Patent Number: 046702130
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 20. Basically, the fuel assembly 20 includes a lower end structure or bottom nozzle 22 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 24 which project upwardly from the bottom nozzle 22. The assembly 20 further includes a plurality of transverse grids 26 axially spaced along the guide thimbles 24 and an organized array of elongated fuel rods 28 transversely spaced and supported by the grids 26. Also, the assembly 20 has an instrumentation tube 30 located in the center thereof and an upper end structure or top nozzle 32 attached to the upper ends of the guide thimbles 24 which incorporates certain improvements in accordance with the present invention which will be fully described below. With such an arrangement of parts, the fuel assembly 20 forms an integral unit capable of being conventionally handled without damaging the assembly parts. As mentioned above, the fuel rods 28 in the array thereof in the assembly 20 are held in spaced relationship with one another by the grids 26 spaced along the fuel assembly length. Each fuel rod 28 includes nuclear fuel pellets (not shown) and is closed at its opposite ends by upper and lower end plugs 34,36. 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 24 and along the fuel rods 28 of the fuel assembly 20 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 24 located at predetermined positions in the fuel assembly 20. Since the control rods are inserted into the guide thimbles 24 from the top of the fuel assembly 20, the placement of the components forming the improved top nozzle 32 and their attachment to the guide thimbles 24 must accommodate the movement of the control rods into the guide thimbles 24 from above the improved top nozzle 32. Improved Top Nozzle for Aligning Fuel Assembly with Upper Core Plate Turning now to FIGS. 1 to 4, there is shown the preferred embodiment of the improved top nozzle 32 which satisfies the design objectives of, first, eliminating relative moving engagement between the fuel assembly 20 and an upper core plate 38 (FIG. 4) of the reactor and thereby avoiding core plate wear, and, second, preventing coolant cross-flow between fuel assemblies and thereby preventing fatigue inducing vibration of certain components of the improved top nozzle 32. The separate components making up the improved top nozzle 32 which is mounted on the upper end portions 40 of the guide thimbles 24 of the fuel assembly 20 include an upper hold-down plate 42, an enclosure 44 having a lower adapter plate 46 and an upstanding sidewall 48 surrounding and attached to the periphery of the adapter plate, a plurality of tubular sleeves 50 disposed between the upper and lower plates 42,46, and a plurality of hold-down coil springs 52 extending between the upper and lower plates 42,46 and about the respective sleeves 50. The upper hold-down plate 42 has a plurality of passageways 54 defined therethrough, while the lower adapter plate 46 has a plurality of openings 56, the passageways 54 and openings 56 being arranged in respective patterns which are matched to that of the guide thimbles 24 of the fuel assembly 20. More particularly, the upper end portions 40 of the guide thimbles 24 extend upwardly through the openings 56 in the lower adapter plate 46 and above the upper surface 58 thereof. A plurality of lower retainers 60 are attached, such as by brazing, to the guide thimbles 24 below the lower adapter plate 46 for limiting downward slidable movement of the adapter plate 46 relative to the guide thimbles 24 and thereby supporting the adapter plate on the guide thimbles with upper ends 62 (only one of which is shown in FIG. 4) thereof extending above the adapter plate. Each lower retainer 60 on one guide thimble 24 has a series of scallops 64 formed on its periphery which are aligned with those of the fuel rods 28 grouped about the respective one guide thimble so that the fuel rods may be removed and replaced during reconstitution of the fuel assembly 20. Furthermore, the improved top nozzle 32 includes a plurality of upstanding bosses 66 having respective central bores 68 defined therethrough. The bosses 66 are disposed above the upper hold-down plate 42, and each boss 66 is attached to the hold-down plate 42 such that its central bore 68 is aligned with a respective one of the passageways 54 of the hold-down plate. Additionally, each boss 66 is of a cross-sectional size adapted to interfit within one of a plurality of holes 70 (only one of which is seen in FIG. 4) formed in the upper core plate 38 which opens at a lower side 72 of the core plate. The upper circumferential edge 74 of each boss 66 is chamfered for mating with a complementarily chamfered edge 76 or the lower side 72 of the upper core plate 38 at the entrance to each of the holes 70 defined therein. Edges having such shapes act as guiding surfaces which facilitate alignment and insertion of the respective bosses 66 into the corresponding holes 70 in the upper core plate 38 during installatin of the fuel assembly within the reactor core. Still further, the elongated sleeves 50 extending between the upper and lower plates 42,46 are slidably inserted at their respective upper ends 78 into the respective passageways 54 of the upper hold-down plate and the corresponding aligned bores 68 of the upstanding bosses 66. At their lower ends 80, the sleeves 50 are releasably connected to the upper ends 62 of the guide thimbles 24 so as to cooperate with the lower retainers 60 in holding the lower adapter plate 46 at a stationary position on the guide thimbles 24. Specifically, as seen in FIG. 4, each of the lower ends 80 of the sleeves 50 is internally threaded for making a releasable threaded connection with an externally threaded section 82 on each of the upper ends 62 of the respective guide thimbles 24. The sleeve 50 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 or unthreading from the upper end 62 of the guide thimble 24. When threaded on the upper end 62 of the guide thimble 24, the sleeve 50 cooperates with the lower retainer 60 to clamp the adapter plate 46 therebetween. The sleeves 50 slide axially in the passageways 54 of the hold-down plate 42 and the bores 68 of the bosses 66 to accommodate thermal growth between the fuel assembly 20 and the upper core plate 38. However, at no time does the sleeves 50 contact the upper core plate. Also, the sleeves protect the control rods (not shown) from cross-flow and supports the inside diameter of the hold-down springs 52 to prevent them from buckling. When all of the sleeves 50 are unthreaded from the upper ends 62 of the respective guide thimbles 24, the improved top nozzle 32 is in condition for removal from the remainder of the fuel assembly 20 for reconstitution thereof. However, due to the crosssectional size of each of the sleeves 50, it stays in place between the upper and lower plates 42,46 of the top nozzle. Particularly, each sleeve 50 has a lower portion 84 of a cross-sectional diameter that is greater than that of an upper portion 86 thereof and also greater than the size of the diameter of the one passageway 54 of the upper hold-down plate 42. Thus, the sleeve 50 cannot be withdrawn through the passageway 54 and so it remains captured between the upper and lower plates 42,46, as also does the respective hold-down coil spring 52 encompassing the sleeve 50, when each sleeve is released from its threaded connection with its respective guide thimble 24. As mentioned above, the hold-down coil springs 52 are disposed about the respective elongated sleeves 50 within the enclosure 44. Further, the springs 52 extend between the lower adapter plate 46 and the upper hold-down plate 42 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 72 of the upper core plate 38 with the upstanding bosses 66 interfitted within the holes 70 of the upper core plate 38. Also, as seen in FIGS. 1 and 2, the upstanding sidewall 48 of the enclosure 44 substantially surrounds the springs 52 so as to protect them from impingement by coolant cross-flow within the reactor core. In a preferred embodiment such as seen in FIGS. 5 and 6, the upper hold-down plate 42 is composed of an array of hubs 88 and ligaments 90 which extend between and interconnect the hubs. Each of the hubs 88 has one of the passageways 54 defined therethrough. Furthermore, one boss 66 is disposed above and connected to each of the hubs 88 with the bore 68 of the boss aligned with the respective passageway 54 of the hub. Finally, the improved top nozzle 32 includes means interconnecting the spaced upper and lower plates 42,46 so as to accommodate movement of the lower plate 46 toward and away from the upper plate 42 upon axial movement of the guide thimles of the fuel assembly 20, such as due to thermal growth, toward and away from the upper core plate 38. Also, the interconnecting means is effective to limit movement of the lower adapter plate 46 away from the upper hold-down plate 42 so as to maintain the springs 52 in a state of compression therebetween and the sleeves 50 captured between the upper and lower plates. In particular, the interconnecting means includes a plurality of lugs 92 attached to the outer sides of the hubs 88 of the upper hold-down plate 42. The lugs 92 extend downwardly from the hubs 88 at the periphery of the upper plate 42 and are respectively coupled to the upstanding sidewall 48 of the enclosure 44. Specifically, a generally vertical slot 94 is defined in each of four corners 96 of the generally rectangular enclosure sidewall 48. Also, a pin 98 is mounted in the lower end of each lug 92 and extends into one of the slots 94 for slidably movement therealong between upper and lower ends 100,102 thereof which respectively define the limits of movement of the lower adapter plate 46 toward and away from the upper hold-down plate 42. The pin 98 is removable in order to facilitate the assembling and disassembling of the upper plate 42 with and from the enclosure 44. When the fuel assembly 20 is free-standing or is being lifted via its hold-down plate 42, the lugs 92 contact the upper ends 100 of the slots 94 in the enclosure 44 so the load path is from a refueling machine gripper (not shown) to the hold-down plate 42, then via the lugs 92 and pins 98 to the enclosure 44 and adapter plate 46. From the latter, the load is transferred to the guide thimbles 24 of the fuel assembly via the above-described threaded connections between the sleeves 50 and the guide thimble upper ends 62. Given the above-described components comprising the improved top nozzle 32 and cooperatively relationships between them, then it is readily seen that concurrently as alignment of the fuel assembly 20 with the upper core plate 38 is achieved through abutting of the upper hold-down plate 42 against the lower side 72 of the upper core plate and interfitting of the bosses 66 within the upper core plate holes 70, axial movement of the fuel assembly 20 relative to the upper core plate is accommodated. Furthermore, such accommodation is made, without incurring any wear of the upper core plate 38, through movement of the enclosure 44 and the plurality of elongated sleeves 50 relative to the upper hold-down plate 42 without relative sliding engagement of either of the upper hold-down plate 42, the plurality of upstanding bosses 66 and the plurality of elongated sleeves 50 of the improved top nozzle 20 with the upper core plate 38. In particular, during reactor operation, the hold-down springs 52 force the hold-down plate 42 into contact with the lower side 72 of the upper core plate 38 and the bosses 66 extend into the holes 70. The friction between the core plate 38 and hold-down plate 42 prevents differential motion between these components due to vibration. Since the axial motion between the fuel assembly and the upper core plate is accommodated by sliding between the sleeves and the hold-down plate and bosses, there is no wear on the core plate holes. All wear is on top nozzle components which are replacable with the fuel assembly. Other alternative embodiments of the upper hold-down plate 42, enclosure 44 and lugs 92 are illustrated in FIGS. 7 to 14. Since these alternative embodiments are generally similar to the preferred embodiment of FIGS. 2 and 3, only their variations will be pointed out hereinafter. In a first alternative embodiment seen in FIGS. 7 and 8, the hold-down plate 104 has a plurality of peripherally-arranged ligaments 106 interconnecting the hubs 108 with the interconnecting lugs 110 connected to and extending downwardly from the peripheral ligaments 106. The sidewall of the enclosure is discontinuous, that is to say, it is formed by a plurality of spaced upstanding wall portions 112 which each has a vertical slot 114 formed therein which opens at the upper end of the wall portion. A removable locking pin 116 is inserted horizontally into the upper end of the wall portion 112 to close the upper portion of the slot 114. The second alternative embodiment in FIGS. 9 and 10 differs mainly from the first alternative of FIGS. 7 and 8 in that the enclosure 118 is continuous as in the preferred embodiment and the locking pin is now a bar 120 seated in the upper edge of each of the four wall portions of the continuous sidewall 122. The third and fourth alternative embodiments in FIGS. 11 and 12 and FIGS. 13 and 14, respectively, differ mainly from the second alternative of FIGS. 9 and 10 in arrangement of their respective interconnecting means, generally designated 124,126 respectively, generally between and adjacent to and at the corners 128,130 of their respective enclosures 132,134. 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.