Patent Number: 048329053
Section: summary

FIELD OF THE INVENTION This invention relates to nuclear fuel assemblies and in particular those assemblies which include spaced fuel rod support grids mounted in a reactor core as a unit. The fuel rods are held between an upper end fitting and a lower end fitting by means of spacer grids. Guide tubes provide the structural integrity between the lower end fitting, the upper end fitting and the spacer grids intermediate the ends of the fuel assembly The spacer grids define an array of fuel rods which, typically, may be rows and columns of 16 rods each. One such spacer and support grid is disclosed in U.S. Pat. No. 3,481,832. The reactor coolant flows upwardly through holes in the lower end fitting, along the fuel rod lower end caps and upwardly along the fuel rod cladding and through the spacer grids in the active region of the fuel assembly. BACKGROUND OF THE INVENTION Debris in the coolant which collects or is trapped in fuel rod spacer grids is believed responsible for as many as 30% of known fuel rod failures. Laboratory and in-reactor experience indicate that fuel rod cladding failures can be caused by debris trapped in a grid region which reacts against the fuel rod cladding in a vibratory fashion resulting in rapid wear of the cladding. The size and shape of the debris capable of causing severe damage is quite variable. In fact, metal fragments which can only be picked up with tweezers have been known to "drill" a hole in fuel rod cladding in less than 1,000 hours of reactor operation. Since most failures occur either within or below the first spacer grid, the grids apparently provide a rather good screen for collecting debris. In order to prevent damage in the active area of the reactor, applicants set out to design a device which will trap a large percentage of debris before it gets to the active area. Previous attempts to treat the debris problem have involved grids in the region of the lower end fitting and the lower rod support grid. One such attempt is the subject of U.S. patent application Ser. No. 020,816 entitled "Debris Catching Strainer Grid", filed Mar. 2, 1987 by Andrew J. Anthony and assigned to the assignee of the instant invention. This grid is typically welded to the upper side of the lower end fitting. Other examples of debris strainers, or traps and grids are seen in U.S. Pat. Nos.: Reissue 27,950; 4,684,495; 4,684,496; 4,652,425; 4,678,627; 4,427,624; 4,096,032; and Japanese Application No. 53-8277, 1-30-1978 (Kokai No. 54-102493, 8-11-1979); German Auslegeschrift No. 1,211,342 (Anmeldetag Feb. 2, 1960); and British Pat. No. 1,214,998. None of these attempts have been totally effective for the purpose intended for reasons of compromise between cost, pressure drop during coolant flow and debris retainability. Most debris related failures in nuclear reactor fuel are caused by metal shavings, turnings, and pieces of wire that remain in the primary coolant system after maintenance operations. Because of their noncompact shapes, shavings and turnings can be removed from the coolant by passing it through screens having comparatively large holes. More prior art debris interception schemes make use of this technique because it is readily incorporated into existing fuel designs by using a greater number of smaller holes in the lower end fitting. Debris in the form of wires, having comparatively small cross sections, are more difficult to trap in these devices. Even if these wires strike the debris trap broadside, they have a tendency of bouncing off letting them have another opportunity to penetrate the screen. In order to remove wires from the coolant, flow holes would have to be so small that the pressure drop through them would become unacceptably high, as would their production cost. SUMMARY OF THE INVENTION Since the mechanism for failure involves the entrapment of debris in spacer grids, or between fuel rods, an attached grid structure specifically designed to capture debris in a similar manner and located in the lower end fitting of the fuel assembly below the fuel rod cladding provides the filtering required. The filtering device is located below the bottom Inconel grid and fuel rods and is contained in the lower end fitting, which acts as a support member for the grid ligaments. The invention consists of an egg crate type grid of thick strips or bars which typically houses four or five thin strips between upper pairs of thick strips or as in the upper portion of the grid and four or five thin strips between lower pairs of thick strips or bars normal to the upper thick strips or bars in the lower portion of the grid to filter out debris. The bars or thick strips of the egg crate support the fuel rods and guide tubes while holding the thin filter strips in place. The invention is shown in the form of a 16.times.16 lower end fitting. The egg crate, typically, consisting of 0.0625 inch or 0.0937 inch bars, thick strips or ligaments is assembled first. It is made of either a low carbon containing stainless steel or Inconel. Thin strips are then inserted and assembled into the egg crate in slots in the bars, and a continuous weld is run along the top of the egg crate structure securing all of the strips for further processing. This may include further machining or brazing. The grid structure is then secured to a guide tube receiving lower end fitting post casting by welding. One embodiment of the invention has contours at the egg crate ligament or bar intersections, each of which follow the outline of a fuel rod end plug and therefore increase fuel rod support and integrity. By changing the fluid flow conditions around and through the screening device noted above, it is possible to eliminate some of the prior art shortcomings and to reduce production costs. This is accomplished by locating the thinner screening ligaments in two different planes. This causes the debris to be more easily trapped. Debris, entrained in the fluid, tends to rub against the thin ligaments of the lower plane, causing them to turn in such a fashion as to be trapped by the upper plane of thin ligaments normal to those of the lower plane. The further modifications to the novel lower end fitting bar intersections mentioned above allow the lower end caps of the fuel rods to rest within mated cutouts in the thicker lower end fitting ligaments or bars, thereby providing additional fuel rod shoulder gap and lateral support.