Patent Number: 047073306
Section: description

DETAILED DISCLOSURE The present invention relates to zirconium metal matrix-silicon carbide compositions and to nuclear reactor components formed from such compositions. Nuclear reactor components comprise, among others, fuel rod cladding, rod guide thimbles, grids and channels, used in nuclear fuel assemblies. Such assemblies are illustrated in FIGS. 1 to 3. In FIG. 1, a fuel assembly 1 for a boiling water reactor comprises a generally square shaped flow channel 3 having a bottom inlet nozzle 5 and upper outlet section 7, usually having a handle 9 for placement in a reactor. Contained within the channel 3 are a plurality of fuel rods 11 and interspersed rod guide thimbles 13, positioned in arrays about a cruciform shaped divider 15. The fuel rods 11 comprise hollow sealed tubular rods that contain the nuclear fuel, and the rod guide thimbles comprise tubular members sized to accept control rods for use in controlling the nuclear reaction as desired. The array of fuel rods 11 and rod guide thimbles 13 are stabilized in a spaced relationship to each other by means of grids 17, which are in the form of thin metal strips interwoven in an egg-crate configuration, the interwoven strips having openings in which separate fuel rods and guide thimbles are located. A useful type of grid structure is illustrated in U.S. Pat. No. 3,920,515, assigned to the assignee of the present invention, the contents of which are incorporated by reference herein. In the type of grid structure illustrated in U.S. Pat. No. 3,920,515, and in FIGS. 2 and 3 of the present application, the grids 17 have springs 19 and dimples 21 on the grid straps 23 which project into each opening to properly support and cool the fuel rods near the grid-fuel rod interface. During operation of the reactor, liquid coolant flows upwardly around the fuel rods and through the openings in the grids and discharge at the upper end of the fuel assembly either in a partially vaporized state in boiling water reactors, or in essentially all-liquid state in pressurized water reactors. The nuclear reactor components of the present invention are produced from a composite that is formed by homogeneous mixing silicon carbide (SiC) whiskers in a zirconium metal matrix and forming the components from said composite. The metal matrix of the composite can comprise zirconium metal or an alloy or zirconium, containing less than about 5 percent by weight of alloying elements, usable in nuclear reactors. Such zirconium alloys contain additional elements which increase the mechanical properties of zirconium metal or the corrosion resistance of zirconium metal. The elements that are used in the formation of such alloys include niobium, oxygen, tin, iron, chromium, nickel, molybdenum, copper, vanadium and the like. Especially useful alloys are those known as Zircaloy-2 and Zircaloy-4. Zircaloy-2 contains, by weight, about 1.2-1.7 percent tin, 0.07-0.20 percent iron, 0.05-0.15 percent chromium, and about 0.03 to 0.08 percent nickel, the balance being zirconium, while Zircaloy-4 contains, by weight about 1.2-1.7 percent tin, 0.12 to 0.18 percent iron, and 0.05 to 0.15 percent chromium, the balance being zirconium. According to the present invention, the matrix of zirconium metal or alloy has silicon carbide whiskers homogeneously distributed throughout the matrix. Such silicon carbide whiskers are known and commercially available. The silicon carbide whiskers are present in the zirconium metal or alloy in an amount of about 10 to 40 volume percent of the composite. Less than about 10 percent by volume of silicon carbide whiskers would probably not be enough to give the structural benefits that are desired, while an amount in excess of about 40 percent by volume would be too high an amount of silicon carbide whiskers and would result in a material that could not be sufficiently thermally or mechanically worked, such as by extrusion or pilgering to produce the nuclear reactor components in the desired shapes and sizes. The silicon carbide whiskers are of a size of about 0.5 microns in diameter and of a length of between about 50 to 100 microns in length, which will enable intimate mixing of the silicon carbide whiskers and powdered zirconium or zirconium alloys and production of a matrix of zirconium or a zirconium alloy with the silicon carbide whiskers homogeneously distributed therethrough. An example of usable whiskers, which are commercially available, are silicon carbide whiskers sold about the trademark SILAR, by Arco Metals Company. In formation of the composite, a zirconium metal or alloy is provided in powdered form, as a finely divided particulate material less than about 325 mesh, U.S. Standard Sieve. To the powdered zirconium metal or alloy, the silicon carbide whiskers of a size previously defined are added and the two components mixed together to give a homogeneous mixture. The mixture is then formed into billets such as by vacuum hot-pressing, or other conventional processing. The hot-pressing is effected on the homogeneous powder mixture to essentially one hundred percent of theoretical density. Conventional vacuum hot-pressing techniques to achieve such a density would be used. Typically, billets of a size of between 500 to 2000 pounds would be formed, which composite billets can then be processed to form the nuclear reactor components. The fabrication of the nuclear reactor components from the present composite material would generally use the conventional fabrication steps for formation of such components from conventional zirconium metal or alloys. In the formation of fuel cladding, the tubular shells used to contain the nuclear fuel, or rod guide thimbles, the billet of composite material would be forged and/or extruded to a rod shape. A central axial bore would then be drilled in the rod and the resultant tube shell extruded and then pilgered to the final desired size. In the fabrication of channels or grid strip material, the billet of composite material would be forged and then rolled to the desired thickness. Since the fabrication steps from the composite billets would be substantially identical to those used in the formation of tubular cladding, thimbles, channels or grid strips used when forming such components from conventional zirconium metal or alloy starting materials, conventional existing metal working and fabricating equipment could be used. It is believed that the present composites of zirconium or zirconium alloy matrix with homogeneously distributed silicon carbide whiskers therein would have an elastic modulus and strength approximately double existing components without such silicon carbide whisker inclusion. Such improved properties would vastly enhance reliability of the nuclear reactor components. Alternatively, components fabricated from the compsites of the present invention, with properties equal to existing components, could be formed having a decreased thickness which would result in a fuel cycle cost savings. The ability to use thinner structural components in boiling water reactors, such as in channels and grids would also increase the flow through the core and reduce the pressure drop across the core which would enable better efficiency and use of smaller reactor unit size. Under certain conditions, it may be desirable to provide a thin layer of zirconium or zirconium alloy (without the silicon carbide whiskers present in said layer) on either one or both surfaces of the zirconium or zirconium alloy composite for enhanced corrosion resistance. The zirconium alloys of such a layer would be selected from those alloys described for use in the composite. Generally, the thickness of such a layer would be about 15 percent or less of the thickness of the composite.