Patent Number: 048851272
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear fuel assemblies for nuclear reactors and, more particularly, is concerned with a nuclear fuel rod support grid employing separate attachable spring and dimple support spacers and a method of assembling a nuclear fuel assembly. 2. Description of the Prior Art In most nuclear reactors, the reactor core is comprised of a large number of elongated nuclear fuel assemblies. Conventional designs of these fuel assemblies include a plurality of nuclear fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods. The grids of the fuel assembly as well know in the art are used to precisely maintain the spacing between the nuclear fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally clamp and retain the rods against longitudinal movement. One popular conventional fuel rod support grid design, being illustrated and described in U.S. Pat. No. 4,492,844 to Kobuck et al, includes a multiplicity of interleaved inner and outer belt-shaped straps having an egg-crate configuration forming a multiplicity of cells which individually accept the nuclear fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and/or relatively rigid protrusions (called dimples) formed directly into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell and together apply a clamping force directed generally perpendicular to the axial direction of the fuel rod. Additionally, the outer straps are attached together and peripherally enclose the inner straps to impart strength and rigidity to the grid. However, several drawbacks arise from this particular grid design. Scratches are generated in the axial direction on the outer surfaces of the fuel rods by the springs and dimples when the fuel rods are inserted into the grid cells at the time of assembling of the fuel assembly. Also, when the fuel rods are inserted through the grid cells in assembling the fuel assembly, considerable pushing (or pulling) force is required since the fuel rods are being moved while clamped between the springs and dimples. Further, the fabrication of the straps for the grid with springs and dimples formed thereon is complicated. Consequently, a need exists for an alternative approach to constructing the fuel rod support grid and assembling the nuclear fuel assembly which will avoid the above drawbacks without presenting new ones in their place. SUMMARY OF THE INVENTION The present invention provides a fuel rod support grid with separate attachable spring and dimple support spacers and a method of assembling the fuel assembly designed to satisfy the aforementioned needs. The occurrence of scratches experienced with prior art grids is substantially avoided by the provision of the separate attachable spring and dimple support spacers of the present invention since the fuel rods can now be inserted through the grid cells before the spring and dimple support spacers are installed into the grid cells and attachably mounted to portions of the interleaved grid straps defining the walls of the grid cells. Accordingly, the present invention is directed to a nuclear fuel rod support grid which comprises: (a) a plurality of straps being interleaved with one another to form a matrix of hollow cells, each cell for receiving one fuel rod and being defined by intersecting pairs of opposing wall portions of the straps which wall portions are shared with adjacent cells; (b) a plurality of fuel rod clamping springs and dimples being associated with the wall portions of the straps defining each cell and protruding into the cell so as to clamp a fuel rod therebetween; and (c) at least one spacer supporting some of springs and dimples being separate from and detachably mounted on some of the wall portions of the straps defining each cell. More particularly, the spacer is L-shaped in cross section being made from a metal plate that is bent into a ninety-degree configuration. Thus, the spacer is formed of a pair of parts being disposed generally perpendicular to each other and integrally connected together at their inner edges. Each spacer part has a height approximately identical to the height of each of the strap wall portions defining each, cell and a width about equal to one-half of the width of each of the strap wall portions defining each cell. The spacer is lodged in a corner of each cell formed by one intersecting pair of the strap wall portions. Still further, the spacer at a central region of the parts thereof adjacent to opposite unconnected outer edges of the parts has one of either one spring or a pair of the dimples formed in the spacer part so as to protrude therefrom into each cell. Further, if the spacer has the dimples formed therein, they are formed at upper and lower corner regions of each part thereof adjacent to its outer unconnected edge. Also, the spacer at a central region of each of the parts thereof adjacent to respective upper and lower opposite edges of the parts has attachment tabs of a pair thereof formed thereon so as to project outwardly therefrom. The upper tabs extend generally perpendicular to the spacer parts, whereas the lower tabs extend generally within a plane of the spacer parts. The present invention also is directed to a nuclear fuel rod support grid which comprises: (a) a plurality of straps being interleaved with one another to form a matrix of hollow cells, each cell for receiving one fuel rod and being defined by intersecting pairs of opposing wall portions of the straps which wall portions are shared with adjacent cells; (b) a plurality of fuel rod clamping springs and dimples being associated with the wall portions of the straps defining each cell and protruding into the cell so as to clamp a fuel rod therebetween; and (c) a pair of spacers inserted in each cell along the strap walls portions defining each cell, one of the spacers supporting the springs and the other of the spacers supporting the dimples, the spacers being separate from and detachably mounted on the wall portions of the straps defining each cell. Further, the present invention is directed to a method of assembling a fuel assembly which comprises the steps of: (a) positioning, relative to one another and separated by a set distance, support grids of a plurality thereof being composed of interleaved straps defining a multiplicity of cells, the grids being positioned with their cells in longitudinal alignment; (b) inserting guide thimbles of a plurality thereof through a plurality of predetermined spaced ones of the aligned grid cells; (c) attaching a bottom nozzle to lower ends of the guide thimbles; (d) inserting fuel rods in a plurality thereof in a predetermined sequential manner through the grids cells unoccupied by guide thimbles; (e) alternately with the inserting of the fuel rods, installing spacers having fuel rod engaging and clamping elements thereon in the grid cells which have fuel rods inserted therethrough; and (f) once the inserting of fuel rods and installing of fuel rod clamping spacers is completed, attaching a top nozzle to upper ends of the guide thimbles. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.