nuclear fuel rods, and fuel rod containment means for containing the plurality of nuclear fuel rods therein, the fuel rod containment means having a shape so that inner and outer diameters at a portion thereof adjacent to a portion of the fuel rods containing driver fuel pellets thereat is smaller than inner and outer diameters of other portions of the fuel rod containment means which are adjacent portions of the fuel rods not containing driver fuel pellets thereat. 9. A reactor core of a fast breeder reactor comprising a plurality of fuel assemblies disposed adjacent one another and extending vertically in a reactor vessel containing coolant, at least one of the fuel assemblies being a fuel assembly according to one of claims 1, 2, 6 and 8. 10. A reactor core according to claim 9, wherein the at least one of the fuel assemblies is disposed in a region of the reactor core generating relatively high thermal output power. 11. Apparatus for containing a plurality of nuclear fuel rods for a fuel assembly of a fast breeder reactor comprising an elongated tubular member having an hourglass shape, the tubular member having inner and outer diameters at a middle portion of an axial length thereof smaller than inner and outer diameters at axial end portions thereof. 12. A method of regulating distribution of coolant in a reactor core of a fast breeder reactor comprising the steps of: composing the reactor core of the fast breeder reactor with a plurality of fuel assemblies adjacent one another and extending vertically in a reactor vessel which contains coolant;  distributing coolant flow among each of the fuel assemblies in the reactor core in dependence upon a difference of thermal output power of respective regions in the reactor core;  optimizing the difference of thermal output power by regulating the distribution of coolant flow by providing at least one of the fuel assemblies with a fuel rod containment means for containing fuel rods therein and having a shape so that the inner and outer diameters at a middle portion of an axial length of the fuel rod containment means is smaller than inner and outer diameters at axial end portions thereof whereby a width of a coolant flow path at least at the middle portion of the fuel containment means is relatively reduced with respect to a width of a coolant flow path at axial end portions of the fuel rod containment means; and  regulating the distribution of coolant flow by changing a rate of enlargement of the width of the coolant flow path of the fuel containment means of respective fuel assemblies.