Patent Number: 046876316
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: 1. "Nuclear Reactor" by Harry M. Ferrari et al, assigned U.S. Ser. No. 732,200 and filed May 9, 1985. 2. "Bow Resistant Structural Member For Fuel Assemblies In Non-Control Rod Locations Of A Nuclear Reactor Core" by John F. Wilson et al, assigned U.S. Ser. No. 748,855 and filed June 26, 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 reusable fastener devices mounted on a fuel assembly top nozzle for releasably attaching structural members thereto in a manner which simplifies removal and replacement of the top nozzle during reconstitution of the fuel assembly. 2. Description of the Prior Art The cores of nuclear reactors conventionally include a plurality of fuel assemblies. In a typical pressurized water nuclear reactor (PWR), all fuel assemblies are geometrically alike. Each fuel assembly includes 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 of the fuel assembly are secured to opposite ends of the guide thimbles which extend above and below the opposite ends of the fuel rods. The guide thimbles together with the top and bottom nozzles rigidly attached thereto compose the structural skeleton of the fuel assembly. To control the fission process created by nuclear fuel contained in the fuel rods, typically a number of control rods are reciprocally positioned for movement in the guide thimbles of the fuel assembly. However, not all of the fuel assembly locations of a reactor core use control rods. Only about one-third of the fuel assemblies are in control rod locations. But since heretofore all PWR fuel assemblies have been constructed to be alike geometrically, this means that the fuel assemblies for control rod locations have been the same as those for non-control rod locations. A departure from this prior practice of constructing all PWR fuel assemblies alike has been proposed recently. As described and illustrated in the first patent application cross-referenced above, a separate fuel assembly design for non-control rod locations includes a bottom nozzle, a number of longitudinally extending structural members which contain a burnable poison, and a top nozzle. It also includes a number of grids which are axially spaced and attached to the longitudinal structural members and support an array of fuel rods. The top and bottom nozzles are attached to the longitudinal structural members, and an instrumentation tube is located in the center of the assembly and supported by the top and bottom nozzles and by the grids. One important difference in this non-control rod fuel assembly over the conventional control rod fuel assembly lies in the design of the longitudinal structural members which interconnects the top and bottom nozzles to form the structural skeleton of the assembly. In the conventional PWR assembly, the structural members are the hollow guide thimble tubes which are open at the top and closed at the bottom (except for small holes for coolant flow). These tubes are positioned within the fuel assembly to align with the control rods. During reactor operation, the control rods move reciprocally in the tubes. On the other hand, in the non-control rod fuel assembly intended for use in non-control rod core locations, the structural member also in the form of tubes do not receive control rods. Therefore, different functional as well as structural use can be made of the tubes. Functionally, this non-control rod structural member contains burnable absorber material. Burnable absorbers, such as a suitable compound of boron, are used in modern reactors to provide an additional means for controlling reactivity especially at the beginning of life of the nuclear fuel. Structurally, the elongated tube of the structural member is closed at each end by end plugs which are welded to the tube. The tube and end plug material is preferably Zircaloy-4. A spring holds the absorber material in place in the tube and provides a plenum for accumulation of helium gas which is released when a neutron interacts with a boron atom. To assemble the non-control rod structural members into the fuel assembly, the tubes must be empty and open at one end. After the grids are bulge fitted to the tubes, the absorber material and springs are loaded into the tubes and the remaining one end plugs welded in place. The fuel rods are then loaded and the top and bottom nozzles are connected to the structural members. The top end plug of each structural member has a threaded end extension which extends upwardly through one of a plurality of holes in the top nozzle adapter plate. The top nozzle is secured to the structural members, being preferably eight in number, by eight nuts which engage the end plug extension threads. This design is satisfactory from a conceptual design standpoint; however, during reconstitution of the fuel assembly, wherein the top nozzle is removed from and then replaced on the structural member, the nuts become loose parts. From a practical standpoint, this is not desirable. Consequently, a need exists for an alternative approach to attaching the top nozzle on the structural members which will eliminate the problem of loose parts and thereby facilitate reconstitution of the non-control rod fuel assembly described above. SUMMARY OF THE INVENTION The present invention provides a top nozzle mounted reusable fastener device designed to satisfy the aforementioned needs. The fastener device of the present invention makes it possible to reconstitute the fuel assembly from its top end without loose parts. The device includes a retainer mounted on the top nozzle adapter plate which captures an attachment nut even when it is unfastened from the structural member upper end. The nut is thereby rendered reusable and the mechanics of removing and replacing the top nozzle greatly simplified. While the fastener device of the present invention is adapted for use in the non-control rod fuel assemblies of both cross-referenced applications, it is not so limited to such use only. It can also be used in other fuel assembly applications. Accordingly, the present invention sets forth in a fuel assembly including a top nozzle with an adapter plate having at least one hole and at least one longitudinal structural member with a threaded upper end portion extendible through the adapter plate hole so as to project therefrom, a reusable fastener device which comprises: (a) attaching means threadable onto and from the upper end portion of the structural member between fastened and unfastened positions therewith for attaching and detaching the structural member to and from the top nozzle adapter plate; and (b) retaining means mounted on the top nozzle adapter plate so as to extend about the adapter plate hole and project outwardly from the adapter plate, the retaining means capturing the attaching means when in both the fastened and unfastened positions thereof and maintaining the attaching means with the top nozzle upon removal from the upper end portion of the structural member. More particularly, the attaching means is in the form of a nut which includes a central tubular stem having upper and lower opposite ends and being internally threaded for mating with the threaded upper end portion of the structural member. Also, the nut includes upper and lower flanges connected to and extending radially outwardly from the respective upper and lower ends of the central stem. The upper flange has a conical-shaped lower surface and a periphery adapted for engagement in order to rotate the nut for threading onto and unthreading from the structural member upper end portion between fastened and unfastened positions. The lower flange is in the form of a plurality of radial segments extending outwardly from the central stem and angularly spaced from one another so as to defined a plurality of cutouts therebetween which alternate with the segements. Still further, the retaining means is in the form of a housing which includes a tubular hollow body having upper and lower opposite ends, a plurality of upper sectors connected to and extending radially inwardly from the upper end of the tubular body and together defining a central opening, and a plurality of lower tabs connected to and extending radially outwardly from the lower end of the tubular body, the tabs resting upon and rigidly connected to the top nozzle adapter plate so as to generally align the central opening of the housing with the hole through the adapter plate. The upper sectors are angularly spaced from one another so as to define a plurality of openings sized to receive the radial segments of the lower flange of the nut therethrough. Also, the sectors extend radially inwardly from the tubular body so as to define the central opening sized to receive the central stem of the nut and to define an interrupted conical-shaped upper surface surrounding the central opening which matches the conical-shaped lower surface on the upper flange of the nut. The tubular body of the housing is attached by the lower tabs on the top nozzle adapter plate so as to dispose the upper sectors in a location spaced from the adapter plate at which their interrupted conical-shaped upper surface will be contacted by the conical-shaped lower surface of the upper flange of nut when the central stem thereof is threaded onto the upper end portion of the structural member to the fastened position. Additionally, the upper sectors are yieldably deflectible upon being contacted by the upper flange of the nut such that inner edges of the upper sectors engage the central stem of the nut so as to rotationally lock the lower flange of the nut to the adapter plate. Furthermore, the retaining housing also includes means in the form of a plurality of auxiliary tabs attached to the upper sectors and being movable between open and closed positions for respectively allowing and preventing passage of the lower segments of the lower flange of the nut through the openings defined by the upper sectors of the retaining housing. The auxiliary tabs, being bendable, are connected to and extend outwardly from the upper sectors. The tabs extend adjacent to the openings between the upper sectors when disposed in the open positions and extend in overlying relation to the openings when disposed in the closed positions. 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.