Patent Number: 048329053
Section: description

PREFERRED EMBODIMENTS OF THE INVENTION The numeral 10 generally designates a fuel assembly unit. The fuel assembly 10 includes an upper end fitting 12, guide tubes 14, spacer grids 16 supporting fuel rods 17, and a skirt portion 18 shown partially broken away in FIG. 1 to illustrate a lower end fitting debris collector constructed primarily of Inconel according to the principles of the invention, generally designated by the numeral 20. Behind the skirt 18 are debris trapping compartments defined by the lower end fitting debris collector grid 21 and solid fuel rod end caps 22. Each fuel rod end cap 22 is located at the intersection of first intersecting and slottedly interlocked grid forming bars 24, known as top bars because of their lower slots 27. Second grid forming bars 26, which are known as bottom bars because of their upper slots 27 for slottedly interlocking with the grid forming bars 24 along their upper margin. When assembled, the bars 24 in the area of intersection are above the bars 26. The pairs of first and second intersecting and slottedly interlocked grid forming bars 24 and 26 are attached to the perimeter strip 28. Members 24, 26 and 28 may be of Inconcel 718. The upper bars or thick strips 24 are parallel to each other and the lower bars or thick strips 26 are parallel to each other and normal to the upper bars 24. All of the bars 24 and 26 are welded or brazed to the rectangular perimeter strip 28. The grid 21 is completed by thin Inconel or other suitable metal strips 30 which are parallel to and mounted between the bars 24 and 26, typically, in groups of two to four parallel strips 30. The strips 30 mounted in slots 32 of bars 24 are normal to the strips 30 mounted in slots 32 of bars 26. As shown, the strips 30 mounted in slots 32 of bars 24 are in engagement with the strips 30 mounted in slots 32 of bars 26, but this is not necessary. All of the strips 30 are welded or brazed to the perimeter strip 28. It has been found advantageous in welding to lay a weld bead 34 along the length of the upper edges of bars 26 to hold the bars 24 and 30 of the top side of the grid 21 in assembled relation therewith and to lay a weld bead 36 along the length of the lower edges of bars 24 to hold the bars 26 and 30 of the bottom side of the grid 21 in assembled relation therewith, as shown in FIG. 3. A lower end fitting debris collector base member 40 is provided which is, typically, a casting of a suitable stainless steel alloy such as A.I.S.I. 304. The base member 40 is welded to bars 24 and 26. In the form illustrated, the base 40 has hollow legs 42 with fastener receiving interior cavities or bores 44 and fastener seats 46. The legs 42 are joined by web members 48 and each leg is welded or brazed into an opening cut into or formed in the grid 21. A flat upper surface 48 of each leg 42 is basically flush with the grid 21 and supports a guide tube 14. A fastener 50 with a slotted head seated against fastener seat 46 is threaded into the end of an internally threaded end of guide tube 14 to secure the guide tube 14 and lower end fitting debris collector 20. Typically, four guide tubes 14 are thus secured. In the central portion of grid 21, an insert 51 is provided instead of a leg 42 for providing means to fasten the central guide tube 14. Insert 51 is hollow and has a fastener seat 46a for retaining the head of a fastener 50. It is welded or brazed within an opening in grid 21. As seen in the embodiment illustrated by schematic FIG. 7, where modified grid parts have like numerals to those of the embodiment of FIGS. 1-6 but with the subscript "a", the intersections of bars 24a and 26a form contoured seats 60 for fuel rod end caps 22. Otherwise, the end caps merely sit on the intersecting strips 24 and 26. The contoured seats for fuel rod end caps 22 illustrated in FIG. 7, are formed by bevels 62 at the open ends of slots 27a of bars 26a. The bars 24a have notches 64 with the same taper as bevels 62 and at locations along the upper edges of bars 24a at the points of intersection with bars 26a. Striss 30 are not affected by seats 60. In all filtering concepts, it must be noted that filterings will not be 100%. Certain deleterious debris can still escape. The effectiveness of the filter is determined by the amount of debris it will filter. To determine filtering effectiveness, a series of tests of proof of principal wer conducted to measure other devices' filtering ability as compared to the instant invention. The filtering effectiveness of the novel modified lower end fitting of the invention proved to be better than most other devices with the advantage of not affecting fuel rod plenum volume, fuel rod expansion shoulder gap, fuel rod reconstitutability and fuel rod support. Therefore, while it is not possible to entirely eliminate fuel rod failure due to debris, the risk of failures can be greatly reduced by means of the instant invention, thereby increasing fuel assembly reliability without compromising design integrity in the abovementioned ways.