Patent Number: 
Section: claims

1. A nuclear fuel assembly having an elongated dimension and comprising:a plurality of interconnected components wherein at least some of the interconnected components comprise:a top nozzle;a bottom nozzle;a plurality of guide thimbles extending between the top nozzle and the bottom nozzle;a plurality of fuel rods extending between the top nozzle and the bottom nozzle; anda plurality of grids arranged in a tandem spaced relationship that extends between the top nozzle and the bottom nozzle along the elongated dimension, with each of the grids having a plurality of cells some of which support the fuel rods and others through which the guide thimbles respectively pass and attach to the plurality of grids;wherein at least some of the interconnected components have a peripheral surface area that extends in a plane a distance along the elongated dimension;wherein one of the at least some of the interconnected components has a bimetallic spring that moves between a first and second position relative to the plane as the fuel assembly transitions from a reactor core shutdown temperature to a reactor core operating temperature, with one of the first and second positions placing the bimetallic spring in contact with an adjoining component when the nuclear fuel assembly is placed in a reactor core;wherein the bimetallic spring comprises a base material coated with a material having a different coefficient of thermal expansion than the base material,wherein the bimetallic spring is configured in a round disc shape,wherein the bimetallic spring is provided proximate one corner of the one of the at least some of the interconnected components, andwherein a tip of the disc shape fits in a recess in a side of the adjoining component. 2. A nuclear fuel assembly having an elongated dimension and comprising:a plurality of interconnected components wherein at least some of the interconnected components comprise:a top nozzle;a bottom nozzle;a plurality of guide thimbles extending between the top nozzle and the bottom nozzle;a plurality of fuel rods extending between the top nozzle and the bottom nozzle; anda plurality of grids arranged in a tandem spaced relationship that extends between the top nozzle and the bottom nozzle along the elongated dimension, with each of the grids having a plurality of cells some of which support the fuel rods and others through which the guide thimbles respectively pass and attach to the plurality of grids;wherein at least some of the interconnected components have a peripheral surface area that extends in a plane a distance along the elongated dimension;wherein one of the at least some of the interconnected components has a bimetallic spring that moves between a first and second position relative to the plane as the fuel assembly transitions from a reactor core shutdown temperature to a reactor core operating temperature, with one of the first and second positions placing the bimetallic spring in contact with an adjoining component when the nuclear fuel assembly is placed in a reactor core; wherein the bimetallic spring comprises a base material coated with a material having a different coefficient of thermal expansion than the base material,wherein the bimetallic spring is configured in a round disc shape, and wherein the bimetallic spring is supported on at least the top nozzle. 3. The nuclear fuel assembly of claim 2 including relief holes spaced around the disc shape. 4. The nuclear fuel assembly of claim 2 wherein the bimetallic spring is provided proximate one corner of the one of the at least some of the interconnected components. 5. The nuclear fuel assembly of claim 4 wherein a tip of the disc shape fits in a recess in a side of the adjoining component. 6. The nuclear fuel assembly of claim 2 wherein the bimetallic spring comprises stainless steel and either FeNi36 or 64FeNi. 7. The nuclear fuel assembly of claim 2 wherein the bimetallic spring is configured from a base metal coated with a low-to negative coefficient of thermal expansion material. 8. The nuclear fuel assembly of claim 2 wherein the bimetallic spring is formed on a corner of at least one of the plurality of grids. 9. The nuclear fuel assembly of claim 6 wherein the bimetallic spring is formed on each corner of the at least one of the plurality of grids. 10. The nuclear fuel assembly of claim 2 wherein the bimetallic spring does not extend substantially out of the plane at temperatures substantially below the reactor core operating temperature and protrudes outwardly from the nuclear fuel assembly at the reactor core operating temperature to an extent to contact the adjoining component. 11. The nuclear fuel assembly of claim 2 wherein the bimetallic spring has a dome. 12. The nuclear fuel assembly of claim 11 wherein a portion of the bimetallic spring is a peak of the dome; and wherein the peak of the dome is seated in a recess of one of a top nozzle and a bottom nozzle of the adjacent one of said plurality of fuel assemblies. 13. A nuclear reactor having a reactive core comprising a plurality of fuel assemblies, each having an elongated dimension and comprising:a plurality of interconnected components wherein at least some of the interconnected components comprise:a top nozzle;a bottom nozzle;a plurality of guide thimbles extending between the top nozzle and the bottom nozzle;a plurality of fuel rods extending between the top nozzle and the bottom nozzle; anda plurality of grids arranged in a tandem spaced relationship that extends between the top nozzle and the bottom nozzle along the elongated dimension, with each of the grids having a plurality of cells some of which support the fuel rods and others through which the guide thimbles respectively pass and attach to the grid;wherein at least some of the interconnected components have a peripheral surface area that extends in a plane a distance along the elongated dimension;wherein on some of the fuel assemblies one of the at least some of the interconnected components has a bimetallic spring that moves between a first and second position relative to the plane as the fuel assembly transitions from a reactor core shutdown temperature to a reactor core operating temperature, with one of the first and second positions placing the bimetallic spring in contact with an adjoining component of the reactive core;wherein the bimetallic spring comprises a base material coated with a material having a different coefficient of thermal expansion than the base material,wherein the bimetallic spring is configured in a round disc shape, andwherein the bimetallic spring is supported on at least the top nozzle. 14. The nuclear reactor of claim 13 wherein the bimetallic spring does not extend substantially out of the plane at temperatures substantially below the reactor core operating temperature and protrudes outwardly from the nuclear fuel assembly at the reactor core operating temperatures to an extent to contact the adjoining component of the reactive core.