Patent Number: 047073268
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. "Reconstitutabale 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 copending U.S. patent application Ser. No. 457,790 filed Jan. 13, 1983, now abandoned. 2. "Improved Removable Top Nozzle Subassembly For A Reconstitutable Nuclear Fuel Assembly" by John F. Wilson et al, assigned U.S. Ser. No. 701,049 and filed Feb. 12, 1985. 3. "Device And Method For Unfastening And Lifting A Top Nozzle Subassembly From A Reconstitutable Fuel Assembly" by John F. Wilson et al, assigned U. S. Ser. No. 720,208 and filed Apr. 4, 1985, new U.S. Pat. No. 4,646,415. 4. "Integral Reusable Locking Arrangement For A Removable Top Nozzle Subassembly Of A Reconstitutable Nuclear Fuel Assembly" by Robert K. Gjertsen et al, assigned U.S. Ser. No. 857,675 and filed Apr. 30, 1986. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to reconstitutable fuel assemblies for nuclear reactors and, more particularly, is concerned with an arrangement and method for attaching and reattaching a top nozzle in a reconstitutable fuel assembly. 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. Nos. 3,770,583 and 3,814,667 to Klumb et al and 4,269,661 to Kmonk et al, and in the first patent application cross-referenced above. Due to occasional failure of some fuel rods during normal reactor operation and in view of the high cost associated with replacing fuel assemblies containing failed fuel rods, the trend is currently toward making fuel assemblies reconstitutable in order to minimize operating and maintenance expenses. Conventional reconstitutable fuel assemblies incorporate design features arranged to permit the removal and replacement of individual failed fuel rods. 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, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in a neutron-absorbing liquid. Once removal and replacement of the failed fuel rods have been carried out on the irradiated fuel assembly submerged at a work station and after the top nozzle has been remounted on the guide thimbles of the fuel assembly, the reconstituted assembly can then be reinserted into the reactor core and used until the end of its useful life. One recently proposed design for a reconstitutable fuel assembly top nozzle is described and illustrated in the second patent application cross-referenced above. The proposed top nozzle overcomes certain problems associated with the removable top nozzle designs of the above-cited U.S. patents and first cross-referenced application. Specifically, it includes improved structures which eliminate relative sliding engagement between the upper core support plate and the hold-down structure of the top nozzle while providing removable mounting of the top nozzle as a unitary subassembly on the guide thimbles of the reconstitutable fuel assembly. The fourth cross-referenced application builds on the highly satisfactory design of the second application by providing an integral reusable locking arrangement for the removable top nozzle which requires no special tooling to actuate it and, as a result, greatly reduces the complexity of the fuel assembly reconstitution operation. While the proposed removable top nozzle designs of the second and fourth cross-referenced patent applications, as just briefly described, were considered to be highly satisfactory solutions to the problems existing previously with regard to earlier designs, it has been recently recognized that even these do not provide optimum solutions to some of the earlier as well as other problems. Specifically, it has been found that protection of the individual hold-down springs of the top nozzle is inadequately provided for by the designs of either of the second and fourth cross-referenced applications. Further, in view that a fuel assembly typically will not be reconstituted more than two times (one time being a realistic estimate), the reusable looking arrangement of the fourth application is now considered to involve fabrication costs which are too high. Consequently, a need exists for a different approach to spring protection from coolant cross flow and to top nozzle attachment and reattachment, one with the objective of retaining the beneficial design features of the top nozzle disclosed in the cross-referenced applications while avoiding their short-comings. SUMMARY OF THE INVENTION The present invention provides an arrangement and method for attaching and reattaching a top nozzle in a reconstitutable fuel assembly which is designed to satisfy the aforementioned needs. The arrangement of the present invention eliminates the costs of machining threads and of providing a reusable locking structure on each of the alignment sleeves and guide thimble upper end portions. It also provides individual spring protection from cross flow present in a transition core between coil spring and leaf spring fuel assembly designs. (inter-assembly cross flow in the top nozzle region could occur in a core with both coil spring and leaf spring fuel assembly designs.) The changes introduced by the present invention are compatiable with the basic design of the top nozzle of the cross-referenced applications and, thus, can be readily implemented without affecting the upper hold-down plate and lower adapter plate castings. Accordingly, the present invention is set forth in a reconstitutable fuel assembly having at least one control rod guide thimble and a top nozzle wherein the guide thimble includes an upper end portion and the top nozzle includes a lower adapter plate and an upper hold-down plate. The lower and upper plates have respectively an opening and passageway defined therethrough and aligned with one another. The opening in the lower plate receives the guide thimble therethrough with its upper end portion extending above the lower plate and toward the passageway of the upper plate. The present invention provides an improved arrangement for mounting the top nozzle on the guide thimble comprising; (a) alignment means extending between the plates and receiving the guide thimble upper end portion, the alignment means at an upper end being inserted into the passageway of the upper hold-down plate and at a lower end resting on the adapter plate; and (b) complementary means formed on and interconnecting the alignment means and the guide thimble upper end portion so as to connect the alignment means and the guide thimble together, the complementary means including a primary interior annular groove formed on the alignment means and a primary exterior bulge formed on the guide thimble upper end portion and extending into the primary annular groove. Furthermore, the complementary means includes a secondary interior annular groove formed on the alignment means at a location spaced below the primary annular groove. The secondary groove is adapted to receive a secondary exterior bulge, which is formed on the guide thimble upper end portion after severance of an upper segment of the guide thimble upper end portion containing the primary bulge followed by removal and receipt of the severed guide thimble upper end portion from and back in the alignment means, for reconnection of the alignment means and the severed guide thimble together. Additionally, the complementary means also includes a primary interior section on the alignment means which contains the primary annular groove, and a secondary interior section on the alignment means which contains the secondary annular groove. The secondary section is disposed below the primary section and has an interior diameter larger than that of the primary section for facilitating receiving of the severed guide thimble upper end portion back into the alignment means for reconnection of the alignment means and the severed guide thimble together. Still further, the arrangement includes an elongated shroud having a lower portion resting on the adapter plate and underlying a hold-down coil spring of the top nozzle. Also, the shroud has an upper portion extending along and surrounding a portion of the spring for protecting the spring from damage by coolant cross flow from adjacent fuel assemblies. Also, in a method of making a fuel assembly reconstitutable, the present invention comprises the steps of (a) providing the alignment sleeve of the top nozzle with at least a pair of internal upper and lower annular grooves; (b) inserting the upper end portion of the guide thimble into the sleeve such that the upper end portion thereof extends adjacent the upper annular groove; and (c) bulging an annular part of the guide thimble upper end portion outwardly into the upper annular groove in the alignment sleeve so as to connect the sleeve and guide thimble together. Further, the present invention includes the steps of: (d) circumferentially cutting the guide thimble upper end portion at a location below the level of its annular part bulged into the upper annular groove in the alignment sleeve and above the level of the lower annular groove in the alignment sleeve to sever an upper segment of guide thimble upper end portion containing the bulged annular part from the remainder thereof; and (e) removing the top nozzle, including the alignment sleeve with the upper guide thimble segment connected thereto, from the severed guide thimble upper end portion for facilitating reconstitution of the fuel assembly. Finally, the present invention includes the steps of; (f) reinserting the severed upper end portion of the guide thimble into the sleeve such that the severed upper end portion thereof extends adjacent the lower annular groove; and (g) bulging another annular part of the severed guide thimble upper end portion outwardly into the lower annular groove in the alignment sleeve so as to connect the sleeve and guide thimble together.