Patent Number: 053612825
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of manufacturing a fuel channel of a zirconium-based alloy for a BWR in accordance with the preferred embodiment of the invention comprises three steps performed in the following sequence: (1) high-temperature heat treatment (heating and quenching) of channel strip material to impart excellent corrosion resistance and to randomize the crystallographic texture; (2) warm forming the heat-treated channel strip into fuel channel components under conditions in which surface oxidation is minimized and then seam welding the components to form the fuel channel; and (3) thermal sizing of the fuel channel to eliminate manufacturing stresses and ensure accurate size control. The strip material is formed by any conventional channel strip fabrication technique, for example, forging, extrusion or rolling. Each strip has a width such that the strip can be bent to form one of two components conventionally welded together to form the desired fuel channel. The heat treatment of the strip is carried out utilizing an induction heating coil or any other conventional heating means, such as radiation, convection or resistance heating. Heating is preferably done in a vacuum. The fuel channel is heated to a temperature of 1800.degree. F. or greater (with a practical upper temperature limit of 2050.degree. F.) and held at a temperature equal to or greater than 1800of for a period in the range of 0.25 sec to 30 min. The purpose of this high-temperature heating is to induce a full transformation of the crystalline structure of the zirconium-based alloy of the channel strip material from the alpha phase (hexagonal close-packed) to the beta phase (body-centered cubic). After the strip material has been maintained at a temperature equal to or greater than 1800.degree. F. for a time duration equal to at least the minimum, i.e., 0.25 sec, the strip material is quenched with a fluid to a temperature of 1500.degree. F. or less at a rate in the range from 10.degree. F./sec to 400.degree. F./sec. The strip may be cooled by inert gas, air, water or other suitable quenching medium. The preferred quenching fluid is an inert gas such as helium, rather than water which is prone to contain contaminants such as dissolved O.sub.2. During rapid quenching, the crystallographic structure of the zirconium-based alloy is transformed from grains of beta phase to grains of alpha phase having a high-f.sub.L crystallographic texture. In particular, the structure of the zirconium-based alloy obtains a crystallographic orientation with a texture factor f.sub.L approximately equal to 1/3, i.e., f.sub.L =0.28-0.38. The rapid cooling rate (10.degree.-400.degree. F./sec) is required to yield a highly corrosion-resistant material. After heat treatment, the surface condition of the strip material is brought to a condition suitable for subsequent fabrication. In particular, any lubricants or any contaminants deposited from the air onto the surface are removed during subsequent processing steps so that no contamination will be baked into cracks, fissures and other microscopic mechanical damage to which the surface is prone. The strip material is then bent into the desired shape by any one of various conventional techniques, e.g., by the use of a bending brake, to form a channel component of substantially U-shaped cross section. Each component has a base wall and two mutually facing side walls. Each side wall includes a longitudinal edge surface. Prior to bending, the channel strip is heated to a temperature in the range of 50.degree. F. to 800.degree. F. (preferably 250.degree. F. to 400.degree. F.), under conditions where surface oxidation is minimized, for example, using temperature and time controls, either alone or in conjunction with an inert gas protective atmosphere. The temperature and time of exposure are minimized to reduce the risk of cracks, fissures and other microscopic mechanical damage to the Zircaloy surface. Warm forming (i.e., heating before bending) serves two purposes. The primary purpose is to improve ductility, thereby facilitating the bending operation. A secondary purpose is to lower residual stresses, thereby improving fabricability. However, in accordance with an alternative preferred embodiment of the invention, bending can be done without prior heating. After the warm forming operation, a pair of channel components are positioned so that their longitudinal edge surfaces abut along their entire length. Then the abutting edge surfaces are welded together, e.g., by tungsten inert gas welding or any other conventional channel welding technique, to form a hollow fuel channel having a substantially uniform rectangular cross section. Upon completion of the welding operation, any weld beads formed along the weld seam inside or outside the fuel channel are reduced to the extent desired by a conventional technique known as planishing. After the channel is assembled by welding, the fuel channel undergoes thermal sizing. In the thermal sizing operation, a conventional mandrel consisting of a material with a thermal coefficient of thermal expansion larger than that of the zirconium-based alloy is inserted into the fuel channel. The mandrel has geometric dimensions corresponding to the final dimensions of the fuel channel. The mandrel/fuel channel assembly is then heated to a temperature of 1000.degree. F. to 1250.degree. F., held in this temperature range for a time duration of 15 min to 10 hr and then cooled. After cooling, the mandrel is removed from the fuel channel. It is critical to avoid contamination of the Zircaloy surface and the formation of oxides and nitrides thereon during thermal sizing. Due to the high reactivity of Zircaloy with oxygen and nitrogen, the fuel channel is preferably protected from extensive oxidation during thermal sizing by an inert gas medium or by enclosing the channel/mandrel assembly in a vacuum system. The resulting fuel channel has a fully annealed (no internal stresses) high-f.sub.L crystallographic structure and precise dimensions. The fuel channel is thereafter subjected to a combination of conventional chemical and mechanical surface conditioning steps as required to meet design and quality requirements. In particular, chemical etching and grit blasting are used to remove oxides from the fuel channel surface. Zircaloy BWR fuel channels manufactured according to the above-prescribed processing limits will exhibit an excellent corrosion resistance and a high-f.sub.L crystallographic texture that provides in-reactor dimensional stability. The method is especially suited for the manufacture of fuel channels made from Zircaloy for a BWR. However, the method could also be used to make fuel rod spacers for use in a fuel channel or any other component made from zirconium-based alloy. Moreover, the product by process of the present invention is not limited to components made from zirconium-based alloy. On the contrary, the invention encompasses any component made from a metal which can be heated to a body-centered cubic (beta) phase, then quenched to a hexagonal close-packed (alpha) phase having high-f.sub.L crystallographic texture and subsequently annealed. Further, benefit is derived from heat treatment of the channel strip material and thermal sizing of the assembled channel regardless of whether warm forming or cold forming is used to form the channel components. These and other variations and modifications of the disclosed preferred embodiment will be readily apparent to practitioners skilled in the art of fuel channel manufacture. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.