Patent Number: 047028815
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactor fuel assemblies and more particularly to an improved spacer grid assembly for spacing and cradling fuel rods in a nuclear fuel assembly. 2. Background In a nuclear fuel assembly, spacer grid assemblies are used to precisely maintain the spacing between the fuel rods in a nuclear reactor core, to prevent rod vibration, and to provide lateral support for the fuel rods. Conventional spacer grid assembly designs include a multiplicity of metal grid straps, interlocked into an egg-crate configuration designed to form cells through which fuel rods (standard cells) and control rod guide thimbles (thimble cells) pass. Slots are often utilized to effect the interlocking engagement between grid straps. Each standard cell provides support for one fuel rod at a given axial location through the use of relatively resilient grid springs and relatively rigid protuberancies (dimples) formed in or attached to the metal grid straps. A peripheral strap may be used to enclose the interlocked grid straps in order to impart strength and rigidity to the fuel assembly and to provide grid spring and dimple locations for the peripheral cells. In order to minimize the lateral displacement of fuel rods during operation and to improve the fuel characteristics of a fuel assembly, a number of such spacer grid assemblies may be spaced along the fuel assembly length. These grids are generally held in place by attachment to the control rod guide thimbles by well known bulging techniques or the like. Examples of such spacer grid assemblies may be found in U.S. Pat. Nos. 3,389,056 to E. Frisch, 3,713,971 to Van Santen et al., 3,944,467 to Biermann et a1., or 4,224,107 to Delafosse et al. During operation in a nuclear reactor, the grid springs and dimples undergo intense radiation which cause the grid springs to tend to lose the initial spring force exerted against the fuel rods, thus permitting the fuel rods to vibrate and chatter against the dimples. The result may be fretting of the fuel rod cladding. In a typical fuel rod, the cladding comprises a metal tube that confines nuclear fuel pellets in the fuel rod. As used herein, the term "fretting" means a rubbing action between the fuel rod cladding and the grid springs and dimples. If severe enough, fretting is believed to result in fuel rod failure. In a nuclear fuel assembly, fretting may be caused by vibration of the rods induced by coolant and/or moderator fluid flow in the reactor core and possibly by vibration forces during shipping and insertion forces during fuel rod loading. Further, as will be understood by the artisan, fuel rods are generally long (several yards), slender cylinders. The round surface of these slender cylinders, when inserted into conventional spacer grid assemblies during fuel loading, typically rests on a flat or raised surface of conventional dimples and grid springs resulting in a point or line contact between the fuel rod surface and those springs and dimples. When loading fuel rods into such spacer grid assemblies, scratches known as loading scratches are a common problem. It is believed that when the fuel rods are inserted into such conventional spacer grid assemblies, the sharp edge on the dimples and springs tends to carve longitudinal scratches into the cladding. Some conventional spacer grids contribute to an additional problem in that the fuel rod springs and dimples may not accurately position the fuel rod at the center of a standard cell. Deviations from that center position can result in adverse nuclear characteristics in the fuel assembly as well as hydraulic flow maldistribution and mechanical damage such as bent dimples and grid springs. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide an improved spacer grid assembly which distributes forces between the fuel rod and the grid springs and dimples over a larger surface area as compared to conventional spacer grid designs. It is a further object of the invention to provide a spacer grid assembly design which will reduce any tendency for the fuel rods to become scratched during loading and/or transport. It is yet a further object of the invention to provide a spacer grid assembly design which will more accurately position a fuel rod within the standard cells of the spacer grid assembly. It is further object of the present invention to provide an improved spacer grid assembly design which will reduce any tendency for fretting between fuel rod cladding and spacer grid springs and dimples. To achieve these as well as other objects, and in accordance with the present invention, a nuclear fuel assembly containing a plurality of generally cylindrical fuel rods is provided which comprises a top and bottom nozzle and at least one control rod guide thimble extending between the top and bottom nozzle. A plurality of spacer grid assemblies are axially disposed along the control rod guide thimble for maintaining the fuel rods in a spaced lateral array. Each of the spacer grid assemblies comprises a plurality of grid straps interlocked into an egg-crate configuration to form standard cells through which the fuel rods pass. Each of the standard cells has at least one, and preferrbly two, grid springs and each grid spring has one, and preferably two, opposing dimples. Each of the grid springs and dimples has a surface portion contoured to cradle the generally cylindrical fuel rod as it passes through a standard cell. In accordance with another aspect of the present invention, a spacer grid assembly for supporting generally cylindrical fuel rods in a nuclear fuel assembly is provided which comprises a plurality of grid straps interlocked into an egg-crate configuration to form cells through which fuel rods are adapted to pass. At least one, and preferably two, grid springs and at least one, and preferably two, dimples opposing each grid spring are disposed in each of the cells in order to support the fuel rods as they pass through the cells. The grids springs and opposing dimples have arcuate face portions adapted to cradle the generally cylindrical fuel rod as the fuel rod passes through the cell. Additional objects, advantages and novel features of the invention will be set forth, in part in the description which follows, and in part will become apparent to those skilled in the art by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the invention as particularly pointed out in the appended claims.