Patent Number: 042785014
Section: summary

The invention relates to a spring element for holding down and bracing or supporting fuel assemblies especially of water-cooled neclear reactors, against a hold-down plate upwardly limiting the reactor core. Bracing the individual fuel assemblies of a nuclear reactor against a hold-down plate in this manner has become generally known heretofore. This is necessary because, otherwise, the flow forces of the reactor coolant might lift the fuel assemblies upwardly out of the normal position thereof. A rigid mounting for the individual fuel assemblies by means of this hold-down plate cannot be provided because variations in length due to thermal expansion and, also, radiation-induced length increase must be afforded. On the other hand, since a loose seating of the fuel assemblies below the hold-down plate must be avoided at all costs, springs are normally disposed in such a manner that length increase and thermal expansion can nevertheless occur against the force of these springs during operation of the fuel assemblies without subjecting the mountings to excessive stresses. While such spring elements known in the state of the art generally fulfill, in practice, the purposes for which they are intended, when other structural materials are used in fuel assemblies, such as Zircaloy guide tubes, for example, a wider operating range which is difficult to attain i.e. spring travel, is necessary to assure that the fuel assemblies will be properly held or supported under all operating conditions. Due to the assembly geometry, which is presupposed in most cases, variations in the operating characteristic could heretofore be attained only to an insufficient extent by modified spring elements. The problem therefore arose of developing a spring element construction, the assembly and disassembly of which from a fuel assembly can be accomplished more simply than heretofore and the spring travel i.e. the operating range, thereof is adjustable and can be kept considerably wider or greater than heretofore. It is accordingly an object of the invention to provide a spring element for holding down nuclear reactor fuel elements which is more simply assemblable with and disassemblable from a fuel assembly than heretofore and has a greater spring travel or operating range than heretofore, due to the adjustability thereof. With the foregoing and other objects in view, there is provided, in accordance with the invention, a spring element for holding down and bracing a fuel assembly against a hold-down plate upwardly limiting the reactor core of a nuclear reactor, comprising a spring-loaded rod-shaped member separately formed independently of the fuel assembly and being slidable axially and form-lockingly into the fuel assembly. Should replacement or removal of the spring element, in the case of a previously irradiated spring element, be desired, this can be effected by means of a simple gripper device, after the reactor core has been made accessible, without requiring rotation of the fuel assembly. At any other place of deposit for fuel assemblies, this can be effected without any special preparations. In accordance with another feature of the invention, the rod-shaped member is telescopically slidable together, and including a compression spring urging the telescopically slidable rod-shaped member, in neutral position thereof, into the greatest possible assumed length thereof, the rod-shaped member, in installed condition thereof, being mounted, at one end thereof, in at least one bore formed in a head of the fuel assembly and engaging the hold-down plate, at the other end thereof. In accordance with a further feature of the invention, the compression spring is disposed around and extends along the length of the rod-shaped member. In accordance with an added feature of the invention, the rod-shaped member is formed of three parts including a threaded bolt formed with a shaft and a head, an upper sleeve telescopically guidable on the shaft of the threaded bolt and engageable with the head of the threaded bolt, and an internally threaded lower sleeve wherein the threaded bolt is threadedly received for adjusting the combined length thereof, the lower sleeve and the threaded bolt being securable against relative rotation, the one end of the rod-shaped member being an end of the threaded bolt projecting beyond the lower sleeve and engageable in the fuel-element head. In accordance with a concomitant feature of the invention, the compression spring surrounds the rod-shaped member, and both the upper and lower sleeves are formed with a respective collar at ends thereof forming opposite ends of the rod-shaped member, the collars serving as stops for the compression spring. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in spring element for holding down nuclear reactor fuel assembly, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.