Patent Number: 046997610
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. 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 attached to the upper end portions 24 of the guide thimbles 14 which together incorporate certain locking features in accordance with the present invention which will be fully described below. With such 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 along the fuel assembly length. Each fuel rod 18 includes nuclear fuel pellets (not shown) and is closed at its opposite ends by upper and lower end plugs 26,28. The fuel pellets composed of fissile material are responsible for creating the reactive power of the reactor. A liquid moderator-coolant such as water, or water containing boron, is pumped upwardly through the guide thimbles 14 and along the fuel rods 18 of the fuel assembly 10 in order to extract heat generated therein for the production of useful work. To control the fission process, a number of control rods (not shown) are reciprocally movable in the guide thimbles 14 located at predetermined positions in the fuel assembly 10. Since the control rods are inserted into the guide thimbles 14 from the top of the fuel assembly 10, the placement of the components forming the top nozzle 22 and their attachment to the upper end portions 24 of the guide thimbles 14, along with the integral locking features of the present invention, must accommodate the movement of the control rods into the guide thimbles 14 from above the top nozzle 22. TOP NOZZLE REMOVABLY MOUNTED ON GUIDE THIMBLES Turning now to FIGS. 2 and 3, as well as Fig. 1, there is shown in greater detail the separate components making up the top nozzle 22 which is removably mounted on the upper end portions 24 of the guide thimbles 14 of the fuel assembly 10. The top nozzle 22 basically includes an upper hold-down plate 30, an enclosure 32 having a lower adapted plate 34 and an upstanding discontinuous sidewall 36 formed by a plurality of spaced upstanding wall portions 38 surrounding and attached to the periphery of the adapter plate, a plurality of tubular alignment sleeves 40 disposed between the upper and lower plates 30,34, and a plurality of hold-down coil springs 42 extending between the upper and lower plates 30,34 and about the respective sleeves 40. The upper hold-down plate 30 has a plurality of passageways 44 defined therethrough, while the lower adapter plate has a plurality of openings 46, the passageways 44 and openings 46 being arranged in respective patterns which are matched to that of the guide thimbles 14 of the fuel assembly 10. More particularly, the upper end portions 24 of the guide thimbles 14 extend upwardly through the openings 46 in the lower adapter plate 34 and above the upper surface 48 thereof. A plurality of lower retainers 50 are attached, such as by brazing, to the guide thimbles 14 below the lower adapter plate 34 for limiting downward slidable movement of the adapter plate 34 relative to the guide thimbles 14 and thereby supporting the adapter plate on the guide thimbles with the upper end portions 26 thereof extending above the adapter plate. Each lower retainer 50 on one guide thimble 14 has a series of scallops 52 formed on its periphery which are aligned with those of the fuel rods 18 grouped about the respective one guide thimble so that the fuel rods may be removed and replaced during reconstitution of the fuel assembly 10. Furthermore, the top nozzle 22 includes a plurality of upstanding bosses 54 having respective central bores 56 defined therethrough. The bosses 54 are disposed above the upper hold-down plate 30, and each boss is attached to the hold-down plate 30 such that its central bore 56 is aligned with a respective one of the passageways 44 of the hold-down plate. Additionally, each boss 54 is of a cross-sectional size adapted to interfit within one of a plurality of holes 58 (only one of which is seen in FIG. 5) formed in the upper core plate 60 which opens at a lower side 62 of the core plate. The upper circumferential edge 64 of each boss 54 is chamfered for mating with a complementarily chamfered edge 66 on the lower side 62 of the upper core plate 60 at the entrance to each of the holes 58 defined therein. Edges having such shapes act as guiding surfaces which facilitate alignment and insertion of the respective bosses 54 into the corresponding holes 58 in the upper core plate 60 during installation of the fuel assembly 10 within the reactor core. As mentioned above, the hold-down coil springs 42 are disposed about the respective elongated alignment sleeves 40 within the enclosure 32. Further, the springs 42 extend between the lower adapter plate 34 and the upper hold-down plate 30 and support the upper plate in a spaced relation above the lower plate at a stationary position in which the upper plate abuts the lower side 62 of the upper core plate 60 with the upstanding bosses 54 interfitted within the holes 58 of the upper core plate 60. Also, the upper hold-down plate 30 is composed of an array of hubs 68 and ligaments 70 which extend between and interconnect the hubs. Each of the hubs 68 has one of the passageways 44 defined therethrough. Furthermore, one boss 54 is disposed above and connected to each of the hubs 68 with the bore 56 of the boss aligned with the respective passageway 44 of the hub. Finally, the top nozzle 22 includes means interconnecting the spaced upper and lower plates 30,34 so as to accommodate movement of the lower plate 34 toward and away from the upper plate 30 upon axial movement of the guide thimbles 14 of the fuel assembly 10, such as due to thermal growth, toward and away from the upper core plate 60. Also, the interconnecting means is effective to limit movement of the lower adapter plate 34 away from the upper hold-down plate 30 so as to maintain the springs 42 in a state of compression therebetween. In particular, the interconnecting means includes a plurality of lugs 72 connected to and extending downwardly from peripheral ones of the ligaments 70. The lugs 72 are respectively coupled to the upstanding wall portions 38 of the discontinuous sidewall 36 of the enclosure 32. Specifically, a generally vertical slot 74 is formed in each wall portion 38 and opens at the upper end thereof. A removable locking pin 76 is inserted horizontally into the upper end of the wall portion 38 to close the upper end of the slot 74 and a pin 78 mounted in the lower end of each lug 72 extends into the slot 74 below the locking pin 76 for slidable movement therealong as the upper and lower plates 30,34 move relative to one another. In such arrangement, the locking pin 76 and the lower end of the slot 74 respectively define the limits of movement of the lower adapter plate 34 toward and away from the upper hold-down plate 30. Integral Reusable Locking Arrangement for Top Nozzle Referring now to FIGS. 4 and 5, there is shown one of the elongated tubular alignment sleeves 40 extending through one of the hold-down coil springs 42 between the upper and lower plates 30,34 and the threaded features on the sleeve 40 and on the upper end portion 24 of the one guide thimble 14 illustrated in the figures for attaching the sleeve and guide thimble together. Also illustrated in these figures is the reusable locking arrangement, generally designated as 80, integrally associated with both the sleeve 40 and the guide thimble upper end portion 24 for locking the attached sleeve and guide thimble together. With respect to the threaded features on the guide thimble 14 and sleeve 40, the upper end portion 24 of the guide thimble 14 has an annular externally threaded section 82, whereas the tubular alignment sleeve 40 has a lower annular internally threaded section 84. The sleeve 40 is mounted through the passageway 44 and bore 56 of the hold-down plate hub 68 and boss 54 for rotatable movement relative to the guide thimble upper end portion 24 between lowered and raised positions, as depicted respectively in FIGS. 5 and 4, for threading and unthreading its internally threaded section 84 onto and from externally threaded section 82 of the guide thimble upper end portion 24 in order to attach and detach the top nozzle 22 onto and from the guide thimble 14. The sleeve 40 is hollow so that, in addition to accommodating insertion of a control rod through it, a suitable tool (not shown) can be inserted into the sleeve for gripping it internally to rotate it in either direction for threading on and unthreading from the upper end portion 24 of the guide thimble 14. When threaded on the guide thimble upper end portion 24, the sleeve 40 cooperates with the lower retainer 50 to clamp the adapter plate 34 therebetween. The integral reusable locking arrangement 80 for the top nozzle 22 includes inner means in the form of a thin-walled tubular section 86 on the guide thimble upper end portion 24 above its externally threaded section 82 and outer means in the form of an axial section 88 on the alignment sleeve 40 above its internally threaded section 84. The tubular section 86 has an enlarged region in the form of an annular circumferential protrusion 90 defined thereon. The protrusion 90 has an external diametric size which is greater than the external diametric size of the remainder of the tubular section 86. The protrusion 90 can be machined around the entire tubular section 86, or can alternately be formed by an expansion (or bulge) operation or take the form of local dimples. The axial section 88 on the alignment sleeve 40 has an internal diametric size which is greater than that of the tubular section 86 but less than that protrusion 90 on the tubular section. With such diametric relationships between the axial section 88 and the protrusion 90, rotational movement of the alignment sleeve 40 relative to the guide thimble upper end portion 24 from its raised toward its lowered position causes interference contact of the axial section 88 with the tubular section protrusion 90 so as to produce a locking force. Additionally, it will be noticed in FIG. 4 that diametric sizes of the tubular section 86 and protrusion 90 are less than that of the exterior diametric size of the externally threaded section 82 of the guide thimble upper end portion 24, and, similarly, the diametric size of the axial section 88 is less than that of the interior diametric size of the internally threaded section 84 of the alignment sleeve 40. Thus, the protrusion 90 on the tubular sleeve 40 coacts with the interior surface 92 of the axial section 88 on the alignment sleeve 40 by creating an interference fit therewith as the internally threaded section 80 of the sleeve 40 is threaded on the externally threaded section 82 of the guide thimble upper end portion 24 when attaching the top nozzle 22 to the guide thimble 14. The interference fit causes the locking force which must be overcome in order to unthread the internally threaded section 84 of the alignment sleeve 40 from the externally threaded section 82 of the guide thimble upper end portion 24 and detach the top nozzle 22 from the guide thimble 14. The locking force takes the form of a constant torsional drag produced between the tubular section protrusion 90 and the axial section interior surface 92 as the alignment sleeve 40 is rotatably moved relative to the guide thimble upper end portion 24 between its lowered and raised positions. During in-core operation there are no appreciable loads acting on these threaded joints. Hence, the drag force, which prevents loosening of the sleeve 40, can be relatively low. It will be noted that unlike a lock washer where total loosening occurs upon small rotation, the interference drag force will act throughout the unthreading operation. An examination of FIG. 4 reveals that the circumferential protrusion 90 on the tubular section 86 is axially displaced from the axial section 88 when the internally threaded section 84 on the alignment sleeve 40 is initially rotatably moved into threaded engagement with the externally threaded section 82 on the guide thimble upper end portion 24. In such arrangement where engagement between the threaded ends of the sleeve 40 and guide thimble 14 leads interference contact between the protrusion 90 and the interior surface 92 of the axial section 88, the mechanical advantage produced by threading the internally threaded section 84 of the sleeve 40 on the externally threaded section 82 of the guide thimble 14 can be used to overcome the torsional drag and force the sleeve onto the guide thimble. When all of the sleeves 40 are unthreaded from the upper end portions 24 of the respective guide thimbles 14, the top nozzle 22 is in condition for removal from the remainder of the fuel assembly 10 for reconstitution thereof. However, due to the cross-sectional size of each of the sleeves 40, it stays in place between the upper and lower plates 30,34 of the top nozzle. Particularly, each sleeve 40 has a lower portion 94 of a cross-sectional diameter that is greater than that of an upper portion 96 thereof and also greater than the size of the diameter of the one passageway 44 of the upper hold-down plate 30. Thus, the sleeve 40 cannot be withdrawn through the passageway 44 and so it remains captured between the upper and lower plates 30,34, as also does the respective hold-down coil spring 42 encompassing the sleeve 40, when each sleeve is released from its threaded connection with its respective guide thimble 14. 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 of sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.