Patent Number: 047626763
Section: summary

CROSS REFERENCE TO RELATED APPLICATION Reference is hereby made to the following copending application dealing with related subject matter and assigned to the assignee of the present invention: "Integral Reusable Locking Arrangement For A Removable Top Nozzle Subassembly Of A Reconstitutable Nuclear Fuel Assembly" by Robert K. Gjertsen et al, assigned U.S. Ser. No. 857,675 and filed Apr. 30, 1985. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an adapter plate of a fuel assembly top nozzle which has a fuel rod capture grid with an adjustable flow feature for tailoring the top nozzle pressure drop to the specific reactor core location of the fuel assembly. 2. Description of the Prior Art In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the lower adapter plate of the top nozzle. It is conventional practice to design the adapter plate of the fuel assembly top nozzle to accommodate three requirements. First, the adapter plate must satisfy a structural criteria, that is, it must be capable of lifting the fuel assembly under a 6 g load. Second, it must serve a fuel rod capture function in which the fuel rods are mechanically restrained by the adapter plate from ejections upwardly from the core. Third, from a functional standpoint, the adapter plate must have sufficient open area to permit reactor coolant flow to pass through the top nozzle with minimum pressure drop. U.S. Pat. No. 4,427,624 proposes a composite nozzle for a fuel assembly adapted for installation on the upper or lower end thereof which is constructed from two components. The first component of the nozzle is a casting weldment or forging designed to carry handling loads, support fuel assembly weight and flow loads, and interface with structural members of both the fuel assembly and reactor. In short, the first component is designed to satisfy the structural criteria. The second component of the nozzle is a thin stamped bore machine flow plate adapted for removable attachment to the first component. The plate is designed to limit upward movement of the fuel rods and thus ejection thereof from the core. The plate also has multiple openings or orifices of varying size and configuration to help direct coolant flow in a predetermined path through the fuel assembly and to assure that a pressure drop of predetermined magnitude will take place across the assembly. In short, the second component is intended to fulfill the two requirements of rod capture and coolant flow with minimum pressure drop. Notwithstanding the overall acceptability of the above-described basic approach to end nozzle construction, the second flow plate component proposed in the aforesaid patent has been found incapable of achieving both functions. In order to provide sufficient coolant flow through it to attain the minimum pressure drop desired, the plate must be built with insufficient structure to perform the fuel rod capture function. Consequently, a need remains for an alternative design of a component which will satisfy the dual requirements of minimum pressure drop and fuel rod capture. SUMMARY OF THE INVENTION The present invention provides a top nozzle adapter plate construction designed to satisfy the aforementioned needs. The present invention provides an adapter plate which has separate components for carrying out the structural and functional features of the adapter plate. The functional requirements of fuel rod capture and coolant flow with low pressure drop are carried out by a grid of interleaved straps which form a large number of coolant flow channels and a large number of intersections for restraining fuel rod movement, and by means for adjusting the pressure drop to tailor it to the specific distribution desired across the fuel assembly. The pressure drop through the grid can be changed without affecting the structural design and integrity of the adapter plate which is established by another component thereof with which the grid is connected. The top nozzle adapter plate of the present invention is really important in reload operations for matching coolant flow output of different fuel assemblies. In each reload operation, approximately one-third of the fuel assemblies are changed. Thus, the reactor will ordinarily contain different fuel assemblies with different amounts of spent fuel. With the ability of changing the direction of flow through the top nozzle and thereby the pressure drop, one can match the outputs of two different fuel assemblies which are located adjacent to one another. Accordingly, the present invention is directed to a top nozzle adapter plate for use in a fuel assembly of a nuclear reactor. The fuel assembly has a plurality of elongated structural members and a multiplicity of fuel rods disposed in a predetermined array. The fuel rods are supported in a manner which permits the possibility of upward movement thereof from the fuel assembly when acted upon by hydraulic forces occurring in upward coolant flow through the fuel assembly in the reactor. The adapter plate comprises: (a) an upper structural component capable of rigid connection to the elongated structural members; and (b) a lower functional component connected to the upper structural component. The lower component includes a grid composed of a plurality of spaced and interleaved straps which are capable of restraining upward movement of the fuel rods from the fuel assembly while defining open channels through the grid which are capable of allowing passage of coolant flow therethrough. The lower component also includes coolant flow directing means being operable to establish a predetermined desired pressure drop across the top nozzle of the fuel assembly. More particularly, the interleaved straps of the grid cross one another to form intersections capable of alignment with individual fuel rods in the array thereof. Further, the coolant flow directing means can take either of two embodiments. First, it can be in the form of a plurality of tabs connected to predetermined ones of the grid straps and extending outwardly therefrom, with the tabs being adjustable into various desired positional relationships with respect to the grid channels for controlling coolant flow therethrough. Or, the coolant flow directing means can be in the form of a thin flat plate having holes of predetermined desired sizes and shapes formed therein, with the plate extending along the interleaved straps of the grid and its holes generally aligned with the open flow channels of the grid. Additionally, the upper structural component includes a plurality of spaced and interconnected hubs and ligaments arranged to define substantial open areas for coolant flow therethrough while providing a rigid framework capable of transmitting lifting loads imposed by the fuel assembly. The hubs are capable of connection to the elongated structural members of the fuel assembly. Also, the grid includes void areas through which the hubs of the upper component extend when the grid is connected to the upper component. Finally, the upper component includes a plurality of open flanges connected to and extending outwardly of the hubs, whereas the grid includes upstanding corner strips for attachment to the flanges of the upper component.