Patent Number: 050248065
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following copending patent applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "Debris Trap For A Pressurized Water Nuclear Reactor" by John F. Wilson et al, assigned U.S. Ser. No. 672,040 and filed Nov. 16, 1984. 2. "Fuel Assembly Bottom Nozzle With Integral Debris Trap" by John F. Wilson et al, assigned U.S. Ser. No. 672,041 and filed Nov. 16, 1984. 3. "Wire Mesh Debris Trap For A Fuel Assembly" by William Bryan, assigned U.S. Ser. No. 679,511 and filed Dec. 7, 1984. 4. "Debris-Retaining Trap For A Fuel Assembly" by John A. Rylatt, assigned U.S. Ser. No. 720,109 and filed Apr. 4, 1985. 5. "Bottom Grid Mounted Debris Trap For A Fuel Assembly" by Harry M. Ferrari et al, assigned U.S. Ser. No. 763,737 and filed Aug. 8, 1985. 6. "Nuclear Fuel Assembly Debris Filter Bottom Nozzle" by John M. Shallenberger et al, assigned U.S. Ser. No. 211,150 and filed June 22, 1988, a continuation of U.S. Ser. No. 046,219, filed May 5, 1987, and now abandoned. 7. "Nuclear Fuel Assembly Debris Resistant Bottom Nozzle" by Edmund E. DeMario, assigned U.S. Ser. No. 104,748 and filed Oct. 5, 1987. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors and, more particularly, is concerned with an enhanced debris filter bottom nozzle for a nuclear fuel assembly. 2. Description of the Prior Art During manufacture and subsequent installation and repair of components comprising a nuclear reactor coolant circulation system, diligent effort is made to help assure removal of all debris from the reactor vessel and its associated systems which circulate coolant therethrough under various operating conditions. Although elaborate procedures are carried out to help assure debris removal, experience shows that in spite of the safeguards used to effect such removal, some chips and metal particles still remain hidden in the systems. Most of the debris consists of metal turnings which were probably left in the primary system after steam generator repair or replacement. In particular, fuel assembly damage due to debris trapped at the lowermost grid has been noted in several reactors in recent years. Debris enters through the fuel assembly bottom nozzle flow holes from the coolant flow openings in the lower core support plate when the plant is started up. The debris tends to become lodged in the lowermost support grid of the fuel assembly within the spaces between the "egg-crate" shaped cell walls of the grid and the lower end portions of the fuel rod tubes. The damage consists of fuel rod tube perforations caused by fretting of debris in contact with the exterior of the tube. Debris also becomes entangled in the nozzle plate holes and the flowing coolant causes the debris to gyrate which tends to cut through the cladding of the fuel rods. Several different approaches have been proposed and tried for carrying out removal of debris from nuclear reactors. Many of these approaches are discussed in U.S. Pat. No. 4,096,032 to Mayers et al. Others are illustrated and described in the U.S. patent applications cross-referenced above. While all of the approaches described in the cited patent and patent applications operate reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, a need still exists for a fresh approach to the problem of debris filtering in nuclear reactors. The new approach must be compatible with the existing structure and operation of the components of the reactor, and at least provide overall benefits which outweigh the costs it adds to the reactor. SUMMARY OF THE INVENTION The present invention provides an enhanced debris filter bottom nozzle in a fuel assembly designed to satisfy the aforementioned needs. The bottom nozzle of the present invention has increased strength and provides for finer filtration by incorporating a consolidated joined array of tubular and solid right cylindrical sections in place of a flat plate full of holes. A major drawback to increased filtration and strength of a plate-type debris filter is the practical limit on reducing flow hole diameter and spacing, while increasing plate thickness and thus hole depth. The enhanced debris filter bottom nozzle of the present invention overcomes these limits by replacing the flat plate and its holes with an array of tubes and bars of appropriate inside and outside diameters, length, and pattern, enclosed and joined to form the debris filter bottom nozzle. Accordingly, the present invention is directed to a debris filter bottom nozzle useful in a fuel assembly for a nuclear reactor wherein the fuel assembly includes a plurality of nuclear fuel rods, at least one grid supporting the fuel rods in an organized array, and at least one guide thimble supporting the grid. The debris filter bottom nozzle is spaced below the grid, supports the guide thimble, and is adapted to allow flow of liquid coolant into the fuel assembly. The debris filter bottom nozzle comprises: (a) an enclosure defining a coolant flow chamber therethrough; and (b) a consolidated array of elongated cylindrical sections disposed across the chamber of the enclosure in side-by-side relation to one another, extending axially in the direction of coolant flow through the chamber, and rigidly connected together and to the enclosure. At least some of the cylindrical sections have tubular cross-sectional configurations defining passages for the coolant flow through the chamber. Some other of the cylindrical sections are solid in cross-sectional configuration. The diameters of the tubular cylindrical sections are substantially equal or, alternatively, are of at least two different sizes. The cylindrical sections are shorter in axial length than the height of the enclosure. The enclosure of the bottom nozzle includes a plurality of interconnected upstanding side walls which define the coolant flow chamber therethrough. The enclosure also includes a plurality of legs connected to the side walls for supporting the bottom nozzle in the reactor. Further, the interconnected side walls define a plurality of corners in the enclosure and each of the support legs is disposed at one of the corners of the enclosure. These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.