Patent Number: 046997563
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 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 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. Control Rod with Axially Inhomogeneous Absorber Material Turning now to FIG. 2, there is shown the improved control rod of the present invention, generally designated 40, which is adapted to be used in EOL leading up to reactor shutdown. For example, at the fifteen year control rod changeover of some reactor cores, the control rods 32 used in earlier core cycles would be replaced by the control rod 40 of the present invention. The improved control rod 40 which employs axially inhomogeneous absorber material basically includes an elongated hollow tubular member 42 having upper and lower opposite ends 44,46 and a hermetically sealed chamber 48 defined within the tubular member between its opposite ends. The lower end 46 is the leading end, whereas the opposite upper end 44 is the trailing end of the member 42 upon insertion of the control rod 40 into the fuel assembly 10. Further, the control rod 40 includes a first neutron absorber material 50, preferably in the form of pellets of boron carbide, contained in the chamber 48 and located nearer to the upper trailing end 44 than to the lower leading end of the tubular member 42. Also, a second neutron absorber material 52, preferably in the form of pellets of silver-indium-cadmium, is contained in the chamber 48 and located nearer to the lower leading end 46 than to the upper trailing end 44 of the tubular member 42. The first neutron absorber material 50 (boron carbide) has a higher neutron absorption cross section absorbing capacity than that of the second neutron absorber material 52 (silver-indium-cadmium), specifically approximately twenty-five percent higher. Further, the second neutron absorber material 52 is greater in quantity than that of the first neutron absorber material 50. In particular, the quantities of the first and second neutron absorber materials 50,52 are represented by the respective lengths thereof. The length of the second material 52 is approximately three times longer than that of the first material 50. That is, the first absorber material 50 extends the upper approximately twenty-five percent of the combined length of the absorber material within the tubular member chamber 48, whereas the second absorber material 52 extends the lower approximately seventy-five percent of the combined length. Thus, the second neutron absorber material 52 has a length approximately three times longer than that of the first neutron absorber material 50. Like other control rods, the tubular member 42 of the improved control rod 40 is formed by an elongated, thin-walled metallic cladding or tube 54 having respective upper and lower end plugs 56,58 for sealing the opposite upper and lower ends 44,46 of the member 42. The upper end plug 56 has an upwardly extending integrally formed stem section with an externally threaded end 60 for connection to the control mechasism 34. The lower end plug 58 is cone-shaped. The absorber material pellets 52,50 which, as seen in FIG. 2, preferably have the same diameter, are slidably disposed within the chamber 48 and rest on the lower end plug 58 in a tandemly arranged stack. A plenum spring 62 is interposed between the upper end of the pellet stack and the upper end plug 56 to maintain an axial spaced relationship therebetween to define a space within the tubular member 42 for receiving gases generated by the pellets 50,52 as they absorb neutrons in the control reaction. Since the boron carbide absorber material is just in the upper twenty-five percent of the absorber material, there is essentially no concern with boron carbide swelling, since it never enters the core until the reactor is at low power or shutdown. Also, there will be no impact on rodded peaking factor at high power level either because (a) rods are not allowed to be inserted deeply, and (b) the same material, i.e., silver-indium-cadmium in this case, is not changed from what is used previously for a particular reactor. It is thought that the improved control rod of 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.