Patent Number: 046648801
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 wate 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 axially 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 relationsihp 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 FIGS. 2 and 3, the debris trap 42 includes a hollow enclosure 46 having upper and lower walls 48,50 and a continuous side wall 52 which interconnects the upper and lower walls at their respective peripheries and spaces them apart so as to define a debris capturing and retaining chamber, generally indicated 54, within the enclosure 46. The walls 48,50,52 of the enclosure 46 are composed of any suitable material permeable to the liquid coolant but impermeable to debris carried by the coolant. For instance, in the illustrated embodiment, the walls 48,50,52 are composed of a wire mesh material in screen or layer form. As the liquid coolant flows upwardly from the openings 30 in the lower core plate 14 through the debris trap enclosure 46, means on the lower wall 50 of the hollow enclosure 46 in the form of a plurality of flap-like wall sections 56 define a plurality of openings 58 into the hollow enclosure 46 through which debris carried by the coolant flow can enter the trap chamber 54. In the illustrated embodiment, being an example of one way in which to form the openings 58, each of the wall sections 56 is a portion of the lower wall 50 which has been partially severed therefrom and then bent inwardly into the chamber 54 so as to extend at an acute angle, for instance thirty degrees, to the remainder of the lower wall 50. The wall sections 56 forming the openings 58 are matched in number and alignment with the plurality of coolant flow openings 30 in the lower core plate 14 so as to place the openings 58 into the trap enclosure chamber 54 directly above the lower core plate openings 30. The angle at which each of the wall sections 56 is stationarily disposed relative to adjacent portions of the lower wall 50 places the wall section 56 in a generally transverse or inclined position across the direction of the coolant flow path, as indicated by arrow A in FIG. 3, from the lower core plate openings 30 through the trap enclosure 46. Because of such positional relationship of the wall section 56 to the remainder of the lower wall 50, each of the entry openings 58 defined between the inner edge 60 of the wall section 56 and adjacent portions of the lower wall 50 lie in a plane extending generally parallel to the direction of coolant flow through the hollow enclosure 46. Thus, in order for debris carried by the coolant flow to enter the debris capturing and retaining chamber 54, the debris must impact one of the wall sections 56 and be deflected laterally therefrom through one of the openings 58. Once the debris has entered the chamber 54, it will be substantially detered from exiting back through one of the openings 58 due to their orientation parallel to the coolant flow path. Instead, once the debris is within the chamber 54, the coolant flow will tend to press the debris against the upper wall 48 of the trap enclosure 46. Parenthetically, it will be noticed that the inner edge 60 of each wall section 56 is U-shaped and thereby is made up of multiple edge portions. Thus, the opening 58 defined between such multiple portions of the inner edge 60 and the adjacent portions of the remainder of the lower wall 48 lies in multiple planes which all extend parallel to the direction A of coolant flow through the hollow enclosure 46. As seen in FIGS. 1 and 2, the debris trap enclosure 46 has overall cross-sectional dimensions sized to allow the enclosure to fit within the peripheral skirt 62 of the bottom nozzle 12 between the corner legs 44 thereof and extend generally coplanar with the adapter plate 34 of the nozzle. Generally arcuate-shaped depressions 64 are defined in the corners of the enclosure 46. One diagonal pair of the depressions 64 provide adequate space for a diagonal pair of alignment pins 66 which extend upright from the lower core plate 14 and fit through openings 68 formed through flanges 70 of one diagonal pair of the corner legs 44. The other diagonal pair of the depressions 64 provide adequate space for means in the form of a pair of leaf springs 72 disposed in the depressions 64 and anchored on the trap enclosure 46 to engage the flanges 70 of the other diagonal pair of corner legs 44 for locking the enclosure 46 within the bottom nozzle 12 upon installation of the trap 42 therein. Preferably, in the installed position of the trap 42, the upper wall 48 of its enclosure 46 is spaced a short distance below the adapter plate 34 so that water flow through the holes 32 of the adapter plate is not obstructed. The trap 42 is installed from the bottom of the fuel assembly 10 when the assembly has been removed from the reactor core. The retaining or locking leaf springs 72 deflect inwardly due to contact with the bottom flanges 70 of the nozzle legs 44 as the trap 42 is inserted into the bottom nozzle 12. The springs 72 then snap outwardly over the flanges 70 when the springs have cleared the top thereof. The trap 42 is then locked in place in the sense that it will not drop out of the bottom nozzle 12 when the fuel assembly 10 is moved. The hollow enclosure 46 of the debris trap 42 also has a central annular sleeve 74 mounted between the upper and lower walls 48,50 of the enclosure for two purposes. First, the sleeve 74 which rests on the lower core plate 14 serves to bolster the structural integrity of the hollow enclosure 46. Second, it allows access to the lower end of the instrumentation tube 22 to where it is attached to the bottom nozzle adapter plate 34. 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.