Patent Number: 046845046
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 adapted for use in non-control rod locations of a nuclear reactor core (not shown), represented in vertically foreshortened form and being generally designated by the numeral 10. Basically, the fuel assembly 10 includes a lower end structure or bottom nozzle 12 for supporting the assembly on the lower core plate (not shown) in the reactor core, and a number of longitudinally extending structural members 14 which at their lower ends are attached to and project upwardly from the bottom nozzle 12. The assembly 10 further includes a plurality of transverse grids 16 axially spaced along the structural members 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 structural members 14. With such 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 axially along the fuel assembly length. Each fuel rod 18 includes nuclear pellets 24 and is closed at its opposite ends by upper and lower end plugs 26,28. The fuel pellets composed of fissile material are responsible for creating the reactive power of the reactor. A liquid moderator-coolant such as water, or water containing boron, is pumped upwardly along the fuel rods 18 of the fuel assembly 10 in order to extract heat generated therein for the production of useful work. Pretensioned Longitudinal Structural Members Turning now to FIGS. 2 and 3, there is seen two slightly different embodiments of the longitudinal structural member 14 of the present invention either of which can be used in the fuel assembly 10. As indicated at the beginning, the fuel assembly 10 is designed to be used at non-control rod core locations so there are no control rods operatively associated with the assembly. In contradistinction to control rod guide thimbles in fuel assemblies at control rod core locations, which reciprocally receive control rods downwardly through their upper ends and also receive coolant flow upwardly through their lower ends, the longitudinal structural members 14 in the non-control rod fuel assembly 10 are sealed at their opposite ends. Each embodiment of the longitudinal structural member 14, being depicted in FIGS. 2 and 3, includes an elongated hollow cladding tube 30 closed at each end by upper and lower end plugs 32,34 which are welded to the tube. The tube and end plug material is preferably Zircaloy-4. The upper and lower end plugs 32,34 have respective threaded studs 38,40 fixed thereto and extending axially therefrom which are inserted through openings in the respective adapter plates 42,44 of the top and bottom nozzles 22,12. Nuts 46,48 are tightened down on the threaded studs 38,40 for rigidly attaching the opposite ends of the structural member 14 to the respective nozzles 22,12. Typically, the tubes 30 of the structural members 14 have a substantially greater thickness than the tubes of control rod guide thimbles and there are substantially fewer structural members 14 in each non-control rod fuel assembly 10 than guide thimbles in each control rod fuel assembly. The remaining locations in the non-control rod fuel assembly 10 which correspond to those occupied by guide thimbles in the control rod fuel assembly are occupied by fuel rods 18. For example, there are typically twenty-four guide thimbles in the control rod fuel assembly, whereas in the non-control rod fuel assembly 10 there are only eight structural members 14 and so the sixteen remaining locations are occupied by additional fuel rods. The improvement provided by the present invention in the structural members 14 herein over those disclosed in the above-referenced patent application relates to modifications made to the structural members 14 for making them resistant to bowing. Each structural member 14 is made resistant to bowing by preloading its cladding tube 30 in pretension. To preload the tube 30 in tension, the central portion thereof must be loaded in compression. The material loaded in compression must not be subject to thermal or irradiation-induced creep or it will creep to a permanently bowed condition. Thus, the tube 30 of the member 14 contains a thermal or irradiation-induced creep resistant material 50, preferably being in pellet form and stacked in the tube 30. Ceramic materials, such as zirc oxide or alumina are typical examples of materials which are very creep resistant. They can also be coated with burnable absorber material, such as boron carbide, so they also fulfill the same function as that of the structural members of the referenced application. For applying a compressive load to the ceramic pellets 50 and reacting the load in such a way as to load the tube 30 in pretension, either one of two embodiments of pretensioning means depicted in FIGS. 2 and 3 can be used. In FIG. 2, the pretensioning means is a bellows type device, generally designated 52, being positioned in the tube 30 in the upper plenum region of the structural member 14. The bellows device 52 is connected, such as being welded, to the upper end plug 32. The outside diameter of the bellows device 52 is radially supported by the cladding tube 30 and the bottom end 54 of the device 52 presses against the stack of pellets 50. A pressurization passage 56 in the side of the upper end plug 32 allows pressurization of the inside of the bellows device 52. After pressurization the passage 56 is sealed and an axial force then exists in the bellows device 52 which puts the pellet stack 50 in compression and the tube 30 in pretension. A bellows pressure of approximately 600 psi (cold) will provide an axial force of about 120 pounds during hot operating conditions. It should be noted that achievement of acceptable fuel assembly bow does not require zero axial stress so a lower pressure could be used. If desired, the inside of the member tube 30, in the pellet region, can be pressurized through a passage 58 in the lower end plug 34. It should be noted that if this is pressurized, it counteracts the pressure in the bellows so the bellows pressure must be increased accordingly. Alternatively, as seen in FIG. 3, the pretensioning means is in the form of an arrangement of belleville springs 60. The belleville springs 60 can be stacked in parallel, as shown, to achieve higher spring rates and in series, as shown, to obtain greater deflection range. Regardless of which embodiment of the pretensioning means is used, its function is to apply a predetermined compressive load to the creep resistant material 50 in the member 14 and react the load so as to preload the tube 30 in a state of pretension. The pretension should be of a magnitude sufficient to substantially counteract an axial load typically transmitted through the unitary structure of the fuel assembly 10 when installed in the reactor core and thereby greatly reduce the compressive stress in the tube 30 of the structural member 14. To some minimal extent, the coil spring as used heretofore applied a compressive load to the stack of absorber pellets. However, the magnitude of the force available from a coil spring which will fit inside the tube is far less than that required to produce any significant pretensioning of the tube. Thus, any pretensioning of the tube provided by the coil spring was incidental and inadequate. For example, a 17.times.17 fuel assembly has a hot BOL hold-down spring force of approximately 953 pounds. Since there are eight structural members 14, each member carries approximately 120 pounds. To obtain a stress free cladding tube during operation, a tension preload of 120 pounds per structural member 14 is required. This is much higher than can be achieved by a coil spring which will fit inside of the tube. In view of the present invention, when the core plate compressive forces are applied to the hold-down springs 62 of the fuel assembly 10, the axial compressive stresses on the cladding tubes 30 of the structural members 14 are greatly reduced. This is shown in FIG. 4 which is a load-stress diagram showing the structural member design of the cross-referenced application compared to the structural member design of the present invention. (All cladding and dimensional parameters are the same in both cases.) The diagram shows that the pretensioned tube of the structural member of the present invention has a much lower compressive stress for a given axial load than that of the prior non-pretensioned tube of the referenced application. 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.