Patent Number: 046844961
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 an elevational view of a fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 10. The fuel assembly 10 is the type used in a pressurized water reactor and basically includes a lower end structure or 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 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 supported by the grids 18. Also, the assembly 10 has an instrumentation tube 22 located in the center thereof and an upper end structure or top nozzle 24 attached to the upper ends of the guide thimbles 16. With such an arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conveniently handled without damaging the assembly parts. 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 (not shown) and is closed at its opposite ends by upper and lower end plugs 26,28. 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 a plurality of flow openings 30 in the lower core plate 14 to the fuel assemblies. The bottom nozzle 12 of each assembly 10 has a series of flow holes 32 defined in its upper central adapter plate 34 through which the coolant flows upwardly 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. Specifically, the top nozzle 24 includes a rod cluster control mechanism 36 having an internally threaded cylindrical member 38 with a plurality of radially extending flukes or arms 40. Each arm 40 is interconnected to a control rod such that the control mechanism 36 is operable to move the control rods vertically in the guide thimbles 16 to thereby control the fission process in the fuel assembly 10, all in a well-known manner. Debris Trap Mounted in Bottom Nozzle 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 a debris trap, generally indicated by the numeral 42, mounted inside or within the bottom nozzle 12 adjacent to and below its upper central adapter plate 34 and between its corner legs 44, as illustrated in FIG. 1. The trap 42 is positioned across the path of coolant flow from the lower core plate openings 30 to the adapter plate holes 32 so as to 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 42 which permits removal of the debris along with the trap 42 and fuel assembly 10 at the next refueling. Turning now to FIG. 2, the debris trap 42 includes a structure 46 being composed of a plurality of straps 48 aligned with respect to each other in a cross-laced or crisscross interlocking arrangement. The straps 48 in such an arrangement define a plurality of interconnected wall portions 50 which form a multiplicity of cells, each being generally designated as 52, with any given interior one of the wall portions 50 being shared by two adjacent ones of the cells 52. Each four interconnected wall portions 50, which form a given one of the cells 52, define the cell 52 with open opposite ends and a central channel 54 extending generally parallel to the path of coolant flow for passage of coolant therethrough to the adapter plate 34 of the bottom nozzle 12. Preferably, each cell 52 has a length to width ratio that is less than one. As seen in FIGS. 1 and 2, the debris trap structure 46 has cross-sectional dimensions sized to allow the structure to fit within the peripheral skirt 56 of the bottom nozzle 12 between the corner legs 44 thereof and extend generally coplanar with the adapter plate 34 of the nozzle. Notches 58 are defined in the corners of the structure 46. One diagonal pair of the notches 58 provide adequate space for a diagonal pair of alignment pins 60 which extend upright from the lower core plate 14 and fit through openings 62 formed through flanges 64 of one diagonal pair of the corner legs 44. The other diagonal pair of the notches 58 provide adequate space for means in the form of a pair of leaf springs 66 disposed in the notches and anchored on the trap structure 46 to engage the flanges 64 of the other diagonal pair of corner legs 44 for locking the structure 46 within the bottom nozzle 12 upon installation of the trap 42 therein. The trap 42 is installed by lowering the fuel assembly 10 over the trap which has been positioned on a suitably raised fixture (not shown). The retaining or locking leaf springs 66 are deflected inwardly, as seen in FIG. 3, by the bottom flanges 64 of the diagonal pair of legs 44 of the bottom nozzle 12 as the fuel assembly 10 is lowered. The springs 66 then snap outwardly over the flanges 64 when the springs have cleared the top thereof. The trap 42 is then locked in place and can be moved with the fuel assembly 10. In operation, the coolant flow holds the trap 42 in contact with the underside of the adapter plate 34 of the bottom nozzle 12. The debris trap 42 also includes means in the form of a plurality of spring-like fingers 68 defined in each of the cells 52 for capturing and retaining within the structure 46 any debris carried into the cells 52 by coolant flowing therethrough. Each wall portion 50 which is common to two adjacent cells 52 has at least two fingers 68 attached to it at two levels, one above the other. Each finger 68 is punched out of material of the strap 48 forming the wall portion 50 and bent so as to extend into the cell channel 54 and point downstream toward the bottom nozzle adapter plate 34. Specifically, with respect to the two fingers 68 punched out of any common wall portion 50, one finger 68 extends into one of the adjacent cells 52 sharing the common wall portion 50, while the other finger 68 extends into the other of the adjacent cells. FIGS. 4, 5 and 6 show various combinations of arrangements of the fingers 68 in the respective cells 52. In one form of the finger arrangement shown in FIGS. 4 and 5, the fingers 68 on oppositely-facing wall portions 50 of a given cell 52 are disposed at two different levels. Thus, in this arrangement, the two fingers 68 at the same level are displaced ninety degrees from each other on wall portions 50 which are connected to one another. In another form of the finger arrangement shown in FIG. 6, the fingers 68 on oppositely-facing wall portions 50 of a given cell 52 are disposed at the same level. Thus, the two fingers 68 at the same level in this arrangement are displaced one hundred eighty degrees from each other. However, in both arrangements, the two fingers 68 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 52. In such manner, the channel 54 through the cell 52 is obstructed by the fingers 68 and the overlapping and spaced apart relationship between the upper and lower sets of fingers 68 provides several regions in the cell channel 54 within which pieces of debris can be trapped and retained in the structure 46. The resilient or spring-like nature of each finger 68 gives it the capability of imposing a lateral force on a piece of debris so as to force it against the wall portion 50 and retain the debris in the trap 42. Even though the fingers 68 obstruct the cell channel 54, they only minimally impede the flow of coolant through the trap 42. First, each wall portion 50 has an opening 70 formed therein at the region where the finger was punched out which facilitates cross flow of coolant through the wall portions 50 between individual ones of the cells 52. Also, each finger 68 has a hole 72 formed through it which allows increased flow of water through the cell. Further, as seen in FIG. 1, and in greater detail in FIGS. 3--6, the underside or upstream end of the trap structure 46 has a plurality of lower tabs 74 which project outwardly toward the lower core plate 14, while the upperside or downstream end of the structure 46 has a plurality of upper projections 76 which extend outwardly toward the bottom nozzle adapter plate 34. The upper projections 76 ensure that there will be some space between the downstream end of the trap structure 46 and the underside of the adapter plate 34 to permit some cross flow of coolant between the trap 42 and the adapter plate. The purpose of the lower tabs 74 is to grasp large pieces of debris to prevent them from moving horizontally along the bottom of the trap 42 to a point of reduced flow velocity, where they would fall back into the flow stream and impact the underside of the trap structure 46 again. These tabs 74, in conjunction with coolant flow, keep very large pieces of debris substantially stationary and in place on the underside of the trap, so they don't circulate around under the trap and repeatedly impact the trap. It is recognized that whenever coolant flow is stopped, most of the very large pieces of debris will fall back onto the lower core plate 14. However, large pieces of debris that land on the core plate can be seen when the fuel assembly 10 is removed, and then can be removed by remote means. Pieces that fall back through the core plate flow openings 30 may be circulated back into the bottom nozzle 12 on a future cycle, but again stand a high probability of being retained by the core plate since there is much more surface area than hole area in the core plate so these pieces eventually can be removed also. Meanwhile, they will not damage the fuel assemblies. It is thought that the debris trap 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.