Patent Number: 047449420
Section: claims

1. A nuclear fuel assembly containing a plurality of generally cylindrical fuel rods, comprising: a top nozzle;  a bottom nozzle;  a control rod guide thimble extending between said top and bottom nozzles;  a plurality of fuel rod spacer grid assemblies axially disposed along said control rod guide thimble for maintaining the said fuel rods in a spaced lateral array, each of said spacer grid assemblies comprising a plurality of grid straps interlocked into an egg-crate configuration to form cells through which said fuel rods pass, each of said cells having a grid spring and at least one opposing dimple, each said grid spring and opposing dimple having a first selected spring force for a lowermost grid, a second selected spring force for an intermediate grid, and a third selected spring force for an uppermost grid less than said first and second spring forces, said first, second and third spring forces allowing the fuel rods to expand in their respective cells.  a plurality of grid straps interlocked into an egg-crate configuration to form a cell through which said fuel rod is adapted to pass;  a grid spring and an opposing dimple disposed in said cell for supporting said fuel rod;  said grid spring and opposing dimple in each grid having a selected spring force, a lowermost grid being formed of a material having a spring force of about 2 to 6 kg and which material is resistant to radiation-induced relaxation, at least next superadjacent intermediate spacer grid being formed of a material having a spring force of about 1.5-6 kg and which material has a relatively low neutron absorption cross-section, and an uppermost spacer grid formed of a material having a spring force of about 0.5 to 2 kg.  a plurality of grid straps interlocked into an egg-crate configuration to form cells through which a corresponding one of said fuel rods is adapted to be located;  spring means disposed in each cell for supporting said fuel rod therein;  said spring means in each cell having a selected spring force, the first of said grids being formed of a material having a relatively high spring force and which material is resistant to radiation-induced relaxation for securing the fuel rods axially in place, at least one axially adjacent intermediate spacer grid being formed of a material having a spring force less than the first grid and which material has a relatively low neutron absorption cross-section, and a remote spacer grid axially adjacent and furthest removed from the first grid, said remote grid being formed of a material having a spring force less than the first grid, said remote grid allowing the fuel rods to axially expand and slip in the cells. 2. The nuclear fuel assembly of claim 1, wherein said first spring force is between about 2 to 6 kg. 3. The nuclear fuel assembly of claim 2, wherein said first spring force is about 3.5 kg. 4. The nuclear fuel assembly of claim 1, wherein said second spring force is between about 1.5 to 6 kg. 5. The nuclear fuel assembly of claim 4, wherein said second spring force is about 5 kg. 6. The nuclear fuel assembly of claim 1, wherein said third spring force is between about 0.5 to 2 kg. 7. The nuclear fuel assembly of claim 6, wherein said third spring force is about 1.5 kg. 8. An assembly for supporting a generally cylindrical fuel rod in a nuclear fuel assembly including at least three superadjacent spacer grids, comprising: 9. An assembly as set forth in claim 8, wherein said lowermost grid secures the fuel rod axially and laterally, and wherein said uppermost superadjacent grid supports the fuel rods with diminishing axial and lateral force so as to permit axial expansion of the fuel rods with diminishing compressive force thereon. 10. An assembly as set forth in claim 8 wherein said uppermost and lowermost grids contain springs fabricated from Inconel and said at least one intermediate grid is formed of at least one of zirconium and zirconium alloy. 11. An assembly for supporting rods in a nuclear fuel assembly, which rods axially expand during use, and including at least three superadjacent spacer grids, comprising: