Patent Number: 046876316
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views. 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 fuel assembly adapted for use in non-control rod locations of a nuclear reactor core (not shown), 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 structural members 14 which at their lower ends are attached to and project upwardly from the bottom nozzle 12. The assembly 10 further includes a plurality of transverse grids 16 axially spaced along the structural members 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 structural members 14. With such an arrangement 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 axially along the fuel assembly length. Each fuel rod 18 includes nuclear fuel pellets 24 and is closed at its opposite ends by upper and lower end plugs 26,28 to hermetically seal the rod. Commonly, a plenum spring 30 is disposed in the rod 18 between the upper end plug 26 and the pellets 24 to maintain the pellets in a tight, stacked relationship therein. The fuel pellets 24 composed of fissile material are responsible for creating the reactive power of the nuclear reactor. A liquid moderator-coolant such as water, or water containing boron, is pumped upwardly along the fuel rods 18 of the fuel assembly 10 in order to extract heat generated therein for the production of useful work. Each of the longitudinal structural members 14 includes an elongated hollow cladding tube 32 closed at each end by upper and lower end plugs 32,34 which are welded to the tube. The tube and end plug material is preferably Zircaloy-4. The upper and lower end plugs 32,34 have respective threaded studs or extensions 38,40 fixed thereto and extending axially therefrom which are inserted through holes 42,44 in the respective adapter plates 46,48 of the top and bottom nozzles 22,12. Whereas a nut 50 is tightened down on the threaded extension 40 for rigidly attaching the lower end of the structural member 14 to the bottom nozzle 12, the upper end of the structural member 14 is releasably attached to the top nozzle adapter plate 46 by the reusable fastener device of the present invention, generally designated 52, which will be described shortly. The structural member 14 may be pretensioned in accordance with the invention described and illustrated in the second application cross-referenced above in order to make it resistant to bowing. Reusable Fastener Device Turning now to FIGS. 2 to 11, there is seen the reusable fastener device 52 of the present invention which releasably attaches the threaded extension 38 on the upper end plug 34 of each of the structural members to the adapter plate 46 of the fuel assembly top nozzle 22. Basically, the reusable fastener device 52 includes an attachment nut 54 and a retainer housing 56. The attachment nut 54 includes a central tubular stem 58 and upper and lower flanges 60,62 connected to and extending radially outwardly from opposite upper and lower ends 64,66 of the stem. The retainer housing 56 includes a thin-walled tubular hollow body 68, a plurality of upper sectors 70 and a plurality of lower tabs 72. The upper sectors 70 and lower tabs 72 are connected to and extend respectively radially inwardly and outwardly from opposite upper and lower ends 74,76 of the tubular body. Referring particularly to FIGS. 4 to 6, there is shown the attachment nut 54 having the central stem 58, upper flange 60 and lower flange 62. The central stem 58 of the nut 54 is internally threaded for mating with the threaded upper end extension 38 of the structural member upper end plug 34. The upper flange 60 of the nut 54 has a conical-shaped underside or lower surface 78 and a periphery 80, preferably hexagonal in configuration. The hexagonal periphery 80 adapts the nut 54 for engagement by a suitable socket tool (not shown) in order to rotate the nut for threading it onto and unthreading it from the structural member threaded extension 38 and thereby moving it between fastened and unfastened positions, depicted respectively in FIGS. 2 and 11. The lower flange 62 of the nut 54 is in the form of a plurality of radial segments 82 extending outwardly from the stem 58. The segments 82 are angularly spaced from one another so as to define a plurality of cutouts 84 therebetween which alternate with the segments. Turning now to the FIGS. 8 to 10, there is shown the retainer housing 56 having the tubular body 68, the upper sectors 70 and the lower tabs 72. The upper sectors 70 of the retainer housing 56 are angularly spaced from one another so as to define a plurality of openings 86 therebetween which alternate with the sectors and are sized to receive therethrough the radial segments 82 of the attachment nut lower flange 62. Additionally, the upper sectors 70 extend radially inwardly from the tubular body 68 of the housing 56 so as to define a central opening 88 sized to receive the central stem 58 of the attachment nut 54. The upper sectors 70 together also define an interrupted conical-shaped upper surface 90 surrounding the central opening 88 which matches the conical-shaped lower surface 78 on the upper flange 60 of the attachment nut 54. The lower tabs 72 of the retainer housing 56 rest upon and are rigidly connected, such as by spot welding, to an upper surface 92 of the top nozzle adapter plate 46 so as to align the central opening 88 of the retainer housing 56 with the respective hole 42 through the adapter plate 46. The tubular body 68, in being so attached to the adapter plate 46, disposes the upper sectors 70 in a location spaced above the adapter plate at which their interrupted conical-shaped upper surface 90 will be contacted by the conical-shaped lower surface 78 of the upper flange 60 of the nut 54 when the 58 central stem thereof is threaded to its fastened position onto the upper end extension 38 of the structural member 14, as seen in FIG. 2. The upper sectors 70 are yieldably deflectible radially inwardly upon being contacted by the upper flange 60 of the attachment nut 54, as the nut is tightened down on the structural member extension 38, such that inner edges 94 of the upper sectors 70 engage the central stem 58 of the nut 54 and rotationally lock its radial segments 82 to the adapter plate 46 when the nut reaches its fastened position. Additionally, as seen best in FIGS. 3, 8 and 9, the retainer housing 56 also includes means in the form of a plurality of auxiliary tabs 96 attached to the upper sectors 70 and being movable between open and closed positions for respectively allowing and preventing passage of the lower radial segments 82 of the nut 54 through the openings 86 defined by the upper sectors 70. Each of the auxiliary tabs 96, being bendable, is connected to and extends outwardly from a radial edge 98 of one of the upper sectors 70. The tabs 96 extend adjacent to the openings 86 between the upper sectors 70 when disposed in their open positions, as seen in solid line form in FIG. 8 and in FIG. 9, so as to allow passage of the radial segments 82 on the nut 54 through the openings 86 and installation thereof into the housing 56. On the other hand, the tabs 96 extend in overlying relation to the openings 86 when disposed in their closed positions, as seen in FIG. 3 and in dotted outline form in FIG. 8, so as to obstruct passage of the nut segments 82 therethrough. By bending the tabs 96, they can be readily moved between their open and closed positions. It is also readily apparent that the auxiliary tabs 96 could be attached on the upper edges 100 of the tubular body 68 to perform the same function. Preferably, the threads on the upper end plug extension 38 end at a given distance below the upper sectors 70 of the retainer housing 56 so as to terminate upward axial movement of the nut 54 as it is rotated during unthreading from the structural member extension 38. In such manner, any tendency for the lower flange 62 to force the auxiliary tabs 96 outwardly from their closed to open positions is eliminated. Also, as an added feature seen in FIG. 7 as well as in FIGS. 4 and 5, to prevent galling of the nut 54 and end plug extension 38, the nut can have small flutes or slots 102 defined in its central stem 58. The slots 102 will collect any chips or balls of material which may form at the threaded interface and prevent them from galling the threads. Further, by proper design of the retainer housing 56 and attachment nut 54 so that the inside diameter of the hollow body 68 and the outside diameter of the lower flange 62 are disposed in an interference fit, the nut 54 will remain in the raised and unfastened position seen in FIG. 11 when detached from the structural member extension 38. This makes it easier to reassemble the nut, although it is not absolutely necessary. Finally, the structural member upper end plug 34 can be rotationally locked to the underside or lower surface 104 of the adapter plate 46 by using a conventional tongue and groove arrangement. To detach and remove the top nozzle 22, a socket wrench (not shown) is engaged with the upper flange periphery 80 of the attachment nut 54 which is always disposed above the retainer housing 56. As the nut is rotated, its lower flange 62 is forced upwardly within the retainer housing 68 from the fastened position of FIG. 2 to the unfastened position of FIG. 11. Then, to reassemble and reattach the top nozzle 22, the top nozzle is put back in place such that the extension 38 on the structural member 14 extends upwardly through the adapter plate hole 42. The nut 54 is then threaded back on the extension 38 and screwed down to the adapter plate 46. The downward motion of the nut 54 with respect to the retainer housing sectors 70 forces them radially inward into hard contact with the stem 58 of the nut, as seen in FIGS. 2 and 3, which again locks the nut against undergoing further rotation. 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.