Patent Number: 049903047
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like, are words of convenience and are not to be construed as limiting terms. In General Referring now to the drawings, and particularly to FIG. 1, there is shown an elevational view of a fuel assembly, represented in vertically or longitudinally foreshortened form and being generally designated by the numeral 10. The fuel assembly 10 basically includes a lower end structure or bottom nozzle 12 for supporting the assembly on the lower core plate (not shown) in the core region of a reactor (not shown), and a number of longitudinally extending guide tubes or thimbles 14 which project upwardly from the bottom nozzle 12. The assembly 10 further includes a plurality of transverse grids 16 axially spaced along the guide thimbles 14 and an organized array of elongated fuel rods 18 transversely spaced and supported by the grids 16. Also, the assembly 10 has an instrumentation tube 20 located in the center thereof and an upper end structure or top nozzle 22 attached to the upper ends of the guide thimbles 14. With such an arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conventionally handled without damaging the assembly parts. As mentioned above, the fuel rods 18 in the array thereof in the assembly 10 are held in spaced relationship with one another by the grids 16 spaced along the fuel assembly length. Each fuel rod 18 includes nuclear fuel pellets 24 and the opposite ends of the rod are closed by upper and lower end plugs 26,28 to hermetically seal the rod. Commonly, a plenum spring 30 is disposed between the upper end plug 26 and the pellets 24 to maintain the pellets in a tight, stacked relationship within the rod 18. The fuel pellets 24 composed of fissile material are responsible for creating the reactive power of the nuclear reactor. A liquid moderator/coolant such as water, or water containing boron, is pumped upwardly through the fuel assemblies of the core in order to extract heat generated therein for the production of useful work. To control the fission process, a number of control rods 32 are reciprocally movable in the guide thimbles 14 located at predetermined positions in the fuel assembly 10. Specifically, the top nozzle 22 includes a rod cluster control mechanism 34 having an internally threaded cylindrical member 36 with a plurality of radially extending flukes or arms 38. Each arm 38 is interconnected to a control rod 32 such that the control mechanism 34 is operable to move the control rods 32 vertically in the guide thimbles 14 to thereby control the fission process in the fuel assembly 10, all in a well-known manner. Turning also to FIG. 2, the instrumentation tube 20 is provided to enable actual flux measurements to be taken under reactor operating conditions. The instrumentation tube 20 is hollow and located in the center of the fuel assembly 10. Also, the instrumentation tube 20 is seated at its bottom end 20A in a recess 40 in the bottom nozzle 12 and located at its top end 20B within a recess in the top nozzle 22. The instrumentation tube 20 is open at its bottom end 20A through the adapter plate 12A of the bottom nozzle 12. The flux measurements are taken by a flux thimble tube 42. The thimble tube 42 is inserted through a coolant flow-limiting seal device 43 attached and extending between a nozzle 44 on the core support plate (not shown) and the bottom nozzle adapter plate 12A, and therefrom into the instrumentation tube 20 through its open bottom end 20A. The thimble tube 42 runs upwardly through most of the entire length of the instrumentation tube 20. Coolant flow passes upward through an annulus 45 formed between the exterior surface 42A of the flux thimble tube 42 and the interior surface 20C of the instrumentation tube 20. Coolant enters this annulus 45 from the underside of the bottom nozzle adapter plate 12A. The coolant exits through the top end of the instrumentation tube 20B and through a bleed orifice 46 formed in the adapter plate 22A of the top nozzle 22. Coolant flow-induced vibration of the thimble tube 42 occurs due to the presence of radial clearance and lack of mechanical connection between the instrumentation and thimble tubes 20, 42. Vibration of the flux thimble tube 42 results in wall degradation and eventual perforation. Vibration-Reducing Features of the Present Invention Referring now to FIGS. 3-7, the present invention provides vibration reducing features for the instrumentation tube 20 in the form of mechanical elements, for instance as seen in FIG. 3, dimples 48 formed in the tube 20, such as by being bulged from the tube wall, so as to protrude or project radially inwardly toward the axis of the tube. The dimples 48 engage and constrain the flux thimble tube 42 within the instrumentation tube 20 such that it maintains physical contact with the interior surface 20C of the instrumentation tube 20 at the dimples 48 thereon. The dimples 48 are formed at a plurality of points being staggered, such as at 0 degrees and 180 degrees, on a single-diametral plane and spaced substantially throughout the length of the instrumentation tube 20. The dimples 48, preferably staggered on a pitch which is compatible with the fuel assembly grid pitch, induce a controlled elastic sinuous deflection of the thimble tube 42, as seen in FIG. 3. The obstructive configuration of the mechanical elements, i.e., dimples 48, and the sinusoidal thimble tube 42 inhibit coolant flow, however, this is negligible. The mechanical elements of the present invention can take other suitable forms. For instance, in FIGS. 4-6, the mechanical elements are in the form of cantilevered spring fingers 50 (only one being shown) formed in the instrumentation tube 20, such as by being cut lengthwise from of the instrumentation tube wall to project radially inwardly therefrom similar to the dimples 48. Preferably, in FIG. 7, the spring fingers 50 are located at the elevation of the grids 16. Then, the grid sleeve 52 will cover the hole in the instrumentation tube 20 created by cutting of the spring fingers 50 and hence prevent the jetting of coolant onto adjacent fuel rods 18. FIGS. 8-10 are longitudinal elevational views, in an enlarged exaggerated form, of instrumentation tubes having vibration-reducing features of still other configurations in accordance with the present invention. In these three embodiments, it's the configuration of the instrumentation tube itself which provides the mechanical elements that engage the flux thimble tube to reduce the aforementioned vibration. In FIG. 8, instrumentation tube 20a has an undulating longitudinal configuration which defines the mechanical elements so as to reduce the vibration of the flux tube when inserted therein. In FIG. 9, instrumentation tube 20b has a spiral configuration which defines the mechanical elements. And in FIG. 10, instrumentation tube 20c has a zig-zag configuration which defines the mechanical elements. The vibration-reducing features on the instrumentation tube 20 have the following advantages. First, vibration is minimized through the relatively small "beam" span with friction damping. Second, no new components are introduced; only existing parts are modified. Third, there is no impact on interfacing of fuel assembly components. Fourth., there is negligible effect on annular cooling flow rate. Fifth, there is no impact on skeleton design and fabrication. Sixth, there are no highly stressed components as a result of the features. Seventh, design parameters are not as critical for dimples 48 as they are for spring fingers 50, so the dimples are preferred. Eighth, the necessity to use the flow-limiting device 43 may be eliminated. Finally, the features are simple to manufacture and performance reliability is 100%. It is thought that the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.