Patent Number: 048287911
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. 046,219 and filed May 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 a debris resistant bottom nozzle in 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. 4,096,032 to Mayers et al. Others are illustrated and described in the first five 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 a debris resistant bottom nozzle in a fuel assembly designed to satisfy the aforementioned needs. The bottom nozzle of the present invention includes a substantially solid flat plate having spaced cut-out regions aligned directly above inlet holes of the lower core plate and open criss-cross structures fixed to the plate and extending across the regions. The criss-cross structures define individual openings small enough in cross-sectional size to filter out debris of damaging-inducing size which otherwise collects primarily in the sections of the fuel assembly between the bottom nozzle and the lowermost grid and in the unoccupied spaces of the lowermost grid and causes fuel rod fretting failures. The criss-cross structures also have structural portions which support the lower ends of guide thimbles aligned with these regions of the bottom nozzle plate. Accordingly, the present invention is directed to a debris resistant bottom nozzle useful in a fuel assembly for a nuclear reactor wherein the fuel assembly includes a plurality of nuclear fuel rods, at least a lowermost grid supporting the fuel rods in an organized array and having unoccupied spaces defined therein allowing flow of liquid coolant therethrough. The debris resistant bottom nozzle is disposed adjacent to and below the grid and below lower ends of the fuel rods. The bottom nozzle comprises: (a) support means adapted to rest on the lower core plate of the nuclear reactor; (b) a plate fixed on the support means and being of a substantially solid configuration with a plurality of spaced cut-out regions therein adapted to align directly above inlet holes in the lower core plate; and (c) a plurality of open separate criss-cross structures, each of the criss-cross structures fixed to the plate and extending across one of the cut-out regions therein. The criss-cross structures define individual openings small enough in cross-sectional size to filter out debris of damage-inducing size which otherwise collects in unoccupied spaces of a lowermost grid of the fuel assembly, but large enough in size to let pass debris of nondamage-inducing size which otherwise passes through the unoccupied spaces of the lowermost grid. More particularly, each of said cut-out regions is approximately of the same size as each of the inlet holes in the lower core plate. Also, at least one guide thimble lower end supporting structure is provided, extending into the cut-out region and fixed to the plate and cross-cross structure. In one embodiment, the cross-cross structure is in the form of a plurality of interleaved straps forming an open grid structure. In another embodiment, the cross-cross structure is in the form of a plurality of interconnected crossed wires forming an open mesh structure. Optionally, at least one pressure drop reducing flow hole can be defined in the plate at a location spaced from the cut-out regions. These and other advantages and attainments 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 .