Patent Number: 046876308
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 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 joint structure interconnecting the top nozzle 32 and the guide thimbles 24 must accommodate the movement of the control rods into the guide thimbles from above the top nozzle. Improved Top Nozzle and Guide Thimble Joint Structure Turning now to FIGS. 1 to 4, there is shown in FIG. 4 a preferred embodiment of the improved joint structure of the present invention, generally designated 38, which flexibly interconnects the top nozzle 32 with the guide thimbles 24. The top nozzle 32, which is mounted to upper hollow tubular extension members 40 of the respective guide thimbles 24. includes an upper hold-down plate 42, a lower adapter plate 44, and a plurality of hold-down coil springs 46 disposed between the upper and lower plates 42,44. The upper hold-down plate 42 has a plurality of passageways 48 defined therethrough, while the lower adapter plate 44 has a plurality of openings 50, the passageways 48 and openings 50 being arranged in respective patterns which are matched to that of the guide thimbles 24 of the fuel assembly 20. More particularly, the upper tubular extension members 40 of the guide thimbles 24 extend upwardly and are slidably inserted through the respective openings 50 in the lower adapter plate 44 and the respective passageways 48 in the upper hold-down plate 42. The hold-down springs 46 are inserted about the respective extension members 40 and displace the upper hold-down plate 42 above the lower adapter plate 44. A plurality of lower retainers 52 are attached, such as by brazing, to the guide thimble extension members 40 below the lower adapter plate 44 for limiting downward slidable movement of the adapter plate 44 relative to the guide thimbles 24 and thereby supporting the adapter plate at a stationary position along the guide thimbles. Each lower retainer 52 on one guide thimble 24 has a series of scallops 54 formed on its periphery which are aligned with those of the fuel rods 28 grouped about the respective one guide thimble 24 so that the fuel rods may be removed and replaced during reconstitution of the fuel assembly 20. As seen particularly in FIGS. 2 and 3, the upper hold-down plate 42 is composed of an array of hubs 56 and radially-arranged spokes or ligaments 58 which extend between and interconnect the hubs. Each of the hubs 56 has one of the passageways 48 defined therethrough. For transfer of the fuel assembly 20 to and from the reactor core, a fuel assembly handling machine (not shown) having a gripper 60, such as seen in FIG. 5, is utilized. The gripper 60 is a cylinder with four inverted J-slots 62 cut in it. The gripper 60 is applied on the four ligaments 58 of the hold-down plate 42 by lowering it so that the ligaments fit through the slots 62 and then it is rotated about its axis to lock the ligaments in place through the slots. When the gripper is raised upwardly, it lifts the upper hold-down plate 42 with it. In the past, the lifting load passed from the gripper to the hold-down plate 42 and therefrom to the guide thimbles 24 via the upper extension members 40 due to rigid or inflexible contact established between the hold-down plate 42 and the guide thimble extensions members 40 at the upper limit of slidable movement of the hold-down plate along the guide thimbles. Consequently, the energy of high impulse loads which occasionally occurred as the fuel assembly 20 was being lifted from or lowered into the reactor core also propagated directly to the guide thimbles of the fuel assembly. However, now in view of the provision of the improved joint structure 38 between the upper hold-down plate 42 of the top nozzle 32 and the upper extension members 40 of the guide thimbles 24, an effective energy absorbing means is interposed between the fuel assembly handling machine gripper 60 and the guide thimbles 24 of the fuel assembly structure which substantially prevents transfer of the impulsive, impact energy. As seen in FIG. 4, the improved joint structure 38 includes first overlapping means in the form of an internal annular ledge 64 defined on each hub 56 of the upper hold-down plate 42 in a lower portion of the passageway 48 thereof so as to surround the extension member 40, and second overlapping means, generally designated 66, on an upper end portion 68 of each guide thimble upper extension member 40 which is disposed through the passageway 48 of the hold-down plate 42. The second overlapping means 66 includes an annular recess 70 defined on the upper end portion 68 of the extension member 40, and an annular spring member 72 fitted on the upper end portion 68 within the recess 70. The spring member 72 extends outwardly beyond the outside diameter of the extension member 40 so as to overlie the internal ledge 64 in the hold-down plate passageway 48. With such arrangement, the ledge 64 of the first overlapping means and the spring member 72 of the second overlapping means are disposed to interfere with one another so as to limit upward movement of the hold-down plate 42 along the guide thimble extension member 40. However, while the inside diameter of the passageway 48 at the ledge 64 is less than the outside diameter of the spring member 72, the inside diameter of the passageway over the ledge is greater than the outside diameter of the spring member. Therefore, there is no interference between the first and second overlapping means which would limit movement of the hold-down plate 42 downwardly along the guide thimble upper extension member 40 during its performance of the hold-down function when the fuel assembly 20 is disposed within the reactor core. In the preferred embodiment of FIG. 4, the spring member 72 takes the form of a stack of several belleville springs 74. The belleville springs 74 can deflect axially upon application of a large impulse load thereto. The springs 74 cooperate with the internal ledge 64 to provide a flexible connection between the hold-down plate 42 and guide thimble extension members 40. The springs 74 are resiliently yieldable so as to bias the hold-down plate 42 toward the lower adapter plate 44 and against the bias of the hold-down springs 46. The springs 74 thus serve as an energy absorber having a simple and reliable construction for absorbing the energy of an impulse load applied to the hold-down plate in the direction upwardly along the guide thimble extension member so as to thereby limit transfer of the load to the extension member. Under normal lifting for lowering loads, the springs 74 are only slightly deflected. If a large impulse load is applied, the springs 74 will then deflect greater and thus absorb the energy and limit its transfer from the hold-down plate 42 to the guide thimbles 24 via the extension members 40. The recess 70 is defined between a lower upwardly-facing annular shoulder 76 on the upper end portion 68 of the guide thimble extension member 40, and an upper detachable member in the form of a retainer nut 78 releasably threaded to the externally-threaded terminal end 80 of the extension member upper end portion 68. The shoulder 76 is defined by an upper annular section 82 which contains the terminal end 80 and has an outside diameter reduced from that of the remainder of the extension member upper end portion 68. Thus, the belleville springs 74 are held in place against downward movement by the shoulder 76 on the extension member 40 and against upward movement by the retainer nut 78. It is further readily seen that the top nozzle 32 including the flexible joint structure 38 can be disassembled by removing the each retainer nut 78 from the terminal end 80 of the each extension member 40. 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.