Uranium metal production

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... 

The reaction takes place in a closed steel vessel, 45 in. long by 12 in. internal diameter, which is lined with electrically fused dolomite or lime, as in the American uranium metal production process. Initiation is carried out in a gas-fired furnace at a temperature of 640°C. A thorium/zinc alloy is formed, from which the zinc is removed by distillation under vacuum, between 1000°C and 1100°C in graphite pots. About 85 per cent of the zinc can be recovered for re-use. 

An efficient electrolysis process must run under substantially irreversible conditions, i.e. the gas evolved at the anode must be unable to react again, either directly or indirectly, with the rare metal product. To this end, the gas (usually chlorine) is often led away in a fairly direct manner without contacting the cathode. It is particularly important to avoid processes where the chlorine can effectively dissolve in the melt and subsequently migrate to the cathode. This might occur, for example, in the electrolysis of uranium trichloride, where the liberated chlorine is capable of reacting with excess uranium trichloride to form uranium tetrachloride, which is freely soluble in the melt. If the tetrachloride is allowed to diffuse to the cathode it can attack the uranium metal product to re-form uranium trichloride, i.e. 

Figure 9.2 presents a number of less commonly used stages, but all of them have commercial application in special cases. Pressure carbonate leaching of ores, which contain limestone can be cheaper than acid leaching and ion-exchange. This is followed by TBP extraction of the concentrate for final purification since any alternative would normally be based upon the same general chemical engineering principles and would only involve a different solvent. The older type of dryway process is then shown, with ammonia precipitation as the first step, since this still has applications for the production of special types of uranium dioxide. The final calcium reduction of oxide finds application on a relatively small scale, where the uranium metal product is required in powder form. 

Nonferrous Metal Production. Nonferrous metal production, which includes the leaching of copper and uranium ores with sulfuric acid, accounts for about 6% of U.S. sulfur consumption and probably about the same in other developed countries. In the case of copper, sulfuric acid is used for the extraction of the metal from deposits, mine dumps, and wastes, in which the copper contents are too low to justify concentration by conventional flotation techniques or the recovery of copper from ores containing copper carbonate and siUcate minerals that caimot be readily treated by flotation (qv) processes. The sulfuric acid required for copper leaching is usually the by-product acid produced by copper smelters (see Metallurgy, extractive Minerals RECOVERY AND PROCESSING). 

The main technological uses for UO2 are found in the nuclear fuel cycle as the principal component for light and heavy water reactor fuels. Uranium dioxide is also a starting material for the synthesis of UF [10049-14-6] 6 (both critical for the production of pure uranium metal and... 

Fluorides. Uranium fluorides play an important role in the nuclear fuel cycle as well as in the production of uranium metal. The dark purple UF [13775-06-9] has been prepared by two different methods neither of which neither have been improved. The first involves a direct reaction of UF [10049-14-6] and uranium metal under elevated temperatures, while the second consists of the reduction of UF [10049-14-6] by UH [13598-56-6]. The local coordination environment of uranium in the trifluoride is pentacapped trigonal prismatic with an 11-coordinate uranium atom. The trifluoride is... 

Uranium hexafluoride [7783-81-5], UF, is an extremely corrosive, colorless, crystalline soHd, which sublimes with ease at room temperature and atmospheric pressure. The complex can be obtained by multiple routes, ie, fluorination of UF [10049-14-6] with F2, oxidation of UF with O2, or fluorination of UO [1344-58-7] by F2. The hexafluoride is monomeric in nature having an octahedral geometry. UF is soluble in H2O, CCl and other chlorinated hydrocarbons, is insoluble in CS2, and decomposes in alcohols and ethers. The importance of UF in isotopic enrichment and the subsequent apphcations of uranium metal cannot be overstated. The U.S. government has approximately 500,000 t of UF stockpiled for enrichment or quick conversion into nuclear weapons had the need arisen (57). With the change in pohtical tides and the downsizing of the nation s nuclear arsenal, debates over releasing the stockpiles for use in the production of fuel for civiUan nuclear reactors continue. 

Bromides and Iodides. The red-brown tribromide, UBr [13470-19-4], and the black tniodide, Ul [13775-18-3], may both be prepared by direct interaction of the elements, ie, uranium metal with X2 (X = Br, I). The tribromide has also been prepared by interaction of UH and HBr, producing H2 as a reaction product. The tribromide and tniodide complexes are both polymeric soflds with a local bicapped trigonal prismatic coordination geometry. The tribromide is soluble in H2O and decomposes in alcohols. 

In only 30 months, the Manhattan Project built 554 buildings including reactors, separation plants, laboratories, craft shops, warehouses, and electrical substations. The Hanford Site plutonium production reactors (B, D, and F) were rectangular, measured 36 feet long by 28 feet wide by 36 feet high, used 200 tons of uranium metal fuel and 1200 tons of graphite, were water cooled, and operated at an initial power level of 250 million watts (thermal). They dwarfed the reactors at other sites. 

Iodine is also given off to a small extent in dissolving the uranium metal in nitric acid, but larger amounts may be obtained on steam distillation after dissolution (5). Ruthenium is often removed from the fission products by distillation of the volatile tetroxide formed by oxidation with potassium permangate, sodium bismuthate, periodic acid (38) etc. The distillation goes readily and gives a product of good purity. 

The major characteristic of technetium is that it is the only element within the 29 transition metal-to-nonmetal elements that is artificially produced as a uranium-fission product in nuclear power plants. It is also the tightest (in atomic weight) of all elements with no stable isotopes. Since all of technetiums isotopes emit harmful radiation, they are stored for some time before being processed by solvent extraction and ion-exchange techniques. The two long-lived radioactive isotopes, Tc-98 and Tc-99, are relatively safe to handle in a well-equipped laboratory. 

Promethium does not occur in metallic form in nature. Minute quantities are associated with other rare earths. It also is detected in uranium fission products. It is probably the rarest of the lanthanide elements. 

Uranium dioxide occurs in mineral uraninite. Purified oxide may be obtained from uraninite after purification. The commercial material, however, also is recovered from other uranium sources. Uranium dioxide is obtained as an intermediate during production of uranium metal (See Uranium). Uranyl nitrate, U02(N03)2, obtained from digesting the mineral uraninite or pitchblende with concentrated nitric acid and separated by solvent extraction, is reduced with hydrogen at high temperatures to yield the dioxide. 

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. 

Can failures occur from time to time. The release of fission products from them depends on the temperature and type of fuel. If the fuel is uranium metal, as in the Windscale and Magnox reactors, and the can fails, the uranium will oxidise in air or C02. In laboratory experiments, the mass median aerodynamic equivalent diameter (MMAD) of the particles produced by oxidation of uranium increased from about 40 ptm when the temperature of oxidation was 600°C to 500 jum at 1000°C (Megaw et al., 1961). At high temperature, a coherent sintered oxide layer formed on the uranium and this hindered the formation of particles. 

The release of 131I and other fission products in reactor accidents has been considered in the previous chapter. In the Windscale accident, the temperature in the fire zone reached an estimated 1300°C and 8 tonne of uranium metal melted. Over 25% of the 1311 in the melted fuel escaped to atmosphere. In the Chernobyl accident, the fuel was U02, the temperature exceeded 2000°C, and about 25% of the total reactor inventory of 131I was released to atmosphere, as vapour or particulate aerosol. In the Three Mile Island accident, 131I remained almost completely in the reactor coolant. The activities of 131I released in reactor accidents, including that at Chernobyl, have totalled much less than the activities released from weapons tests (Table 2.3). 

Uranium fuel preparation takes the UF6 and is converted to either (a) aluminum-clad uranium metal for the weapons plutonium production reactors or (b) to Zirconium-clad U02 for electricity production in the light and heavy water power reactor (see Fig. 21.13). 

Fig. 21.17. Uranium metal reduction process a similar process is used for plutonium metal production. (Courtesy USDOE.)...

Mantz, E., Production of Uranium Tetrafluoride and Uranium Metal, USAEC Report NCLO 1068, U.S. Atomic... 

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