Patent Number: 051475987
Section: summary

CROSS REFERENCE TO RELATED APPLICATION Reference is hereby made to the following copending patent application dealing with related subject matter and assigned to the assignee of the present invention: 1. "Fuel Assembly Containing Fuel Rods Having Standardized-Length Burnable Absorber Integral With Fuel Pellets And Method Of Customizing Fuel Assembly" by Barry F. Cooney, U.S. Ser. No. 07/270,560, filed Nov. 14, 1988. Abandoned on Aug. 2, 1990. 2. "Nuclear Fuel With Helium Release-Reducing Burnable Absorber Coating" by Charles A. Bly, U.S. Ser. No. 345,859, filed May 1, 1989. Abandoned on Aug. 20, 1991. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors and, more particularly, is concerned with a nuclear reactor core having nuclear fuel and composite burnable absorber arranged for power peaking and moderator temperature coefficient control. 2. Description of the Prior Art In a typical nuclear reactor, such as a pressurized water reactor (PWR), the reactor core includes a large number of fuel assemblies each of which is composed of a plurality of elongated fuel elements or rods. The fuel rods each contain fissile material in the form of a stack of nuclear fuel pellets The fuel rods are grouped together in an array which is organized 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, such as water, is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work. In the operation of a PWR it is desirable to prolong the life of the reactor core as long as feasible to better utilize the uranium fuel and thereby reduce fuel costs. To attain this objective, it is common practice to provide an excess of reactivity initially in the reactor core and, at the same time, maintain the reactivity relatively constant over its lifetime. In a PWR, initial excess reactivity is controlled primarily by use of soluble boron in the coolant water and power peaking is controlled primarily by use of burnable absorber. For long cycles, the control of initial excess reactivity by soluble boron alone would require high boron concentrations in water, which would lead to positive moderator coefficient Therefore, in addition to power peaking control, burnable absorber is used to hold down some of the excess reactivity, so that the soluble boron concentration is appropriate to maintain the moderator temperature coefficient within the technical specifications. In one prior art approach, a burnable absorber is mixed directly with the fissionable material of the fuel pellets and integrated therewith to enable the use of an excessive amount of fuel in the reactor core during the initial life of the fuel. In another prior art approach, a burnable absorber coating is applied to the surface of fuel pellets. For example, in U.S. Pat. No. 3,427,222 to Biancheria et al, assigned to the assignee of the present invention, the fuel pellets have a fusion-bonded coating on the surface of each pellet Each fuel pellet is a cylindrical body composed of sintered particles of fissionable material, such as enriched uranium oxide, and an outer coating of predetermined thickness containing a burnable absorber or poison material, such as boron, cadmium, gadolinium, samarium, and europium Examples of boron-containing compounds used are boron carbide, boron nitride and zirconium boride or zirconium diboride. The burnable absorber coating approach has been successfully applied in an integral fuel burnable absorber (IFBA) rod, manufactured and marketed by the assignee of the present invention and used in a PWR fuel assembly known commercially as the VANTAGE 5. Up to the present, the same burnable absorber, such as zirconium diboride employed in IFBA rods, has been used for controlling both power peaking and moderator temperature coefficient. For long cycles, with high initial excess reactivity, a number of IFBA rods are used for power peaking control and oftentimes additional IFBA rods are needed for moderator temperature coefficient control. The latter is done indirectly by reducing the concentration of boron in water (used to surpress excess core reactivity) by providing for increased absorption through burnable absorber rods. This situation leads to the use of a large number of IFBA rods and a higher residual penalty. Consequently, a need exists for a different approach to controlling both power peaking and moderator temperature coefficient than by use of a large number of IFBA rods in the nuclear reactor core as has been the practice heretofore. SUMMARY OF THE INVENTION The present invention provides a nuclear reactor core having nuclear fuel rods and composite fuel and burnable absorber rods in an arrangement designed to satisfy the aforementioned needs. In accordance with the present invention, power peaking and moderator temperature coefficient are controlled by using two different absorber materials in the composite fuel and burnable absorber rods, one material tailored primarily for controlling power peaking and the other material tailored primarily for controlling moderator temperature coefficient. The result is a significant reduction in the number of composite fuel and burnable absorber rods, and reduction in the residual penalty without any loss in peaking factor or moderator temperature coefficient control. Boron in the zirconium diboride coated on the nuclear fuel is the preferred material for power peaking control in view of its well-known advantages of no moderator displacement and very low residual penalty. Erbium has nuclear absorption resonances around 0.5 ev, providing effective moderator temperature coefficient control through increased absorption in the resonances as moderator temperature rises leading to reduction in moderator density Erbium coated on or mixed in the nuclear fuel is the preferred material for moderator temperature coefficient control. It eliminates the need to use additional zirconium diboride or other burnable absorber material for moderator temperature coefficient control. The combination of zirconium diboride and erbium takes advantage of the strength of both of these absorbers. For power peaking control, erbium by itself would require its use in high concentrations, with the attendant residual poison penalty. Boron by itself controls moderator temperature coefficient indirectly by reducing the soluble boron concentration in the coolant water, thus requiring a large number of absorber rods. The combination of the two, on the other hand, uses each one for the control of the parameter that it is most effective for, i.e., Erbium controlling the moderator temperature coefficient directly and effectively through resonance absorption and zirconium diboride controlling power peaking utilizing high absorption in Boron. The combination is thus better than the sum of each or the use of each separately. Accordingly, the present invention is directed to a nuclear reactor core having a first group of fuel rods containing fissionable material and no burnable absorber, and a second group of fuel rods containing fissionable material and two burnable absorber materials. The groups of fuel rods are arranged in the core for controlling power peaking and moderator temperature coefficient. The number of fuel rods in the first group are greater than the number in the second group. More particularly, the two burnable absorber materials can be provided as separate coatings or a mixture. One burnable absorber material is an erbium-bearing material such as erbium oxide and the other is a boron-bearing material such as zirconium diboride. Alternatively, the erbium-bearing material can be interspersed or mixed with the fissionable material. These and other features and advantages 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.