ULTRA-PURE ZIRCONIUM-TIN LINER MATERIAL FOR NUCLEAR REACTOR FUEL ELEMENTS

Abstract: Zirconium alloys for use in liners of reactor fuel element claddings in which the zirconium is alloyed with 0.1-0.4 weight percent tin. A very low metallic impurity level contributes to the reduction of solid solution strengthening, second phase formation, property variability from lot to lot and susceptibility to steam corrosion. Preferably, oxygen is controlled to a very low level as well, to provide a low, but fabricable, hardness in the alloyed liner material.

Description: This invention relates to zirconium alloys for use in liners of reactor fuel elements. In the commercial production of zirconium and hafnium metal, the ore is generally initially subjected to a chlorination step which produces a relatively impure, hafnium containing, zirconium tetrachloride and by-product silicon tetrachloride (which by-product is relatively easily separated). The zirconium and hafnium containing material is then subjected to a number of purifying operations and also a complex hafnium separation operation. These operations result in purified oxides of zirconium and hafnium which, of course, are thereafter maintained separate. The purified oxides are then separately chlorinated. Zirconium and hafnium are reduced from the chloride by means of a reducing metal (generally magnesium). Excess reducing metal and by-product salt, (e.g. magnesium and magnesium chloride) are removed from the so-called zirconium "sponge" by a distillation step. The zirconium metal is then generally double or triple vacuum arc melted to produce an ingot, which is then further processed (e.g. into Zircaloy tubing for reactor fuel element cladding). Ultrapure ("crystal-bar") zirconium has been proposed for a liner for the inside surface of Zircaloy tubing for use as a cladding for reactor fuel, as described in, for example, U.S. Patent Specification No. 4,372,871 (Armijo et al.). A similar use, but with moderate purity material, is proposed in U.S. Patent Specification No. 4,200,492 (Armijo et al.). Ultra-pure zirconium has been produced in iodide cells by the so-called "crystal bar" process, (a very expensive process which produces a very soft, 90 Brinell product) as discussed, for example, in U.S. Patent Specification No. 4,368,072 (Siddall). Material for lining cladding for reactor fuel elements by electron beam (EB) melting is disclosed in Japanese Patent Application No. 1979-144,789 (Kawakita et al.), published June 8, 1981. That application discloses utilizing electron beam melting as the final melting, in a quite small laboratory (rather than commercial EB furnace). Commercial reactors generally use either Zircaloy-2 or Zircaloy-4. The history of the development of Zircaloy-2 and Zircaloy-4 is summarized in: Kass, "The Development of the Zircaloys", ASTM Special Technical Publication No. 368 (1964), pages 3-27. Also of interest with respect to Zircaloy development are U.S. Patent Specification Nos. 2,772,964; 3,097,094; and 3,148,055. Zircaloy-2 is a zirconium alloy having about 1.2-1.7 weight percent (all percentages herein are weight percent) tin, 0.07-0.20 percent iron, about 0.05-0.15 percent chromium, and about 0.03-0.08 percent nickel. Zircaloy-4 generally contains about 1.2-1.7 percent tin, about 0.18-0.24 percent iron, and about 0.07-0.13 percent chromium. U.S. Patent Specification No. 4,675,153 discloses a zirconium alloy having generally somewhat less alloying agent content (typically 0.2-0.6 Sn, 0.03-0.11 Fe, less than 0.02 Cr, less than 350 ppm oxygen, with the balance essentially Zr) and U.S. Patent Specification No. 4,613,479 discloses an example of a niobium zirconium alloy with less than 59% Zr. EB (electron beam) melting of materials, including zirconium, has been discussed in a number of patents. EB melting has been used to consolidate crushed particles or chips in so-called hearth furnaces and to separate impurities by either overflowing floating inclusions (U.S. Patent Specification No. 4,190,404, Drs et al.) or to produce an electrode for arc melting (U.S. Patent Specification No. 4,108,644, Walberg et al.). A number of U.S. patents have used EB melting of powders or granules, often producing an ingot in a chilled mold. These powder melting EB patents include U.S. Patent Specification Nos. 2,942,098 (Smith); 2,960,331 (Hanks); 2,963,530 (Hanks et al.); 2,997,760 (Hanks et al.); 2,935,395 (Smith); and 4,482,376 (Tarasescu et al.). Electron beam zone refining using multiple passes is described in U.S. Patent Specification No. 3,615,345 (King). EB melting using a consumable feed "electrode" to produce an ingot collected in a chilled mold has also been disclosed in a number of patents, including U.S. Patent Specification Nos. 3,087,211 (Howe); 3,226,223 (Bussard et al.); 2,880,483 (Hanks et al.); and 4,130,416 (Zaboronok et al.). U.S. Patent Specification No. 3,219,435 (Gruber et al.) shows a commercial type EB furnace utilizing multiple beams. Typically the beams are directed to the surface of the molten pool and are continually swept across the pool surface to avoid overheating of any single portion of the pool surface. U.S. Patent Specification No. 3,091,525 (D'A. Hunt) describes adding a small amount of zirconium, for example, to hafnium, for example and melting in an EB furnace to deoxidize the hafnium. Accordingly, the present invention resides in a zirconium alloy for use in liners of reactor fuel elements characterized in that said alloy consists of zirconium alloyed with 0.1-0.4 weight percent tin, said alloy containing as impurities less than 400 ppm of oxygen, less than 100 ppm of iron, and less than 100 ppm total of other metallic and metalloid impurities. Apart from a significant reduction in metallic impurity content, the alloys of the present invention give rise to a more consistent product due to reduced solid solution strengthening and second phase formation. Liners made of these alloys exhibit better corrosion resistance than unalloyed zirconium, but generally have the ductility of the commonly used unalloyed liner material. In addition, these alloys being strengthened by tin, instead of oxygen, are believed to be more crack propagation resistant than unalloyed material of the same hardness, especially under irradiation. The alloys of the present invention can be produced by a process utilizing electron beam melting of sponge zirconium, at a very slow feed rate, to reduce metallic impurities (especially aluminum and iron). The electron beam melted zirconium is then alloyed in a vacuum arc furnace by means of an alloying charge. The alloys preferably contain less than 300 and most preferably less than 175 ppm of oxygen and preferably less than 50 ppm of iron. Thus, there are provided an ultra-pure alloys of tin and zirconium from which ductile, but reliably fabricatible liners can be made having somewhat better corrosion resistance than unalloyed zirconium. Such alloys are believed to give better crack propagation resistance under irradiation at a reliably fabricatible hardness than any other material. Zircaloy reactor fuel claddings lined with liners made from alloys of the present invention are similar to the lined cladding described in the aforementioned U.S. Patent Specification Nos. 4,372,817 and 4,200,492, except that the liner material is not unalloyed pure or ultra-pure zirconium. 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 too soft for reliable production, 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, and the only other thing removed in the double or triple vacuum melting is a very small amount of manganese). Thus the purity of 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 copending United States patent application Serial Nos. 871,182 and 871,183, both filed on June 5, 1986). This EB material has been significantly further purified by melting in an electron beam furnace at about 10,2-40,6 cm (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. Patent Specification No. 4,675,153, which alloy contains from 0.2-0.6 weight percent tin, from 0.03-0.11 percent iron, and up to about 350 ppm oxygen. 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 the copending United States patent application Serial No. 113,841, filed October 28, 1987, can give aluminum of less than 5 ppm (experimental runs produced zirconium containing less than 2 ppm of aluminum). In addition, that process 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. Silicon is reduced to less than 10 ppm, also reducing second phase formation. The aluminum reduction reduces solid solution strengthening. The reduced aluminum is combined with low oxygen content, as produced, for example, by the afore-mentioned copending United States patent application Serial No. 871,182 and/or United States Patent Application Serial No. 017,301, filed February 20, 1987 or by the ultra-slow EB melting or combinations thereof, such that the hardening produced by the alloying agents is generally compensated by the softening effect of the lesser oxygen and lesser aluminum. This provides a material which is soft and ductile enough to substantially impede crack propagation (generally less than 125 Brinell) while being hard enough for reliable fabrication (generally at least about 105 Brinell) and having corrosion properties somewhat superior to non-alloyed zirconium. The ultra-slow EB melting provides some oxygen removal (as well as generally removing aluminum, iron, chromium and other metallic impurities). The oxygen removal in a commercial EB furnace is very surprising as, although previously reported in a very small laboratory furnace, there had previously been no indication of any oxygen reduction in a commercial EB furnace. The so-called "pellet-cladding-interaction" has caused crack initiation on the inside surface of cladding, and the zirconium lined cladding of the prior art greatly reduces such cracking. The alloys of this invention give as good or better resistance to crack propagation combined with somewhat improved (as compared to unalloyed zirconium) resistance to steam corrosion. In particular, this material has a very low metallic impurity content (especially aluminum and iron), combined with very low oxygen content. Metalloids, such as phosphorous and silicon are also reduced. For example, this material can be made by reducing zirconium tetrachloride to metallic zirconium utilizing low oxygen magnesium (e.g., magnesium treated by the process described in the afore-mentioned copending United States patent application Serial No. 017,301) and, after distillation, prebaking the low oxygen sponge to remove absorbed water (generally the process of the afore-mentioned copending United States patent application Serial No. 871,182) and ultra-slow electron beam melting the material at less than 2,5 cm (1 inch) (and generally less than about 1,9 cm (3/4 inch) and preferably about 0,25-1,3 cm (1/10-1/2 inch) per hour and then double or triple vacuum arc melted (generally EB and vacuum arc melting as taught in copending application Serial No. 871,183, except that the EB melting is at a much slower rate and that an alloying charge is added to the vacuum arc melting electrode). The alloying charge added during vacuum arc melting contains tin to provide 0.1-0.4 weight percent of tin in the final product (excess may be required to allow for some loss of tin during melting). 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 material (as used herein, the term "essentially aluminum-free" means having less than 5 ppm aluminum), having less than 400 ppm oxygen. More preferably, the process is controlled to provide material containing less than 300 ppm oxygen (and most preferably less than 175 ppm). In addition, the material preferably contains less than 100 ppm (and most preferably less than 50 ppm) iron. The material also preferably contains less than 10 ppm chromium and most preferably less than about 5 ppm chromium and preferably less than 10 ppm silicon. Other than iron and oxygen, the material contains less than 100 ppm total of metallic and metalloid impurities. Thus, it can be seen that this invention provides zirconium alloys from which liners can be made (rather than liners of unalloyed zirconium) having an extremely low metallic impurity level (especially aluminum and iron) and a very low oxygen level and provides a consistent and lower corrosion (as compared to unalloyed zirconium) liner. The liner material generally has a Brinell hardness of about 105-125 (similar to unalloyed sponge having 500-600 ppm of oxygen), thus combining fabricability, crack arresting capability and corrosion resistance. It is believed that such material, being hardened to 105-125 Brinell by tin, rather than oxygen, is less subject to irradiation embrittlement and thus gives better crack propagation resistance under irradiation. At extremely low oxygen levels (less than about 175 ppm) somewhat higher tin levels (0.2-0.4%) are utilized to provide a hardness in the desired range. At somewhat higher (although still very low by prior standards) oxygen levels of 200-300 ppm, for example, somewhat lower tin is generally utilized to provide the desired hardness. Generally the higher tin (and thus the lower oxygen) is preferred to give improved corrosion performance.

Claims: A zirconium alloy for use in liners of reactor fuel elements, characterized in that said alloy consists of zirconium alloyed with 0.1-0.4 weight percent tin, said alloy containing as impurities less than 400 ppm of oxygen, less than 100 ppm of iron, and less than 100 ppm total of other metallic and metalloid impurities. An alloy according to claim 1, characterized in that said alloy contains less than 5 ppm aluminum. An alloy according to claim 1 or 2, characterized in that said alloy contains less than 10 ppm chromium. An alloy according to claim 1, 2 or 3, characterized in that said alloy contains less than 300 ppm oxygen. An alloy according to claim 4, characterized in that said alloy contains less than 175 ppm oxygen. An alloy according to any of the preceding claims, characterized in that said alloy has a Brinell hardness of 105-125. An alloy according to claim 6 when dependent on claim 4, characterized in that said alloy contains 0.1-0.3 weight percent tin. An alloy according to claim 6 when dependent on claim 5, characterized in that said alloy contains 0.2-0.4 weight percent tin. An alloy according to any of the preceding claims, characterized in that said alloy contains less than 50 ppm iron.

Inventors: WOODS, CHARLES ROBERT, WORCESTER, SAMUEL AUSTIN
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1988
Main classification: C22C16
IPC codes: C22C16, B32B15, G21C3