Patent Number: 048141364
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention provides an process for producing liner material for zircaloy reactor fuel cladding similar to the lined cladding described in the aforementioned U.S. Pat. Nos. 4,372,817 and 4,200,492, except the liner material, rather than being pure or ultrapure zircoium, is, in the invention hereindescribed, produces a zirconium alloy. While so-called crystal bar material has been proposed for lining of fuel element cladding, such material is generally too expensive for commercial use, and so-called "sponge" zirconium has generally been used for such lined cladding. The sponge material is typically selected from the lower oxygen containing batches of normal zirconium production, and generally contains 500-600 ppm of oxygen. Such material is referred to as "sponge" or "selected sponge" as there is practically no purification of the metal after reduction (the distillation and vacuum melting are generally viewed as the separation of magnesium chloride by-product and excess magnesium left over from reduction, rather than as purification, although the double or triple vacuum melting also removes some small amount of manganese). Thus the purity the metal in the final product in such material is generally the same as the purity of the metal in the sponge configuration produced by reduction (the reduction product, although metallic, has a sponge-like appearance). Recently, so-called "EB" material has been used to line cladding (as noted in the aforementioned copending applications Ser. Nos. 871,182 and 871,183). This EB material has been significantly further purified by melting in an electron beam furnace at about 4-16 inches per hour, generally to reduce the iron content. Typically, the iron content of sponge is in the 500-800 ppm range, and EB melting in one or two passes at such speeds is utilized to lower the iron content to the 200-300 ppm range. In addition to crystal bar, sponge, and EB melted zirconium, it has also been proposed that zirconium alloyed with 0.5-1.5 percent tin be utilized for liner material. A zirconium alloy liner is also described in the aforementioned U.S. Pat. No. 4,675,153, which alloy contains about 0.2-0.6 weight percent tin, about 0.03-0.11 percent iron, and up to about 350 ppm oxygen. The process of this invention can be used to produce the improved alloy of that patent. In particular, typical sponge has an aluminum content of 40-50 ppm (the ASTM Spec B349-80, cited in that patent prescribes a 75 ppm maximum). The process of this invention will give aluminum of less than 5 ppm (our experimental runs produced zirconium containing less than 2 ppm of aluminum). In addition, this invention will reduce the chromium content from typically about 100 ppm (the aforementioned specification calls for 200 ppm chromium max) to less than 10 ppm chromium (typical measured numbers were about 5 ppm chromium). While chromium, unlike aluminum, is not generally considered detrimental in many alloys, reducing the chromium reduces lot-to-lot property variability due to second phase formation. The aluminum reduction reduces solid solution strengthening. Preferably, the reduced aluminum is combined with low oxygen content, as produced, for example, by the aforementioned copending application Ser. Nos. 871,182 and/or 017,301, such that the hardening produced by the alloying agents is at least partially compensated by the softening effect of the lesser oxygen and lesser aluminum. This can provide a material which is softer and more ductile than other zirconium alloys to substantially impede crack propagation. The so-called "pellet-cladding-interaction" has caused crack initiation on the inside surface of cladding, and while the zirconium lined cladding of the prior art greatly reduces such cracking, such a zirconium liner is susceptible to steam corrosion. The alloys of the process of this invention combine the resistance of crack propagation with resistance to steam corrosion. In particular, this process provides for very low metallic impurity content (especially aluminum), preferably combined with low oxygen content. In a preferred embodiment, zirconium tetrachloride is reduced to metallic zirconium utilizing low oxygen magnesium (e.g., magnesium treated by the process described in copending application Ser. No. 017,301) and, after distillation, the low oxygen sponge is prebaked to remove absorbed water (generally the process of the aforementioned copending application Ser. No. 871,182) and the material is electron beam melted and then double or triple vacuum arc melted (generally EB and vacuum arc melting as taught in copending application Ser. No. 871,183, except that an alloying charge is added to the vacuum arc melting electrode). The alloying charge added during vacuum arc melting contains 0.1-2.0 weight percent of alloying agent selected from tin or iron or a combination of the two, and may in addition, contain 0.02-1.0 weight percent of additional alloying element, the additional alloying element being selected from the group consisting of niobium, chromium, molybdenum, copper, and combinations thereof. Preferably the alloying elements are either tin and niobium or 0.1-0.6 tin and 0.03-0.30 iron. The ingot of vacuum arc melted zirconium alloy can then be fabricated into the liner of reactor fuel element cladding, providing an essentially aluminum-free (as used herein, the term "essentially aluminum-free" means having less than 5 ppm aluminum), and preferably less than 400 ppm oxygen, material. More preferably, the process is controlled to provide material containing less than 300 ppm oxygen. In addition, when iron is not used as an alloying agent, the material preferably contains less than 300 ppm iron (and most preferably less than 100 ppm iron). When chromium is not used as an alloying agent, the material also preferably contains less than 10 ppm chromium and most preferably less than about 5 ppm chromium. Other than iron and oxygen, the material preferably contains less than 100 ppm of impurities. Thus, it can be seen that the process of this invention produces an alloy liner (rather than a liner of unalloyed zirconium) having an extremely low metallic impurity level (especially aluminum and also preferably a low oxygen level) and provides a consistent and low corrosion liner. The invention is not to be construed as limited to the particular examples described herein as these are to be regarded as illustrative, rather than restrictive. The invention is intended to cover all processes which do not depart from the spirit and the scope of the invention.