Patent Number: 047298677
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following copending U.S. patent applications dealing with related subject matter and assigned to the assignee of the present invention: "Apparatus and Method For Loading Fuel Rods Into Grids Of A Fuel Assembly" by E. E. DeMario et al, assigned U.S. Ser. No. 717,263 and filed Mar. 28, 1985, now U.S. Pat. No. 4,651,403. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors and, more particularly, is concerned with spring retainer apparatus and method for facilitating loading of fuel rods into grid of a fuel assembly without incurring scratches on the exterior surfaces of the fuel rods. 2. Description of the Prior Art Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work. In nuclear reactors generally, a plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a nuetron moderator. Specifically, in a boiling water reactor (BWR) the fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rod is insertable within the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assemblies surrounding a control rod is commonly referred to as a fuel cell of the reactor core. A typical BWR fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods. The bundle of fuel rods are supported in laterally spaced-apart relation and encircled by an outer tubular channel having a generally rectangular cross-section. The outer flow channel extends along substantially the entire length of the fuel assembly and interconnects a top nozzle with a bottom nozzle. The bottom nozzle fits into the reactor core support plate and serves as an inlet for coolant flow into the outer channel of the fuel assembly. Coolant enters through the bottom nozzle and thereafter flows along the fuel rods removing energy from their heated surfaces. In a fuel assembly of this type the fuel rods in the central region of the bundle thereof may be undermoderated and overenriched. In order to remedy this condition by increasing the flow of moderator water through this region of the assembly, an elongated centrally-disposed water cross is frequently used in the assembly. The central water cross has a plurality of four radial panels which together form a cruciform water flow channel which divides the fuel assembly into four, separate elongated compartments, with the bundle of fuel rods being divided into mini-bundles disposed in the respective compartments. The water cross thus provides a centrally-disposed cross-shaped path for the flow of subcooled neutron moderator water within the channel along the lengths of, but separated from, adjacent fuel rods in the mini-bundles thereof. The fuel rods of each mini-bundle extend in laterally spaced apart relationship between an upper tie plate and a lower tie plate and connected together with the tie plates to comprise a separate fuel rod subassembly within each of the compartments of the channel. The water cross has approximately the same axial length as the fuel rod subassemblies, extending between the upper and lower tie plates thereof. Additionally, a plurality of spacers or grids, for example six in number, are disposed at axially displaced positions along the fuel rods of each fuel rod subassembly to maintain the fuel rods in their laterally spaced relationships. The grids have interleaved straps which define a multiplicity of cells disposed in vertical alignment. The fuel rods are supported in an organized and transversely spaced array in the vertically aligned cells of the grids by springs and dimples on the straps which extend into the cells. Thus, the grids are introduced to maintain the desired fuel mini-bundle configuration and to prevent excessive fuel rod bow and flow induced vibrations, and thereby provide significant benefits from a structural standpoint. In the common way of loading fuel rods into the fuel assembly grids containing the springs and dimples for holding the fuel rods in place, the fuel rods are ordinarily scraped by the springs and dimples as they enter the grid cells. Such scraping produces scratches which provide sites for accelerated corrision of the exterior surfaces of the fuel rods and debris for release into the coolant flow system of the reactor. Devices are known for deflecting the springs of the fuel assembly grids for facilitating insertion of the fuel rods without marring the surfaces thereof. Representative of the prior art are the devices disclosed in U.S. Pat. Nos. 3,982,994 and 4,058,224 to Jabsen. While these prior art devices would appear to operate reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, they seem to embody shortcomings which make them less than an optimum solution to the problem of scratch-free loading of the fuel rods. The devices require openings in the straps of the grid through which their actuating parts must be inserted to position them for deflecting the grid springs. Such openings may tend to weaken the grid structure. Also, operation of such devices in deflecting the grid springs appears to be unduly complicated. Consequently, a need exists for a fresh approach to insertion of fuel rods into the grids which will avoid or reduce the damage to the exterior surfaces of the fuel rods and at the same time be less complicated in operation. SUMMARY OF THE INVENTION The present invention provides a spring retainer apparatus and method which are designed to satisfy the aforementioned needs. Specifically, the spring retainer apparatus facilitates loading the fuel rods into the fuel assembly grids in a scratch-free way. The springs of the grids are held in retracted positions so as to provide sufficient clearance for entry of the fuel rods without contacting the springs and dimples of the grid and thus without incurring scratching of the fuel rod exterior surfaces. Accordingly, the present invention relates to a spring retainer apparatus for facilitating the loading of fuel rods into a fuel assembly. The fuel assembly has at least one grid formed of interleaved straps defining a plurality of hollow cells for respectively receiving the fuel rods. At least some of the straps are disposed in pairs thereof so as to form springs in pairs thereof positioned in back-to-back relationships between adjacent ones of the cells. The springs in each pair thereof are configured to normally assume expanded positions in which they are displaced away from one another to engage fuel rods received in the respective cells and are deflectible to retracted positions in which they are displaced toward one another to allow loading of the fuel rods in the respective cells without engaging the springs. The spring retainer apparatus comprises: (a) a plurality of elongated holder bars, each holder bar being alignable with one of the pairs of the straps of the grid which defines the pairs of springs and extendible along one strap pair and between and spaced from positions occupied by fuel rods when received in the cells of the grid; and (b) a plurality of members supported by each of the holder bars corresponding to the pairs of springs defined by the pair of straps aligned with the holder bar; (c) each of the members having a terminal end configured to engage and retain the springs of one of the pairs thereof in their retracted positions when the respective holder bar supporting the member is aligned with and moved toward the pair of straps aligned with the holder bar. More particularly, each of the members is an elongated post with the terminal end of the member being a bifurcated end on the post. The bifurcated post end defines at least one pair of spaced apart fingers adapted to receive the pair of springs therebetween for retaining the springs in their retracted positions. Preferably, the bifurcated end on each of the posts defines two spaced pairs of spaced apart fingers adapted to receive the pair of springs therebetween and engage the spring pair at two displaced locations therealong for retaining them in their retracted positions. Also, each member in the plurality thereof supported by a respective one holder bar is rigidly connected to and extends from the bar in a generally parallel relationship with respect to the other members supported by the bar. Specifically, the terminal end of each member defines a pocket adapted to receive the pair of springs therein and retain the springs in their retracted positions and a convergently-tapered entrance to the pocket for facilitating insertion of the springs when in their retracted positions into the pocket. Preferably, the convergently-tapered entrance to the pocket also causes deflection of the springs from their normal extanded positions to their retracted positions as the springs are inserted into the pocket. The present invention also relates to a spring retainer method for facilitating the loading of fuel rods into a fuel assembly, comprising the steps of: (a) aligning a plurality of elongated holder bars with pairs of the straps of the grid which define the pairs of the springs such that the holder bars extend along the strap pairs and between and spaced from positions occupied by fuel rods when received in the cells of the grid; and (b) moving the holder bars toward the pairs of the straps to position terminal ends of members supported by each of the holder bars in engagement with the springs to retain the springs in their retracted positions. More particularly, moving of the holder bars toward the strap pairs causes deflection of the springs from normal expanded positions to retracted positions and retention of the retracted springs in their retracted positions. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.