Patent Number: 046648801
Section: summary

CROSS REFERENCE TO RELATED APPLICATION Reference is hereby made to the following co-pending United States patent applications dealing with subject matter related to the present invention: 1. "Debris Trap for a Pressurized Water Nuclear Reactor" by John F. Wilson et al, U.S. Ser. No. 672,040, filed Nov. 16, 1984 (W.E. 52,222). PA0 2. "Fuel Assembly Bottom Nozzle with Integral Debris Trap" by John F. Wilson et al, U.S. Ser. No. 672,041, filed Nov. 16, 1984 (W.E. 52,223). BACKGROUND 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 loading thereof in the fuel assembly where the debis can cause cladding perforations in the fuel rods and other damage to the fuel assembly. 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 out weigh the costs it adds to the reactor. SUMMARY OF THE INVENTION The present invention provides a debris trap designed to satisfy the aforementioned needs. The debris trap has a hollow wire mesh enclosure adapted to fit the present existing bottom nozzle of the fuel assembly without modification thereof. The trap is positioned between the adapter plate of the bottom nozzle and the upper surface of the lower core support plate such that entry openings of the trap which correspond in number and relative positions to the coolant flow openings in the lower core plate are aligned over the core plate openings. The entry openings of the trap are defined by angularly-extending wall sections which substantially allow only one-way movement of debris through the openings concurrently with the flow of coolant through the wire mesh enclosure of the trap. Thus, the debris is retained within the hollow trap which permits removal of the debris along with the trap and fuel assembly at the next refueling. 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, and 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 hollow enclosure disposed adjacent the end nozzle on an opposite side thereof from the grid, the enclosure being composed of a material which is permeable to liquid coolant but impermeable to debris carried by the coolant; (b) the enclosure having upper and lower walls spaced apart and interconnected at their peripheries so as to define a debris capturing and retaining chamber within the enclosure; and (c) means on the hollow enclosure defining at least one opening into the chamber of the enclosure through the lower wall, the opening defining means being disposed in a predetermined positional relationship with respect to the direction of coolant flow such that debris carried by the liquid coolant flow which enters the chamber of the enclosure through the opening will be substantially detered from exiting through the opening. More particularly, the opening defining means on the lower wall of the debris trap hollow enclosure includes a wall section which extends into the chamber at an angle to the direction of coolant flow. The wall section has an inner edge displaced inwardly from adjacent portions of the lower wall so as to define the opening to the chamber at a position located generally between the upper and lower walls of the chamber and in a plane extending generally parallel to the direction of coolant flow. Preferably, the trap includes a plurality of entry openings in the hollow debris trap enclosure which are matched in number and position to the coolant flow openings of the bottom core plate of the reactor so that the trap entry openings are located directly above the core plate openings when the trap is mounted in the bottom nozzle of the fuel assembly. The trap also includes a central annular sleeve mounted between the upper and lower wall of the trap enclosure for bolstering the structural integrity of the trap enclosure and providing access to the central instrumentation tube of the fuel assembly. Furthermore, the trap enclosure 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. 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.