Patent Number: 053316789
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to nuclear fuel assemblies, and more particularly, to grid strips which form an egg crate type fuel rod support grid in a pressurized water nuclear fuel assembly. Conventionally, pressurized water nuclear reactors have a core of nuclear fuel assemblies, in which the fuel rods are supported and spaced relative to each other, by spacer grids. Each cell of the grid utilizes a system of fuel rod support features including low stiffness springs and opposing high stiffness arches to contact, locate, and stabilize the fuel rod. At the time of fabrication of the fuel assembly, the geometry of each grid and associated cell is intended to provide lateral forces between the fuel rod and the support features. This type of fuel rod support is represented by U.S. Pat. Nos. 4,803,043, 4,879,090 and 5,188,797. Typically, the separation distance in the as built grid, between each spring and its opposing arches, is less than the outside diameter of a new fuel rod. Thus, when a rod is inserted into a grid cell, the spring is compressed and the rod is held between the support features with a preload force. Since these support or contact features project into the flow stream that passes through the cell during operation in the reactor, the rod support structure should present a low cross section to minimize pressure drop losses. The effect of the reactor environment on the initial grid/rod interface is to cause the preload force to diminish through a combination of short-term and long-term mechanisms, followed by gap formation in cases where the force drops to zero. Short-term mechanisms include the effects of initial heatup of the fuel (partial relaxation of forming stresses in the grid strip and possible yielding of the grid strip due to reduced yield strength at higher temperatures) and initial pressurization of the reactor (spring compression is reduced because the fuel rod diameter decreases due to differential pressure). Long-term mechanisms include spring stress relaxation due to temperature and neutron flux, fuel rod cladding diametral creepdown, and, in the case of spacer grids made of Zircaloy, radiation growth of the grid cell. Full scale flow tests have shown that inadequate rod support by the grid results in excessive rod vibration, leading to wear and possibly rod failure. These tests have shown that the key to limiting rod wear is the prevention of significant gaps between the grid contact features and the rod. Based on the mechanisms described above there are two general ways to minimize gaps. One can lessen the creepdown of the fuel rod cladding or improve the short-term and long-term responses of the grid features to reactor operating conditions. It is the latter approach which is used in the present invention. SUMMARY OF THE INVENTION It is, accordingly, an object of the present invention to improve the short-term and long-term response of the fuel rod support structures in the fuel assembly grid, by resisting the inherent tendency of the spring preload to decrease during reactor operation. This is accomplished in the present invention, by stamping the grid strips to form beam springs such that each end of the beam spring is rigidly supported by a pedestal or the like which projects from the flat base region of the grid strip. In particular, a grid strip in accordance with the present invention, comprises a substantially flat plate having length, height and width dimensions, and a plurality of slots extending along the height dimension at regular intervals along the length dimension, thereby defining successive cell walls between successive slots along the length dimension. Each cell wall has upper, central and lower regions along the height dimension, each region including a substantially flat base area and fuel rod support structure projecting integrally from the base area along the width dimension of the strip. The support structure in each of the upper and lower regions includes a relatively stiff, arched stop which projects in a first direction and the support structure in the central region includes a relatively soft, arched spring which projects in a second direction opposite the first direction. The spring includes spaced apart pedestals formed in the base area of the central region and projecting in the second direction, and a resilient beam extending between and rigidly supported by the pedestals, so as to project in the second direction beyond the projection of the pedestals. Preferably, the pedestals project into each cell, a distance that puts them in close proximity to a fuel rod of the fabricated assembly. The crown of the beam spring projects slightly beyond the pedestal projection, in preload contact with the fuel rod. Preferably, each pedestal forms an arch that curves along the length dimension of the strip and the beam forms an arch that curves along the height dimension of the strip. The arch stops are preferably formed between a pair of longitudinal cut-outs that extend along the length dimension of the strip. The beam is formed between a pair of transverse cut-outs that extend along the height dimension of the strip, and each pedestal is formed between one longitudinal cut-out and the pair of transverse cut-outs. In a normal operating configuration in a nuclear reactor core, the length dimension of the strip would be horizontally oriented, the height dimension vertically oriented, and the width dimension oriented horizontally in a direction perpendicular to the length dimension. The present invention provides a number of significant advantages over conventional springs. The horizontal pedestals at the base of the beam, limit the deflection of the beam to prevent damage by fuel rods that are excessively deflected during handling when the grid is fabricated or reconstituted. The flow blockage area of the spring is minimized, thus reducing the pressure drop associated with the spring. The shape of the beam spring against the fuel rod as loaded in the cell is almost flat, Especially in a fuel assembly grid in which the strips are made from a zirconium alloy material, e.g., Zircaloy, long-term exposure to radiation in the reactor core causes the material to elongate, with the amount of elongation directly related to the amount of straining (cold-working) the material experienced after annealing. Therefore, the elongated shape of the beam spring amplifies the differential growth between the base region of the strip and the beam spring, thereby minimizing or eliminating gap formation between the spring and the fuel rod despite prolonged core operation. Additional straining of the spring (overform, then force back) and creating a slight cant of the pedestal projections away from each other during the forming process of the spring, can further enhance the amplification effect.