Patent Number: 049869590
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. Referring now to the drawings, and particularly to FIG. 1, there is shown a fuel assembly, generally designated by the numeral 10, having an expandable top nozzle subassembly 12 constructed in accordance with the principles of the present invention. In addition to the top nozzle subassembly 12, the fuel assembly 10 basically includes a bottom nozzle 14 for supporting the fuel assembly on the lower core support plate (not shown) in the core region of a nuclear reactor (not shown) and a number of longitudinally extending control rod guide tubes or thimbles 16 projecting upwardly from the bottom nozzle 14 and attached at their upper and lower ends to the top nozzle subassembly 12 and bottom nozzle 14. Further, an organized array of fuel rods 18 are held in spaced relationship to one another by a number of transverse grids 20 spaced along the fuel assembly length and attached to the guide thimbles 16. An instrumentation tube 22 is located at the center of the fuel assembly 10. The top nozzle subassembly 12, bottom nozzle 14 and guide thimbles 16 together form an integral assembly capable of being conventionally handled without damaging the assembly parts. Referring to FIGS. 1-3, the expandable top nozzle subassembly 12 of the present invention has a construction which permits improved utilization of space for accommodating greater growth of fuel rods 18 of the fuel assembly 10 and higher fuel rod burnup. At the same time, the top nozzle subassembly 12 continues to allow the use of a conventional handling system for installing and removing the fuel assembly 10 in and from the reactor core. The expandable top nozzle subassembly 12 basically includes an upper structure 24, a lower adapter plate 26 and a plurality of resiliently-yieldable biasing devices 28. As shown alone and in greater detail in FIGS. 4-7, the upper structure 24 of the top nozzle subassembly 12 is composed of a top plate 30 and a sidewall enclosure 32 rigidly connected to and depending from the outer peripheral edge 30A of the top plate 30. The top plate 30 has an annular configuration defining a large central opening 34. Two diagonal ones of a plurality of corner portions 30B of the top plate 30 each has a hole 36 defined therethrough which permit insertion of components of the fuel assembly handling system (not shown) for engaging the lower surface 30C of the top plate 30 in order to lift the fuel assembly 10 in installing and removing it from the core. One of the other corner portions 30B has a hole 38 which provides a reference for properly orientating the fuel assembly 10 in the core. Also, a pair of holes 40 is defined through each corner portion 30B of the top plate 30 with each hole 40 surrounded on the lower surface 30C of the top plate by an annular recess 40A formed in the lower surface of the top plate. The sidewall enclosure 32 of the upper structure 24 is composed of four generally planar vertical wall portions 42 rigidly interconnected together at their opposite vertical edges to define the enclosure 32 in a generally square box-like configuration. At the lower peripheral edge of the sidewall enclosure 32, each wall portion 42 has a narrow inwardly-projecting retaining structure 44 composed of a series of spaced fingers 44A defining the retaining structure in a generally scalloped or serrated configuration. As shown alone and in greater detail in FIGS. 8-10, the lower adapter plate 26 of the top nozzle subassembly 12 is of generally square configuration and flat construction. The adapter plate 26 is formed of a plurality of cross-laced ligaments or bars 46 defining a plurality of coolant flow openings 48 of oblong shapes. Also, a plurality of circular through holes 50 corresponding in number and pattern to that of the guide thimbles 16 are provided through the adapter plate 26. The through holes 50 are of sufficient dimensional size to permit the adapter plate 26 to be installed over the upper ends of the guide thimbles 16. Further, a pair of circular recesses 52 are defined at each corner portion 26A of the lower adapter plate 26 in the upper surface 26B of the adapter plate. The pairs of recesses 52 are aligned below the pairs of holes 40 and annular recesses 40A in the top plate 24. In each recess 52, a series of coolant flow openings 54 in a generally square pattern are defined through the adapter plate 26. The outer peripheral edge of the adapter plate 26 is undercut to define a ledge or seat structure 56 overlying a continuous recess or groove 58. Referring to FIGS. 2, 3, 11 and 12, the resiliently-yieldable biasing devices 28 take the form of pairs of coil springs 60 being seated at their opposite upper and lower ends 60A, 60B in the respective recesses 44A, 52 formed at aligned corner portions of the 30A, 26A of the top plate 30 and lower adapter plate 26 located outside of an outer perimeter of the guide thimbles 16. The outer perimeter of the guide thimbles 16 is also aligned below an inner peripheral edge 30C defining the central opening 34 of the top plate 30. Also, a stabilizing or guide member 62 in the form of a short cylindrical plug or bar is secured to the top plate 30 in each of the holes 40 and extends within the corresponding coil spring 60. The guide members 62 provide lateral stabilization of the springs 60 and direct their expansion and compression along a generally vertical path. In the expanded states of the springs 60 seen in FIG. 11 the length of the guide members 62 is less than half the length of the springs. In the compressed state of the springs 60 as seen in FIG. 12, the lower ends of the guide members 62 are located adjacent to the adapter plate 26. FIG. 11 shows the top nozzle subassembly 12 in an expanded condition, whereas FIG. 12 shows it in a compressed condition. In both conditions of the top nozzle subassembly 12, the adapter plate 26 is stationarily secured in the same position on the upper ends of the guide thimbles 16 in a conventional manner by locking tubes 64. By way of example, the adapter plate 26 is disposed approximately 1 inch to 1.5 inches higher above the upper ends of the fuel rods 18 than is a conventional adapter plate heretofore. Also, the adapter plate 26 is slidably movably mounted within the interior of the sidewall enclosure 32. The sidewall enclosure 32 and adapter plate 26 are not rigidly connected to one another but instead are slidably movable relative to one another. The retaining structure 44 of the sidewall enclosure 32 and the seat structure 56 of the adapter plate 26 provide interengagable means on the lower edge of the enclosure 32 and on a peripheral edge of the adapter plate 26 for capturing and retaining the adapter plate 26 within the enclosure 32 upon movement of the enclosure relative to the adapter plate which, in turn, moves the top plate 30 away from the adapter plate 26. With such arrangement, the adapter plate 26 cannot become separated from the upper structure 24. To place the top nozzle subassembly 12 in the expanded condition seen in FIG. 11, the upper core support plate (not shown) is removed from imposing a downward bearing contact force upon the top plate 30 of the upper structure 24 of the top nozzle subassembly. The springs 60 are thus allowed to assume their expanded states in which they force the upper structure 24 away from adapter plate 26 and thus force the latter in resting or abutting relation at its annular seat structure 56 upon the fingers 44A of the retaining structure 44 on the enclosure 32 of the upper structure 24. The fingers 44A capture and retain the adapter plate 26 within the enclosure 32. The adapter plate 26 and top plate 30 are now spaced their maximum distance apart and provide sufficient space between them for insertion of the components of the fuel assembly handling system through the corner holes 36 in the top plate. To place the top nozzle subassembly 12 in the compressed condition seen in FIG. 12, the upper core support plate is installed upon the top plate 30 of the upper structure 24 of the top nozzle subassembly so as to reimpose the downward bearing contact force thereon. The top plate 30 is thus moved downward toward the lower adapter plate 26 forcing the springs 60 to their compressed states and slidably moving the sidewall enclosure 32 downwardly along and relative to the adapter plate 26 and lowering the retaining structure 44 on the enclosure away from the seat structure 56 on the adapter plate 26. The space between the top plate 30 and the adapter plate 26 is now reduced below that needed for insertion of the components of the fuel assembly handling system. However, this does not matter since the fuel assembly is never handled by the system while it is in the core with the upper core support plate placed on the top nozzle subassembly. Thus, the extra or "dead" space previously existing between the top plate 30 and adapter plate 26 has now been eliminated and is instead now being utilized by the higher mounting position of the adapter plate 26 on the guide thimbles 16 permitting greater distance between the adapter plate 26 and upper ends of the fuel rods 18 for increased growth and greater burnup of the fuel rods in the core. Later when the fuel assembly 10 is to be handled, the upper core plate is removed and the springs 60 moves the upper structure 24 upward to its position in FIG. 11 returning the top plate 30 and adapter plate 26 to their maximum spacing for providing the necessary space therebetween for the fuel assembly handling system components. The scalloped shape of the retaining structure 44 permits them to move downwardly past the upper ends of the fuel rods 18 to the position seen in FIG. 12. The central opening 34 of the top plate 30 accommodates passage of control rods (not shown) into the guide thimbles 16 in a conventional manner. The coil springs 60 transmit the necessary hold-down force from the upper core plate directly to the adapter plate 26. The number and arrangement of the coil springs 60 have the advantage of allowing the load from the upper core plate to be distributed much more evenly to the adapter plate, then in the case of prior hold-down arrangements, thus helping to prevent bow in the fuel assembly. It will be noted also that the sidewall enclosure 32 of the upper structure 24 completely encloses the springs 60 in both expanded and compressed conditions of the top nozzle subassembly 12, thus protecting and shielding the springs from imposition of lateral forces thereon by coolant flow. It should be realized, however, that other forms of biasing devices can be used, such as elongated leaf springs. In comparison of FIGS. 11 and 12, it can be understood that only the springs 60 extend between and engage both the top plate 30 and the lower adapter plate 26 and that the amount of reduction in the height of the top nozzle subassembly 12 in moving from its expanded to compressed condition is only limited by the amount of displacement the springs 60 can undergo in moving from their expanded to compressed states. It is thought that the expandable top nozzle subassembly of 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.