Patent Number: 046844953
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 a partially sectioned elevational view, with parts broken away for clarity, of a fuel assembly constructed in accordance with well-known practices, being generally designated by the numeral 10, which incorporates a preferred embodiment of the invention. The fuel assembly 10 basically includes a lower end structure in the form of an improved bottom nozzle 12 for supporting the assembly on a lower core support plate 14 in the core region of a reactor (not shown), and a number of longitudinally extending guide tubes or thimbles 16 which project upwardly from the improved bottom nozzle 12. The assembly 10 further includes a plurality of transverse grids 18 axially spaced along the guide thimbles 16 and an organized array of elongated fuel rods 20 transversely spaced and axially supported by the grids 18. Also, the assembly 10 has an instrumentation tube 22 located in the center thereof. Attached to the upper end portions of the guide thimbles 16 is an upper end structure or top nozzle with hold-down means incorporated therewithin, generally designated by the numeral 24. With such an arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conveniently handled without damaging the assembly parts. To form the fuel assembly 10, the transverse grids 18 are first attached to the longitudinally extending guide thimbles 16 at predetermined axially spaced locations. Next, the fuel rods 20 are inserted from below through the grids 18, and the improved bottom nozzle 12 is then suitably attached, such as by machine screws 26 (only one being shown), to the lower end portions of the guide thimbles 16. Finally, the top nozzle 24 is attached to the upper end portions of the guide thimbles 16. As mentioned above, the fuel rods 20 in the array thereof in the assembly 10 are held in spaced relationship with one another by the grids 18 spaced along the fuel assembly length. Each fuel rod 20 includes nuclear fuel pellets 28 and is closed at its opposite ends by upper and lower end plugs 30,32. The fuel pellets 28 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 a plurality of flow openings 34 in the lower core plate 14 to the fuel assemblies. The coolant flows upwardly into the fuel assembly 10 through its improved bottom nozzle 12 and therefrom through the guide thimbles 16 and along the fuel rods 20 of the fuel assembly 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 16 located at predetermined positions in the fuel assembly 10. IMPROVED BOTTOM NOZZLE WITH INTEGRAL DEBRIS TRAP As mentioned above, fuel assembly damage due to debris trapped at the lowermost one of the grids 18 has been noticed in recent years. Therefore, to prevent occurrence of such damage, it is highly desirable to trap and remove this debris before it reaches the lowermost grid 18. The present invention relates to the improved bottom nozzle 12 of the fuel assembly 10 which uniquely combines a debris trap, generally indicated by the numeral 36, and a modified central adapter plate 38 so as to carry out the functions of capturing and retaining debris, limiting fuel rod displacement and supporting the guide thimbles without increasing the overall pressure drop across the bottom nozzle above that experienced in the case of the pre-existing bottom nozzle which did not have a debris trap. Referring again to FIG. 1 and also to FIG. 2, the improved bottom nozzle 12 includes a skirt or housing 40 formed by four interconnected, upstanding side walls 42, which surround the modified central adapter plate 38 of the nozzle, and four legs 44 extending downwardly from the four corners of the housing 40 (see also FIG. 7). In an exemplary embodiment, the bottom nozzle 12 is fabricated as a stainless steel casting. The corner legs 44 of the bottom nozzle 12 seat in holes 46 provided in the lower core plate 14 along with corresponding legs from the three adjacent fuel assemblies. Also, in the lower core plate 14, the series of openings 34, usually four in number, are aligned with the modified central adapter plate 38 of the bottom nozzle 12 and provide paths for coolant flow through the lower core plate to the fuel assembly 10. As depicted in FIG. 7, the modified adapter plate 38 of the improved bottom nozzle 12 is made up of a network of interconnected members in the form of crisscross and diagonally-extending ligaments 50 which extend across the space within the housing 40 and are connected at their outer ends at the inside of the housing side walls 42. The ligaments 50 of the adapter plate 38 support attaching means such as a plurality of bosses 52 for anchoring the lower end portions of the guide thimbles 16 and a central guide sleeve 54 which extends downward to the elevation of the bottom peripheral edge of the housing 40 to where it contacts the lower core plate 14. The guide thimble bosses 52 have countersunk through-holes 56 which receive the machine screws 26 for attaching and anchoring the lower end portions of the guide thimbles 16 to the adapter plate 38. The guide sleeve 54 receives the lower end portion of the instrumentation tube 22 and also serves to support the center of the modified adapter plate 38 so as to minimize the structural strength requirement of its network of ligaments 50. While the housing 40 and legs 44 of the improved bottom nozzle 12 are per se substantially the same as in the pre-existing bottom nozzle, the modified adapter plate 38 in being formed by the network of ligaments 50 is not only structurally different but also provide much more open space for coolant flow than heretofore and thus has a lower pressure drop across it. As seen in FIGS. 1 and 2, the debris trap 36 of the improved bottom nozzle 12 is disposed above the modified adapter plate 38. Specifically, the ligaments 50 of the adapter plate 38 are attached to the housing side walls 42 a short distance below the upper peripheral edge 58 of the housing 40 so as to define an open region 60 within the upper portion of the housing above the adapter plate. The trap 36 is dimensioned to fit within the open region 60 and thus be positioned across the path of coolant flow from the lower core plate openings 48 and through the open space of the ligament network of the modified adapter plate 38. In such position, the trap 36 will capture debris, such as small loose parts or pieces, from the flowing coolant and thereby prevent it from entering the fuel assembly 10. Instead, the debris is retained within the trap 36 which permits removal of the debris along with the fuel assembly 10 at the next refueling cycle. Turning now to FIG. 3, the debris trap 36 includes a structure 62 being composed of a plurality of straps 64, for instance made of stainless steel material, which are aligned with respect to each other in a cross-laced or crisscross interlocking arrangement. The straps 64 in such an arrangement define a plurality of interconnected wall portions 66 which form a multiplicity of cells, each being generally designated as 68, with any given interior one of the wall portions 66 being shared by two adjacent ones of the cells 68. Each four interconnected wall portions 66, which form a given one of the cells 68, define the cell 68 with open opposite ends and a central channel 70 extending generally parallel to the path of coolant flow for passage of coolant therethrough to the interior of the fuel assembly 10. Each cell 68 has a width to depth ratio that is less than one. As also seen in FIG. 3, the intersections 72 formed by the crisscross arrangement of the straps 64 making up the debris trap structure 62 underlie and are aligned with the respective fuel rods 20, which are illustrated in dashed outline form in the figure. In such way, the debris trap structure 62 acts as a platform for catching any one of the fuel rods 20 which might inadvertently become free of the grids 18 and drop downward during operation of the reactor core. As mentioned earlier, in the pre-existing bottom nozzle, its adapter plate served this function. Further in FIG. 3, there is shown a plurality of hubs 74 supported by the crisscross straps 64 in alignment with the guide thimble bosses 52 in the adapter plate 38. The hubs 74 permit passage of the thimbles 16 through the structure 62 of the debris trap 36 and into engagement with the bosses 52 in the adapter plate 38. The debris trap 36 includes means in the form of a plurality of leaf springs 76 integrally formed on and bent slightly outward from the peripheral wall 78 of the trap structure 62. When the trap structure 62 is disposed in the open region 60 of the housing 40 above and resting upon the ligaments 50 of the modified adapter plate 38, the springs 76 extend within and are engageable with a plurality of inwardly-facing recessed shoulders 80 formed in the upper peripheral portion 82 of the housing 40 which defines the open region 60. In the position of the spring 76 shown in FIG. 4, it locks the structure 62 within the open region 60. To remove the trap structure 62 from the housing 40, the bottom nozzle 12 must be removed from the fuel assembly 10 and a tool (not shown) must be inserted through a hole 84 extending through the housing portion 82 and into the recessed shoulder 80 in order to deflect the spring 76 inwardly away from the recessed shoulder. The debris trap 36 also includes means in the form of a plurality of spring-like fingers 86 defined in each of the cells 68 for capturing and retaining within the structure 62 any debris carried into the cells 68 by coolant flowing therethrough. Each wall portion 66 which is common to two adjacent cells 68 has at least two fingers 86 attached to it at two levels, one above the other. Each finger 86 is punched out of material of the strap 64 forming the wall portion 66 and bent so as to extend into the cell channel 70 and point downstream toward the interior of the fuel assembly 10. Specifically, with respect to the two fingers 68 punched out of any common wall portion 66, one finger 86 extends into one of the adjacent cells 68 sharing the common wall portion 66, while the other finger 86 extends into the other of the adjacent cells. FIGS. 5, 6 and 8 show various combinations of arrangements of the fingers 86 in the respective cells 68. In one form of the finger arrangement shown in FIGS. 5 and 6, the fingers 86 on oppositely-facing wall portions 66 of a given cell 68 are disposed at two different levels. Thus, in this arrangement, the two fingers 86 at the same level are displaced ninety degrees from each other on wall portions 66 which are connected to one another. In another form of the finger arrangement shown in FIG. 8, the fingers 86 on oppositely-facing wall portions 66 of a given cell 68 are disposed at the same level. Thus, the two fingers 86 at the same level in this arrangement are displaced one hundred eighty degrees from each other. However, in both arrangements, the two fingers 86 at each level overlap at their respective tip ends one slightly above the other. Further, in both arrangements, the overlapped tips of the two fingers at the upper level, in turn, overlap the overlapped tips of the two fingers at the lower level in each cell 68. In such manner, the channel 70 through the cell 68 is obstructed by the fingers 86 and the overlapping and spaced apart relationship between the upper and lower sets of fingers 86 provides several regions in the cell channel 70 within which pieces of debris can be trapped and retained in the structure 62. The resilient or spring-like nature of each finger 86 gives it the capability of imposing a lateral force on a piece of debris so as to force it against the wall portion 66 and retain the debris in the trap 36. Even though the fingers 86 obstruct the cell channel 70, they only minimally impede the flow of coolant through the trap 36. First, each wall portion 66 has an opening 88 formed therein at the region where the finger was punched out which facilitates cross flow of coolant through the wall portions 66 between individual ones of the cells 68. Also, each finger 86 has a hole 90 formed through it which allows increased flow of water through the cell. It is thought that the improved bottom nozzle 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.