Patent Number: 051805499
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 a double enclosure 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 and 2, the double enclosure top nozzle subassembly 12 of the present invention has a construction which permits improved utilization of space for accommodating greater thermal 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. Basically, the top nozzle subassembly 12 includes an upper structure 24, a lower structure 26, interengaging means 28 on the lower and upper peripheral edges of the respective upper and lower structures 24, 26, and a plurality of resiliently-yieldable biasing devices 30 disposed between the upper and lower structures 24 26. As shown alone and in greater detail in FIGS. 2-11, the upper structure 24 of the top nozzle subassembly 12 is composed of a top plate 32 and an outer sidewall enclosure 34 rigidly connected to and depending from the top plate 32. Referring to FIGS. 3-8, the top plate 32 is generally rectangular in configuration having four sides 32A defining four corners 32B. The top plate 32 includes an annular body 36 and an annular rim 38 integrally attached to and projecting outwardly from an upper outer peripheral edge of the body 36. The body 36 has an inner peripheral edge defining a large central opening 40. Two diagonal ones of the corner 32B of the top plate body 36 each has a hole 42 defined therethrough which permit insertion of components of the fuel assembly handling system (not shown) for engaging the underside surface 32C of the top plate 32 in order to lift the fuel assembly 10 in installing and removing it from the core. One of the other corners 32B has a hole 44 which provides a reference for properly orientating the fuel assembly 10 in the core. The annular rim 38 on the body 36 of the top plate 32 defines an annular cavity 46 surrounding the annular body 36 below an outer peripheral edge of the top plate 32 defined by the rim 38. The annular cavity 46 receives an upper peripheral edge portion 34A of outer sidewall enclosure 34. The top plate 32 also has a plurality of indentations 48 defined in spaced relation from one another along the periphery and underside surface 32C of the annular body 36 of the top plate 32. The indentations 48 face outwardly and downwardly of the annular body 36. Portions of the annular body 36 between the indentations 48 form downwardly protruding tabs 50 along the periphery of the body 36 for attaching the upper peripheral edge 34A of the outer sidewall enclosure 34 to the top plate 32. Referring to FIGS. 9 and 10, the outer sidewall enclosure 34 of the upper structure 24 is composed of four generally planar vertical wall portions 34B rigidly interconnected together at their opposite vertical edges to define the outer enclosure 34 in a generally square box-like configuration. As shown in FIGS. 19-22, a series of aligned holes 52, 54 are formed in the annular body 36 below the rim 38 and in the upper peripheral edge portion 34A of the outer sidewall enclosure 34. Pins 56 are inserted through the aligned holes 52, 54 for securing the outer sidewall enclosure 34 and the annular body 36 together. The holes 52 in the annular body 36 can either extend partially into the body 36 as shown in FIG. 20 and complete through the body 36 as shown in FIG. 21. As shown alone and in greater detail in FIGS. 12-15, the lower structure 26 of the top nozzle subassembly 12 is composed of a lower adapter plate 58 and an inner sidewall enclosure 60 rigidly connected to and upstanding from the lower adapter plate 58. Referring to FIG. 15, the lower adapter plate 58 is generally rectangular in configuration having four sides 58A defining four corners 58B. The lower adapter plate 58 is formed of a plurality of cross-laced ligaments or bars 62 defining a plurality of coolant flow openings 64 of oblong shapes. Also, a plurality of circular through holes 66 corresponding in number and pattern to that of the guide thimbles 16 are provided through the adapter plate 58. The through holes 66 are of sufficient dimensional size to permit the adapter plate 58 to be installed over the upper ends of the guide thimbles 16. Referring to FIGS. 12 and 13, the inner sidewall enclosure 60 of the lower structure 26 is composed of four generally planar vertical wall portions 60A rigidly interconnected together at their opposite vertical edges to define the inner enclosure 60 in a generally square box-like configuration and integrally connected at their lower edges to the periphery of the lower adapter plate 58. The inner sidewall enclosure 60 has a plurality of upper edge portions 60B spaced apart by notches 68 defined between the upper edge portions 60B. The alternating upper edge portions 60B and the notches 68 of the inner sidewall enclosure 60 of the lower structure 26 are capable of mating respectively with the alternating indentations 48 and tabs 50 of the annular body 36 of the top plate 32 of the upper structure 24 when the top nozzle subassembly 12 is in the compressed condition, as depicted in FIG. 18. Thus, the lower adapter plate 58 of the lower structure 26 is disposed below the top plate 32 of the upper structure 24 with the inner sidewall enclosure 60 being disposed within the outer sidewall enclosure 34. Further, the inner and outer sidewall enclosures 60, 34 are movable in sliding contacting relationship relative to one another so as to permit movement of the top plate 32 toward and away from the lower adapter plate 58 and thereby the top nozzle subassembly 12 between compressed condition of FIG. 18 and the expanded condition of FIG. 17. Referring to FIGS. 1, 9-14 and 16-22, the interengaging means 28 on respective upper and lower peripheral edge portions 60B, 34A of the inner and outer sidewall enclosures 60, 34 define stops which limit the movement of the top plate 32 and lower adapter plate 58 away from each other so as to retain the outer and inner sidewall enclosures 34, 60 in the sliding contacting relationship with one another. In addition, the interengaging means 28 define sliding contact surfaces between the outer and inner sidewall enclosures 34, 60. The interengaging means 28 on the respective upper and lower structures 24, 26 include an inwardly-projecting continuous annular flange 70 on the lower peripheral edge of the outer sidewall enclosure 34, and an outwardly-projecting interrupted annular flange 72 on the upper edge portion of the inner sidewall enclosure 60. The inwardly-projecting flange 70 defines a contact surface 70A engaged with an exterior surface 60C of the inner sidewall enclosure 60. The outwardly-projecting flange 72 defines a contact surface 72A engaged with an interior surface 34C of the outer sidewall enclosure 34. As seen in FIGS. 1, 16, 17 and 19-22, the flanges 70, 72 provide the stops by overlapping with one another so as to prevent the outer and inner enclosures 34, 60 from pulling apart. Referring to FIGS. 1 and 16-18, there is illustrated the resiliently-yieldable biasing devices 30 disposed within the outer and inner sidewall enclosures 34, 60 and extending between and engaging the top plate 32 and the lower adapter plate 58. The devices 30 are composed of resiliently and yieldable flexible material, such as a metal material, and are movable between compressed and expanded states, as shown in FIGS. 17 and 16, in response respectively to application and removal of a hold-down force on the upper structure 24 in the direction of the lower structure 26 for respectively permitting and causing movement of the top nozzle subassembly 12 between compressed and expanded conditions. More particularly, preferably the biasing devices 30 are a plurality of leaf springs 74 arranged in separate stacks thereof and disposed between the top plate 32 and the lower adapter plate 58 and flexible between expanded and compressed states. The lower adapter plate 58 has a depression 76 formed in a topside surface 58C of the lower adapter plate along each of the sides 58A and approximately midway between the corners 58B thereof. The top plate 32 has a pair of elongated guide grooves 78 defined in the underside surface 32C of the top plate 32 along each of the sides 32A and adjacent the corners 32B thereof. The leaf springs 74 each has a generally U-shaped configuration composed of a lower bight portion 74A and upper end portions 74B connected to and extending upwardly from the lower bight portion 74A. Each leaf spring 74 is seated at the lower bight portion 74A within one of the depressions 76 of the lower adapter plate 58 and at the opposite upper end portions 74B within the guide grooves 78 of the top plate 32. It should be realized, however, that other forms of the biasing devices 30 can be used, such as elongated coil springs. The coil springs would be mounted between the top plate 32 and the lower adapter plate 58 in the same way as illustrated and described in the patent application cross-referenced above, the disclosure of which is incorporated herein by reference. FIGS. 16 and 17 depict successive stages in the assembly of the double enclosure top nozzle subassembly 12. FIG. 16 shows the top nozzle subassembly 12 after the leaf springs 74 have been installed in the telescoping outer and inner enclosures 34, 60, but before the top plate 32 is applied and secured to the outer enclosure 34. FIG. 17 shows the top nozzle subassembly 12 after the top plate 32 and pins 56 has been installed to secure the top plate to the upper peripheral edge portion 34A of the outer enclosure 34. FIGS. 19-21 depict successive stages in the installation and securement of the top plate 32 and pins 56 to the upper peripheral edge portion 34A of the outer enclosure 34. In summary, FIG. 17 shows the top nozzle subassembly 12 in an expanded condition, whereas FIG. 18 shows it in a compressed condition. In both conditions of the top nozzle subassembly 12, the lower adapter plate 58 is stationarily secured in the same position on the upper ends of the guide thimbles 16 in a conventional manner by locking tubes 80. By way of example, the lower adapter plate 58 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 inner sidewall enclosure 58 is slidably movably mounted within the interior of the outer sidewall enclosure 34. The overlapping flanges 70, 72 provide stops which prevent separation of the upper structure 24 from the lower structure 26. To place the top nozzle subassembly 12 in the expanded condition seen in FIG. 17, the upper core support plate (not shown) is removed from imposing a downward bearing contact force upon the top plate 32 of the upper structure 24 of the top nozzle subassembly. The leaf springs 74 are thus allowed to assume their unflexed, or expanded, states in which they force the upper structure 24 away from the lower structure 26 to the limit defined by engagement between the flanges 70, 72. The lower adapter plate 58 and top plate 32 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 42 in the top plate 32. To place the top nozzle subassembly 12 in the compressed condition seen in FIG. 18, the upper core support plate is installed upon the top plate 32 of the upper structure 24 of the top nozzle subassembly so as to reimpose the downward bearing contact force thereon. The top plate 32 is thus moved downward toward the lower adapter plate 58 forcing the leaf springs 60 to their flexed, or compressed, states and slidably moving the outer sidewall enclosure 34 downwardly along and relative to the inner sidewall enclosure 60 and moving the flanges 70, 72 away from one another. The space between the top plate 32 and the adapter plate 58 is now reduced below that needed for insertion of the components of the fuel assembly handling system. 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 32 and adapter plate 58 has now been eliminated and is instead now being utilized by the higher mounting position of the adapter plate 58 on the guide thimbles 16 permitting greater distance between the adapter plate 58 and upper ends of the fuel rods 18 for increased thermal 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 leaf springs 74 moves the upper structure 24 upward to its position in FIG. 17 returning the top plate 32 and adapter plate 58 to their maximum spacing for providing the necessary space therebetween for the fuel assembly handling system components. The lower peripheral edge portion of the upper structure 24 does not move downwardly past the lower adapter plate 58 and so any possible fretting of the fuel rods 18 is eliminated. The central opening 40 of the top plate 32 accommodates passage of control rods (not shown) into the guide thimbles 16 in a conventional manner. The leaf springs 74 transmit the necessary hold-down force from the upper core plate directly to the adapter plate 58. It will be noted also that the telescoping outer and inner sidewall enclosures 34, 60 of the upper and lower structures 24, 26 completely enclose the leaf springs 74 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 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.