Patent Number: 046997598
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. "Reconstituting A Nuclear Reactor Fuel Assembly" by Robert K. Gjersten et al, assigned U.S. Ser. No. 564,056 and filed Dec. 31, 1983 (W.E. 49,189). 2. "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 (W.E. 51,311I) which is a continuation-in-part of U.S. Ser. No. 537,775, filed Sept. 30, 1983, now abandoned. 3. "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 (W.E. 52,507). 4. "Guide Thimble Captured Locking Tube In A Reconstitutable Fuel Assembly" by Gary E. Paul, assigned U.S. Ser. No. 717,991 and filed Mar. 29, 1985 (W.E. 52,508). 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 double lock joint for attaching the top nozzle of a fuel assembly to its guide thimbles in a manner which allows easy removal and replacement of the top nozzle. 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 nozzle, 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 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 early method of reconstituting the fuel assembly by removing and replacing its top nozzle is illustrated and described in the first patent application cross-referenced above. The top nozzle is removed by severing the guide thimbles just below where they are welded to the adapter plate of the top nozzle. Then, before the top nozzle is replaced just below where they are welded to the adapter plate of the top nozzle. Then, before the top nozzle is replaced back on the guide thimbles, annular grooves are formed in the passageways of the adapter plate. After the top nozzle is placed back on the guide thimbles with the latter inserted into the adapter plate passageways, circumferential portions of the upper ends of the guide thimbles are bulged into the passageway grooves so as to secure the top nozzles to the guide thimbles. An alternative approach to the above reconstitution method is illustrated and described in the second patent application cross-referenced above. An attaching structure for removably mounting the top nozzle on the upper ends of the guide thimbles is disclosed. The attaching structure includes a plurality of outer sockets defined in the 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 formed therein. 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, 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. Notwithstanding the overall acceptability of the above-described approaches, one disadvantage is that each requires the machining of annular grooves in the passageways of the top nozzle adapter plate. Consequently, a need remains for still another alternative approach to fuel assembly reconstitution which may further 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 need. The present invention introduces a double lock joint concept which provides a tighter and stronger connection between the top nozzle and guide thimbles than heretofore. In particular, positive locking is provided at two elevations by axially spaced bulges on the guide thimbles which capture the top nozzle adapter plate therebetween. Excellent axial strength and resistance to transverse loading are built into the double lock joint. Furthermore, the design provides easy insertion and removal of the guide thimbles and locking tubes, eliminates the need for machining of internal annular grooves in the adapter plate passageways, and exhibits superior joint alignment and protection against joint slippage. Accordingly, the present invention sets forth in a reconstitutable fuel assembly including at least one guide thimble with an upper end portion having a central axis and a top nozzle with an adapter plate having top and bottom spaced apart surfaces and at least one passageway extending between the surfaces, a double lock joint structure for attaching the top nozzle adapter plate in releasable locking engagement upon the guide thimble upper end portion. The double lock joint structure comprises: (a) means defined in the upper end portion of the guide thimble to permit inward elastic collapse thereof to a compressed position upon application of forces directed radially inward toward the axis of the upper end portion and outward elastic return thereof to an expanded position upon removal of the radially inward directed forces; (b) upper means formed in the upper end portion of the guide thimble so as to provide the upper end portion at the location of the upper means with a diametric size greater than that of the adapter plate passageway when the guide thimble upper end portion is at its expanded position and a diametric size less than that of the adapter plate passageway when the upper end portion is collapsed to its compressed position upon application of the radially inward directed forces during insertion and withdrawal of the upper end portion into and from the adapter plate passageway; and (c) lower means formed in the upper end portion of the guide thimble so as to provide the upper end portion at the location of the lower means with a diametric size greater than that of the adapter plate passageway when the guide thimble upper end portion is at either one of its expanded and collapsed positions. The upper means is axially displaced from the lower means through a distance approximately equal to that between the top and bottom surfaces of the adapter plate such that after insertion of the upper end portion of the guide thimble through the adapter plate passageway the adapter plate is placed in a captured position between the upper and lower means. Also, the double lock joint structure includes a locking tube insertable into and removable from the upper end portion of the guide thimble between a locking position which maintains the upper end portion in the expanded position and the adapter plate in the captured position between the upper and lower means and an unlocking position which permits the upper end portion to inwardly collapse to the compressed position upon insertion and removal of the adapter plate onto and from the upper end portion. More particularly, the means defined in the upper end portion of the guide thimble to permit inward elastic collapse thereof to the compressed position is at least one axially extending slot formed in the upper end portion. Further, the upper and lower means each takes the form of bulges, preferably circumferentially, formed in the upper end portion of the guide thimble. Additionally, the locking tube includes upper and lower axially, and preferably circumferentially, displaced protuberances adapted to mate with the upper and lower annular bulges of the guide thimble upper end portion when the locking tube is inserted at its locking position therein and unmate from the upper and lower bulges when the locking tube is removed from the guide thimble upper end portion. Also, the locking tube includes a top annular flange located above the upper protuberance for facilitating insertion and removal of the tube into and from the guide thimble 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.