Patent Number: 046844961
Section: summary

CROSS REFERENCE TO RELATED APPLICATION Reference is hereby made to the following co-pending U.S. patent applications dealing with subject matter related to the present invention: 1. "Fuel Assembly Bottom Nozzle with Integral Debris Trap" by John F. Wilson, U.S. Ser. No. 672,041, filed Nov. 16, 1984. 2. "Wire Mesh Debris Trap for a Fuel Assembly" by William Bryan, U.S. Ser. No. 675,511, filed Dec. 7, 1984. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors and, more particularly, is concerned with a trap mounted in the bottom nozzle of a fuel assembly for capturing and retaining debris left in the reactor after assembly, repair and/or replacement operations and thereby preventing entry and lodging thereof in the fuel assembly where the debris can cause cladding perforations in the fuel rods and other damage to the 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. In particular, fuel assembly damage due to debris trapped at the bottom grid has been noted in several reactors in recent years. The damage consists of fuel rod tube perforations caused by fretting of debris in contact with the exterior of the tube. The debris tends to be relatively thin rectangular pieces, as opposed to pieces which are spherical in shape. Specifically, most of the debris consists of metal turnings which were probably left in the primary system after steam generator repair or replacement. The debris lodges in the region of the lowermost grid within the spaces between its "egg-crate" shaped cell walls and the lower end portions of the fuel rod tubes. Almost all of the debris is deposited just above the four coolant flow openings in the lower core support plate. 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. While all of the approaches described in this patent 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 removal in nuclear reactors. The new approach must be compatiable with the existing structure and operation of the components of the reactor, be effective throughout the operating cycle 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 trap designed to satisfy the aforementioned needs. Underlying the present invention is the recognition of the mechanism by which debris is captured and retained in the bottom grid of the fuel assembly and application of that mechanism to the design of the trap structure. The entrapment mechanism by which small pieces of debris are trapped and retained by the lowermost grid centers around the interaction between the cell walls and compliant members (springs and dimples) of the grid and the fuel rod tube. The coolant flow seems to wedge the debris into the grid cell, and the compliance of the cell and debris holds it in place. The trap of the present invention seeks to simulate this mechanism but at a different location. Specifically, the trap is mounted within the bottom nozzle where coolant flow first enters the fuel assembly and employs a multiplicity of small cells with means for providing a lateral force on the debris to retain it in the cells. Thus, whenever coolant flow stops, the debris will not fall back into the system only to impact the trap structure again upon restart of coolant flow. By being mounted within the bottom nozzle, the trap structure and debris captured therein are removed from the reactor system with the fuel assembly. Also, the trap is designed to be backfittable on existing fuel assemblies. Accordingly, the present invention sets forth in a fuel assembly for a nuclear reactor including a plurality of nuclear fuel rods, at least one grid supporting the fuel rods in an organized array, an end nozzle disposed adjacent the grid and liquid coolant flowing through the end nozzle and into the fuel assembly, a trap for capturing and retaining debris carried by the flowing coolant to prevent entry of debris into the fuel assembly. The debris trap includes: (a) a structure disposed adjacent the end nozzle on an opposite side thereof from the grid, the structure forming a multiplicity of small cells each being open at opposite ends and defining a central channel for passage of coolant flow therethrough to the end nozzle; and (b) means defined in each of the cells for capturing and retaining within the structure any debris carried into the cells by the coolant flowing therethrough. More particularly, the debris trap structure includes interconnected wall portions forming each of the cells and defining the central channel thereof, with the capturing and retaining means being in the form of a spring-like finger attached to one of the wall portions and extending into the channel of the cell and downstream toward the end nozzle. Specifically, the structure is composed of a plurality of straps aligned with respect to each other in a crisscross interlocking arrangement and defining the wall portions of each cell in oppositely-disposed interconnected pairs thereof. One spring-like finger is punched out of each wall portion of each pair thereof, and the fingers on one pair of wall portions are disposed in the cell downstream of and in overlapping relation with the fingers on the other pair of wall portions. The structure is sized to fit within the end nozzle and includes means in the form of a pair of leaf springs for releasably locking it within the end nozzle. A plurality of tabs are mounted on the upstream end of the structure and project outwardly therefrom for grasping and holding pieces of debris generally stationary so as to prevent lateral circulation thereof along, and repeated impact thereof against, the debris trap structure. 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 wherein there is shown and described an illustrative embodiment of the invention.