Patent Number: 046844953
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS 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. 2. "Wire Mesh Debris Trap for a Fuel Assembly" by William Bryan, U.S. Ser. No. 679,511, 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 debris trap built into the bottom nozzle of a fuel assembly so as to combine the fuel rod retention and debris trap functions into a single component and thereby provide a lower overall pressure drop and a more compact bottom nozzle design. 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 an improved bottom nozzle integrally incorporating a debris trap designed to satisfy the aforementioned needs. Heretofore, the bottom nozzle of the fuel assembly had an upper adapter plate which performed two basic functions. First, the pre-existing bottom nozzle adapter plate served as an essential bottom structural component of the fuel assembly. The adapter plate incorporated sufficient solid framework to support and rigidly anchor the guide thimbles of the assembly thereon. Second, the adapter plate acted as a fuel rod retainer while directing the distribution of coolant flow to the rest of the fuel assembly. The adapter plate included additional solid framework sufficient to provide a platform which limited downward displacement of the fuel rods of the fuel assembly should the fuel rods become detached from the fuel assembly grids. The total overall solid framework of the pre-existing adapter plate, thus needed to enable the plate to carry out these two functions, made it a rather thick mass with small coolant flow holes and high pressure loss. To now add a backfittable debris trap to the pre-existing bottom nozzle would increase the pressure drop of the nozzle to an even more unacceptably high level. Underlying the present invention is the recognition that the pre-existing bottom nozzle adapter plate need not perform what has heretofore been its dual functional role: guide thimble support and fuel rod retention. Instead, in arriving at the present invention, it was perceived that by strategic positioning of an appropriate debris trap in the bottom nozzle, the trap, in addition to carrying out its debris capturing and retaining function, would also inherently perform the fuel rod retention function provided heretofore by the adapter plate. Thus, by eliminating the necessity for the adapter plate to carry out the fuel rod retention function, the adapter plate could then be modified to a simplier design, one which would decrease the overall complexity of its framework and thereby decrease the overall pressure drop across the nozzle by at least as much as the incorporation of the debris trap increased it. As it turned out, the improved bottom nozzle design of the present invention, in combining the fuel rod retention function with the debris capturing and retaining function of the trap, achieved a lower overall pressure drop and a more compact structure than the pre-existing bottom nozzle. It should be mentioned that the debris trap per se has much the same structure as that of the debris trap disclosed in the first U.S. patent application cross-referenced above. However, the improved bottom nozzle of the present invention is distinguished from the bottom nozzle of the first cross-referenced application by the combination of the debris trap and modified adapter plate as employed in the improved bottom nozzle disclosed herein. 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 at least one guide thimble supporting the grid, an improved end nozzle disposed adjacent the grid, supporting the guide thimble and adapted to allow flow of liquid coolant into the fuel assembly. The improved end nozzle includes: (a) means supporting the guide thimble and allowing flow of coolant into the fuel assembly; (b) means mounted about the supporting means so as to define therewith an open region above the supporting means; and (c) a trap disposed within the open region and adapted for passage of the guide thimble therethrough to the supporting means and the flow of coolant therethrough for capturing and retaining debris carried by the flowing coolant to prevent entry of debris into the fuel assembly. More particularly, the supporting means is a network of interconnected structural members and the debris trap includes a structure disposed above the network of members and within the open region on the same side of the network of members as the grid. The debris trap structure forms a multiplicity of small cells each being open at opposite ends and defining a central channel for passage of coolant flow therethrough to the fuel assembly. The structure also includes 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. Further included are 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 away from the network of members. More 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. Additionally, the means mounted about the network of members defining the open region is a housing formed by interconnected side walls having a plurality of inwardly-facing recessed shoulders formed therein. The structure of the debris trap is sized to fit within the open region of the interconnected side walls and includes means in the form of a plurality of leaf springs engagable within the recessed shoulders for releasably locking the structure within the open region above the network of members in the housing. The intersections formed by the crisscross arrangement of straps making up the structure underlie and are aligned with the respective fuel rods of the fuel assembly such that a platform is provided for catching any fuel rod which should happen to drop in the fuel assembly. The straps also incorporate at least one hub disposed in alignment with guide thimble attaching and anchoring means in the network of members and the guide thimble of the fuel assembly to permit passage of the thimble through the structure of the trap and into engagement with the attaching means such as a boss on the network of members. 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.