Patent Number: 050531913
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to nuclear reactors, and more particularly, to holddown springs for nuclear reactor fuel assemblies. In most commercial nuclear power reactors, the space allocated to fuel assembly components is constrained by the other internal components of the reactor, so that only the type of fuel assembly holddown springs supplied by the original equipment manufacturer, can be used on replacement fuel assemblies. In the reactors of interest herein, each fuel assembly is vertically supported between an upper core plate and a lower core plate, and cantilever-type holddown springs are interposed between the upper core plate and the upper end fitting of the fuel assembly for biasing the assembly against the lower core plate. This bias accommodates differential thermal expansion between the assemblies and the reactor core and core plates, the upward forces imposed on the assemblies resulting from the flow of primary coolant through the core, and transient loads that can result from a variety of both normal and abnormal plant operating conditions. In nuclear reactors where the holddown springs are of the cantilever, rather than coil or leaf spring type, it is possible that certain transients can overload the cantilever springs, such that, upon the return of the reactor to normal operation, the performance of the springs has been permanently degraded. SUMMARY OF THE INVENTION It is, accordingly, an object of the present invention to modify the typical cantilever spring to improve its performance under peak loading, by increasing its spring rate as increased load is applied. In accordance with the present invention, the cantilevered spring has a secondary, or auxiliary loading point in the form of a dimple or thickened region projecting toward the upper core plate. The dimple contacts the core plate after the core plate has deflected the spring a predetermined amount via the primary loading point. The transferred, secondary loading effectively shortens and stiffens the spring against further movement of the core plate and upper end fitting toward each other. The cantilevered spring itself, in accordance with the invention, preferably comprises a unitary, elongated metal bar having a substantially straight, long legged portion with one end adapted to be mounted to the fuel assembly end fitting, and an arcuate transition portion at the other end of the long leg. A straight short legged portion extends from the transition portion at an acute included angle with the long legged portion and has an unrestrained free end. The load transfer projection is located on the long leg intermediate the transition portion, which is the spring loading point during normal operation, and the first end adapted for attachment of the long leg to the assembly.