Uranium—(Planet Uranus), U; atomic weight 238.029; atomic number 92; melting point 1132.3 ± 0.8°C; boiling point 3818°C; specific gravity ~ 18.95; valence 2, 3, 4, 5, or 6. Yellow-colored glass, containing more than 1 % uranium oxide and dating back to 79 A.D., has been found near Naples, Italy. Klaproth recognized an unknown element in pitchblende and attempted to isolate the metal in 1789. The metal apparently was first isolated in 1841 by Peligot, who reduced the anhydrous chloride with potassium. Uranium is not as rare as it was once thought. It is now considered to be more plentiful than mercury, antimony, silver, or cadmium, and is about as abundant as molybdenum or arsenic. It occurs in numerous minerals such as pitchblende, uraninite, carnotite, autunite, uranophane, davidite, and tobernite. It is also found in phosphate rock, lignite, monazite sands, and can be recovered commercially from these sources. The A.E.C. purchases uranium in the form of acceptable U3O8 concentrates. This incentive program has greatly increased the known uranium reserves. Uranium can be prepared by reducing uranium halides with alkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminum, or carbon at high temperatures.
The metal can also be produced by electrolysis of KUF5 or UF4, dissolved in a molten mixture of CaCl2 NaCl. High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament.
Uranium exhibits three crystallographic modifications as follows:
Uranium is a heavy, silvery-white metal which is pyrophoric when finely divided. It is a little softer than steel, and is attacked by cold water in a finely divided state. It is malleable, ductile, and slightly paramagnetic. In air, the metal becomes coated with a layer of oxide. Acids dissolve the metal, but it is unaffected by alkalis. Uranium has fourteen isotopes, all of which are radioactive. Naturally occurring uranium nominally contains 99.2830% by weight U238, 0.7110% U235, and 0.0054% U234. Studies show that the percentage weight of U235 in natural uranium varies by as much as 0.1%, depending on the source. The A.E.C. has adopted the value of 0.711 as being their "official" percentage of U235 in natural uranium. Natural uranium is sufficiently radioactive to expose a photographic plate in an hour or so.
Much of the internal heat of the earth is thought to be attributable to the presence of uranium and thorium. U238 with a half-life of 4.51 × 109 years, has been used to estimate the age of igneous rocks. The origin of uranium, the highest member of the naturally occurring elements—except perhaps for traces of neptunium or plutonium—is not clearly understood, although it may be presumed that uranium is a decay product of elements of higher atomic weight, which may have once been present on earth or elsewhere in the universe. These original elements may have been created as a result of a primordial "creation," known as "the big bang," in a supernovae, or in some other stellar processes.
Uranium is of great importance as a nuclear fuel. U238 can be converted into fissionable plutonium by the following reactions:
This nuclear conversion can be brought about in "breeder" reactors where it is possible to produce more new fissionable material than the fissionable material used in maintaining the chain reaction. U235 is of even greater importance, for it is the key to the utilization of uranium. U235, while occurring in natural uranium to the extent of only 0.71%, is so fissionable with slow neutrons that a self-sustaining fission chain reaction can be made to occur in a reactor constructed from natural uranium and a suitable moderator, such as heavy water or graphite, alone. U235 can be concentrated by gaseous diffusion and other physical processes, if desired, and used directly as a nuclear fuel, instead of natural uranium, or used as an explosive. Natural uranium, slightly enriched with U235 by a small percentage, is used to fuel nuclear power reactors for the generation of electricity. Natural thorium can be irradiated with neutrons as follows to produce the important isotope U233
While thorium itself is not fissionable, U233 is, and in this way may be used as a nuclear fuel. One pound of completely fissioned uranium has the fuel value of over 1500 tons of coal. The uses of nuclear fuels to generate electrical power, to make isotopes for peaceful purposes, and to make explosives are well known. Uranium in the U.S.A. is controlled by the Atomic Energy Commission. New uses are being found for "depleted uranium, i.e., uranium with the percentage of U235 lowered to about 0.2%. It has found use in inertial guidance devices, gyro compasses, counterweights for aircraft control surfaces, as ballast for missile reentry vehicles, and as a shielding material. Uranium metal is used for X-ray targets for production of high-energy X-rays; the nitrate has been used as photographic toner, and the acetate is used in analytical chemistry. Crystals of uranium nitrate are triboluminescent. Uranium salts have also been used for producing yellow "vaseline" glass and glazes. Uranium and its compounds are highly toxic, both from a chemical and radiological standpoint. Finely divided uranium metal, being pyrophoric, presents a fire hazard. The maximum recommended allowable concentration of soluble uranium compound in air (based on chemical toxicity) is 0.05 mg/M3 (8-hr time-weighted average—40-hr week); for insoluble compounds the concentration is set at 0.25 mg/M3 of air. The maximum permissible total body burden of natural uranium (based on radiotoxicity) is 0.2μCi for soluble compounds.