Patent Number: 047028823
Section: description

DETAILED DESCRIPTION OF THE INVENTION Reference will not be made in detail to the presently preferred embodiment of the invention, examples of which are illustrated in the accompanying drawings. In the following description, like reference 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 partially sectioned elevational view with parts broken away for clarity of a fuel assembly constructed in accordance with well known practices, generally indicated by the numeral 10, which incorporates a preferred embodiment of the invention. The fuel assembly 10 basically comprises 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). A number of longitudinally extending control rod guide thimbles 14 project upwardly from the bottom nozzle 12. A plurality of transversely extending fuel rods spacer grids 16 are axially spaced along the guide thimbles 14. An organized array of elongated fuel rods 18 are transversely spaced and supported by the spacer grids 16. An instrumentation tube 20 is located in the center of the assembly. An upper end structure top nozzle, generally designated by the numeral 22, is attached to the upper ends of the guide thimbles 14 in a manner more fully described below to form an integral assembly capable of being conventionally handled without damaging the assembly components. The top nozzle 22 includes a transversely extending adapter plate 24 having upstanding sidewalls 26 secured to the peripheral edges thereof and defining an enclosure or housing. An annular flange 28 is secured to the top of the sidewalls 26. Suitably clamped to the annular flange 28 are holddown springs 30 (only one of which is illustrated in FIG. 1 for clarity) which cooperate with the upper core plate (not shown) in a conventional manner to prevent hydraulic lifting of the fuel assembly caused by upward flow of coolant through the assembly while also allowing for changes in the fuel assembly length due to core-induced thermal expansion and the like. Disposed within the opening defined by the annular flange 28 is a conventional rod cluster control assembly 32 having radially extending flukes 34 connected to the upper end of the control rods 36 for vertically moving the control rods in the control rod guide thimbles 14 in a well known manner. With the exception of the top spacer grid 38, each of the spacer grids 16 may be of any suitable, conventional design for laterally spacing and supporting the fuel rods 18. The fuel assembly 10 depicted in the drawings is of the type having a square array of fuel rods 18 with the control rod guide thimbles strategically arranged within the fuel rod array. Further, the bottom nozzle 12 and likewise the top nozzle 22 are generally square in cross section. The specific fuel assembly represented in the drawings is for illustration only; it is to be understood that neither the shape of the nozzles nor the number and configuration of the fuel rods and guide thimbles are to be limiting and that the invention is equally applicable to shapes, configurations, and arrangements other than the ones specifically illustrated. To form the fuel assembly 10, the transverse spacer grids 16 are attached to the longitudinally extending guide thimbles 14 at predetermined axially spaced locations. The fuel rods 18 are inserted through the spacer grids 16 in order to form the fuel rod array. The lower nozzle 12 is suitably attached to the lower ends of the guide thimbles 14 and the top nozzle 22 is attached to the upper ends of the guide thimbles 14 in the manner described below in accordance with the improved attaching structure of the present invention. Referring now to FIGS. 2, 2a and 3, a first preferred embodiment of the improved attaching structure for removably mounting the top nozzle 22 on the upper end of the guide thimbles 14 and the top spacer grids 38 will be discussed. Although each of the guide thimbles 14 compressively supports the top nozzle 22, the description that follows is directed to the support arrangement for only one of the guide thimbles, the other guide thimbles supporting the top nozzle in the same manner. Similarly, although each side of the top fuel rod spacer grid 38 has an skirt extension 40 for tensively supporting the top nozzle 22, the description which follows is directed to the arrangement between the top nozzle 22 and only one of the spacer grid skirt extensions 40. It should however be understood that each of the four available skirt extensions 40 are preferably used. The improved structure for removably supporting and attaching the top nozzle 22 includes thimble collars 44 which are welded or otherwise secured to the guide thimbles 14 and which are radially dimensioned to support the top nozzle 22 by bearing against the adapter plate 24 under compressive loading, and skirt extensions 40 formed in the top spacer grid 38 which removably attach, preferably without any loose attachment parts, to the sidewall 26 of the top nozzle 22 in order to support the fuel assembly under tensile loading. Details of these elements and connections as well as another preferred embodiment of a quick disconnect top nozzle fuel assembly will now be described. According to a preferred embodiment of the present invention, compressive loads from the top nozzle 22, such as loads imposed by the holddown springs 30, are transmitted via the load collars 44 on the guide thimbles 14, while tensive loads, such as lifting loads, are transferred through the top nozzle 22 onto upwardly extending skirt extensions 40 of the top spacer grid 38. The top spacer grid assembly 38 may be fastened in any conventional manner, for example, by bulging techniques, to the guide thimbles 14. Thus, any tensive loads on the grid skirt extensions 40 are transferred through the spacer grids 38 to the guide thimbles 14 eliminating many of the costly, complex and loose components previously used to connect the guide thimbles to the top nozzle. As alluded to above, the guide thimbles 14 are clearance fitted into apertures 46 in the adapter plate 24. The amount of radial clearance is preferably small, on the order of about two mils. Preferably, at least the portion of the guide thimble 14 in the vicinity of the top nozzle 22 is formed of stainless steel and the load collar 44 is formed from a coaxial stainless steel sleeve brazed, welded, or otherwise attached on to the guide thimble in the vicinity of its top end. The load collar 44 is radially dimensioned to be larger than the aperture 46, thereby any compressive load on the top nozzle 22 will be borne by the guide thimble 14 via the load collar 44. However, the clearance fit between the guide thimble 14 and the apertures 46 permits the top nozzle to be removed from the guide thimbles in the manner described below and require no unlocking, unscrewing, or other detachment operations between the guide thimble 14 and the top nozzle 22. The grid skirt extension 40 may be of any desired geometry for providing mechanical support to the fuel assembly while permitting adequate coolant flow through the fuel assembly. The skirt extention 40 extends along the sidewall 26 of the top nozzle 22. It should be understood that the sheet metal skirt extensions 40, while strong under tensive stresses, will buckle relatively easy under compressive loading and are therefore not primarily relied upon to provide compressive strength. Each of the grid skirt extensions 40 includes means for securing the top spacer grid assembly 38 to the top nozzle 22 in a manner whereby it can support tensive loads. Such means may include aperture 42 in the grid skirt extension which aligns with apertures 48 in the sidewalls 26. Each sidewall 26 has a spring steel tang 50 extending generally parallel to the sidewall 26 to form therebetween a space for the skirt extension 40. A number of generally orthgonally locking pins 52, corresponding to the number of aligned apertures 42 and 48, are provided in the grid skirt extensions 40. The tangs 50 may be welded, integrally formed with, or otherwise secured to the sidewall 26 or to the annular flange 28. As best seen in FIG. 2a, the tang 50 preferably includes a notched end 54 which may be easily gripped by the end 56 of a pull-back tool 58. As best seen in FIG. 3, the end 56 of the pull back tool is complementary shaped with respect to the notched end 54 of the tang 50. In use, the top nozzle 22 may be removed by pulling back the tang 50, i.e. to the left as viewed in FIG. 2a, until the locking pin 52 clears the aperture 42 whereupon the top nozzle may be simply lifted off of the clearance fitted guide thimbles 14. For reassembly, the tang 50 need only be pulled back with respect to the sidewall 26 enough to provide sufficient clearance between the sidewall and the locking pin 52 for passage of the grid skirt extension 40. Thereupon, the apertures 46 in the adapter plate 24 can be aligned with the guide thimbles 14 and the apertures 42 aligned with the apertures 48 and the locking pin 52. Upon release of the tang 50, the locking pins 52 will lock the sidewall 26 to the grid skirt extensions 40. Turning now to FIGS. 4, 5a, and 5b, a second embodiment of the invention will be described. In the embodiment of FIG. 4, the spacer grid skirt extensions 40 terminate in a tang 58 which is designed to engage the complementary slot 60 formed in the sidewall 26 of the top nozzle 22. Each tang 58 preferably has an upstanding flange portion 62 designed to be engaged by a combination lift and release tool 64 as described below. The sidewall 26 of the top nozzle is preferably provided with a hole 68 through which a skirt extention deflecting portion 66 of the lift release tool 64 is designed to protrude. The protruding portion 66 of the tool 64 may simply comprise a small cylindrical member sized to clearance fit through the hole 68 and protrude far enough to deflect the tang 58 out of engagement with the slot 60. This is best seen in FIG. 5a. In this position, the top spacer grid 38 is unlatched from the top nozzle 22. The tool 64 further comprises a tang capture portion 70 having a notched end 72 designed to capture a flange 62 on the tang 58 and hold the tang in a position deflected away from the sidewall 26 and out of mating engagement with the slot 60 so that when the protruding portion 66 of the tool 64 is withdrawn from contact with the tange 58, i.e. moved to the right as viewed in FIG. 5b, the tang capture portion 70 of the tool 64 can be lowered to engage the the flange 62 allowing the top nozzle to be lifted. During lifting, the portion of the tool 64 which bears against the annular flange 28 may be used to support the top nozzle. Thus, by modifying the top spacer grid assembly to support tensive loads on the fuel assembly and by providing load collars on the guide thimbles to support compressive loads, a fuel assembly according to the present invention can be quickly and simply constituted and reconstituted and individual fuel rods in a fuel assembly can be handled on a routine basis at the end of each fuel cycle merely by removing the top nozzle in the manner described above. In addition to the other advantage described above, the quick disconnect top nozzle permits the enrichment of fuel rods within each fuel assembly to be more precisely tailored to more closely approximate the optimum hydrogen to uranium ratio for a given burnup. Further, the quick disconnect top nozzle permits rapid access to the fuel rods while eliminating the many costly, intricate, and loose attaching components of prior art attachment designs. The foregoing description of a preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. Other quick disconnect latching schemes between the top grid assembly and the top nozzle can be used and other compressive load supporting devices than simple load collars can be employed. The embodiments presented were choosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use comtemplated. It is intended that the scope of the invention be defined by the claims appended hereto.