Patent Number: 047073306
Section: summary

Metallic structures that are present in nuclear reactors, such as boiling water reactors and pressurized water reactors must have specific properties. These structures must be formed from a material that is resistant to corrosion in an aqueous environment at elevated temperatures, have a low neutron absorption capacity, and high mechanical strength. In order to provide the requisite properties, the use of zirconium alloy components has been widely accepted. Such zirconium alloys, generally Zircaloy-2 and Zircaloy-4, are commercially available. Even with the use of these alloys, however, problems still exist. One problem associated with the use of zirconium alloys is their tendency to "creep" or slowly deform, under load, over a period of time. The problem of creeping of the zirconium alloy material exhibits itself in fuel tubes or cladding that is used to contain the nuclear fuel in a reactor. This problem can lead to the disappearance of the desired clearance between fuel pellets and the cladding in which they are contained, and pellet-clad interaction leading to the initiation of cracks in the cladding. Another problem associated with the use of zirconium alloys in nuclear reactor components is the tendency of articles to "bow" or bend when subjected to long exposure to reactor operating conditions. This bowing for example is a problem with the fuel rods, with rod guide thimbles which are used to contain control rods, and in boiling water reactors with the channels used to direct the coolant through the reactor core. Also, in many water reactors, the grids that are provided to stabilize and prevent vibration of the fuel rods during passage of coolant through the core, have spring-like projections which are subject to irradiation-induced stress relaxation which results in a decrease in spring force, which can cause abrasion or "fretting" of the fuel rod component. Numerous attempts have been made to enhance the properties of zirconium alloys for use in nuclear reactor components, either by specific mechanical or thermal treatment of the material or alteration of the material. U.S. Pat. No. 3,775,823, for example, teaches a method of producing dispersion-strengthened zirconium products containing fine particles of yttria, magnesia, cerium oxide or beryllium oxide in a matrix of zirconium or a zirconium alloy. The process described in that patent involves mixing of zirconium or a zirconium alloy in a hydrated, powdery state with powder of yttrium oxide, magnesium oxide, cerium oxide or beryllium oxide, and heating the mixture in vacuum for the purpose of expelling hydrogen and for simultaneous or subsequent sintering in a compressed condition, the heating effected at a temperature of below about 800.degree. C. Products prepared by that process, containing dispersed yttria, are described as having substantially higher flow stress and ultimate tensile strength as compared to prior art zirconium alloy articles containing dispersed yttria. In U.S. Pat. No. 3,782,924, a method is described for making zirconium with inhibited grain growth characteristics, which involves vacuum melting of the zirconium, adding 0.5 to 1.0 percent by weight carbon and stirring, homogenizing, and cooling the material for solidification. The carbon can be added as graphite or as zirconium carbide. The addition of silicon carbide whiskers to aluminum to form reinforced material, with the silicon carbide whiskers contained in a matrix of aluminum, has been reported and is believed to be close to commercialization. Such metal matrix composites have been described by Atlantic Richfield Co.'s Arco Metal Co. Div. at its Silar operation in Greer, S.C. It has been reported that other metals, including titanium, magnesium, and copper can also be reinforced. SUMMARY OF THE INVENTION Metallic components for use in nuclear reactors comprise a composite of 90-60 volume percent of a zirconium matrix which contains, homogeneously incorporated therein, 10-40 percent by volume of silicon carbide whiskers. The whiskers are preferably of a length of about 50-100 microns and have a diameter of about 0.5 microns. Nuclear reactor components, such as channels, fuel rod cladding, rod guide thimbles, and grids are formed from the composite material by conventional forming techniques and exhibit high elastic modulus and strength compared to conventional zirconium-based such components, or if only comparable such properties are desired, the components may be formed as thinner metallic components when compared to conventional zirconium-based such components.