Uranium metal powder

Metallurgically, uranium metal may exist in three allotropic forms orthorhombic, tetragonal, or body-centered cubic (EPA 1991), and may be alloyed with other metals to alter its structural and physical properties to suit the application. Like aluminum metal powder, uranium metal powder is autopyrophoric and can burn spontaneously at room temperature in the presence of air, oxygen, and water. In the same manner, the surface of bulk metal, when first exposed to the atmosphere, rapidly oxidizes and produces a thin surface layer of UO2 which resists oxygen penetration and protects the inner metal from oxidation. 

Uranium carbide UC (UC2 and U2C also exist) has a melting point of about 2300 °C and is an important nuclear fuel for high-temperature reactors. It is prepared by reduction of UO2 with carbon, followed by pressing and sintering. It can also be made by hot pressing of mixtures of uranium metal powder with graphite at 1000 to 1100 °C. A mixed carbide with ThC is manufactured in the form of spheroids by melting. As the product is hydrolyzed on exposure to air, it is coated with a protective carbon layer. 

The yield of uranium metal powder is about 93 per cent of the uranium content of the original oxide. The loss is entirely into the leaching solution, from which it is recoverable. 

After water-washing and vacuum drying for 40 hr, uranium metal powder is sometimes coated with a thin film of paraffin wax to stabilize it against surface oxidation on storage. The wax is applied in the molten condition at a temperature just above its melting-point. An alternative is to store the powder under an atmosphere of argon in steel containers. 

The hot cathode, with its adhering uranium metal powder, is withdrawn from the melt, placed in a narrow vessel and smothered with fine dry sodium chloride. This excludes air and prevents the product from catching fire during cooling. It is sometimes possible to remove the electrode by twisting and withdrawing it from the plastic mixture of metal powder and salts. If this is not possible, chipping of the cold mixture of salt and powder away from the electrode by means of an air hammer is resorted to. The mixture contains about 2 lb of salt per lb of uranium. 

The major losses of uranium in this process occur in the various grinding, washing, drying and handling stages after electrolysis itself. Additional losses and hazards can also be introduced as a result of fires, caused by the pyrophoric nature of uranium metal powder, unless great care is taken during the post-electrolysis operations. 

The reaction of Ca or Mg with UO2 produces uranium metal powder, a consequence of the refractory character of the CaO or MgO reaction products that interfere with the formation of a molten metal billet. Ca is preferred over Mg , which results in the formation of a U powder of finer particles at a lower chemical yield the smaller powder size increases the pyrophoricity hazard. Metal powder is also formed in the electrolysis of UCI3 or UF4 dissolved... 

Because of the commercial importance of uranium, a number of methods for generating finely divided chemically reactive uranium metal have been developed. Pyrophoric uranium metal powders have been prepared by thermal decomposition of uranium amalgam [21-23] or uranium hydride [24, 25]. Many methods have involved reduction of uranium oxides [26]. Other methods employed are melt electrolysis [26] and potassium reduction of (i/ -C6H5)4U [27]. 

Both zirconium hydride and zirconium metal powders compact to fairly high densities at conventional pressures. During sintering the zirconium hydride decomposes and at the temperature of decomposition, zirconium particles start to bond. Sintered zirconium is ductile and can be worked without difficulty. Pure zirconium is seldom used in reactor engineering, but the powder is used in conjunction with uranium powder to form uranium—zirconium aUoys by soHd-state diffusion. These aUoys are important in reactor design because they change less under irradiation and are more resistant to corrosion. 

Properties. Uranium metal is a dense, bright silvery, ductile, and malleable metal. Uranium is highly electropositive, resembling magnesium, and tarnishes rapidly on exposure to air. Even a poHshed surface becomes coated with a dark-colored oxide layer in a short time upon exposure to air. At elevated temperatures, uranium metal reacts with most common metals and refractories. Finely divided uranium reacts, even at room temperature, with all components of the atmosphere except the noble gases. The silvery luster of freshly cleaned uranium metal is rapidly converted first to a golden yellow, and then to a black oxide—nitride film within three to four days. Powdered uranium is usually pyrophoric, an important safety consideration in the machining of uranium parts. The corrosion characteristics of uranium have been discussed in detail (28). 

Strangely enough, conversion of massive irradiated uranium metal to the hydride (producing a very finely divided powder) followed by decomposition of the hydride to the very finely divided metal, releases very little of the inert gases formed by fission of the uranium (38), (112). 

When Klaproth dissolved some pitchblende in nitric acid and neutralized the acid with potash, he obtained a yellow precipitate which dissolved in excess potash. Klaproth concluded correctly that the mineral must contain a new element, which he named in honor of the new planet, Uranus, which Herschelhad recently discovered (12). He then attempted to obtain metallic uranium just as Hjelm had prepared metallic molybdenum. By strongly heating an oil paste of the yellow oxide in a charcoal crucible, he obtained a black powder with a metallic luster, and thought he had succeeded in isolating metallic uranium (29). For over fifty years the elementary nature of his product was accepted by chemists, but in 1841 Peligot showed that this supposed uranium metal was really an oxide. 

Bains ME, Rowbury PW. 1969. The biological excretion pattern from a person involved in the inhalation of a mixture of enriched uranium oxide and lead metal powders. Health Phys 16 449-453. 

RIeke, R. D., Rhyne, L. D. Preparation of highly reactive metal powders. Activated copper and uranium. The Ullmann coupling and preparation of organometalllc species. J. Org. Chem. 1979, 44, 3445-3446. 

Production of uranium metal suffidently pure for use in nuclear reactors is difficult. Uranium forms very stable compounds with oxygen, nitrogen, and carbon, and it reduces the oxides of many common refractories. Methods that yield uranium metal at temperatures below its melting point result in a fine powder that oxidizes rapidly in air and is difficult to consolidate into massive metal. Uranium cannot be deposited electrolytically from aqueous solution. It is not practical to purify uranitun by distillation because of its very high boiling point, 3900° C. Any nonvolatile impurities introduced into uranium during production will remain in it during subsequent operations and contaminate the final product. 

As in the case of uranium metal, production of pure plutonium metal presents many difficulties. It forms very stable compounds with oxygen and carbon, it oxidizes rapidly in air when in the form of powder, it cannot be deposited electrol3 ically from aqueous solution, and it boils at too high a temperature to be purified by distillation. Additionally, the extreme radiotoxicity of plutonium and neutron production from (a, n) reactions require that production operations be carried out in airtight and shielded enclosures. Nuclear criticality limits the amoimt of plutonium produced in any production operation to batch sizes of no more than a few kilograms. Methods that have been used to produce plutonium metal are... 

Sintered membranes are made on a fairly large scale from ceramic materials, glass, graphite and metal powders such as stainless steel and tungsten.9 The particle size of the powder is the main parameter determining the pore sizes of the final membrane, which can be made in the form of discs, candles, or fine-bore tubes. Sintered membranes are used for the filtration of colloidal solutions and suspensions. This type of membrane is also marginally suitable for gas separation. It is widely used today for the separation of radioactive isotopes, especially uranium. 

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