US2894889A - Jacketed uranium slugs and method - Google Patents

Description

2,894,889 IACKETED SLUGS AND METHOD Nb Drawing. Application July 19, 1949 Serial No. 105,667

4 Claims, (Cl. 204-4932) This invention relates to the fabrication of composite metal bodies. The invention is particularly concerned with a process for the fabrication of molybdenum-clad uranium bodies.

This invention further relates to a process for cladding or jacketing uranium metal so as to effect a composite body of uranium metal substantially enclosed within a corrosion-protective layer of nonfissionable material wherein the protective coating metal is firmly bonded to the base metal.

In the operation of neutronic reactors, it has been found necessary to enclose uranium metal within a solid nonfissioning barrier in order to inhibit the introduction of radioactivity into the coolant and moderator materials with which the uranium would otherwise be in direct contact. A prime requisite for the jacketed uranium body used in a neutronic reactor is that the jacket of such a body shall be absolutely free from ruptured or penetrated regions so that the canned or jacketed piece will be able to withstand prolonged exposure to water or steam at high temperature without the slightest trace of water coming in contact with the base metal enclosed by the protective metal jacket.

In the construction of neutronic reactors and of the fuel units used therein, it is desirable to so limit the thickness of the nonfissionable covering material that the escape of fission fragments into the moderator and coolant'will be avoided and also to use an amount of nonfissionable material such that the neutron absorption thereof will not prevent the establishment of a self-sustaining neutron chain reaction by absorbing a greater number of neutrons than would be permissible in terms of the design and geometry of the reactor. Therefore, the nonfissionable jacketing material enclosing the uran ium should have the minimum thickness requisite to maintain the necessary strength, resistance to corrosion and erosion and ability to inhibit passage of fission productsfrom the fuel material into the surrounding coolant and moderator materials. Moreover, the permissible quantity of the nonfissionable cladding material relative to the uranium enclosed therein is dependent partly upon the size of the'base metal body.

In a neutronic reactor the nonfissionable jacketing material must also possess the necessary properties with respect to the transfer of heat between the material undergoing fission and the surrounding moderator and coolant materials. In many cases, it has been found that after prolonged operation, the metallic enclosure tends to become detached from the component base metal because of an insecure bond formed between the uranium metal and the metal coating thereon. For an understanding of the theory of a reactor and disclosure of neutronic reactorsin which jacketed uranium slugs of this invention can be used, reference is made to Fermi et al. Patent 2,708,656, dated May 17, 1955.

An object of this invention is to provide a method for bonding one metal body to another metal body by casting. H

A further object of this invention is to provide a method whereby a body of uranium metal is cast and bonded during said casting to the protective metal sheet which substantially surrounds the uranium body and wherein said metal sheet consists of a nonfissionable metal having a low neutron capture cross-section suitable for use in a neutronic reactor as well as having the properties necessary for jacketing such a cast body.

A further object of this invention is to provide a method for casting uranium metal so as to effect a composite body consisting of cast uranium substantially enclosed within and bonded to molybdenum.

Other objects and advantages will be apparent upon further examination of this specification.

In accordance with the process of this invention, a suitably prepared charge of uranium metal, or an alloy containing a predominant proportion of uranium therein, is heated until the uranium metal or its alloy becomes sufliciently molten to permit pouring. The pouring temperature is always in excess of the melting point of the uranium, suitably between 1200 and 1400" C. While in the molten state the uranium is then cast into a cooled die. The entire process ispreferably carried out under sufiicient vacuum or in the presence of an inert gas to preclude oxidation of the cast metal and the jacketing material.

The die container is shaped so as to impart the contour desired for the final product to the cast metal and said die container is lined with a metal sheet material such as molybdenum sheet, the inner face of which is substantially free of oxides, which is crimped in place within the die container to which contour it is closely conformed. A suitable material for the die container is stainless steel.

In accordance with the process of this invention, molybdenum is used as the jacketing material for the uranium cast therein, which molybdenum also serves as a lining material for the die. I have selected molybdenum because, used in accordance with the process of this invention, it is capable of forming a solid solution with uranium metal and thus effects a strongly adherent diffusion bond between the uranium base material and the protective molybdenum coating which also possesses the necessary nuclear characteristics and has a melting point in excess of the temperature necessary for casting uranium. Molybdenum is a particularly suitable jacketing material for the purpose of this invention, since in addition to form: ing a solid solution with uranium, molybdenum does not form low melting intermetallic compounds or eutectics fracturable on thermal stress such as are formed when ing of nickel having a thickness of between about 0.3 and 1.0 mil and preferably about 0.5 mil is sulficient to serve as a flux to wet both the molybdenum surface and the uranium surface and prevents formation of oxide,

particularly molybdenum oxide. Molybdenum oxide has i been found to inhibit the formation of a permanent bond between uranium and molybdenum in those areas where molybdenum oxide is present. Although nickel can be applied to either metal to effect a diffusion bond between solid uranium and solid molybdenum, according to the process of this invention, it is obviously impractical to add the nickel to the molten uranium prior to casting and shape the molybdenum jacketing material is suitably between 3 and 8 mils in thickness. The nickel coating is plated on the molybdenum sheet surface which comes in contact with the molten uranium during the casting operation and is suitably applied by a flash plating technique in order to effect a uniformly distributed nickel coating which measures preferably about 0.5 mil in* thickness.

In applying to the molybdenum sheet such a uniform, thin, discrete nickel coating of the order of 1 mil thickness, the following flash plating procedure may be followed. The molybdenum sheet is first degreased in acetone and subjected to cathodic electrocleansing at a current density between 40 and 50 amperes per square foot in a solution containing 60 grams per liter of a conventionaltype cleanser which consists essentially of various silicates, trisodium phosphate, caustic soda and a bufier to maintain constant pH. During, such cleansing operation the cleansing solution is maintained at a temperature between 85 and 95 C. which is followed by a cold water rinse. in a concentrated nitric acid solution held at a temperature between 20 and 40 C. and the current applied is between 40 and 50 amperes per square foot. After a second cold water rinse the nickel plating is accomplished by immersing the molybdenum sheet in a high chloride nickel solution containing 240 grams of nickel chloride and 100 cc. of concentrated hydrochloric acid per liter. The flux plating is accomplished within. a striking time of 1 minute at a current density between 40 and 60 amperes per square foot. The resultant deposit of nickel adheres uniformly to the molybdenum sheet, thus permitting the washing of said nickel coating into the uranium matrix during the casting operation so that the amounts of nickel used according to the process of this invention are insuflicient to form brittle compounds in any part of the uranium-molybdenum bond effected by the castingv of molten uranium within molybdenum sheet prepared with the above-described flux coating of nickel. Even in the absence of any metallurgical bonding, the nickel coating is sufficiently adherent to the molybdenum to resist both blistering when severely scratch brushed and scaling or separation from the base metal when a knife is used to induce such sealing or separation. The above method of coating the molybdenum surface with a nickel flux is only illustrative, and any method of obtaining a nickel coating of the desired thickness may be used.

After the uranium metal is obtained in the completely molten state, it is cast by pouring in vacuo directly from the furnace in which the melt is obtained into a cooled mold formed of a suitable material, such as stainless steel. The mold or die is fabricated in the contour desired for the finished uranium body and is lined with a thin discrete layer of nickel-plated molybdenum sheet which is crimped into place within the die so as to conform to the contour of the die and'completely cover the die surface, thereby preventing any direct contact between the molten uranium and the supporting member which can therefore be fabricated from a material having a melting point lower than the casting temperature for uranium or capable offorming a low melting eutectic with said uranium at the casting temperature. After pouring in vacuo into a suitably cooled moldor die, the uranium metalthuscast forms a diffusion bond with the nickel plated molybdenum sheet lining material. During mold cooling in vacuo, the molybdenum-sheet metal conforms to "said uranium body-as a firmly adherent molyb The sheet material is then subjected to etching 4 denum coatingand the resultant assembly consisting of the cast uranium bonded to a molybdenum coating shrinks from the die support.

An apparatus in which the previously described operation may be carried out is constructed, as shown in Wilhelm et a1. Patent 2,782,475, dated February 26, 1957, of a melting or casting furnace suitably heated by. induction means and connected to a vacuum system. The

melting furnace thus described is arranged in close association with a mold cooled by: suitable means such as water, which mold is also connectedtoa vacuum source. The uranium is reduced to the molten state within the induction furnace which consists of a quartz enclosure surrounding a shield made of fused alumina which shield in turn contains the graphite inductor which'surrounds a beryllia crucible to contain the melt. The furnace in which the uranium is reduced to the molten state is connected directly with a cooled mold locatedimmediately below the furnace. A graphite funnel is located and con tained within the furnace and mold at the junctiombetween said furnace and mold. The opening to this graphite funnel-is fittedwith a graphite plug, which ex tends through the entire length andin the center of the beryllia crucible from the opening in the bottom of the furnace to the top of the crucible containing the molten" charge. This plug serves to obstruct passage of the uranium melt into the mold until the uranium has been heated sufliciently to permit pouring at which timethe plug is removed from the funnel thereby permittingthe molten:

uranium to'flow directly into 'the mold. When in place within the funnel the graphite plug also serves to prevent heat radiation from the furnace to the mold during formation of the melt. The plug is so shaped as to form a guide for the molten uraniumduring the pouring operation in order to insure adequate contact of the melt with.

the lining material for the die and to wash the lining during filling. Since the plug as well as the charge is heated during the formation of the uranium melt, the plug also serves to inhibit premature cooling at the top of the mold thereby further concentrating the molten metal in the main portionof the slug as the molten metal is fed down intothe body of the metal beingcasto The stainless steel die or supporting member for the molyb denum sheet rests on a water-cooled upper base plate and the entire die assembly is surrounded by fused alumina.

In casting uranium which has a marked tendency to form carbides, it is preferred to paint all graphite surfaces which would come in contact with the uranium during the casting operation with a beryllia slurry which is oven-dried on the graphite suitably at a temperature of C.

The process of the present invention is not limited to being carried out in the above-described type of casting furnace, but the process may be carried out in any type of uranium casting apparatus which is capable of furnishing, the necessary conditions for carrying out this process as set forth in the appended claims.

The following examples will further illustrate the process of this invention.

Example I The charge, consisting of four uranium rods each measuring x 3 /2 and having atotal weight ofabout 1 pound, was cleanedwith abrasive paper (Nos. 400 The stainless steeldie or supportingcylinder to 600). was lined with molybdenum sheet measuring 3 -mils in thickness and having approximately 0.5 mil nickel coating plated thereon. The nickel-plated molybdenum sheetwas degreased in acetone and crimped in place within the stainless steel moldso that contact between the stainlesssteel mold and the meltwas not possible.

The previously cleaned uranium charge was placedin escapee in 35 minutes and a temperature maximum of 1320 C. was reached during a heating period totaling 1 hour and minutes during which time vacuum was maintained at 10- microns. Heating was discontinued and the melt was poured from a temperature of 1300' C. and cooled under vacuum within the mold.

The nickel plating served as a flux between the molten uranium metal and the molybdenum base lining material and the resultant bond between the uranium and molybdenum was firm on all sides and could not be removed or scaled from the cast slug by mechanical means.

Example 11 The charge consisted of 440 grams of uranium metal which was cleaned with abrasive paper. The stainless steel mold was lined with 3 mils size molybdenum sheet which had been plated previously with 0.5 mil nickel, which nickel was not bonded in any special manner to the molybdenum sheet. The nickel plated molybdenum sheet material was degreased in acetone and crimped in place within the plain carbon steel tube which served as the mold. The charge was heated under vacuum at approximately l 10- microns which vacuum was determined at the point of discharge. The temperature was raised to 1090 C. within about 50 minutes and reached 1155 C. after 60 minutes heating; 1205 C. after 65 minutes heating; 1240 C. after 80 minutes heating. The temperature of the metal was then maintained at 1240 C. for about 5 minutes after which time the metal was poured into the plain carbon steel mold lined with the nickel-plated molybdenum sheet. The assembly within the steel mold was cooled under vacuum and unloaded after 14 hours.

The molybdenum sheet was firmly bonded to the cast uranium over the entire surface. The molybdenum jacket appeared smooth and free from blisters and could not be stripped from the cast uranium when tested by application of a chisel to the canned slug. Metallographic examination corroborated the firm bond formed between the molybdenum and uranium, which junction appeared to be due to the formation of a difiusion bond between the two metals.

The examples I and II comprise the preferred embodiments of this invention. However, many alterations and changes therein may be made without departing from 6 the spirit and scope of this invention which is set forth in the appended claims which are to be construed as broadly as possible in view of the prior art.

What is claimed is:

1. A process comprising the steps of lining the entire surfaces of a mold with a composite liner of nickel-plated molybdenum, the molybdenum being in contact with the mold, filling the mold with molten uranium, and cooling the mass to below the melting point of the uranium.

2. A process comprising the steps of lining the entire interior surfaces of a cylindrical mold with a liner of molybdenum having a nickel coating of about 0.5 mil thickness, chilling the mold, pouring molten uranium at a temperature between 1200 and 1400 C. into the chilled mold, cooling the assembly to below the melting point of the uranium, and sustaining an inert atmosphere during the pouring and cooling.

3. An article of manufacture comprising a body of uranium having its entire surface except one end covered with a composite coating of nickel and molybdenum, the nickel being contiguous to the uranium.

4. An article of manufacture comprising a uranium body, a coating of molybdenum covering the entire body except one end and having a thickness of between 3 and 8 mils, and a bonding liner of nickel between the body and the coating having a thickness of between 0.3 and 1.0

References Cited in the file of this patent UNITED STATES PATENTS 1,162,340 Coolidge Nov. 30, 1915 1,343,842 Piersol June 15, 1920 1,579,779 Rentschler Apr. 6, 1926 1,733,744 Marden et al. Oct. 29, 1929 1,930,088 Foulke Oct. 10, 1933 2,116,927 Germer May 10, 1938 2,203,679 Edwards June 11, 1940 OTHER REFERENCES Hackhs Chemical Dictionary, 3rd ed., p. 349 (1944), Blakiston.

Smyth Report: Atomic Energy for Military Purposes, p. 27, paragraph 2.27, August 1945. Copy may be purchased from the Supt. of Documents, Washington 25, DC.

Claims (2)

1. A PROCESS COMPRISING THE STEPS OF LINING THE ENTIRE SURFACES OF A MOLD WITH A COMPOSITE LINER OF NICKEL-PLATED MOLYBDENUM, THE MOLYBDENUM BEING IN CONTACT WITH THE MOLD, FILLING THE MOLD WITH MOLTEN URANIUM, AND COOLING THE MASS TO BELOW THE MELTING POINT OF THE URANIUM.

3. AN ARTICLE OF MANUFACTURE COMPRISING A BODY OF URANIUM HAVING ITS ENTIRE SURFACE EXCEPT ONE END COVERED WITH A COMPOSITE COATING OF NICKEL AND MOLYBDENUM, THE NICKEL BEING CONTIGUOUS TO THE URANIUM.