Patent Number: 046844988
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, which is a continuation-in-part of U.S. Ser. No. 537,775, filed Sept. 30, 1983, now abandoned. 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 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. 5. "Locking Tube Insertion Fixture And Method In A Reconstitutable Fuel Assembly" by John M. Shallenberger et al, assigned U.S. Ser. No. 689,656 and filed Jan. 8, 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 improvements for releasably locking the top nozzle on the upper ends of the control rod guide thimbles and a method of carrying out the locking and unlocking of the top nozzle to and from the guide thimbles. 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 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 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 inderted 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, to prevent inadvertent escape due to vibration forces and the like, heretofore the locking tubes have been secured in their locking positions. After insertion of the locking tubes into their locking positions within the inner sockets of the hollow upper end portions of the guide thimbles, a pair of bulges are formed in the upper portion of each locking tube. These bulges fit into the circumferential bulge in the upper end portion of the guide thimble and provide an interference fit therewith. When reconstitution of the fuel assembly is undertaken, these locking tubes must first be removed from the top nozzle. The locking tubes must be handled as "loose parts", either individually or together using an appropriate removal fixture, which require storage, retention and accountability during performance of underwater nuclear fuel assembly reconstitution activities. Then, after the failed fuel rods have been removed and replaced and following remounting of the top nozzle, handling is again required when either the same locking tubes are reused a second time by inserting them back into the guide thimble upper ends and re-deforming them to secure them at their locking positions or a full complement of new locking tubes are inserted on the guide thimble upper ends and secure them by bulging. This practice has a number of disadvantages. First, a large number of locking tubes must be handled and a large inventory thereof must be maintained. Second, provision must be made for disposal of the discarded irradiated locking tubes. Third, after each locking tube is inserted, a deforming operation must be carried out remotely to produce the bulges in each tube. And, fourth, an inspection of bulges must be carried out remotely to ascertain whether the bulges were made to the correct dimension. Consequently, notwithstanding the overall acceptability of the use of the above-described attaching structure in reconstitutable fuel assemblies, these recently recognized disadvantages have created a need for further improvement of the reconstitution operation so as to enhance commercial acceptance thereof. SUMMARY OF THE INVENTION The present invention provides a reconstitutable fuel assembly with improved features for locking the top nozzle upon and unlocking it from the guide thimbles which are designed to satisfy the aforementioned needs. The present invention introduces a push-down locking tube concept in which the locking tube is not removed from the fuel assembly guide thimbles during underwater reconstitution activities. This push-down concept improves the reconstitution operation in several ways. First, it eliminates the need to track separate pieces during the reconstitution operation. Second, it reduces the time required to reconstitute the fuel assembly by eliminating the locking tube deforming operation after remounting the top nozzle. Third, by retaining the locking tubes within the guide thimbles and reusing them, provisions are not necessary for handling and disposal of the irradiated locking tubes. To summarize, by the design change in the locking tube concept contemplated by the present invention, an easier, more trouble-free removal and remounting of the top nozzle during reconstitution is achieved which result in an overall more efficient and reliable reconstitution operation without impacting the highly desirable basic design and integrity of the top nozzle/guide thimble attaching structure described and illustrated in the first patent application cross referenced above. Accordingly, the present invention sets forth in 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 for mounting the top nozzle adapter plate in releasable locking engagement upon the guide thimble upper end portion, the improvement which comprises: (a) a push-down locking tube mounted within the guide thimble upper end portion for movement relative thereto between an upper locking position wherein the adapter plate and guide thimble upper end portion are maintained in the locking engagement and a lower unlocking position wherein the adapter plate is releasable from the guide thimble upper end portion; and (b) cooperating means defined on the locking tube and the guide thimble for retaining the locking tube at either of its upper and lower positions in the guide thimble. More particularly, the cooperating means on the guide thimble includes a pair of upper and lower circumferential bulges defined on the guide thimble upper end portion and located below the adapter plate when the latter is in locking engagement upon the guide thimble upper end portion. The bulges are axially spaced from one another along the guide thimble upper end portion such that the upper bulge is located to retain the locking tube at its upper locking position and the lower bulge is located to retain the locking tube at its lower unlocking position. Still further, the cooperating means on the locking tube includes a circumferential bulge defined on a lower portion of the locking tube which is seatable in either of the upper and lower circumferential bulges defined on the guide thimble upper end portion. Additionally, the locking tube cooperating means defines at least one slot in the locking tube extending axially upwardly along the lower portion of the locking tube from a lower edge thereof. The axial slot allows radial compression and expansion of the lower portion of the locking tube upon movement of the locking tube between and positioning of the tube at its upper and lower positions for seating and unseating of the circumferential bulge of the locking tube into and from the respective upper and lower circumferential bulges of the guide thimble upper end portion. The present invention also relates to a method of locking the top nozzle adapter plate on and unlocking it from the guide thimble upper end portion, which comprises the steps of: (a) selectively moving a locking tube within the guide thimble upper end portion to an upper locking position wherein the adapter plate and guide thimble upper end portion are maintained in locking engagement; and (b) selectively moving the locking tube within the guide thimble upper end portion to a lower unlocking position displaced below the upper locking position wherein the adapter plate is releasable from the guide thimble upper end portion.