Patent Number: 047028831
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following copending applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "Reconstitutable Nuclear Reactor Fuel Assembly With Unitary Removable Top Nozzle Subassembly" by John M. Shallenberger, assigned U.S. Ser. No. 673,681 and filed Nov. 20, 1984, a continuation-in-part of U.S. application Ser. No. 457,790, filed Jan. 13, 1983. 2. "Improved Top Nozzle And Guide Thimble Joint Structure In A Nuclear Fuel Assembly" by John F. Wilson et al., assigned U.S. Ser. No. 711,433 and filed Mar. 13, 1985. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with improved features for removably attaching the top nozzle on the guide thimbles of a fuel assembly for facilitating reconstitution thereof. 2. Description of the Prior Art Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Such hold-down devices have included the use of springs surrounding the guide thimbles, such as seen in U.S. Pat. No. 3,770,583 (U.S. Pat. No. Re. 31,583) and U.S. Pat. No. 3,814,667 to Klumb et al. and U.S. Pat. No. 4,269,661 to Kmonk et al. During operation of such assembly in a nuclear reactor, the fuel rods may occasionally develop cracks along their length resulting primarily from internal stresses, thus establishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. In view of the high costs associated with replacing fuel assemblies containing failed fuel rods, both domestic and foreign utilities have indicated an interest in reconstitutable fuel assemblies in order to minimize their operating and maintenance expenses. Conventional reconstitutable fuel assemblies incorporate design features arranged to permit the removal of individual failed fuel rods, the option to replace rods, followed by the additional use in the reactor and/or normal handling and storage of the affected fuel assembly. Reconstitution has been made possible by providing a fuel assembly with a removable top nozzle. The top nozzle is mechanically fastened usually by a threaded arrangement to the upper end of each control rod guide thimble assembly, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in neutron-absorbing liquid. With rod removal/replacement and after the top nozzle has been remounted on the control rod guide thimbles, the reconstituted assembly can then be reinserted into the reactor and used until the end of its useful life, and/or stored in spent fuel pools or other places in a safe, normal manner. The above-cross referenced patent applications describe and illustrate reconstitutable fuel assemblies having different arrangements for removably attaching the top nozzle to the upper ends of the guide thimbles. When given the task to modify a preexisting fuel assembly wherein the top nozzle is not readily removable, prior top nozzle removable attachment arrangements such as those of the cross-referenced applications are instructive of possible approaches, but do not necessarily point toward the direction one should take to make the required modification in the simplest and least costly way. Consequently, a need exists for a fresh approach tailored to the particular preexiting top nozzle attachment structure, one which will achieve removability of the top nozzle with the minimum change in the design of preexisting parts. SUMMARY OF THE INVENTION The present invention provides improved features designed to make a preexisting top nozzle attachment structure removable in a manner which satisfies the aforementioned needs. In the preexisting structure, axially extending recesses having upper and lower ledges were machined into the upper portions of the guide thimble extensions and received radial pins inserted in the upper hold-down plate of the top nozzle to form an upper limit to hold-down plate travel and to support the weight of the fuel assembly during lifting. In order to remove the top nozzle, these pins had to be removed which was a tedious operation. The present invention introduces a simple change to each of the guide thimble extensions which makes the upper hold-down plate and thus the top nozzle readily removable from the guide thimbles. The upper end of each guide thimble extension is modified to form a removable stop which defines the upper ledge of the recess. Each stop interacts with one of the hold-down plate radial pins to form the upper limit for travel of the hold-down plate along the respective guide thimble extension. The lower portion of the stop having a reduced diameter compared to the upper portion thereof is externally threaded in order to be threadably received into an internally threaded section on the upper end of the remainder of the guide thimble extension. By unthreading the stop from the respective guide thimble extension, the upper hold-down plate can be removed form the guide thimbles. Accordingly, the present invention is provided in a nuclear fuel assembly having at least one control rod guide thimble and a top nozzle, wherein the guide thimble includes an upper extension member and the top nozzle includes an upper hold-down plate having a passageway slidably receiving an upper end portion of the extension member. The present invention is directed to an improved structure for removably attaching the upper hold-down plate on the guide thimble upper extension member. The improved attaching structure basically comprises: (a) means defining a recess on the upper end portion of the extension member; (b) a stop member having upper and lower portions, with the stop member lower portion adapted to connect on the extension member upper end portion and the stop member upper portion having an outside diameter greater than that of the stop member lower portion and extending above the extension member upper end portion when the stop member lower portion is connected on the same, and the stop member also having a ledge formed thereon at a transition between its upper and lower portions which defines an upper limit of the recess when the stop member is connected on the extension member upper end portion; and (c) an element mounted in the upper hold-down plate and extending therefrom into the passageway of the plate and the recess of the extension member upper end portion, with the element being positioned to slide upwardly along the recess until making engagement with the ledge on the stop member when the stop member is connected on the extension member upper end portion for limiting upward movement of the upper hold-down plate along the guide thimble and the element being positioned to slide upwardly along and past the recess when the stop member has been disconnected from the extension member upper end portion for allowing removal of the upper hold-down plate from the guide thimble. More particularly, the upper end portion of the extension member has means defining a threaded section on the interior thereof, whereas the stop member lower portion has a complementary threaded section defined on the exterior thereof adapted to threadably fit into the threaded section of the extension member upper end portion for releasably connecting the stop member on the guide thimble extension member. Further, the recess terminates at an upper terminal edge of the guide thimble extension member upper end portion. 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.