Combined electron beam and vacuum arc melting for barrier tube shell material

Description

[0001] This invention relates to the production of purified zirconium.

[0002] The conventional process for making zirconium metal utilizes a fluidized bed process in which the ore is 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 hafnium and zirconium 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 maintained separate. The purified oxides are separately chlorinated. Zirconium and hafnium are commonly reduced from the chlo­ride by means of a reducing metal, typically magnesium. At the present time, the commercial processes are batched-type processes. U.S. Patent Specification No. 3,966,460, for example, describes a process of introducing zirconium tetrachloride vapor onto molten magnesium, with the zirco­nium being reduced and traveling down through the magnesium layer to the bottom of the reactor and forming a metallic sponge. The metalic sponge (containing remaining chloride and some remaining excess reducing metal) is then placed in a distillation vessel for removal of the remaining salt and reducing metal by high temperature vacuum distillation. The sponge material is generally crushed, screened and pressed into electrodes for vacuum arc melting. Particu­larly, the material is multiple (typical double or triple) vacuum arc melted to provide ingots which are then further fabricated into various shapes. Most of the zirconium currently is used to produce Zircaloy.

[0003] Commercial nuclear reactors generally have used Zircaloy tubes as cladding material to contain the uranium dioxide fuel. Generally a Zircaloy ingot is processed into a so-called "trex" and pilgering operations are used to reduce the trex inside diameter and wall thickness to size. Ultra-pure zirconium has been proposed for a liner for the inside surface of Zircaloy tubing which is used as a cladding for nuclear fuel and is described in, for example, U.S. Patent Specification No. 4,372,817 (Armijo et al.). A similar use of moderate purity material is proposed in U.S. Patent Specification No. 4,200,492 (Armijo et al.). The ultra-pure zirconium material described has been purified by iodide cells to produce so called "crystal bar" materi­al. This rather expensive crystal bar processing is performed after reduction and is described, for example, in U.S. Patent Specification No. 4,368,072 (Siddal).

[0004] EB (electron beam) melting of materials, includ­ing zirconium has been discussed in a number of U.S. patent specifications. 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. patent specifications have described the EB melting of powders or granules, often producing an ingot in a chilled mold. These powder melting EB patents include U.S. Patent Specification No. 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).

[0005] EB melting using a consumable feed "electrode" to produce an ingot collected in a chilled mold has also been discussed in a number of U.S. patent specifications, including U.S. Patent Specifications 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. Japanese application 1979-144789 Kawakita, published as patent publication 1981-67788 describes the use of a very small ingot with a high power density and ultra slow melting to produce a deep molten pool to produce a high purity ingot directly usable for lining of Zircaloy tubing for nuclear reactor applications. Such laboratory sized apparatus with its high powered consumption and very low throughput is, of course, not practical for commercial production.

[0006] Accordingly, a process for producing zirconium in purified form comprises reducing zirconium tetrachloride to produce a sponge of metallic zirconium which is distilled to generally remove residual magnesium and magnesium chloride, and melting the distilled sponge to produce an ingot, characterized by forming said distilled sponge into a consumable electrode; melting said consumable electrode in a multiple swept beam electron furnace with a feed rate of from 1 to 20 inches per hour to form an intermediate ingot; and vacuum arc melting said intermediate ingot to produce a final ingot.

[0007] This is a process for making very pure and very homogeneous material for use in the lining of the interior of zirconium alloy fuel element cladding. Generally this process provides material much purer than the so called sponge material and almost as pure as the crystal bar material, at a fraction of the cost of crystal bar materi­al. Generally purified zirconium produced according to the present invention has oxygen in the 250-450 ppm range (and preferably less than about 350) and iron in the 50-300 ppm range. Total impurities are generally in the 500-1000 ppm range (total impurities for these purposes generally comprise the elements listed in the afore-mentioned U.S. Patent Specification No. 4,200,492).

[0008] Preferably the energy input via the electron beams is maintained to a moderate level such that the molten pool on the upper portion of the intermediate ingot has a depth of less than about one fourth of the ingot diameter, thus lowering power costs. Preferably an argon sweep is provided in the electron beam furnace during melting. Multiple passes may be made both through the EB furnace and the vacuum arc furnace.

[0009] The distilled zirconium sponge is formed into a consumable electrode for use in a production EB furnace. A production furnace is generally shown in the afore-mentioned U.S. Patent Specification No. 3,219,435, but with the multiple beams being constantly swept across the surface of the molten pool (as defined herein, a production EB furnace has an output "intermediate" ingot having a diameter greater than five inches, and generally greater than six inches. Generally, this consumable electrode for EB melting is formed by pressing crushed virgin sponge (not recycle scrap). The compact and an appropriate end fitting are welded to form the consumable electrode.

[0010] The consumable EB electrode is melted in a production electron beam furnace with a feed rate of from 1 to 20 inches per hour. It has been found that small amounts of residual magnesium chloride remain in the electrode and absorb some moisture. Melting at faster than 20 inches per hour results in this moisture reacting to oxidized zirconium and thus causing an unacceptably high oxygen level in the product. Conversely too slow a melting rate, while possibly removing some oxygen from the molten pool (as described in the afore-mentioned Japanese patent publication 1981-67788) is uneconomical. It should be noted that significant oxygen removal from the molten pool takes considerable superheating of the molten pool and much slower melting rates and thus this invention provides for no significant oxygen removal from the molten pool. It has also been found that the iron impurity level is generally reduced by about a factor of two, each pass through the EB furnace (that is, when the intermediate ingot formed during the first EB melting pass is used as the consumable elec­trode for a second EB melting, the iron level is reduced by another factor of approximately 2). It has also been found that the level of other common impurities, for example aluminum and chromium, are also reduced by each pass through the EB furnace. It should also be noted that, as the residual magnesium chloride is generally removed during the first EB melting, there is minimal absorbed moisture on the second pass and thus somewhat faster speeds may be used after the first EB pass.

[0011] Generally an argon sweep is provided in the electron beam furnace during melting. It is felt that this helps remove moisture which has been vaporized off the electron from the furnace, minimizing contamination of the output intermediate ingot. Preferably the argon sweep is at a flow of 10,000-1,000,000 liters per second, with the liters measured at a pressure of 10⁻⁵ Torr (rather than at standard conditions). The argon sweep can be established, for example, with pumps capable of handling 60,000 liters per second and with a pressure of 10⁻⁵ Torr measured with no argon flow, by controlling argon introduction to a rate to raise the pressure to approximately 10⁻⁴ Torr.

[0012] It should be noted that the sponge used to form the consumable electrode is generally virgin material (as opposed to recycled scrap or turnings) and preferably is selected high quality material and generally selected for low oxygen content.

[0013] Generally, after EB melting, the material is arc melted (and preferably double arc melted or even triple arc melted) to homogenize the impurity distribution. It has been found that in production EB furnaces, with their relatively shallow molten pool (the molten pool being shallow both in comparison to arc melting, where the molten pool is typically about twice the ingot diameter and in comparison to non-multiple swept beam, laboratory type furnaces where the fixed single beam covers essentially the entire surface of the molten pool and produces molten pools of about one diameter in depth) do not produce a homoge­neous product. The zirconium material beneath the molten pool is, of course, solid, and can be slowly withdrawn as material from the electrode drips into the pool, as it is known in the prior art.

[0014] Thus, on a production EB furnace, the shallow molten pool results in a non-homogeneous product, and only by following such melting with vacuum arc melting can a homogeneous product be obtained. Conversely, non-swept beam EB furnaces having very high power costs for very low throughput, are impractical for commercial applications. This invention lowers oxygen by removing at least some of the moisture prior to melting while the laboratory type of EB furnace is generally removing oxygen from the molten pool.

Claims

1. A process for producing zirconium in purified form which comprises reducing zirconium tetrachloride to produce a sponge of metallic zirconium which is distilled to generally remove residual magnesium and magnesium chloride, and melting the distilled sponge to produce an ingot, characterized by forming said distilled sponge into a consumable electrode; melting said consumable electrode in a multiple swept beam electron furnace with a feed rate of from 1 to 20 inches per hour to form an intermediate ingot; and vacuum arc melting said intermediate ingot to produce a final ingot.

2. A process according to claim 1, characterized in that the intermediate ingot has on its upper portion a molten pool of less than one fourth of an ingot diameter in depth.

3. A process according to claim 1 or 2, charac­terized in that an argon sweep is provided in the electron beam furnace during said melting.

4. A process according to claim 3, characterized in that the argon sweep is at a flow of 10,000-1,000,000 liters per second, measured at a pressure 10⁻⁵ Torr.

5. A process according to nay of claims 1 to 4, characterized in that multiple passes are made through the electron beam furnace.

6. A process according to nay of claims 1 to 5, characterized in that multiple passes are made through the vacuum arc melting.

7. A process according to nay of claims 1 to 6, characterized in that virgin sponge material is utilized.

8. A process according to any of the preceding claims, characterized in that the intermediate ingot has a diameter of greater than 5 inches.