Patent Number: 046997601
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, now U.S. Pat. No. 4,631,168. PA0 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, now U.S. Pat. No. 4,639,998. PA0 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. PA0 4. "Locking Tube Insertion Fixture And Method In A Reconstitutable Fuel Assembly" by John M. Shallenberger et al, assigned U.S. Ser. No. 689,696 and filed Jan. 8, 1985, now U.S. Pat. No. 4,638,556 PA0 5. "Locking Tube Removal Fixture And Method In A Reconstitutable Fuel Assembly" by John M. Shallenberger et al, assigned U.S. Ser. No. 694,762 and filed Jan. 28, 1985. PA0 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. PA0 7. "Fixture And Method For Rectifying Damaged Guide Thimble Insert Sleeves In A Reconstitutable Fuel Assembly" by John M. Shallenberger et al, assigned U.S. Ser. No. 797,337 and filed Nov. 13, 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 a reconstitutable fuel assembly skeleton which after being repaired has both structural and non-structural top nozzle/guide thimble joints. 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 insert or 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. In response to the high costs associated with replacing fuel assemblies containing defective fuel rods, recently, reconstitutable fuel assemblies have been developed. The general approach to making a fuel assembly reconstitutable is to provide it with a removable top nozzle. One approach to top nozzle removal is illustrated and disclosed in European Patent Application No. 0 036 821. In one embodiment of this approach, as shown in FIGS. 3, 6 and 7 of the application, the guide thimbles are severed just below the top nozzle adapter plate to allow removal of the top nozzle. Then, a new top nozzle equipped with short tubes having diameters slightly larger than the outside diameter of the guide thimbles are applied to the upper severed ends of the guide thimbles and bulged fitted thereto. However, this approach disadvantageously requires the use of a new top nozzle and only allows the fuel assembly to be reconstituted once. Another approach to reconstitutable fuel assembly construction is illustrated and described in the first application cross-referenced above. It incorporates a releasable attaching structure for removably mounting the top nozzle on the upper ends of the control rod guide thimbles. Various tools and fixtures used in releasing and installing the attaching structure for removing and replacing the top nozzle are illustrated and described in the second through fifth cross-referenced applications. A recently devised, reusable locking tube for the attaching structure is illustrated and described in the sixth application cross-referenced above. 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. Although unlikely, the possibility exists that during a fuel assembly reconstitution operation, while the top nozzle is removed from the irradiated fuel assembly to provide access to any defective fuel rod, the upper end portion, or insert sleeve, of the guide thimble could be inadvertently damaged by tooling or by a dropped object which strikes the guide thimble upper portion. Should such a mishap occur, the fuel assembly could be rendered unacceptable for further reactor service if the damaged portion cannot enter the respective top nozzle adapter plate passageway and be properly secured by the respective locking tube. Consequently, there is a need for a workable repair approach to rectify (i.e., repair or remove) and accommodate damaged guide thimble upper end portions which effectively addresses potential customer concerns related to improbable, but possible, mishaps which could jeopardize successful fuel reconstitution. SUMMARY OF THE INVENTION The present invention together with the invention of the seventh application cross-referenced above are designed to satisfy the aforementioned needs by providing contingency reconstitution equipment and a top nozzle/guide thimble joint modification designed to rectify (i.e., repair or remove) and accommodate damaged guide thimble upper end portions. The invention of the last cross-referenced application relates to a fixture and method for rectifying damaged guide thimble upper end portions. In particular, a remotely-operated fixture is operable to repair moderately damaged guide thimble portions and cut off irreparably damaged portions, thus permitting the remounting of the top nozzle on the repaired and severed portions of the guide thimbles and the continued use of the reconstituted fuel assembly in the reactor. In cases where the damaged guide thimble portions are irreparable and must be cut off, the present invention presents a modified top nozzle attachment concept compatible with fuel assembly skeletons containing several severed guide thimble upper end portions, or insert sleeves. Specifically, in the modified top nozzle, a non-structural joint is substituted for the severed guide thimble portion which previously provided a structural joint with the top nozzle adapter plate. Depending upon the particular fuel assembly configuration, at least four to six removable top nozzle/guide thimble structural joints can be eleminated without increasing the load carried by the remaining structural joints to an unacceptable level. Accordingly, the present invention sets forth in a fuel assembly skeleton including a top nozzle and a plurality of longitudinally-extending guide thimbles having upper ends, means interconnecting the top nozzle and upper ends of the guide thimbles which includes: (a) a plurality of structural joints; and (b) at least one non-structural joint. The non-structural joint is composed of a member, such as a hollow sleeve, rigidly connected to one of the top nozzle and the upper end of one of the guide thimbles and is disposed in a slip fit relationship to the other of the top nozzle and the guide thimble upper end. The sleeve has an upper portion joined to the top nozzle and of a diameter size substantially the same as that of the guide thimble upper end, and a lower portion of a diameter size slightly larger than that of the guide thimble upper end for slidably receiving the guide thimble upper end therein in a close fitting relationship. The present invention also relates to a fuel assembly skeleton, which comprises the combination of: (a) a top nozzle; (b) a plurality of longitudinally-extending guide thimbles having upper ends; and (c) a plurality of joints interconnecting the top nozzle and the upper ends of the guide thimbles. All but at least one of the joint are structural joints providing rigid connections between all but at least one of the guide thimble upper ends and the top nozzle. The one joint is a non-structural joint providing a non-rigid connection between the one guide thimble upper end and the top nozzle. More particularly, the number of structural joints constitutes at least a substantial majority of the joints and, in any event, is at least three times the number of the non-structural joints.