Patent Number: 046997563
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following copending U.S. applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "A Nuclear Reactor Control Rod Having A Reduced Worth Tip" by John F. Wilson et al, assigned U.S. Ser. No. 633,774 and filed July 24, 1984 (W.E. 51,888). 2. "Light Water Moderator Rod For A Nuclear Reactor" by P. K. Doshi et al, assigned U.S. Ser. No. 654,709 and filed Sept. 26, 1984 (W.E. 52,140). 3. "Soluble Burnable Absorber Rod For A Nuclear Reactor" by P. K. Doshi et al, assigned U.S. Ser. No. 654,625 and filed Sept. 26, 1984 (W.E. 52,141). BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors, and more particularly, is concerned with an improved control rod for use with a nuclear fuel assembly in reactor shutdown. 2. Description of the Prior Art In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending between the nozzles and a plurality of tranverse grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work. Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. One common way of doing this is by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles. Representative of the prior art are the control rods and systems disclosed in U.S. Pats. Nos. to Busby et al (3,088,898), Hitchcock (3,230,147 and 3,255,086), Eich (3,485,717), French et al (3,519,535), Schabert et al (3,734,825), Radkowsky et al (4,123,328), Bevilacqua (4,169,759) and Anthony et al (4,172,762). One common arrangement utilizing control rods in association with a fuel assembly can be seen in U.S. Pat. No. 4,326,919 to Hill and assigned to the assignee of the present invention. This patent shows an array of control rods supported at their upper ends by a spider assembly, which in turn is connected to a control rod drive mechanism that vertically raises and lowers (referred to as a stepping action) the control rods into and out of the hollow guide thimbles of the fuel assembly. The typical construction of the control rod used in such an arrangement is in the form of an elongated metallic cladding tube having a neutron absorbing material disposed within the tube and with end plugs at opposite ends thereof for sealing the absorber material within the tube. Generally, the neutron absorbing material is one having a high neutron absorption cross section, such as boron carbide, tantalum, a combination of silver-indium-cadmium, or many others well known in the art. The material is ordinarily in the form of a stack of closely packed ceramic or metal pellets which only partially fill the tube, leaving a void space or axial gap between the top of the pellets and the upper end plug which defines a plenum chamber for receiving gasses generated during the control operation. A coil spring is disposed within this plenum chamber and held in a state of compression between the upper end plug and the top pellet so as to maintain the stack of pellets in their closely packed arrangement during stepping of the control rod. At end of cycle life (EOL) and hot zero power (HZP) core condition in reactors, an adverse power (flux) distribution shift to the top of the core commonly occurs. For instance, the axial flux imbalance (AFI) for a typical pressurized water reactor is on the order of 50 to 60%. With such extreme AFI, there is significant reactivity redistribution which nuclear designers have to account for in accident analysis via a reactivity penalty on available rod worth. The penalty, commonly known as reactivity redistribution factor (RRF), is approximately 0.85% delta p at EOL and accounted for in the shutdown margin calculation. The control rod designs used heretofore have failed to adequately alter this imbalance in the core axial power distribution at reactor shutdown. Consequently, a need exists for a control rod design which will conteract this situation so as to substantially reduce RRF during reactor shutdown. SUMMARY OF THE INVENTION The present invention provides a control rod employing axially inhomogeneous absorber material designed to satisfy the aforementioned need. Unlike the prior art control rods, the control rod of the present invention has stronger absorber material placed in the upper, approximately 25%, of the rod than in the lower three-quarters thereof. With this design concept, the heavier worth control rod absorber material will push the HZP, EOL flux shape to the more typical hot full power shape which is reasonably balanced at the middle portion of the reactor core. In such manner, the need for RRF penalty is eliminated. This design is used for all control rods in the core including shutdown banks. As mentioned earlier, the AFI at HZP and EOL with prior art homogeneous control rods is approximately 60%. With the heavier absorber material in the upper 25%, the AFI at the same conditions becomes 4% only. Accordingly, the present invention sets forth in a fuel assembly for a nuclear reactor including a plurality of guide thimbles and a plurality of nuclear fuel rods spaced apart from one another and from the guide thimbles and grouped together in an array organized to generate a neutron flux in the fuel assembly, an improved control rod for insertion into at least one of the guide thimbles for regulating the reactor neutron flux. The improved control rod is composed of: (a) an elongated hollow tubular member having opposite ends and a hermetically sealed chamber defined therein between its opposite ends, one of the member ends being a leading end and the other of the member ends being a trailing end upon insertion of the control rod into the fuel assembly; (b) a first plurality of neutron absorbe material pellets contained in the chamber and located nearer to the trailing than to the leading end of the member; and (c) a second plurality of neutron absorber material pellets contained in the chamber and located nearer to the leading than to the leading end of the member. More particularly, the first neutron absorber material, preferably being boron carbide, has a higher neutron absorption cross section than that of the second neutron absorber material, which preferably is silver-indium-cadmium. Also, the second neutron absorber material is greater in quantity than that of the first neutron absorber material. Particularly, the quantities of the first and second neutron absorber materials are represented by the respective lengths thereof. The length of the second material is approximately three times longer than that of the first material. 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.