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 fuel rods and others through which the guide thimbles respectively pass and attach to the plurality of grids;wherein at least one grid of the plurality of grids comprises a peripheral surface area that extends in a plane a distance along the elongated dimension; andwherein the at least one grid of the plurality of grids comprises 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 the second position placing the bimetallic spring in contact with a corresponding grid of an adjacent nuclear fuel assembly located in a reactor core. 2. The nuclear fuel assembly of claim 1 wherein the bimetallic spring has an elongated dimension and the elongated dimension of the bimetallic spring extends transverse to the elongated dimension of the nuclear fuel assembly. 3. The nuclear fuel assembly of claim 2 wherein the bimetallic spring is continuous and extends across several of the grid cells. 4. The nuclear fuel assembly of claim 1 wherein the at least one grid of the plurality of grids is a mid-grid. 5. The nuclear fuel assembly of claim 1 wherein the at least one grid of the plurality of grids is one of an upper grid or a lower grid or both an upper grid and a lower grid. 6. The nuclear fuel assembly of claim 1 wherein the bimetallic spring is configured in a rectangular shape having an elongated dimension. 7. The nuclear fuel assembly of claim 6 wherein the elongated dimension of the bimetallic spring extends substantially parallel to the elongated dimension of the nuclear fuel assembly. 8. The nuclear fuel assembly of claim 1 wherein the bimetallic spring comprises stainless steel and either FeNi36 or 64FeNi. 9. The nuclear fuel assembly of claim 8 wherein the bimetallic spring is formed on each corner of the at least one grid of the plurality of grids. 10. The nuclear fuel assembly of claim 1 wherein the peripheral surface area of the at least one grid of the plurality of grids comprises a base metal, and wherein the bimetallic spring is formed by coating the base metal with a material having a lower coefficient of thermal expansion than the base metal. 11. The nuclear fuel assembly of claim 1 wherein the bimetallic spring is formed on a corner of the at least one grid of the plurality of grids. 12. The nuclear fuel assembly of claim 1 wherein the bimetallic spring does not protrude outwardly from the plane in the first position and protrudes outwardly from the plane in the second position to contact the corresponding grid of the adjacent nuclear fuel assembly. 13. The nuclear fuel assembly of claim 1 wherein the bimetallic spring is configured in a round disc shape. 14. The nuclear fuel assembly of claim 13 wherein the bimetallic spring has a dome when exposed to the reactor core operating temperature. 15. The nuclear fuel assembly of claim 13 further comprising relief holes spaced around the circumference of the bimetallic spring. 16. The nuclear fuel assembly of claim 1 wherein the bimetallic spring comprises a base material coated with a material having a different coefficient of thermal expansion than the base material. 17. The nuclear fuel assembly of claim 1 wherein the bimetallic spring comprises a lamination of a first material and a second material, wherein the first material has a different coefficient of thermal expansion than the second material.