Patent Number: 047160179
Section: summary

FIELD OF THE INVENTION The present invention relates to nuclear reactor fuel assemblies, and more particularly to a method, apparatus and tool for providing a concentric, reduced inside diameter or otherwise restricting large diameters of structural tubes used in the fabrication of nuclear fuel assemblies. BACKGROUND OF THE INVENTION A typical nuclear reactor fuel assembly includes top and bottom support members having a multiplicity of fuel rods and control rod guide tubes supported therebetween. Each fuel rod and control rod guide tube is separately held against lateral displacement by grids, generally of an egg crate configuration, which are axially spaced along the fuel assembly length. Since the fuel rods and control guide tubes are usually made of Zircoloy and the grid assemblies used for supporting these components are usually made of Inconel, the incompatibility of the materials requires that the grids be held in position along the fuel assembly length by mechanical means, rather than brazing, welding or the like. In one well-known grid design short sleeves, which correspond to the number of control rod guide tubes in the fuel assembly, are brazed at appropriate points in grid assembly cells which are formed by interleaved grid straps. Each sleeve projects on the order of about two inches beyond the edge of a grid strap. During assembly of the fuel assembly, the grids are mounted in an axial predetermined position and after the control rod guide tubes are pulled through the grid sleeves, a bulging tool is moved into the control rod guide tube and stopped at a point just below a grid strap, but still inside the sleeve which extends through the grid cell. The tool is then expanded to cause projections on the tool to plastically deform the control rod guide tube and sleeve. The bulging tool is then moved to a point just above the grid, and the process of plastically deforming the material again repeated with the result being that the grid is mechanically locked and rigidly secured to the control rod guide tubes in the fuel assembly. Prior art bulge tools, such as that described in U.S. Pat. No. 4,182,152, comprise a cylindrical housing having axially extending tines formed by slots cut into the walls of the cylindrical housing. Projections are integrally formed in the outer surface of the tines near the end of the cylindrical housing. These projections are made to move radially outward under the influence of an internally operating ram to form bulges in a sleeve and guide tube while plastically deforming the material thereof. As the sleeve and guide tube material is deformed by the action of the ram riding on the complementary inner surfaces of the tines, the inner diameter of the tubes is held to a predetermined minimum by a coacting effect of other tines, located between the tines having projections, and the ram surfaces. In the above-described prior art fuel assemblies, the sleeve and guide tubes are deformed with dimple-like bulges circumfunctially spaced about 90.degree. apart to capture the guide tube with respect to the grid straps. The guide tubes are therefore subject to local stresses at the bulges. In addition, the prior art expansion tools require several operations to create all of the necessary bulges to securely capture the guide tube at the various axially spaced grid locations in the fuel assembly. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a device for centering an insert within a relatively large diameter structural tube. It is a further object of the present invention to provide a device for centering and capturing an insert within a relatively large diameter structural tube in a nuclear fuel assembly. It is a further object of the present invention to provide an effective reduced diameter inside of a relatively large diameter structural tube in a nuclear fuel assembly. It is a still further object of the present invention to provide a method and apparatus for automatically feeding and locating inserts for producing a reduced, concentric inside diameter in large diameter structural tubes. In accordance with a preferred embodiment of the invention, these and other objects are accomplished by providing a nuclear fuel assembly having a skeletal structure comprising a bottom nozzle assembly, a top nozzle assembly, at least one control rod guide tube extending between said top and bottom nozzle assemblies and at least one grid assembly, disposed in said fuel assembly between said top and bottom nozzle assemblies, the guide tube extending through and being captured with respect to said grid assembly. An instrumentation structural tube is provided which extends at least partially between the top and the bottom nozzle assemblies and through the grid assembly. A restricted inside diameter insert is coaxially disposed within the instrumentation structural tube at a location where the structural tube passes through the grid assembly. The restricted inside diameter insert is operable to provide a central opening having an engineered inside diameter and is further operable to lock the structural tube into the fuel assembly at the grid assembly location. In accordance with another embodiment of the invention, there is provided an insert for providing a reduced inside diameter for a structural tube having a nominal inside diameter. The insert is provided with one or more lobes for centering the insert within the structural tube inside diameter, and forming lobes, which are operable to expand and locally deform the structural tube inside diameter. Projections are provided on the reduced inside diameter of the insert which are operable to cooperate with an expansion tool to accurately position the expansion tool with respect to the forming lobes. In accordance with a further aspect of the present invention, there is provided an expansion tool which comprises a hollow, generally cylindrical housing. A plurality of tines are formed on an end portion of the housing. The end portion includes a head portion which has a forming surface for contacting an insert which is to be expanded. The tool has a shoulder which registers with a projection on the insert to accurately position the forming surface within the insert. A movable expander pin having a tapered section is adapted to be inserted into the cylindrical housing. The expander pin cooperates with the tines to radially expand the tines whereby the forming surface contacts and plastically deforms the insert. Finally, in accordance with another aspect of the present invention there is provided a method of securing structural tubes in grid assemblies having grid straps in a nuclear reactor fuel assembly. The method comprises the steps of loading a plurality of inserts having first and second projections on inside diameters thereof onto an expansion tool. The expansion tool is then inserted into the structural tube and a first of the inserts is aligned with a first grid assembly whereby first and second forming lobes, which are associated with the first and second projections respectively, straddle the grid straps. An expander pin is then inserted into the expansion tool to plastically deform the first expansion lobe against the structural tube to thereby plastically deform and mechanically lock together the insert and structural tube on one side of the grid strap. The expander pin is then withdrawn a distance sufficient to relax the expansion tool and the expansion tool is withdrawn within the structural tube until a shoulder thereof registers with the second projection of the first insert. The expander pin is then reinserted into the expansion tool to plastically deform the second expansion lobe against the structural tube thereby mechanically locking the insert and structural tube together on the other side of the grid strap. The expansion pin is then withdrawn a distance sufficient to relax the expansion tool and the expansion tool is withdrawn until the shoulder thereof registers with the first shoulder of a second insert. The second insert is then aligned with the second grid assembly and the steps are repeated until all the inserts have been expanded at the appropriate grid locations.