Patent Number: 047284870
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 subject application: 1. "Light Water Moderator Filled Rod For A Nuclear Reactor" by P. K. Doshi et al, assigned U.S. Ser. No. 654,709 and filed Sept. 26, 1984. 2. "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. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors and, more particularly, is concerned with a unique design concept for burnable absorber rods which provides rods of an overall standardized length while simulating conventional custom-designed rods which have a variety of reduced lengths. 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 transverse 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. Generally, this is done 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. Also, it is conventional practice to design an excessive amount of neutron flux into the reactor core at start-up so that as the flux is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. In view of this practice, in some reactor applications burnable absorber or poison rods are inserted within the guide thimbles of some fuel assemblies to assist the control rods in the guide thimbles of other fuel assemblies in maintaining the neutron flux or reactivity of the reactor core relatively constant over its lifetime. The burnable poison rods, like the control rods, contain neutron absorber material. They differ from the control rods mainly in that they are maintained in stationary positions within the guide thimbles during their period of use in the core. The overall advantages to be gained in using burnable poison rods at stationary positions in a nuclear reactor core are described in U.S. patents to Rose (U.S. Pat. No. 3,361,857) and to Wood (U.S. Pat. No. 3,510,398). U.S. Pat. Nos. 4,169,759 and 4,169,760 to Bevilacqua describe the use of a first group of full length control rods insertable into a nuclear reactor core for normal control of reactor power and of a second group of part length control rods insertable into the core for control of power oscillations. The part length control rod has first and second ends with a first neutron absorbing material at its first end, a second neutron absorbing material at its second end spaced from the first neutron absorbing material by a distance less than the length of the core, and a third intermediate portion connecting the first and second neutron absorbing material, the intermediate material being substantially non-neutron absorbing. The first neutron absorbing material is normally positioned outside of the reactor core where it has little or no effect on the neutron flux of the reactor core, while the second neutron absorbing material is normally positioned in the central region of the core for control of power oscillation. Upon the requirement for a rapid reactor shutdown, the part length control rod is scrammed or inserted into the core so that both first and second ends of the control rod are present in the core. The current trend in reactor core power distribution control is toward the use of burnable absorber rods having reduced lengths for the maximization of peaking factor margin. This approach involves the provision of custom-designed lengths and placements of the reduced length burnable absorber or poison rods. However, such approach presents some potentially significant manufacturing and handling difficulties. First, it precludes the manufacturing facility from being able to build rods from a standard inventory and can require a significant additional non-standard inventory (to cover manufacturing yield problems) that is unusable after completion of the reduced lenght rods for one particular reload region of the core. Second, handling of reduced length burnable absorber rod clusters when loading or unloading the reactor core entails considerable difficulty. Because of the design of the burnable absorber rod cluster handling tool, it is a very difficult task to be able to deal with clusters of a length different from that originally assumed in the design of the tool. It is doubly difficult because the cluster is sufficiently delicate that handling it improperly can result in damage, which could result in a delay in restarting the reactor. Consequently, a need exists for a different approach to burnable absorber rod design which will retain the flexibility of custom-designed reduced length burnable absorber rods in terms of maximizing reactor core power distribution control capability while, if not eliminating, at least reducing the above-described difficulties encountered in the manufacturing and handling of these custom-designed rods. SUMMARY OF THE INVENTION The present invention provides a standardized length burnable absorber rod designed to satisfy the aforementioned needs. The standardized rod is composed of three separate parts which, when assembled from standardized pre-manufactured parts, provides the reactor core power distribution control capability of the custom-designed reduced length burnable absorber rod that has been available heretofore. The standardized design concept of the present invention provides several advantages over the custom-designed or custom-fit concept. First, the absorber section, which is typically the most expensive part, can be provided as a small set of standard lengths, enabling the manufacturing facility to build from standardized inventory without sacrificing the peaking factor margin of the custom-fit design. Second, all burnable absorber rods will always be of the same overall length, eliminating any problems with handling. Third, providing the right kind of reduced length burnable absorber rod in an emergency situation becomes a matter of the assembly, rather than the manufacture, of the rod which drastically reduces emergency response time. Accordingly, the present invention sets forth for use in a fuel assembly for a nuclear reactor a standardized reduced length burnable absorber rod, comprising: (a) an upper end plug; (b) a lower end plug; (c) an elongated middle tubular section having opposite upper and lower ends and a chamber defined therein between the opposite ends; (d) a burnable absorber material disposed in the chamber of the middle tubular section; (e) an elongated upper tubular extension extending between and rigidly interconnecting the upper end plug and the upper end of the middle tubular extension; and (f) an elongated lower tubular extension extending between and rigidly interconnecting the lower end plug and the lower end of the middle tubular section. Still further, the standardized rod includes a pair of upper and lower end caps, the upper end cap being attached to the upper end of the middle tubular section so as to seal the same and the lower end cap being attached to the lower end of the middle tubular section so as to seal the same. More particularly, the upper tubular extension of the standardized rod has a hollow chamber formed therein which defines an upper space extending between the upper end plug and the upper end cap on the upper end of the middle tubular section. Similarly, the lower tubular extension of the standardized rod has a hollow chamber formed therein which defines a lower space of a predetermined axial length extending between the lower end plug and the lower end cap on the lower end of the middle tubular section. Each of the upper and lower tubular extensions and the middle tubular sections has one of a plurality of known different standard axial lengths being selected so that the upper and lower tubular extensions and middle tubular section when interconnected together will have a combined standard axial length which is the same from rod to rod. The present invention also relates to a cluster of standardized reduced length burnable absorber rods for use in a fuel assembly of a nuclear reactor, comprising: (a) a plurality of middle rod sections having a multiplicity of different axial lengths and containing burnable absorber material; (b) a plurality of upper end spacer sections having a multiplicity of different axial lengths; and (c) a plurality of lower end spacer sections having a multiplicity of different axial lengths; (d) each of the rods being formed of one middle rod section tandemly arranged between and rigidly interconnecting one upper end spacer section and one lower end spacer end section, the rigidly interconnected sections which form each rod being selected so as to provide the same standard combined axial length for each of the rods although the axial lengths of the middle rod sections can vary from rod to rod whereby the axial location of the burnable absorber material along the rod can also vary from rod to rod. Also, each of the middle rod sections has a sealed chamber defined therein for containing the burnable absorber material. Additionally, each of the rods has an upper end plug on the upper end spacer section and a lower end plug on the lower end spacer section. Each of the upper end spacer sections defines an upper empty space between the upper end plug and the middle rod section of the rod, while each of the lower end spacer sections defines a lower empty space between the lower end plug and the middle rod section of the rod.