Patent Number: 048287911
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 has a structural skeleton which at its lower end includes the debris resistant bottom nozzle 12 of the present invention (which will be described later in detail). The bottom nozzle 12 supports the fuel assembly 10 on a lower core support plate 14 in the core region of a reactor (not shown). In addition to the bottom nozzle 12, the structural skeleton of the fuel assembly 10 also includes a top nozzle 16 at its upper end and a number of guide tubes or thimbles 18 which extend longitudinally between the bottom and top nozzles 12,16 and at opposite ends are rigidly attached thereto. The fuel assembly 10 further includes a plurality of transverse grids 20 axially spaced along and mounted to the guide thimbles 18 and an organized array of elongated fuel rods 22 transversely spaced and supported by the grids 20. Also, the assembly 10 has an instrumentation tube 24 located in the center thereof and extending between and mounted to the bottom and top nozzles 12,16. 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 22 in the array thereof in the assembly 10 are held in spaced relationship with one another by the grids 20 spaced along the fuel assembly length. Each fuel rod 22 includes nuclear fuel pellets 26 and is closed at its opposite ends by upper and lower end plugs 28,30. The pellets 26 are maintained in a stack thereof by a plenum spring 32 disposed between the upper end plug 28 and the top of the pellet stack. The fuel pellets 26 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 in the lower core plate 14 to the fuel assembly. The bottom nozzle 12 of the fuel assembly 10 passes the coolant flow upwardly through the guide thimbles 18 and along the fuel rods 22 of the assembly in order to extract heat generated therein for the production of useful work. To control the fission process, a number of control rods 34 are reciprocally movable in the guide thimbles 18 located at predetermined positions in the fuel assembly 10. Specifically, a rod cluster control mechanism 36 positioned above the top nozzle 16 supports the control rods 34. The control mechanism has an internally threaded cylindrical member 37 with a plurality of radially extending flukes or arms 38. Each arm 38 is interconnected to a control rod 34 such that the control mechanism 36 is operable to move the control rods vertically in the guide thimbles 18 to thereby control the fission process in the fuel assembly 10, all in a well-known manner. Debris Resistant Bottom Nozzle As mentioned above, fuel assembly damage due to debris trapped at or below the lowermost one of the grids 20 has been found to be a problem. Therefore, to prevent occurrence of such damage, it is highly desirable to prevent this debris from passing through the bottom nozzle flow holes. The present invention relates to a bottom nozzle 12, which in addition to supporting the fuel assembly 10 on the lower core support plate 14, also contains features which function to filter out potentially damage-inducing size debris from the coolant flow passed upwardly through the bottom nozzle. The bottom nozzle 12 includes support means in the form of a plurality of corner legs 42 for supporting the fuel assembly 10 on the lower core plate 14 and a top, generally rectangular planar flat plate 46 suitably attached, fixed, such as by welding, to the corner legs 42. As seen in FIG. 2, the prior art bottom nozzle 12A has a plate 46A with a large number of relatively large flow holes 48A,48B of two different diameter sizes therein (for instance 0.25 and 0.50 inch). The flow holes 48A,48B are large enough in their respective diameters to pass the damage-inducing size debris typically carried in the liquid coolant flow. Turning now to FIGS. 3 and 4, it can be seen that the top plate 46 of the debris resistant bottom nozzle 12 of the present invention is of a substantially solid configuration, closed to the flow of liquid coolant therethrough, except for a plurality of spaced cut-out regions 50 defined therethrough. The cut-out regions 50 are located in alignment with and directly above the plurality of inlet liquid coolant flow holes 52 of the lower core plate 14. Each of the cut-out regions 50 is approximately of the same size as each of the inlet holes 52 in the lower core plate 14. The debris resistant bottom nozzle 12 also includes a plurality of separate, open criss-cross structures 54. Each of the criss-cross structures 54 is fixed, such as by welding, to the plate 46 and extends across one of the cut-out regions 50 therein within the plane of the plate 46. The criss-cross structures 54 define individual openings 56 therethrough, for example being square-shaped in cross section, which are small enough in cross-sectional size to filter out debris of damage-inducing size which otherwise collects in unoccupied spaces of the lowermost grid 20 of the fuel assembly 10, but large enough in size to let pass debris of nondamage-inducing size which otherwise passes through the unoccupied spaces of the lowermost grid 20. Therefore, any debris being carried by the liquid coolant flow which is small enough to pass through the criss-cross structure openings 56 will also pass through the unoccupied grid spaces, whereas any debris being carried by the liquid coolant which is large enough to not pass through the unoccupied grid spaces and collect in the grid 20 will not pass through the criss-cross structure openings 56. Observations have shown that debris-induced fuel rod failures were at or below the lowermost grid and appeared to be caused by debris somewhat larger than 0.190 inch in width. Thus, openings 56 through the structures 54 of sizes less than 0.190 inch at their longest dimension should substantially reduce the percentage of potential rod-damaging debris carried into the fuel assembly 10 by the primary coolant flow. In the preferred embodiment shown in Figs. 3-10, the criss-cross structure 54 is in the form of a plurality of spaced and interleaved straps 58 forming an open grid structure. In their height dimensions, the horizontal straps 58 extend in vertical planes generally parallel to the direction of coolant flow through the open grid structure. The openings 56 defined by the interleaved straps 58 are in the form of open vertically extending channels which allow passage of coolant flow with very slight pressure drop. The straps 58 also serve to block the path of any fuel rods 22 which happen to loosen from the grip imposed by the springs and dimples (not shown) of the grids 20 and drop down on the top of the bottom nozzle plate 46. As also seen in FIGS. 3, 4, 8, 10 and 12, the criss-cross structure 54 of the debris resistant bottom nozzle 12 includes several double bore-defining structures 60 within the cut-out regions 50 for supporting the lower ends of the ones of the guide thimbles 18 which overlie the cut-out regions 50. The double bore-defining structures 60 are fixed to the plate 46 and structure 54 both at the intersection of the structure 54 and the plate 46 and within the structure 54 itself. The structure 60 fixed totally within the criss-cross structure 54 is in the form of a sleeve, as shown in FIGS. 3, 4 and 10. FIGS. 1, 3, 4, 11 and 12 also illustrate a hole 62 in the center of the plate 46 within which the lower end of the instrumentation tube 24 is anchored. In an alternative embodiment shown in FIG. 5A, the criss-cross structure 54 can take the form of a plurality of interconnected, crossed and spaced wires 64 forming an open mesh structure. The openings 56 are defined between the wires 64 and are square in cross section. The wires 64 might be used in those bottom nozzle configurations wherein the cut-out regions 50 are not aligned with guide thimbles and thus no guide thimble lower end supporting structure would be necessary. Finally, should the pressure drop of the debris resistant bottom nozzle 12 increase more than tolerable upon employment of the cut-out regions 50 and criss-cross structures 54 in the bottom nozzle 12, at least one and probably several pressure drop reducing flow holes 66 can be drilled through the plate 46 at locations spaced from the cut-out regions 50, as shown in FIG. 12 and 13. It is thought that the present invention and many of its attendant advantages will be understood from the forefoing 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 forms hereinbefore described being merely preferred or exemplary embodiments thereof.