Patent Number: 046648755
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. "Nuclear Reactor Fuel Assembly with a Removable Top Nozzle" by John M. Shallenberger et al, assigned U.S. Ser. No. 644,758 and filed Aug. 27, 1984. 2. "Locking Tube Removal and Replacement Tool and Method in a Reconstitutable Fuel Assembly" by John M. Shallenberger et al, assigned U.S. Ser. No. 670,418 and filed Nov. 9, 1984. 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 a fixture and method for removing a top nozzle from and replacing it on the upper ends of a plurality of guide thimbles of a reconstitutable fuel assembly. 2. Description of the Prior Art In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array of grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in passageways provided in the adapter plate of the top nozzle. The guide thimbles may each include an upper sleeve for attachment to the top nozzle. During operation of such fuel assembly in a nuclear reactor, a few of the fuel rods may occasionally develop cracks along their lengths 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. Such products may also be released into a flooded reactor cavity during refueling operations or into the coolant circulated through pools where the spent fuel assemblies are stored. Since the fuel rods are part of the integral assembly of guide thimbles welded to the top and bottom nozzles, it is difficult to detect and remove the failed rods. Until recently, to gain acess to these rods it was necessary to remove the affected assembly from the nuclear reactor core and then break the welds which secure the nozzles to the guide thimbles. In so doing, the destructive action often renders the fuel assembly unfit for further use in the reactor because of the damage done to both the guide thimbles and the nozzle which prohibits rewelding. In view of the high costs associated with replacing fuel assemblies, considerable interest has arisen in reconstitutable fuel assemblies in order to minimize operating and maintenance expenses. The general approach to making a fuel assembly reconstitutable is to provide it with a removable top nozzle. One reconstitutable fuel assembly construction, devised recently, is illustrated and described in the first U.S. Pat. application cross-referenced above. It incorporates an attaching structure for removably mounting the top nozzle on the upper ends of the control rod guide thimbles. The attaching structure includes a plurality of outer sockets defined in an adapter plate of the top nozzle, a plurality of inner sockets with each formed on the upper end of one of the guide thimbles, and a plurality of removable locking tubes inserted in the inner sockets to maintain them in locking engagement with the outer sockets. Each outer socket is in the form of a passageway through the adapter plate which has an annular groove. Each inner socket is in the form of a hollow upper end portion of the guide thimble having an annular bulge which seats in the annular groove when the guide thimble end portion is inserted in the adapter plate passageway. A plurality of elongated axial slots are provided in the guide thimble upper end portion to permit inward elastic collapse of the slotted portion so as to allow the larger bulge diameter to be inserted within and removed from the annular circumferential groove in the passageway of the adapter plate. In such manner, the inner socket of the guide thimble is inserted into and withdrawn from locking engagement with the outer socket. The locking tube is inserted from above the top nozzle into a locking position in the hollow upper end portion of the guide thimble forming the inner socket. When inserted in its locking position, the locking tube retains the bulge of the inner socket in its expanded locking engagement with the annular groove and prevents the inner socket from being moved to a compressed releasing position in which it could be withdrawn from the outer socket. In such manner, the locking tubes maintain the inner sockets in locking engagement with the outer sockets, and thereby the attachment of the top nozzle on the upper ends of the guide thimbles. Furthermore, due to vibration forces and the like, it is desirable to secure the locking tubes in their locking positions. For such purpose, suitable means, such as a pair of bulges, are formed in the upper portion of each locking tube after insertion in its locking position which bulges fit into the circumferential bulge in the upper end portion of the guide thimble. While the reconstitutable fuel assembly construction briefly described above has demonstrated considerable promise as to measure by which domestic and foreign utilities can minimize both operating and maintenance expenses, a need exists for means to effectively and efficiently carry out removal and replacement of the locking tubes and top nozzle of the reconstitutable fuel assembly so as to enhance commercial acceptance thereof. SUMMARY OF THE INVENTION The present invention together with other components, some of which comprise the invention disclosed and claimed in the second U.S. patent application cross-referenced above, provide a system of remotely-operated, submersible equipment designed to satisfy the aforementioned needs. The equipment is operable to remove and subsequently remount or replace the locking tubes and top nozzle of a reconstitutable fuel assembly, such as the one disclosed in the first U. S. patent application cross-referenced above, at a reactor plant. After the locking tubes and top nozzle have been removed, the upper ends of the fuel rods are exposed from the top of the reconstitutable fuel assembly. Thus, access to the fuel rods is gained for any of a variety of purposes: inspecting them for failure, removing and replacing failed rods, transferring partially spent fuel rods from one assembly to another, and/or rearrangement of fuel rods to attain better uranium utilization in the reactor core. Once inspection, removal, replacement and/or rearrangement of the fuel rods is completed, the top nozzle is placed back on the upper ends of the guide thimbles and the locking tubes replaced in their locking positions. The present invention provides a fixture and method for removing the top nozzle from and replacing it on the guide thimbles in a reconstitutable fuel assembly once the locking tubes have been removed. In particular, the fixture provides for safe, positively controlled removal and replacement of the top nozzle from and onto the slotted upper end portions of the guide thimbles comprising the inner sockets of the top nozzle attaching structure. Not only does the fixture include a gear driven mechanism operable to generate sufficient force to cause the collapse of the slotted upper end portions of the guide thimbles for removal and replacement of the top nozzle thereon, in addition thereto the fixture incorporates stop devices for ensuring that the remounted top nozzle is in precisely the same axial position on the fuel assembly as it was prior to removal. This is an important feature of the fixture because of the potential for fuel assembly damage should the remounted top nozzle be driven down an excessive distance over the upper end portions of the guide thimbles. Because the split or slotted guide thimble upper end portions must be collapsed to a smaller diameter while passing through the passageways in the top nozzle adapter plate (i.e. outer sockets of the top nozzle attaching structure), the fixture must be capable of exerting a force in excess of 100 pounds to remove or replace a top nozzle. Accordingly, the present invention sets forth for use with a reconstitutable fuel assembly being held in a fixed position within a work station wherein the fuel assembly includes a top nozzle with an adapter plate having at least one passageway, at least one guide thimble with an upper end portion and an attaching structure releasably mating the upper end portion of the guide thimble within the passageway of the top nozzle adapter plate, a fixture and method for removing the top nozzle from and replacing it back onto the guide thimble of the reconstitutable fuel assembly. The fixture for removing the top nozzle includes: (a) a base; (b) means for movably mounting the base on the work station in alignment with the top nozzle of the fuel assembly; (c) means for locking the top nozzle to the base when the base is movably mounted on the work station; and (d) means for moving the base, and the top nozzle therewith when locked thereto, relative to the work station so as to lift the top nozzle away from the guide thimble and thereby cause release of the attaching structure and removal of the top nozzle from the guide thimble. In addition, the fixture also includes means operable for establishing a reference representing the displacement between the base and the work station when the top nozzle is locked to the base but before the base and top nozzle have been lifted away from the work station for facilitating replacement of the top nozzle back onto the guide thimble at the same axial position on the fuel assembly as it was prior to removal. The method for removing the top nozzle from the guide thimble includes the operative steps of: (a) movably mounting a fixture on the work station in alignment with the top nozzle of the fuel assembly; (b) locking the top nozzle to a base of the fixture; and (c) moving the base, and the top nozzle therewith when locked thereto, away from the work station so as to lift the top nozzle off the guide thimble and thereby cause release of the attaching structure and removal of the top nozzle from the fuel assmebly. Furthermore, the method also includes the step of establishing a reference representing the displacement between the base of the fixture and the work station when the top nozzle is locked to the base but before the base and top nozzle have been lifted away from the work station for facilitating replacement of the top nozzle back onto the guide thimble at the same axial position on the fuel assembly as it was prior to removal. 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.