Metallic Uranium

The phases of metallic uranium and their transition temperatures are listed in Table S.S. 

Uranium metal is very reactive. It tarnishes in air, with the oxide film preventing further oxidation of massive metal at room temperature. However, finely divided uranium ignites spontaneously at room temperature, and massive uranium bums steadily at 700°C, forming 

Water attacks massive uranium slowly at room temperature and rapidly at higher temperatures. UO2 and UH3 are formed, heat is evolved, and the metal swells and disintegrates. In water ooled reactors uranium metal must be clad or canned in nonreacting metals such as aluminum, stainless steel, or zirconium. Nitric acid dissolves uranium readily. 

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Uranium [7440-61-17 is a naturally occurring radioactive element with atomic number 92 and atomic mass 238.03. Uranium was discovered in a pitchblende [1317-75-5] specimen ia 1789 by M. H. Klaproth (1) who named the element uranit after the planet Uranus, which had been recendy discovered. For 50 years the material discovered by Klaproth was thought to be metallic uranium. Pnligot showed that the uranit discovered by Klaproth was really uranium dioxide [1344-57-6] UO2, and obtained the tme elemental uranium as a black powder in 1841 by reduction of UCl [10026-10-5] with potassium (2). 

In 1896, Becquerel discovered that uranium was radioactive (3). Becquerel was studying the duorescence behavior of potassium uranyl sulfate, and observed that a photographic plate had been darkened by exposure to the uranyl salt. Further investigation showed that all uranium minerals and metallic uranium behaved in this same manner, suggesting that this new radioactivity was a property of uranium itself In 1934, Fermi bombarded uranium with neutrons to produce new radioactive elements (4). 

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). 

Preparation of Uranium Metal. Uranium is a highly electropositive element, and extremely difficult to reduce. As such, elemental uranium caimot be prepared by reduction with hydrogen. Instead, uranium metal must be prepared using a number of rather forcing conditions. Uranium metal can be prepared by reduction of uranium oxides (UO2 [1344-59-8] or UO [1344-58-7] with strongly electropositive elements (Ca, Mg, Na), reduction of uranium halides (UCl [10025-93-1], UCl [10026-10-5] UF [10049-14-6] with electropositive metals (Li, Na, Mg, Ca, Ba), electro deposition from molten... 

In the production of metallic uranium the fluoride UF4 is used for magnesium reduction since die reduction of UCI4 does not produce sufficient heat. The chloride can be reduced with calcium, when a signihcantly greater amount of heat is generated to reach die desired temperature. 

In Britain, a population of thermal reactors fuelled by metallic uranium have remained in use, side by side with more modern ones (to that extent. Lander et al. were not quite correct about the universal abandonment of metallic uranium). In 1956, Cottrell (who was then w orking for the Atomic Energy Authority) identified from first principles a mechanism which would cause metallic (ot) uranium to creep rapidly under small applied stress this was linked with the differential expansion of... 

In 1789 M. H. Klaproth examined pitchblende, thought at the time to be a mixed oxide ore of zinc, iron and tungsten, and showed that it contained a new element which he named uranium after the recendy discovered planet, Uranus. Then in 1828 J. J. Berzelius obtained an oxide, from a Norwegian ore now known as thorite he named this thoria after the Scandinavian god of war and, by reduction of its tetrachloride with potassium, isolated the metal thorium. The same method was subsequendy used in 1841 by B. Peligot to effect the first preparation of metallic uranium. 

Hydrogen reduction to form metallic uranium is simply an impossibility. This readily follows from the potential-pH diagrammatic portrayal for uranium. 

Another, more modern, route of processing the yellow cake is shown in Figure 5.38, accomplishes the production of enriched uranium oxide entirely by pyroprocessing. Thus, uranium is finally obtained in three forms metallic uranium, enriched uranium dioxide, and natural uranium dioxide. As the flowsheet shows, and as briefly described herein, these are essentially the products of hydro and pyro-based processing schemes. 

At the end of the eighteenth century, scientists thought that pitchblende was a mixture of iron and zinc compounds. In 1789 Martin Heinrich Klaproth (1743—1817) discovered a new metallic element in a sample of pitchblende, which he named uranus after the recently discovered planet. Although what he actually discovered was the compound uranous oxide (UOj), it was adequate to establish him as the discoverer of uranium. For almost a century, scientists believed that the compound uranous oxide (UO ) was the elemental metal uranium. In 1841 Eugene-Melchoir Pefigot (1811—1890) finally isolated the metal uranium from its compound. Even so, no one knew that both the compounds and metal of uranium were radioactive until 1896, when Henri Becquerel (1852—1908) mistakenly placed apiece of potassium... 

All compounds as well as metallic uranium are radioactive—some more so than others. The main hazard from radioactive isotopes is radiation poisoning. Of course, another potential hazard is using fissionable isotopes of uranium and plutonium for other than peaceful purposes, but such purposes involve pohtical decisions, not science. 

Determination of the D/H ratio of water is performed on H2-gas. There are two different preparation techniques (1) equilibration of milliliter-sized samples with gaseous hydrogen gas, followed by mass-spectrometric measurement and back calculation of the D/H of the equilibrated H2 (Horita 1988). Due to the very large fractionation factor (0.2625 at 25°C) the measured H2 is very much depleted in D, which complicates the mass-spectrometric measurement. (2) water is converted to hydrogen by passage over hot metals (uranium Bigeleisen et al. 1952 Friedman 1953 ... 

Metallic uranium can be prepared from its oxides or hahdes by reduction at high temperature. Uranium dioxide, UO2, or other oxides such as UO3 or UsOs may be reduced to uranium metal by heating with carbon, calcium or aluminum at high temperatures. Similarly, uranium tetrafluoride or other halides can be reduced to metal by heating with sodium, potassium, calcium, or magnesium at high temperatures. Alternatively, uranium tetrafluoride mixed with fused alkali chlorides is electrolyzed to generate uranium metal. 

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. 

Since uranous oxide cannot be reduced with hydrogen or carbon, it had always been mistaken for metallic uranium. 

Eugene Peligot, 1811-1890. Professor of analytical chemistry and glassmaking at the Central School of Arts and Manufactures in Paris. Director of assays at the Paris Mint. Professor of agricultural chemical analysis at the National Agronomic Institute. The first to isolate the metal uranium. 

In 1822 Arfwedson published his paper on uranium (18). More than thirty years before, M. H. Klaproth had heated a paste made with uranic oxide and linseed oil, and obtained a brown powder with a metallic luster, which he regarded as metallic uranium. Although others... 

Two uranium carbides are known, the monocarbide, UC, and the dicarbide, UC2. These can be prepared by direct reaction of carbon with molten uranium, or by reaction of carbon monoxide with metallic uranium at elevated temperatures. The scsqutcarbidc, U,Oj, has been found to exist as a stable compound below about 18 ") U and can be produced by heating a mixture of UC and UC2 between 1.250 and 1,800°C. 

Uranium and nitrogen form an extensive series of compounds that can be prepared by direct action of nitrogen on the metal. Uranium mononiteide,... 

To separate isotopes by this process, they must be in the gaseous form. Therefore, the separation of isolopes of uranium required the conversion of die metallic uranium into a gaseous compound, for which purpose the hexafluoride. UF. was chosen. Since the atomic weight of fluorine is 19, die molecular weight of the hexafluoride of 235 LI is 235 + (6 19) = 349, and the molecular weight of die hexafluoride of 23SU is 238 + (6 x 19) = 352. Since the rate of diffusion of a gas is inversely proportional to die square root of its density (mass per unit volume), the maximum separation factor for one diffusion process of the uranium isotopes is V352/349 = 1.0043, Since only part of the gas can be allowed to diffuse, the, actual separation factoi is even less dian this theoretical maximum. 

Write balanced chemical equations for three different methods to produce metallic uranium. 

Nearly all the methods described in Chapter 2 have been applied for the preparation of uranium derivatives. The reaction of metallic uranium with alcohols in the presence of halogenes (method 1) doesn t lead, as it turned out, to homoleptic alkoxides, but is a facile route to alkoxide halides [79, 1669] ... 

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