Patent Number: 048881511
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like references 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. Being the type used in PWR, the fuel assembly 10 basically 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 to form an integral assembly capable of being conveniently 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 reactor 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. Prior Art Control Rod Spider Assemblies Turning now to FIGS. 2 and 3 as well as FIG. 1, there is shown a typical embodiment of a conventional control rod spider assembly, generally designated 32, designed for use in the conventional fuel assembly 10 of FIG. 1. In its basic components, the control assembly 32 includes a plurality of control rods 34 and a spider structure 36 which supports the control rods at their upper ends. The spider structure 36 holds the control rods 34 in a pattern matched to that of the guide thimbles 14 which adapts them to be inserted through the top nozzle 22 and downward through the guide thimbles 14 of the PWR fuel assembly 10. The spider structure 36 is connected to a control mechanism (not shown) which is operable in a known manner to move the control rods 34 so as to regulate core power. In a typical construction, each control rod 34 of the control assembly 32 is composed of an elongated metallic cladding tube 38 having a neutron absorbing material disposed therein and upper and lower end plugs 40,42 attached at opposite ends of the cladding tube 38 for sealing the absorber material therewithin. The spider structure 36 of the control assembly 32 typically includes a plurality of radially extending flutes or vanes 44 supported on and circumferentially spaced about a central hub 46. Cylindrical shaped control rod connecting fingers 48 are mounted to and supported by the vanes 44. Some of the vanes 44 have only a single connecting finger 48 attached thereon, whereas other vanes 44 have a spaced pair of connecting fingers 48 associated therewith. Turning now to FIGS. 4-9, there is illustrated one prior art attachment joint, generally indicated by the numeral 50, provided between each control rod connecting finger 48 on the vane 44 of the control assembly spider structure 36 and the upper end plug 40 of each control rod 34. Typically, the upper end plug 40 of each control rod 34 has a threaded outer end 52. Each connecting finger 48 is mounted to the vane 44 in a bayonet-type of welded connection, and has an axial bore 54 formed in a lower portion 56 thereof with a smaller-diameter threaded hole 58 tapped therein at the inner end of the bore 54. The threaded outer end 52 of the upper end plug 40 is threadably received in the tapped hole 58 when the plug 40 is received within the axial bore 54. The end plug 40 is secured or locked therein by a key or pin 60 inserted through aligned holes 62,64 in the sides of the finger 48 and end plug 40 and then welded thereto. Parenthetically, it should be pointed out that the axial bore 54 terminates at the start of an upper portion 66 of each connecting finger 48 where the finger connects with the vane 44. A major disadvantage of this conventional control assembly 32 is that it is not reconstitutable; that is, the assembly 32 cannot readily be taken apart and have worn or damaged components thereof replaced. Instead, the whole assembly has to be discarded. However, as mentioned earlier, control rod spider assemblies having removable control rods are known in the prior art. One recent control rod spider assembly that is reconstitutable is disclosed in the aforecited French patent application No. 86/08381. Similar to the above-described prior art control assembly, the French control assembly includes a spider structure with connecting fingers on vanes and a plurality of control rods with upper end plugs having a threaded outer end. However, the attachment joint employed to secure each control rod to one connecting finger is modified somewhat from that described above. Each connecting finger of the French control assembly has an axial bore extending therethrough from end to end. When the upper end plug of one control rod is inserted through the axial bore, its threaded outer end extends above the top end of the finger. A fastener or nut is threaded onto the outer end of the control rod upper end plug until it contacts the top end of the finger. Then a tubular locking cup formed on the control rod upper end plug above the threaded outer end and extending above the threaded nut is deformed radially outward to lock within a groove in the nut to retain the nut thereon. However disadvantageously, to remove the control rod from the spider structure, the portion of the end plug which includes the threaded outer end with the nut fastened thereon must first be severed or cut off. Thus, the fastening nut must be replaced after removal. Control Rod Spider Assembly of the Present Invention Turning now to FIGS. 10-16, there is shown particularly in FIG. 10, in fragmentary form, a reconstitutable control rod spider assembly, generally designated by the numeral 68 and constructed in accordance with the principles of the present invention, which provides an improved arrangement for removing and replacing a control rod. The reconstitutable control assembly 68 basically includes a spider structure 70, a plurality of control rods 72 (only one fragmentarily shown), and a plurality of improved attachment joints 74 (only one shown) for detachable fastening the control rods 72 to the spider structure 70. Except for the parts thereof to be described hereinafter, the spider structure 70 and control rods 72 of the control assembly 68 are identical to that of the prior art control assembly 32 of FIGS. 2 and 3. Each attachment joint 74 includes an elongated connecting finger 76 on the spider structure 70 and an elongated detachable split upper end plug 78 on each control rod 72. The split upper end plug 78 includes a pair of separate upper and lower plug portions 80,82. The upper plug portion 80 has integrally-connected tandemly-arranged upper, middle and lower sections 80A,80B,80C, whereas the lower plug portion 82 has integrally-connected tandemly-arranged upper, middle and lower segments 82A,82B,82C. Being substantially identical to the prior art attachment joint 50 of FIG. 8, the connecting finger 76 of the attachment joint 74 of the present invention has an axial bore 84 formed in a lower portion 76A of the finger 76 so as to open at a lower end 86 of the finger, and an internally-threaded hole 88 at an upper end of the bore 84. Further, the upper section 80A of the upper plug portion 80 has external threads 90 allowing it to be rigidly threadably attached within the threaded hole 88 of the connecting finger 76. Also, the lower segment 82C of the lower plug portion 82 is of a smaller diameter size than an upper open end 92 of an elongated hollow tube 94 of the control rod 72 allowing it to be mated and rigidly connected within the control rod tube upper end 92. However, the remaining middle and lower sections 80B,80C of the upper plug portion 80 and the upper and middle segments 82A,82B of the lower plug portion 82 are different from the upper end plug 40 of the prior art attachment joint 50 of FIG. 8. More particularly, in the attachment joint 74 of the present invention, the lower section 80C of the upper plug portion 80 and the middle segment 82B of the lower plug portion 82 have complementarily threaded means 96,98 defined thereon for rigidly threadably attaching the upper and lower plug portions 80,82 of the split upper end plug 78 to one another. The complementarily threaded means 96 defined on the lower section 80C of the upper plug portion 80 is preferably in the form of external male threads 96. The complementarily threaded means 98 on the middle segment 82B of the lower plug portion 82 preferably includes internal female threads defined in a recess 100 at the inner end of a central opening 102 extending through the upper and middle segments 82A,82B of the lower plug portion 82. The external threads 96 of the upper plug portion 80 are threadably received within the internal threads 98 of the lower plug portion 82 when the portions are attached together. Further, lower section 80C of the upper plug portion 80 has two different diametric sizes. At the smaller one of the two diameters of the lower section 80C is defined the external threads 96. Bridging the two diameter sizes of the lower section 80C is an annular shoulder 104 which faces toward the lower plug portion 82 and is located just above the external threads 96. The central opening 102 of the middle segment 82B of the lower plug portion 82 is larger in diameter than the internally threaded recess 100 thereof so as to define therebetween an annular ledge 106 located just above the recess 100. The upper plug portion shoulder 104 is capable of being bottomed out in a preloaded condition against the lower plug portion ledge 106 when the upper and lower plug portions 80,82 are threadably attached together. Further, in the attachment joint 74 of the present invention, the middle section 80B of the upper plug portion 80 and the upper segment 82A of the lower plug portion 82 have complementary interlocking means 108,110 defined thereon for locking the threadably attached upper and lower plug portions 80,82 together so as to resist unthreading thereof from one another during normal reactor operation. The complementary interlocking means 108 defined on the middle section 80B of the upper plug portion 80 is a plurality of flat surfaces 108 formed on the exterior thereof at locations circumferentially spaced from one another. There are preferably four flat surfaces 108 being circumferentially displaced about ninety degrees from one another. The complementary interlocking means 110 defined on the upper segment 82A of the lower plug portion 82 is a hollow sleeve 110 fitted over and about the flat surfaces 108 and deformably crimped thereagainst so as to resist unthreading of the upper and lower plug portions 80,82 from one another. However, the sleeve 110 is thin-walled so as to reformable back to a generally cylindrical configuration upon application of a predetermined torque to the control rod 72 to rotate the same and unthread the lower and upper plug portions 80,82 of the split upper end plug 78 from one another. As can be seem in FIGS. 10, 15 and 16, the upper segment 82A of the lower plug portion 82 defining the sleeve 110 is much thinner in wall thickness than the middle segment 82B thereof. To summarize, the reconstitutable control assembly 68 of the present invention incorporates a plurality of removable control rods 72 each with a split upper end plug 78 for detachably attaching the control rod 72 to a vane finger 76 of the spider structure 36. Specifically, the split upper end plug 78 is composed of two separate upper and lower plug portions 80,82. The upper and lower plug portions 80,82 of each upper end plug 78 are attached together by screw threading and are torqued to incorporate joint preload. Also, the interlocking flat surfaces 108 and crimped sleeve 110 prevent loosening of the plug portions 80,82 during reactor operation. Locking is accomplished by swaging/crimping the thin-walled sleeve 110 of the lower plug portion upper segment B2A onto flat surfaces 108 of the upper plug portion middle section 80B. The flat surfaces 108 are thus provided on the upper plug portion to prevent subsequent joint loosening. Removal of a control rod 72 is accomplished by unscrewing the rod which will rotate the lower plug portion 82 of the upper end plug 78 being fixedly attached to the control rod tube 94 relative to the upper plug portion 80 thereof being fixedly attached to the spider vane connecting finger 79 and overcome the crimp retainer locking feature provided by the deformed sleeve 110. A replacement control rod can then be reinstalled and crimped for rod retention. The replacement thin-walled sleeve to be crimped is virgin material on the lower plug portion of the upper end plug on the replacement control rod. The detachable attachment features incorporated by the separate upper and lower plug portions 80,82 of the split upper end plug 78 do not adversely impact any existing desirable features, such as the flexure joint 112 on the upper plug portion 80 and the intended clearance gap 114 between the flexure joint 112 and the lower end 86 of the finger 76; these prior art features are retained in the split upper end plug 78 of the present invention. Turning now to FIGS. 17-21, a modified split upper end plug 116 is illustrated having upper and lower plug portions 118,120 and employing a different type of locking feature as an alternative to the above-described crimp retainer-type locking feature. In all respects but the following ones the modified split upper end plug 116 is identical to the earlier-described split upper end plug 78. First, in place of the thin-walled crimpable hollow sleeve 110 of the upper section 82A of the upper end plug lower portion 82 and the flats-bearing middle segment 80B of the upper end plug upper portion 80, a thicker-walled cylindrical upper segment 120A is provided on the lower plug portion 120 and a cylindrical nonflat-bearing middle section 118B is provided on the upper plug portion 118. Thus, where the crimp locking feature was previously employed, the corresponding middle section 118B and upper segment 120A of the respective upper end plug portions 118,120 are of cylindrical configuration and merely form a slip fit type joint instead. Second, the threaded lower section 118C of the upper plug portion 118 and threaded middle segment 120B of the lower plug portion 120 of the modified split upper end plug 116 are greater in length than the corresponding threaded lower section 80C and middle segment 82B of the respective upper and lower portions 80,82 of the split upper end plug 78. Finally, the modified locking feature is provided by a member 122 in the form of a radially flexible and expandable thread-defining coil deployed between the threaded lower section and middle segment 118C,120B of the respective upper and lower end plug portions 118,120. More particularly, the coil 122 threadably fits between and at its respective interior and exterior peripheries makes threaded frictional engagement with external male threads and internal female threads 124,126 on the respective upper plug portion lower section 118C and lower plug portion middle segment 120B. The coil 122 thus provides frictional resistance to relative rotation of the upper and lower plug portions 118,120 once attachment of the plug portions has been completed. However, the coil makes it possible to rotate the control rod against the frictional resistance posed by the coil without the need for overriding or overcoming a crimp retainer-type locking feature. As best seen in FIGS. 22 and 23, the radially flexible and expandable thread-defining coil 122 is a screw thread coil insert which per se is a commercial product marketed under the tradename HeliCoil. The locking action of the coil is achieved by one or more of the windings 128 of the coil 122 having a series of straight segments or "chords" 128A. When the threaded lower section 118C of the upper plug portion 118 enters the "grip" coil 122, these chordal segments 128A flex outward, creating pressure on the threaded section and providing frictional resistance to rotation of the upper plug portion 118 relative to the lower plug portion 120. 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.