Patent Number: 047028831
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are words of convenience and are not to be construed as limiting terms. In General Referring now to the drawings, and particularly to FIG. 1, there is shown an elevational view of a reconstitutable nuclear reactor fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 10. Basically, the fuel assembly 10 includes a lower end structure or bottom nozzle 12 for supporting the assembly on the lower core plate (not shown) in the core region of a reactor (not shown), and a number of longitudinally extending guide tubes or thimbles 14 which project upwardly from the bottom nozzle 12. The assembly 10 further includes a plurality of transverse grids 16 axially spaced along the guide thimbles 14 and an organized array of elongated fuel rods 18 transversely spaced and supported by the grids 16. Also, the assembly 10 has an instrumentation tube 20 located in the center thereof and an upper end structure or top nozzle 22 attached to the upper ends of the guide thimbles 14 which incorporates certain improved features in accordance with the present invention which will be fully described below. With such arrangements of parts, the fuel assembly 10 forms an integral unit capable of being conventionally handled without damaging the assembly parts. As mentioned above, the fuel rods 18 in the array thereof in the assembly 10 are held in spaced relationship with one another by the grids 16 spaced along the fuel assembly length. Each fuel rod 18 includes nuclear fuel pellets (not shown) and is closed at its opposite ends by upper and lower end plugs 24,26. The fuel pellets composed of fissile material are responsible for creating the reactive power of the reactor. A liquid moderator/coolant such as water, or water containing boron, is pumped upwardly through the guide thimbles 14 and along the fuel rods 18 of the fuel assembly 10 in order to extract heat generated therein for the production of useful work. To control the fission process, a number of control rods (not shown) are reciprocally movable in the guide thimbles 14 located at predetermined positions in the fuel assembly 10. Since the control rods are inserted into the guide thimbles 14 from the top of the fuel assembly 10, the placement of the components of the top nozzle 22 which attach to the guide thimbles 14 must accommodate the movement of the control rods into the guide thimbles from above the top nozzle. Top Nozzle and Guide Thimble Mounting Arrangement Turning now to FIGS. 1 to 4, there is shown the components of the top nozzle 22 which interconnect with the guide thimbles 14. The top nozzle 22, which is mounted to upper hollow tubular extension members 28 of the respective guide thimbles 14, includes an upper hold-down plate 30, a lower adapter plate 32, and a plurality of hold-down coil springs 34 disposed between the upper and lower plates 30,32. The upper hold-down plate 30 has a plurality of passageways 36 defined therethrough, while the lower adapter plate 32 has a plurality of openings 38, the passageways 36 and openings 38 being arranged in respective patterns which are matched to that of the guide thimbles 14 of the fuel assembly 10. More particularly, the upper tubular extension members 28 of the guide thimbles 14 extend upwardly and are slidably inserted through the respective openings 38 in the lower adapter plate 32 and the respective passageways 36 in the upper hold-down plate 30. The hold-down springs 34 are inserted about the respective extension members 28 and displace the upper hold-down plate 30 above the lower adapter plate 32. A plurality of lower retainers 40 are attached, such as by brazing, to the guide thimble extension members 28 below the lower adapter plate 32 for limiting downward slidable movement of the adapter plate 32 relative to the guide thimbles 14 and thereby supporting the adapter plate at a stationary position along the guide thimbles. Each lower retainer 40 on one guide thimble 14 has a series of scallops 42 formed on its periphery which are aligned with those of the fuel rods 18 grouped about the respective one guide thimble 14 so that the fuel rods may be removed and replaced during reconstitution of the fuel assembly 10. As seen particularly in FIG. 2, the upper hold-down plate 30 is composed of an array of hubs 44 and radially-arranged spokes or ligaments 46 which extend between and interconnect the hubs. Each of the hubs 44 has one of the passageways 36 defined therethrough. Improved Features for Removably Attaching Top Nozzle on Guide Thimbles Referring now to FIGS. 2 to 4, there is shown some of the preexisting top nozzle attaching structure along as the modifications provided to it by the present invention which have made the top nozzle 22 easily removable and thereby the fuel assembly 10 readily reconstitutable. As designed heretofore, upper end portions 48 of the guide thimble extension members 28 each had an axially extending slot or recess (not shown) machined on the exterior thereof with upper and lower oppositely facing ledges. Also, the hold-down plate 30 had a plurality of inwardly-projecting pins 50 which each extended into one of the passageways 36 in the upper plate and one of the recesses on the extension members. The pins and upper ends or ledges of the recesses interacted to establish an upper limit to upward travel of the hold-down plate 30 and to support the weight of the fuel assembly during lifting. In view that this prior attaching structure has performed satisfactorily in reactor service, is relatively inexpensive to fabricate and has proven reactor experience, it is desirable to retain it in the improved removable attaching structure of the present invention. The improved attaching structure makes the top nozzle 22 removable and the fuel assembly 10 reconstitutable by incorporating several modifications to the preexisting structure and by adding certain new parts thereto. First, the upper end portions 48 of the guide thimble tubular extension members 28 have been modified. They have been reduced in length so that the upper terminal edges 52 thereof now terminate at the tops of the recesses 54 which thereby have also been modified from their configuration heretofore. Also, the interior of the extension member upper end portions 48 now have threaded sections 56 defined thereon. Further, a plurality of stop members 58 are incorporated by the improved attaching structure which each have upper and lower portions 60,62. Each stop member lower portion 62 has a threaded section 64 defined on its exterior which adapts it to threadably fit into the threaded section 56 of one of the extension member upper end portions 48. Also, each stop member upper portion 60 has an outside diameter greater than that of the lower portion 62 so as to form an overhanging ledge 66 on the stop member 58 at a transition between its upper and lower portions 60,62. The ledge 66 on each stop member 58 defines an upper end of one of the modified recesses 54 on the extension member upper end portions 48 when the stop member 58 is threaded thereon. Thus, the upper hold-down plate 30 is trapped between the ledge 66 on the removable stop member 58 and the lower end or ledge 68 of the recess 54 which ledge 68 is constructed the same as before. Therefore, each pin 50 fixed such as by welding in the upper hold-down plate 30 is positioned to slide upwardly along the corresponding one recess 54 until making engagement with the ledge 66 on one of the stop members 58 when the stop member is threaded on the corresponding extension member upper end portion 48. In such manner, the pin 50 coacts with the stop member ledge 66 to limit upward movement of the hold-down plate 30 along the guide thimble 14. Also, each pin 50 is positioned to slide upwardly along and past the corresponding one recess 54 when the one stop member 58 has been unthreaded from the corresponding extension member upper end portion 48. In such way, removal of the upper hold-down plate 30 is allowed from the guide thimble 14. In summary, therefore, to remove the top nozzle 22, the four upper stop members 58 are unthreaded for the guide thimble extension members 28, after which the hold-down plate 30, hold-down coil springs 34 and lower adapter plate 32 can be removed. A conventional fixture can be used to hold these components together during reconstitution of the fuel assembly 10. Parenthetically, it will be noted that the upper portion 60 of each of the stop members 58 has an outside diameter substantially the same as that of the upper end portion 48 of each of the guide thimble extension members 28 which adapts the stop members to be slidably receivable in the upper hold-down plate passageways 36. It will also be noted in reference to FIGS. 2, 3, 5 and 6, that while there is one pin 50 and recess 54 associated with each of the four corner extension members 28, there is a pair of opposing pins 50 and recesses 54 associated with the middle or center extension member 28. One additional advantage brought about by the improved attaching structure of the present invention is that it simplifies the adjustment of the engagement pins 50 during fabrication since they can now be preassembled to the hold-down plate 30 and then assembled onto the thimble extension members 28, rather than having to insert and locate the pins during final assembly. It is thought that the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.