Patent Number: 050376055
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, FIG. 1 is an illustration of a typical nuclear fuel assembly generally designated by the numeral 10. Fuel assembly 10 is typical of that used in a pressurized water reactor and is generally comprised of a plurality of fuel rods 12, grid assemblies 14, guide tubes 16, upper end fitting 18, and lower end fitting 20. Fuel rods 12 are maintained in an array spaced apart by grid assemblies 14. Guide tubes 16 extend through grid assemblies 14 and are attached to end fittings 18,20 and, in addition to providing structural integrity to the entire assembly, also serve as guides for control rods not shown. Upper and lower end fittings 18,20 provide structural and load bearing support to fuel assembly 10 and are also provided with openings therethrough to allow coolant to flow vertically through fuel assembly 10. Lower end fitting 20 rests on the lower core support plate (not shown) of the reactor and directly above coolant inlet openings in the core support plate that direct coolant upward to the fuel assembly. Dimples or tabs in the walls of the grid assemblies allow maximum surface area contact of fuel rods 12 with the coolant as it flows upwardly therethrough. Lower end fitting 20, seen in FIG. 4, is formed from a substantially square base 22 having four side walls. Between the side walls, a plurality of ribs 24 are provided that interconnect, leaving the interior of base 22 substantially open to allow coolant flow therethrough. A plurality of guide tube bosses 26 are provided along ribs 24 in a pattern matching the distribution of guide tubes 16 in grid assemblies 14. Legs 28, seen in FIG. 2 and 3 may be separate parts attached to base 22 or may be integral therewith. Openings 30 provided on two diagonally disposed legs 28 are used to attach lower end fitting 20 to the lower core plate to prevent movement of end fitting 20 and fuel assembly 10 during reactor operations. Debris filter 32, seen separately in FIG. 5 and attached to lower end fitting 20 in FIG. 2 and 3, is formed from a stamped plate in the preferred embodiment for ease of manufacturing and is approximately 0.1 inch thick. The plate is stamped so as to have guide tube openings 34 in a pattern matching guide tube bosses 26 on lower end fitting 20. Triangular shaped flow holes 36 are provided in a square pattern or cluster wherein the solid portions of the plate between each cluster are substantially aligned with the ends of fuel rods 12. This provides support to fuel rods 20 when necessary due to movement and allows maximum coolant flow through flow holes 36, grid assemblies 14, and around fuel rods 20. In the preferred embodiment, each square cluster measures 0.405 inch on each side with the distance from the base of each triangular flow hole 36 to its apex adjacent the center of the square cluster being 0.181 inch. This provides a total flow area through debris filter 32 of approximately 27 square inches. In the preferred embodiment, the portions of the plate or ligaments between flow holes 36 in each square cluster is approximately 0.030 inch in width so as to provide adequate support in preventing failure of debris filter 32 during operations. The ligaments of the plate between the flow holes in each cluster are diagonally oriented relative to the sides of the plate. The diagonal orientation of the filter ligaments is such that the unblocked flow area, when considering the projected area of the fuel rods and the filter flow holes, is maximized. This minimizes pressure drop. As best seen in FIG. 3, ribs 24 in lower end fitting 20 and the solid portions of debris filter 32 between flow holes 36 are arranged in identical configurations so that ribs 24 do not interfere with coolant flow after passing through debris filter 32. Debris filter 32 may be attached to lower end fitting 20 by any suitable means such as welding. An alternate embodiment of the invention is illustrated in FIG. 6-11. In the alternate embodiment, instead of having ribs 24 bearing guide tube bosses 26, lower end fitting 20 is formed from a plate provided with a plurality of flow holes 40. The solid portions of the plate between each cluster of flow holes 40 are substantially aligned with the ends of fuel rods 12. As seen in the bottom view of FIG. 7 and the partial side sectional view of FIG. 11, flow holes 40 are provided with a bottom chamfer as indicated by the letter A of approximately 20 degrees. Positioned around guide tube bosses 26 are semicircular flow holes 42 to provide the maximum flow area possible. The inner diameter of flow holes 40 is approximately 0.405 inch while the chamfered lower edge is approximately 0.495 inch in diameter. As seen in the bottom and side view of FIG. 8 and 10, legs 28 are provided with openings 30 as described above. Debris filter 132, seen in the top view of FIG. 6, is also formed from a stamped plate approximately 0.1 inch thick and is provided with a plurality of flow holes 44. Flow holes 44 are substantially pie-shaped and arranged in circular clusters of four and in semicircular clusters of two around guide tube openings 134 in a pattern matching that of flow holes 40 in lower end fitting 20 (illustrated in FIG. 8). The portions of the plate or ligaments between flow holes 44 in each circular cluster are approximately 0.030 inch in width. This results in each pie-shaped quadrant having a diameter of approximately 0.181 inch with the circular cluster having a diameter equal to that of flow holes 40. The ligaments of the plate between the flow holes in each cluster are diagonally oriented relative to the sides of the plate. The diagonal orientation of the filter ligaments is such that the unblocked flow area, when considering the projected area of the fuel rods and the filter flow holes, is maximized. This minimizes pressure drop. The total flow area provided by debris filter 132 is thus approximately 22 square inches. As seen in FIG. 10, debris filter 132 is attached to the top of lower end fitting 20 by any suitable means such as welding. FIG. 9 illustrates a top view of debris filter 132 as it appears attached to lower end fitting 20. Because many varying and differing embodiments may be made within the scope of the inventive concept taught herein and because many modifications may be made in the embodiment herein detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.