Patent Number: 050858270
Section: summary

FIELD OF THE INVENTION The present invention relates to a spacer usable for positioning fuel rods and water rods in a nuclear fuel assembly and springs therefor, and, in particular, to springs that occupy reduced space in such spacers. BACKGROUND OF THE INVENTION In a known type of nuclear power reactor, for example, a boiling-water reactor, nuclear fuel is provided in elongated rods. The nuclear fuel is typically in the form of uranium oxide and/or plutonium oxide pellets enclosed in zircaloy tubes. A number of such fuel rods are grouped together in an open-ended tubular flow channel. The flow channel with the fuel rods positioned therein is referred to as a "fuel assembly" or "bundle." A plurality of fuel assemblies are removably positioned in the reactor core in a matrix. The reactor core formed in this manner is capable of self-sustained fission reaction. The core is submerged in a fluid, such as light water, which serves both as a coolant and as a neutron moderator. The fuel rods in a fuel assembly are supported between upper and lower tie plates. The fuel rods are typically arrayed in parallel side by side vertical upstanding relation. The fluid coolant flows past the fuel rods in the intersticies between the vertical and parallel fuel rods. To provide proper coolant flow and preserve integrity of the fuel rods, it is important to maintain the rods in a preferred spaced relationship and to restrain them from bowing and vibrating during reactor operation. To maintain spaced apart alignment of its fuel rods, a plurality of fuel rod spacers spaced along the length of the fuel assembly are provided for this purpose. One type of spacer includes a plurality of generally cylindrical ferrule elements. An example of a spacer usable in a fuel assembly is that shown in Matzner, et al U.S. Pat. No. 4,508,679, issued Apr. 2, 1989. As shown in Matzner, et al., one method of positioning a fuel rod within the ferrule elements of a spacer is to provide a spring member for biasing the fuel rod against rigid stops in the ferrules. The spring depicted in U.S. Pat. No. 4,508,679 is in the form of a continuous loop of generally elliptical shape. The springs are positioned in the volume of the spacer area between adjacent fuel rods. Modern fuel bundle design typically includes fuel rods being arrayed in a square sectioned fuel bundle. The arrays originally where in a 7.times.7 matrix. This left relatively large spaces between the fuel rods. Accordingly, the problem of placing springs between the rod to maintain the rods in vertical upstanding relation presented a generally simple mechanical design problem. Unfortunately, modern fuel bundle designs include much denser fuel rod arrays. Such arrays have gone from fuel rod matrices including 8.times.8 fuel rod arrays to 9.times.9 and 10.times.10 fuel rod arrays. This being the case, the interstitial volume (or thickness) between the fuel rods has shrunk. Although the same spring action is required for the most part from the springs acting with the spacers to maintain the fuel rods in vertical upstanding relation, the space in which such spring action can occur is vastly reduced. Previous fuel assemblies having 8.times.8 matricies fuel rods had been constructed with rod-to-rod spacings (i.e., distances between outer circumferences of adjacent rods) of about 0.160 inches (about 4 mm). Modern fuel bundles are being designed with 9.times.9 matricies of fuel rods to have a reduced rod-to-rod spacing, such as about 0.12-0.14 inches (about 3 to about 3.5 mm). This reduction in fuel rod interstitial spacing has had severe constraints on the spring design. It should be noted that the springs, although necessary for positioning the rods, can have certain undesirable effects. These undesirable effects include absorption of neutrons and interference with the coolant circulation. Materials from which the springs are typically formed absorb 20-100 times the number of neutrons absorbed by the spacer material. Accordingly, there is a high motive to maintain a minimum of spring material within the fuel bundle. SUMMARY OF THE INVENTION A spring system is provided for bracing side by side fuel rods in dense (9.times.9) arrays. The spring system is incorporated to the several spacers, typically 6, 7 or 8 positioned at vertically distributed intervals between the upper and lower tie plates in a square sectioned fuel bundle. The spacers and springs disclosed maintain a substantially uniform spacing between the vertically upstanding fuel rods and water rods contained with the fuel bundle. A loop spring is provided as the main fuel rod biasing unit. Like the prior art, the loop spring surrounds portions of the spacer and is thus held in a generally vertically disposed elongate loop by the spacer. Each loop spring acts on typically a pair of fuel rods, one rod being on each side of the spring. Each fuel rod is confined within its own discrete ferrule at the spacer and is spring biased by the spring against paired protuberances. Each loop spring includes upper and lower C-shaped end portions for retaining the spring to the spacer. In between each upper and lower C-shaped end portions there are formed two spring legs. The spring legs begin at the ends of each "C" and flare with gradual reversing curvature to substantially linear spring leg portions. Each spring leg includes in the center of the leg a convex and outwardly disposed arched shaped rod contacting portion for spring biased contact with the fuel rods on either side of the spring. Unlike the prior art, two inwardly disposed U-shaped bends are incorporated in each spring leg immediately on either side of the outwardly disposed arched shaped rod contacting portion. These two U-shaped bends extend the effective length of the spring material over which the spring forces can act and effectively evenly distribute the maximum bending forces on the spring material to the arched shaped rod contacting portion and the upper and lower C-shaped end portions. There results a spring which can provide a wide range of deflections while being confined to the narrow interstices between closely spaced and dense fuel rod arrays such as the 9.times.9 array disclosed. The disclosed 9.times.9 matrix fuel bundle has the seven middle lattice positions occupied by two relatively large and circularly sectioned water rods. Seven central ferrules are removed to provide space for the water rods. Two ferrules adjacent to the water rods are not paired with another ferrule, so that the spring in each of these ferrules acts on only one fuel rod. A special method for mounting these springs is required. A water rod spacer plate is disclosed. This plate has two functions. First, it provides the required mounting for the disclosed loop springs in the locations where the spring bears on one rod only. Second, the water rod spacer plate provides stops for the water rods. The water rod spacer plate has a U-shaped central region for bearing against the two water rods at their adjoining exterior surfaces. One lower portion of the "U" bears against one water rod; the other lower portion of the "U" bears against the other water rod. These contact regions provide stops for the water rods. A spring mounted on a separate plate biases each water rod toward these stops. The water rod spacer plate has a wing at each end of the U-shaped central region. One of these wings is shaped to provide mounting means for the springs which bear against only one fuel rod. Because of the intervention of the water rods, at least one spring must be provided with a fuel rod on one side only. This spring is conveniently mounted on a disclosed interlock between a tab protruding from the specially constructed fuel rod ferrule and a mating tab on the "U" sectioned water rod spacer member. Accordingly, required bias is supplied against all spaced apart vertical fuel rods and water rods of the dense 9.times.9 matrix fuel bundle.