Patent Number: 046997598
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 reconstitutable nuclear 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 removably attached to the upper ends of the guide thimbles 14, in a manner fully described below, to form an integral assembly 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 24 and the opposite ends of the rod are closed by upper and lower end plugs 26,28 to hermetically seal the rod. Commonly, a plenum spring 30 is disposed between the upper end plug 26 and the pellets 24 to maintain the pellets in a tight, stacked relationship within the rod 18. 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 through the fuel assemblies of the core in order to extract heat generated therein for the production of useful work. To control the fission process, a number of control rods 32 are reciprocally movable in the guide thimbles 14 located at predetermined positions in the fuel assembly 10. The top nozzle 22 has a lower adapter plate 34 with a plurality of passageways 36 (only one being shown) formed through the adapter plate. The guide thimbles 14 which receive the control rods 32 have their uppermost end portions 38 coaxially positioned within the adapter plate passageway 36. Specifically, operatively associated with the top nozzle 22 is a rod cluster control mechinism 40 havng an internally threaded cylindrical member 42 with a plurality of radially extending flukes or arms 44. Each arm 44 is interconnected to one or more control rods 32 such that the control mechanism 40 is operable to move the control rods 32 vertically in the guide thimbles 14 to thereby control the fission process in the fuel assembly 10, all in a well-known manner. Double Lock Joint Structure for Releasably Attaching Top Nozzle to Guide Thimbles Referring now to FIGS. 2 through 5, there is seen the improved features of the present invention for releasably connecting the lower adapter plate 34 of the top nozzle 22 to the upper end portions 38 of the guide thimbles 14 for facilitating access to the fuel rods 18 of the fuel assembly 10. The improved features of the present invention comprise components of a double lock joint structure, generally designated 46. As depicted in FIGS. 2 through 5, the double lock joint structure 46 includes axially extending slots 48, being four in number in the illustrated embodiment, defined in each guide thimble upper end portion 38 which permit inward elastic collapse thereof to a compressed position upon application of sufficient forces directed radially inward toward a central axis A of the upper end portion 38. The upper end portion 38 elastically returns outward to an expanded position upon removal of the radially inward directed forces thereto. These forces are imposed on the upper end portion 38 of each guide thimble 14 whenever the adapter plate 34 is applied to it. The joint structure 46 also includes means preferably taking the shape of axially spaced upper and lower annular bulges 50,52 formed circumferentially in the upper end portion 38. The upper annular bulge 50 has an outside diametric size greater than the inside diametric size of a given adapter plate passageway 36 when the guide thimble upper end portion 38 is at its expanded position. On the other hand, the outside diametric size of the upper bulge 50 reduces to less than the inside diametric size of the adapter plate passageway 36 when the upper end portion 38 is collapsed to its compressed position due to application of the radially inward directed forces thereto during insertion and withdrawal into and from the adapter plate passageway 36. However, in the case of the annular bulge 52, it has an outside diametric size which is greater than the inside diametric size of the one adapter plate passageway 36 when the guide thimble upper end portion 38 is at either one of its expanded and collapsed positions. For locking the guide thimble upper end portion 38 to the adapter plate 34, the upper annular bulge 50 is axially displaced from the lower annular bulge 52 through a distance approximately equal to that between top and bottom surfaces 54,56 of the adapter plate 34. Therefore, after insertion of the upper end portion 38 of the guide thimble 14 through the adapter plate passageway 36, as shown in FIG. 5, the upper annular bulge 50 is located above the adapter plate 34 adjacent to and in contact with the top surface 54 thereof, the lower annular bulge 52 is located below the adapter plate 34 adjacent to and in contact with the bottom surface 56 thereof and the adapter plate 34 is placed in a captured position between the upper and lower annular bulges 50,52. Further, the joint structure 46 includes a locking tube 58 removably inserted into each guide thimble upper end portion 38 to a locking position therein which maintains the upper end portion in its expanded position and the adapter plate 34 in the captured position between the upper and lower annular bulges 50,52. Removal of the locking tube 58 withdraws it to an unlocking position which permits the upper end portion 38 to inwardly collapse to its compressed position upon removal of the adapter plate 34 from the upper end portion. The upper end portion 38 can also be collapsed to its compressed position upon insertion of the adapter plate 34 back on the guide thimble 14. For securing the locking tube 58 at its locking position in the upper end portion 38 of the guide thimble 14, the tube includes upper and lower axially, and circumferentially, displaced protuberances 60,62 in the form of short segments adapted to mate with the upper and lower annular bulges 50,52 of the guide thimble upper end portion 38. Also, a top annular flange 64 on the tube 58 is located above the upper protuberance 60 for facilitating gripping of the tube for insertion and removal of the tube into and from the guide thimble upper end portion 38. While the upper and lower bulges 50,52 are illustrated as each extending circumferentially about the upper end portion 38, it should be understood that each bulge need not be continuous about the upper end portion, but can be in the form of a series of short segments similar to the protuberances 60,62 of the locking tube 58. Likewise, it should be understood that when the upper and lower bulges 50,52 are continuous as is illustrated, the upper and lower protuberances 60,62 of the locking tube 58 can extend continuously about the tube similar to the upper and lower bulges, instead of being short segments. 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 adavantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.