Patent Number: 046844988
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 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 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. Specifically, the top nozzle 22 includes a rod cluster control mechanism 34 having an internally threaded cylindrical member 36 with a plurality of radially extending flukes or arms 38. Each arm 38 is interconnected to one or more control rods 32 such that the control mechanism 34 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. As illustrated in FIG. 1, the top nozzle 22 has a lower adapter plate 40 with a plurality of control rod passageways 42 (only one being shown) formed through the adapter plate. The control rod guide thimbles 14 have their uppermost end portions 44 coaxially positioned within the passageways 42 in the adapter plate 40. For gaining access to the fuel rods 18, the adapter plate 40 of the top nozzle 22 is removably connected to the upper end portions 44 of the guide thimbles 14 by an attaching structure, generally designated 46. The attaching structure 46, best seen in FIGS. 2 and 7, includes the improved features of the present invention. Except for certain important improved features associated with each locking tube 48 and each guide thimble upper end portion 44 which allows the locking tube to be captured in the guide thimble 14 and renders it reusable as will be discussed later, the attaching structure 46 contains features generally similar to those depicted in FIGS. 9 and 14 of the first patent application cross-referenced above. The prior features of the attaching structure 46 will be described herein to the extent necessary to facilitate an understanding of the improved features of the present invention. TOP NOZZLE ATTACHING STRUCTURE As best seen in FIGS. 2 and 5, the top nozzle attaching structure 46 of the reconstitutable fuel assembly 10 includes a plurality of outer sockets 50 (only one being shown) defined in the top nozzle adapter plate 40 by the plurality of passageways 42 (also only one being shown) which each contains an annular circumferential groove 52 (only one being shown), a plurality of inner sockets 54 (only one being shown) defined on the upper end portions 44 of the guide thimbles 14, and a plurality of improved push-down locking tubes 48 (only one being shown) inserted in the inner sockets 54 to maintain them in locking engagement with the outer sockets 50. Each inner socket 54 is defined by an annular circumferential bulge 56 on the hollow upper end portion 44 of one guide thimble 14 only a short distance below its upper edge 58. A plurality of elongated axial slots 60 are formed in the upper end portion 44 of each guide thimble 14 to permit inward elastic collapse of the slotted end portion to a compressed position so as to allow the circumferential bulge 56 thereon to be inserted within and removed from the annular groove 52 via the adapter plate passageway 42. The annular bulge 56 seats in the annular groove 52 when the guide thimble end portion 44 is inserted in the adapter plate passageway 42 and has assumed an expanded position. In such manner, the inner socket 54 of each guide thimble 14 is inserted into and withdrawn from locking engagement with one of the outer sockets 50 of the adapter plate 40. More particularly, the axially extending passageway 42 in the adapter plate 40 which defines the outer socket 50 is composed of an upper bore 62 and a lower bore 64. The lower bore 64 is of considerably greater axial length than the upper bore 62 and contains the annular groove 52 which is spaced a short distance below a ledge 66 formed at the intersection of the upper and lower bores 62, 64. The lower bore 64 has a diameter which is greater than that of the upper bore 62; therefore, the ledge 66 faces in a downward direction. The primary purpose of the ledge 66 is to serve as a stop or an alignment guide for proper axial positioning of the upper end portion 44 in the passageway 42 when the inner socket 54 is inserted into the outer socket 50. As seen in FIGS. 5 and 6, the upper edge 58 abuts the ledge 66. IMPROVED TOP NOZZLE LOCKING AND UNLOCKING FEATURES Referring now to FIG. 2 through 7, there is also seen the improved features of the present invention being associated with the locking tube 48 and the guide thimble upper end portion 44. The locking tube 48 has upper and lower portions 68, 70 and is mounted within the guide thimble upper end portion 44 for movement therealong between an upper locking position, as seen in FIG. 5, and a lower unlocking position, as seen in FIG. 6. When the locking tube 48 is at its upper locking position, its upper portion 68, which is an elongated tubular body having a generally uniform diameter, is located coextensive with the inner and outer sockets 54, 50 for retaining the inner socket 54 seated within the outer socket 50 and thereby maintaining the adapter plate 40 and the guide thimble upper end portion 44 in locking engagement. On the other hand, when the locking tube 48 is at its lower unlocking position, its upper portion 68 is located below the inner and outer sockets 54, 50 for allowing unseating of the inner socket 54 from the outer socket 50 and thereby release of the adapter plate 40 from the guide thimble upper end portion 44. The improved features of the present invention relate to cooperating means, generally designated 72 and 74, defined on the guide thimble upper end portion 44 and on the lower portion 70 of the locking tube 48 for retaining the locking tube 48 at either of its upper position (FIG. 5) or its lower position (FIG. 6) within the guide thimble upper end portion 44. The cooperating means 72 on the guide thimble upper end portion 44 includes a pair of upper and lower circumferential bulges 76, 78 which are located below the adapter plate 40 when the inner socket 54 of the guide thimble upper end portion 44 is seated in locking engagement within outer socket 50 of the adapter plate passageway 42. The upper and lower bulges 76, 78 are axially spaced from one another along the guide thimble upper end portion 44 such that the upper bulge 76 is located to retain the locking tube 48 at its upper locking position, while the lower bulge 78 is located to retain the tube 48 at its lower unlocking position. The cooperating means 74 on the locking tube 48 includes a circumferential bulge 80 defined on the lower portion 70 of the locking tube 48 which is seatable in either of the upper and lower circumferential bulges 76, 78 defined on the guide thimble upper end portion 44. In addition thereto, the cooperating means 74 on the locking tube 48 further includes means defining at least one, but preferably, as seen in FIG. 3, four slots 82 in the locking tube 48 extending axially upwardly along the lower portion 70 of the locking tube 48 from a lower edge 84 thereof. The slots 82 allow radial compression and expansion of the lower portion 70 of the locking tube 48 upon movement of the locking tube between and positioning of the tube at its upper and lower positions for seating and unseating of the circumferential bulge 80 of tube into and from the respective upper and lower circumferential bulges 76, 78 of the guide thimble upper end portion 44. To remove the top nozzle 22 from the guide thimbles 14, each locking tube 48 is moved from its upper locking position, as seen in FIG. 5, wherein each outer socket 50 of the adapter plate and each inner socket 54 of the guide thimble upper end portion 44 are maintained in locking engagement, to its lower unlocking position, as seen in FIG. 6. When each locking tube 48 is in its lower position of FIG. 6, the inner socket 54 is now unobstructed by the upper portion 68 of the locking tube and free to collapse inwardly and release its engagement with the outer socket 50 when the adapter plate 40 is lifted upwardly from the guide thimble 14 to the position seen in FIG. 7. The top nozzle 22 is remounted on the guide thimbles 14 by performing the above steps in reverse. 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.