Patent Number: 047073268
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 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 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 SUBASSEMBLY 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 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 adapter 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 sleeve members 40, comprising part of the features of the present invention to be described later, being 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 disposed in a manner to be described below relative to the sleeve members 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 24 thereof extending above the adapter plate. (The upper end portions 24 of the guide thimbles 14 contain the remaining features of the present invention to be described below.) 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 (not shown) formed in the upper core plate (not shown) which open at a lower side thereof. The upper circumferential edge 64 of each boss 54 is chamfered for mating with a complementarily chamfered edge (not shown) on the lower side of the upper core plate at the entrance to each of the holes defined therein. Edges having such shapes act as guiding surfaces which facilitate alignment and insertion of the respective bosses into the corresponding holes in the upper core plate during installation of the fuel assembly within the reactor core. As mentioned above, the hold-down coil springs 42 are disposed within the enclosure 32 and 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 of the upper core plate (not shown) with the upstanding bosses 54 interfitted within the holes of the upper core plate. 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 (not shown). 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. ARRANGEMENT FOR ATTACHING AND REATTACHING TOP NOZZLE IN RECONSTITUTABLE FUEL ASSEMBLY Referring now to FIGS. 4 to 9, there is shown the features of the arrangement for attaching and reattaching the top nozzle, generally designated 80, which together constitute the present invention. Specifically, these features include, first, the elongated sleeve members 40 of which one is shown disposed relative to one of the hold-down coil springs 42 between the upper and lower plates 30,34 and, second, complementary means, generally indicated at 82, on each of the sleeve members and the guide thimble upper end portions 24 which attach the top nozzle 22 and guide thimbles 14 together. Each sleeve member 40 includes an inner tubular alignment sleeve portion 84, an outer tubular shroud portion 86 and an intermediate annular flange portion 88. The inner alignment sleeve portion 84 is disposed within the coil spring 42, extends between plates 30,34 in alignment with the respective passageway 44 and opening 46 thereof, and receives the guide thimble upper end portion 24. Also, at its upper end 90 the inner sleeve portion 84 is inserted into the passageway 44 of the hold-down plate 30, while at its lower end 92 it rests on the adapter plate 34 adjacent the opening 46 therethrough. The outer tubular shroud portion 86 of the sleeve member 40 is disposed in concentric but outwardly spaced relation to the inner alignment sleeve portion 84, as clearly depicted especially in FIGS. 7 and 8. The outer shroud portion 86 has a lower end 94 resting on the adapter plate 34 also. From its lower end 94, the outer shroud portion 86 extends upwardly about a portion of the spring 42 for protecting the spring from damage by coolant cross flow from fuel assemblies located adjacent to the fuel assembly 10. The downward force of the spring 42 retains the sleeve member 40 in a generally stationary position upon and pressed against the adapter plate 34. The intermediate annular flange portion 88, which underlies the coil spring 42, extends between and interconnects the respective lower ends 92,94 of the inner sleeve and outer shroud portions 84,86. The intermediate flange portion 88, together with the lower end 92 of the inner tubular alignment sleeve portion 84 and the lower end 94 of the outer shroud portion 86 being connected to the flange portion, are all disposed in an unattached but contacting relationship with respect to the adapter plate 34 about the opening 46 therethrough. The complementary means 82 is formed on and interconnects the inner alignment sleeve portion 84 of each sleeve member 40 and the upper end portion 24 of each guide thimble 14 so as to attach the individual pairs of the sleeve member 40 and corresponding guide thimbles 14 together. The complementary means 82 includes serially arranged and spaced apart primary, secondary and tertiary annular grooves 96,98,100 formed circumferentially, such as by machining, on the interior 102 of the tubular alignment sleeve portion 84, with the primary groove 96 being located at the highest level and the tertiary groove 100 at the lowest level on the sleeve portion interior. Further, the complementary means 82 includes a progressive series of primary, secondary and tertiary interior sections 104,106,108 on the interior 102 of the alignment sleeve portion 84 which respectively contain the primary, secondary and tertiary annular grooves 96,98,100. The secondary and tertiary sections 106,108 are formed, such as by machining, on the interior 102 relative to the primary section 104 so as to provide regions of increasing relief immediately below each preceding groove. More particularly, the secondary section 106, being disposed below the primary section 104, has an interior diameter larger than that of the primary section, whereas the tertiary section 108, being disposed below the secondary section 106, has an interior diameter larger than that of the secondary section. As will be explained below, these regions of increasing relief on the interior 102 of the alignment sleeve portion 84 facilitate subsequent reconnections of the alignment sleeve portion and the respective guide thimble 14 together after reconstitution of the fuel assembly 10. Finally, the complementary means 82 also includes a primary exterior (360-degree) circumferential bulge 110 (see FIG. 4) formed on the guide thimble upper end portion 24, by a suitable bulging tool which fits within the guide thimble. The primary bulge 110 extends into the primary annular groove 96 so as to rigidly connect the sleeve member 40 and guide thimble 14 together. After an upper segment 112 of the guide thimble upper end portion 24 has been severed (see FIG. 5), by any suitable internal cutter, followed by removal and replacement of the top nozzle 22 at the initial reconstitution of the fuel assembly 10, the secondary groove 98 will receive a secondary exterior bulge 114 formed on the guide thimble upper end portion 24 in the same way as the primary bulge 110. If there should occur a second reconstitution of the fuel assembly 10 (which is unusual), then the tertiary groove 100 will receive a tertiary exterior bulge (not shown). The tertiary bulge would be formed on the guide thimble upper end portion 24 in the same way as the primary and secondary bulges 110,114, after severance of a second upper segment of the upper end portion, which now would contain the secondary bulge 114, followed by removal and receipt of the twice severed guide thimble upper end portion 24 from and back in the alignment sleeve portion 84, for reconnection of the alignment sleeve portion 84 and the twice severed guide thimble 14 together. The purpose for the presence of the relieved secondary and tertiary sections on the interior 102 of the alignment sleeve portion 84 of the sleeve member 40 is to facilitate subsequent insertion of the severed end of the guide thimble 14 back into the sleeve portion. To explain, when a bulge-type joint is made, the material immediately above and below the bulge is pushed radially outward into contact with the outside member (in this case, the sleeve portion). Since this causes plastic deformation of the material, the outside diameter of the guide thimble annular part being bulged from inside the thimble is permanently expanded by about 2 to 5 mils on the diameter immediately above and below the bulge. Therefore, to aid in reinserting the severed upper end portion 24 of the guide thimble 14 into the alignment sleeve portion 84 of the member 40, the sleeve portion is relieved radially outward below each bulge-receiving groove, as best seen in FIG. 9. It would be difficult, if not impossible, to reinsert the severed guide thimble back into the sleeve portion 84 without these relieved sections. It should be noted that 45-degree inward and upward tapered transitions 116,118,120 respectively interconnect the primary, secondary and tertiary sections 104,106,108 with the next lower, relieved section so that there is no hangup when lowering the top nozzle 22 back on the guide thimbles 14. To summarize, the steps carried out in attaching and reattaching the top nozzle 22 to the guide thimbles 14 of the reconstitutable fuel assembly 10 are depicted in FIGS. 4 to 6. In FIG. 4, the guide thimble upper end portion 24 has been inserted into the alignment sleeve portion 84 which includes at least the upper primary and lower secondary annular grooves 96,98 and an annular part of the upper end portion 24 internally formed by a suitable conventional tool as the primary exterior bulge 110. The primary bulge 110 extends outwardly into the primary annular groove 96 in the alignment sleeve portion 84 so as to connect the sleeve member 40 and guide thimble 14 together. Then, as seen in FIG. 5, when initial reconstitution of the fuel assembly 10 is desired, by using a suitable conventional internal cutter, the guide thimble upper end portion 24 is circumferentially cut at a location below the level of its annular part 110 bulged into the upper primary annular groove 96 but above the level of the lower secondary annular groove 98, for instance approximately at the location of the first transition 116 between the primary and secondary sections 104,106. In such manner, the upper segment 112 of the guide thimble upper end portion 24 which contains the primary bulged annular part 110 is severed from the remainder of the guide thimble 14. It will be noted that this segment 112 remains rigidly attached within the sleeve portion 84 and because of its position will not interfere (see FIG. 6) when the top nozzle 22 is reinserted back on the guide thimble 14. By using another suitable fixture, such as disclosed in the third patent application cross-referenced above, the top nozzle, including the alignment sleeve 84 with upper guide thimble segment 112 connected thereto, can be removed from the severed guide thimble upper end portion 24 for exposing the fuel rods 18 of the fuel assembly 10 for reconstitution. Note that there are no loose parts and the hold-down plate 30, hold-down springs 42 and sleeve members 40 remain in place on the removed top nozzle 22. After reconstitution of the fuel assembly 10, the same fixture is used to reinsert the top nozzle 22 back on the severed upper end portion 24 of each guide thimble 14 such that the severed upper end portions are received into the sleeve members 40. Then, as seen in FIG. 6, another upper annular part (secondary bulge 114) of each severed guide thimble upper end portion 24 is bulged outwardly from its interior into the secondary annular groove 98 in the alignment sleeve portion 84 so as to reconnect the sleeve member 40 and guide thimble 14 together. 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.