Patent Number: 046648747
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. 3. "Top Nozzle Removal And Replacement Fixture And Method In A Reconstitutable Fuel Assembly" by John M. Shallenberger et al, assigned U.S. Ser. No. 670,729 and filed Nov. 13, 1984. 4. "Locking Tube Insertion Fixture And Method In A Reconstitutable Fuel Assembly" by John M. Shallengerger et al, assigned U.S. Ser. No. 689,696 and filed Jan. 8, 1985. 5. "Locking Tube Removal Fixture And Method In A Reconstitutable Fuel Assembly" by John M. Shallenberger et al, assigned U.S. Ser. No. 695,762 and filed Jan. 28, 1985. 6. "Reusable Locking Tube In A Reconstitutable Fuel Assembly" by John M. Shallenberger et al, assigned U.S. Ser. No. 719,108 and filed Apr. 2, 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 an insertion and removal fixture and method for installing and removing a reusable locking tube into and from a releasable locking position in a removable top nozzle 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 by 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 access 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 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. patent 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 the 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. Prior to removal of the top nozzle from, and after its replacement back on, the fuel assembly, the locking tubes must be removed from and replaced back at their locking positions. Tools and fixtures for accomplishing either removal or replacement of each locking tube, either individually one at a time or all simultaneously, are illustrated and described in the second through fifth U.S. patent applications cross-referenced above. In carrying out reconstitution of the fuel assembly, it is the common practice to discard the old locking tubes having the above construction because of the presence of partially collapsed bulges thereon which are produced by deformation upon removal of the locking tubes. Then, a full complement of new locking tubes are installed on the guide thimble upper ends and secured thereon by formation of a pair of new bulges. This practice had a number of disadvantages in terms of the large inventory of locking tubes which must be maintained, the provision which must be made for disposal of discarded irradiated locking tubes, the requirement for producing new bulges in the newly installed locking tubes and the need for inspection of the new bulges. In order to substantially eliminate the above-mentioned disadvantages, a reusable locking tube as illustrated and disclosed in the sixth U.S. patent application cross-referenced above was recently originated. However, in order to fulfill the objectives of the reusable locking tube as a viable solution to the problems associated with the prior locking tube, a need does exist for means to effectively and efficiently carry out removal and replacement of the reusable locking tube from and into the top nozzle 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 third U.S. patent application cross-referenced above, provides a system of remote-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 sixth 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 operable to simultaneously remove and subsequently reinstall a full complement of reusable locking tubes, such as the one disclosed in the sixth cross-referenced application, into and from a releasable locking position in a removable top nozzle of a reconstitutable fuel assembly. Components of the fixture remove and replace the reusable locking tubes in a manner which provides positive locking tube engagement and disengagement. With the fixture of the present invention mounted on the removable top nozzle of an irradiated fuel assembly to be reconstituted, the locking tubes can be withdrawn from the removable top nozzle/guide thimble joints and held within the confines of the top nozzle just above the adapter plate in accurate alignment for subsequent reinsertion. Time required to accomplish either removal or replacement of all locking tubes securing a removable top nozzle after placement of the fixture onto the top nozzle is very short, approximately one to two minutes. Accordingly, the present invention sets forth for use with a reconstitutable fuel assembly including 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 having a hollow locking tube for releasably locking the upper end portion of the guide thimble within the passageway of the top nozzle adapter plate, a fixture and method for inserting and removing the locking tube into and from a locking position in the top nozzle. The fixture includes: (a) locking tube engaging means being circumferentially expandable and collapsible; (b) actuating means receivable through the locking tube engaging means and being movable between disengaged and engaged positions relative to the locking tube engaging means for causing the latter to respectively assume circumferentially collapsed and expanded conditions; (c) aligning means interconnecting the actuating means and the locking tube engaging means so as to limit movement of the actuating means along a predetermined linear path between upper and lower limits relative to the locking tube engaging means, the actuating means when located at one of the limits being disposed at its disengaged position in which the tube engaging means is caused to assume its circumferentially collapsed condition in which it is disposed away from a lower end of the locking tube, the actuating means when located at the other of the limits being disposed at its engaged position in which the tube engaging means is caused to assume its circumferentially expanded condition in which it is disposed against the lower end of the locking tube; (d) biasing means interengaging the actuating and locking tube engaging means so as to bias the actuating means for movement relative to the tube engaging means so as to dispose the actuating means at its engaged position and other limit, the biasing means being yieldable for allowing the actuating means to move relative to the tube engaging means so as to dispose the actuating means at its disengaged position and one limit; and (e) operating means supporting the actuating means and being releasably lockable on the top nozzle for disposing the actuating means and therewith the locking tube engaging means, via its interconnection by the aligning means to the actuating means, through the locking tube in the top nozzle, the operating means being operable for moving the actuating means to its disengaged position and one limit to cause the locking tube engaging means to assume the circumferentially collapsed condition for facilitating disposing of the actuating means and the tube engaging means through the locking tube, the operating means also being operable for moving the actuating means to its engaged position and other limit to cause the locking tube engaging means to assume the circumferentially expanded condition for facilitating removal of the locking tube from its locking position, the operating means further being operable when the actuating means is at its engaged position and other limit and said tube engaging means is in its circumferentially expanded condition to move the actuating means and tube engaging means together away from the top nozzle and withdraw the locking tube from its locking position. More particularly, the locking tube engaging means includes a lower traveling plate, and at least one hollow flexure tube attached to and projecting downwardly from the lower plate. The flexure tube has an axially segmented sleeve portion which terminates in a lower segmented rim, with the rim being expandable to a first outside diameter greater than an inside diameter of the locking tube and collapsible to a second outside diameter less than the inside diameter of the locking tube. The actuating means includes an upper traveling plate disposed above the lower plate, and at least one actuating rod attached to and projecting downwardly from the upper plate and through the flexure tube. The actuating rod has a shaft portion which terminates in a lower enlarged nose disposed below the segmented rim of the flexure tube such that movement of the actuating rod to its engaged position forcibly inserts its enlarged nose into the segmented rim sufficiently to expand the same to its first outside diameter size wherein the segmented rim will engage the lower end of the locking tube, whereas movement of the actuating rod to its disengaged position withdraws its enlarged nose from the segmented rim sufficiently to allow contraction of the same to its second outside diameter size wherein the segmented rim will fit through the locking tube. Further, the operating means includes a mounting plate, and a central shaft supporting the upper traveling plate from the mounting plate and being operable for moving the upper plate toward and away from the lower plate and thereby moving the actuating rod between its disengaged position and one limit and its engaged position and other limit and for moving the upper and lower traveling plates together and thereby moving the actuating rod and the flexure tube together toward and away from the locking tube in the top nozzle. Also, the aligning means is a pair of elongated pins extending between and interconnecting the mounting plate and the upper and lower traveling plates. Each of the pins has a lower element on a lower end which defines the maximum limit of movement of the upper plate away from the lower plate. The method for inserting and removing the locking tubes into and from their locking positions includes the operative steps of: (a) positioning a pair of upper and lower plates adjacent the adapter plate of the top nozzle such that a plurality of elongated flexure tubes, being attached to the lower plate and receiving a plurality of actuating rods attached to the upper plate, extend through the locking tubes; (b) moving the upper plate upwardly to a maximum displacement away from the lower plate so as to forcibly insert the enlarged lower noses of the actuating rods into the segmented lower rims of the flexure tubes sufficiently to expand the rims so that they engage the lower ends of the locking tubes; and (c) once the upper plate has been moved to its maximum displacement away from the lower plate, moving the upper and lower plates together so as to move the flexure tubes relative to the top nozzle and cause withdrawal of the locking tubes from their locking positions. The method further includes the steps of: (d) after removal and replacement of the top nozzle, positioning the upper and lower plates, being disposed at their maximum displacement away from one another, adjacent the adapter plate of the top nozzle so as to place the locking tubes engaged on the flexure tubes back into their locking positions; (e) moving the upper plate toward the lower plate so as to withdraw the enlarged lower noses of the actuating rods from the segmented lower rims of the flexure tubes sufficiently to allow contraction of the rims so that they disengage from the lower ends of the locking tubes; and (f) moving the upper and lower plates away from the top nozzle so as to withdraw the flexure tubes and actuating rods through the locking tubes. 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.