Patent Number: 046844996
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 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. In the operation of a nuclear reactor, it is common practice to provide an excess of reactivity initially in the reactor core and, at the same time, provide means to maintain the reactivity relatively constant over its lifetime. Such means commonly takes the form of control rods (not shown) supported for movement into the guide thimbles of some fuel assemblies in the core and burnable absorber rods 32 supported stationarily in the guide thimbles 14 of other fuel assemlies in the core. The stationary absorber rods 32 assist the movable control rods in maintaining a substantially constant level of neutron flux or reactivity in the core throughout its operating cycle. Before describing the means of the present invention for stationarily attaching the burnable absorber rods 32 to the top nozzle 22, the means by which the guide thimbles 14 are attached to the top nozzle will first be briefly described. Top Nozzle Attaching Structure As illustrated in FIGS. 1, 2 and 11, 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. Each guide thimble 14 has its uppermost end portion 38 coaxially positioned within one passageway 36 in the adapter plate 34. For gaining access to the fuel rods 18 in reconstitution of the fuel assembly 10, the adapter plate 34 of the top nozzle 22 is removably connected to the upper end portions 38 of the guide thimbles 14 by an attaching structure, generally designated 40, which provides a plurality of structural joints between the top nozzle 22 and the guide thimbles 14 of the fuel assembly skeleton. The attaching structure 40 is generally the same as illustrated and described in the third and fourth applications cross-referenced above, but will be described herein to the extent necessary to facilitate an understanding of the present invention. As best seen in FIGS. 2 through 11, the top nozzle attaching structure 40 which makes the fuel assembly 10 reconstitutable includes a plurality of outer sockets (only one being shown) defined in the top nozzle adapter plate 34 by the plurality of passageways 38 (also only one being shown) which each contains an annular circumferential groove 42 (only one being shown), a plurality of inner sockets (only one being shown) defined on the upper end portions 38 (only one being shown) of the guide thimbles 14, and a plurality of locking tubes 44 (only one being shown) inserted in the inner sockets to maintain them in locking engagement within the outer sockets. Each inner socket is defined by an annular circumferential bugle 46 on the hollow upper end portion 38 of one guide thimble 14 only a short distance below its upper edge 48. A plurality of elongated axial slots 50 (only one being shown in FIG. 2) are formed in the upper end portion 38 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 46 thereon to be inserted within and removed from the annular groove 42 via the adapter plate passageway 36. The annular bulge 46 seats in the annular groove 42 when the guide thimble end portion 38 is inserted in the adapter plate passageway 36 and has assumed an expanded position. In such manner, the inner socket of each guide thimble 14 is inserted into and withdrawn from locking engagement with one of the outer sockets of the adapter plate 34. More particularly, the axially extending passageway 36 in the adapter plate 34 which defines the outer socket is composed of an upper bore 52 and a lower bore 54. The lower bore 52 is of greater axial length than the upper bore 54 and contains the annular groove 42 which is spaced a short distance below a ledge 56 formed at the intersection of the upper and lower bores 52,54. The lower bore 54 has a diameter which is greater than that of the upper bore 52; therefore, the ledge 56 faces in a downward direction. The primary purpose of the ledge 56 is to serve as a stop or an alignment guide for proper axial positioning of the guide thimble upper end portion 38 in the passageway 36 when the inner socket is inserted into the outer socket. As seen in FIGS. 2 to 11, the upper edge 48 of the guide thimble 14 abuts the ledge 56. Finally, each locking tube 44 is inserted from above the top nozzle 22 into its respective locking position in the hollow upper end portion 38 of one guide thimble 14 forming one inner socket. When the locking tube 44 is inserted in its locking position, as seen in FIGS. 2 and 11, it retains the bulge 46 of the inner socket in the latter's expanded locking engagement with the annular groove 42 and prevents the inner socket from being moved to its compressed releasing position in which it could be withdrawn from the outer socket. In such manner, each locking tube 44 maintains its respective one inner socket in locking engagement with the outer socket, and thereby retains the structural joint formed by the attachment of the adapter plate 34 of the top nozzle 22 on the upper end portion 38 of each guide thimble 14 in an assembled rigid form. Burnable Absorber Rod Releasably Latching Structure Heretofore, the burnable absorber rods and thimble plugs have been combined into an assembly in which they are rigidly attached at their upper ends to a support plate which rests upon the adapter plate of the top nozzle with the rods and plugs extending downwardly through the adapter plate passageways into the guide thimbles. This prior attachment design requires the absorber assembly to be completed at a manufacturing facility normally located away from the reactor site. Such requirement eliminates the capability of specifying the absorber configuration at the latest possible time just before actual refueling of the reactor is to take place and thereby prevents fine tuning of the nuclear design based on the latest reactor operations input. The releasable latching structure of the present invention, as seen in FIGS. 3 and 4 and being generally identified by the number 58, was originated to overcome such obstacles and provide the desired flexibility to decide upon the final configuration of the absorber assembly based on last minute reactor operating information just before fuel reloading is to be carried out. Referring now to FIGS. 3 to 9, there is shown the preferred embodiment of the releasable latching structure 58 for releasably attaching the upper end 60 of each absorber rod 62 and thimble plug (not shown) directly to the top nozzle adapter plate 34. For purposes of brevity and clarity in describing the present invention, it should be understood that, in addition to the absorber rod 62, the latching structure 58 is applicable for use on any other elongated member which is inserted within one of the guide thimbles 14, such as a thimble plug. Therefore, since showing the latching structure 58 affixed on a conventional thimble plug which would merely be duplicative of what is already shown in FIGS. 3, 4, 10 and 11, it should be understood that whenever reference is made to the latching structure being attached on an absorber rod the same would apply to a thimble plug. Basically, the releasable latching structure 58 includes a generally cylindrical mounting body 64 and a generally cylindrical latch spring 66. The mounting body 64 is composed of a generally cylindrical inner plug portion 68 attached to and sealing the upper end 60 of the absorber rod 62, a generally cylindrical outer end portion 70 having an annular circumferential groove 72 defined therein, and a middle body portion 74 integrally interconnecting the inner and outer portions 68,70. The middle body portion 74 has a generally conical configuration tapering inwardly from the outer end portion 70 to the inner plug portion 68 so as to define a tapering recessed void region, indicated generally by the numeral 76, which surrounds the mounting body 64 at the middle portion thereof. The spring latch 66 of the latching structure 58 is composed of a generally cylindrical outer ring portion 78 disposed about the outer end portion 70 of the mounting body 64 and a plurality of circumferentially spaced apart latch fingers 80 connected at their outer ends 82 to the outer ring portion 78 in cantilever fashion and extending downwardly therefrom along the middle body portion 74 of the mounting body 64. The outer ring portion 78 has an annular circumferential bulge 84 formed therein which extends into the circumferential groove 72 in the outer end ptortion 70 so as to connect the sptring latch 66 to the mounting body 64. More particularly, the latch fingers 80 having external latching keys 86 defined on their inner ends 88 and are radially deflectible toward and away from the middle body portion 74 of the mounting body 64 between outer latching positions, as seen with respect to the finger 80a on the right side of FIG. 10, and inner unlatching positions, as seen with respect to the finger 80b on the left side of FIG. 10. The normal relaxed position to which each of the fingers 80 is biased to return is the latching position seen on the right side of FIG. 10. The fingers 80 are resiliently flexible so that they will yield when sufficient force is aptpled to them in a radially inward direction causing each to be deflected inwardly against its bias toward the middle body portion 74. As clearly seen in FIG. 10, when the fingers 80 are disposed in their outer latching positions (as is finger 80a on the right side of FIG. 10), the fingers are disposed generally outside of the recessed region 76 surrounding the mounting body middle portion 74. On the other hand, when the fingers 80 are disposed in their inner unlatching positions (as is finger 80b on the left side of FIG. 10), the fingers are disposed generally within the recessed region 76. Referring now to FIG. 11, the releasable latching structure 58 is shown interconnecting the upper end 60 of the absorber rod 62 to the adapter plate 34 of the fuel assembly top nozzle 22, with the absorber rod 62 being disposed within a guide thimble 14. The upper end portion 38 of the guide thimble extends into adapter plate passageway 36 and is locked therein by the attaching structure 40 which includes the locking tube 44. The mounting body 64 of the latching structure 58 is attached at its inner plug portion 68 to the upper end 60 of the absorber rod 62 and extends axially upward therefrom through the adapter ptlate passageway 36 and locking tube 44 to its outer end portion 70 disposed above the adapter plate 34. The middle body portion 74 of the mounting body 64 extends from above the adapter plate 34 downwardly into the passageway 36 to approximately the depth of the ledge 56. In such manner, a circumferential recess 90 defined in the adapter plate 34 within the upper bore 52 of its passageway 36 is accessible from above the adapter plate 34 through the recessed void region 76 surrounding the middle body portion 74. The spring latch 88 of the latching structure 58 is disposed about the mounting body 64 generally above the adapter plate 34 and its latch fingers 80 extend downwardly along the middle body portion 74 toward and into the adapter plate passageway 36. The latching keys 86 on the inner ends 88 of the latch fingers 80 are disposed within the passageway 36, aligned with the passageway recess 90, and configured to engage the adapter plate 34 within the recess 90. When the latch fingers 80 are disposed in their latching positions as seen in FIG. 11, they are generally outside of the recessed region 76 with their latching keys 86 engaged within the adapter plate recess 90. The absorber rod 62 is then retained in a stationary relationship with respect to the adapter plate 34. By insertion of a sleeve 92 over the latch fingers 80, as shown in FIG. 10, from its uppter position to its lower position, the latch fingers 80 are yieldably deflected radially inward toward the mounting body 64, causing the latching keys 86 thereon to disengage from the adapter plate recess 90. The latch fingers 80 are then generally disposed within the recessed region 76 surrounding the mounting body 64 and their latching keys 86 displaced inwardly to allow sufficient clearance for the absorber rod 62, and the latching structure 58 therewith, to pass upwardly through the passageway 36 for removing the absorber rod 62 from the guide thimble 14. An undercut cavity 94 formed in the top end of the outer end portion 70 of the mounting body 64 is configured to receive a gripper tool (not shown) for use in insertion and removal of the absorber rod 62. The spring latch 66 is preferably machined from cold worked stainless steel tube. The fingers 80 are so formed so that in the relaxed position (no external forces applied) the latching keys 86 extend radially outward well beyond the diameter of the recess 90 formed in the adapter plate 34. Thus, when the absorber rod 62 and latching structure 58 are installed in the guide thimble 14 and adapter plate 34, the keys 86 are forced into the recess 90. Since the upper engaging sides of the keys 86 and the recess 90 are flat, any upward force on the absorber rod 62 does not tend to force the keys 86 and latch fingers 80 inward. Also, since there are multiple spring fingers 80, there is redundancy in the locking structure 58. A single absorber rod 62 requires about twelve pounds of force to hold it in place against the hydraulic flow forces. Twelve pounds of force is an amount well within the capability of the locking structure 58. 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 exemptlary embodiment thereof.