Patent Number: 046845011
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with compliant inserts mounted within holes in the upper tie plate of a BWR fuel assembly for supporting the upper end plug extensions of assembly fuel rods so as to prevent binding and axial loading 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 neutron 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. A hollow water cross extends axially through the outer channel so as to provide an open inner channel for subcooled moderator flow through the fuel assembly and to divide the fuel assembly into four, separate, elongated compartments, each containing an identical mini-bundle of the fuel rods. 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 through the water cross and along the fuel rods removing energy from their heated surfaces. 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 comprises a separate fuel rod subassembly within each of the compartments of the channel. A plurality of grids axially spaced along the fuel rods of each fuel rod subassembly maintain the fuel rods in their laterally spaced relationships. The water cross has approximately the same axial length as the fuel rod subassemblies, extending between the upper and lower tie plates thereof. In each fuel rod subassembly of the BWR fuel assembly, the mini-bundle of fuel rods is composed of standard fuel rods and tie rods. Such use of standard and tie fuel rods is conventional, as can be seen in BWR fuel bundles illustrated in U.S. patents to Qurnell et al (U.S. Pat. No. 3,741,868) and Smith et al (U.S. Pat. No. 4,022,661). Ordinarily, the tie rods have extensions with nuts on the ends thereof which limit movement within holes in the upper tie plate, whereas the standard fuel rods have upper end plug extensions which are slidably received within holes in the plate. The upper tie plate is positioned axially by the tie rods, whereas the top ends of all the fuel rods, including the tie rods, are positioned and supported laterally by the upper tie plate via the pattern of holes defined therein. Since thermal and irradiation growth rates may be different between the tie rod and the standard fuel rod, especially in fuel assembly designs where the fuel cladding is made of cold-worked Ziracaloy, the end plug extension on the standard fuel rod must slide freely in its receiving hole in the upper tie plate to accommodate relative growth in length between the tie and standard fuel rods. Otherwise, binding and an axial load in the fuel rod would result, which leads to bowing of the fuel rod. Consequently, a need exists for a way to prevent binding of the upper end plug extension within the upper tie plate hole in order to avoid axial loading and resultant bowing of the fuel rod. SUMMARY OF THE INVENTION The present invention provides a compliant insert for supporting the end plug extension in the tie plate hole in a manner which is designed to satisfy the aforementioned needs. The compliant insert of the present invention provides a solution to the binding problem in the upper tie plate holes by interposing resiliently flexible spring support members within the respective hole (after being slightly enlarged in diameter) between the end plug extension and the hole sidewall which will accommodate relative angular movement between the end plug extension and tie plate, such as due to tilting of the tie plate relative to the plug extension, while still allowing axial movement of one relative to the other. Although, spring-type members have been incorporated heretofore in fuel assembly grids or spacers for engaging fuel rods to support them in a desired array, such as disclosed in U.S. patents to Ashcroft et al (U.S. Pat. No. 3,361,639), Milburn (U.S. Pat. No. 3,801,452) and Amaral et al (U.S. Pat. No. 4,089,742), the art has failed to either perceive the above-described binding problem existing heretofore between the fuel rod end plug extension and upper tie plate or the possibility of using a compliant insert as the solution thereof. Accordingly, the present invention is set forth in a fuel assembly having a plurality of fuel rods, a plurality of grid structures axially spaced from one another along the fuel rods and supporting the fuel rods in a side-by-side spaced array, and tie plates disposed at opposite ends of the fuel rods. At least one of the tie plates has a plurality of holes defined by endless sidewalls formed therethrough between opposite sides of the tie plate and in an array which matches that of the fuel rods. Each of the fuel rods has a pair of end plugs sealing opposite ends thereof with at least one of the end plugs having an extension member thereon which extends axially outward therefrom. The present invention relates to a compliant insert disposed in each of the holes of the one tie plate and including a plurality of spring members engaged with the tie plate and the end plug extension member so as to support the extension member within the hole in spaced relationship from the hole sidewall. In one embodiment, the spring members are separate from one another, whereas in another embodiment, they are integrally connected to one another. More particularly, each of the spring members has opposite end portions disposed along opposite sides of the tie plate adjacent to the tie plate hole. In one embodiment, the opposite end portions are tabs being bendable between axially-extending releasing and radially-extending securing positions. In addition, means are provided for securing the opposite end portions of each spring member to the respective sides of the tie plate. In one embodiment, the securing means are indentations formed in the respective sides of the tie plate into which the opposite end portions of the spring members extend. In another embodiment, the securing means are welds which interconnect the opposite end portions of the spring members to the respective sides of the tie plate. Still further, each of the spring members has an elongated middle portion extending through the plate hole between the hole sidewall and the extension member, and resilient means defined on each spring member middle portion engaging and positioning the extension member in spaced relationship from the hole sidewall. In one embodiment, the resilient means is a single inwardly-protruding dimple formed on the middle portion of the spring member. In another embodiment, the resilient means is a pair of tandemly-arranged inwardly-protruding dimples formed on the middle portion of the spring member. Also, the present invention relates to the combination in a fuel assembly, comprising: (a) a plurality of elongated fuel rods, each of the fuel rods having a pair of end plugs sealing opposite ends thereof, at least the end plug at one of the opposite ends of each fuel rod having an extension member thereon which extends axially outward from the end plug and is of a diameter less than that of the fuel rod; (b) a plurality of grid structures axially spaced from one another along the fuel rods between the opposite ends thereof and supporting the fuel rods in a side-by-side spaced array; (c) a pair of tie plates disposed at the respective opposite ends of the fuel rods, at least one of the tie plates having a plurality of holes defined by endless sidewalls formed therethrough between opposite sides of the tie plate and in an array which matches that of the fuel rods; and (d) a compliant insert disposed in each of the holes of the one tie plate and engaged with the tie plate and the end plug extension member so as to yieldably support the extension member within the hole in spaced relationship from the hole sidewall. 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.