(Planet Uranus) 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, not as rare as once thought, 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, and tobernite. It is also found in phosphate rock, lignite, monazite sands, and can be recovered commercially from these sources.
The United States Department of Energy 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 and 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: alpha --(688C)--> beta --(776C)--> gamma. 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 sixteen isotopes, all of which are radioactive. Naturally occurring uranium nominally contains 99.28305 by weight 238U, 0.7110% 235U, and 0.0054% 234U. The IUPAC lists the abundance (in % atom ratio) of U235 as 0.7200 (4); where (4) represents the uncertainty in the last digit (See IUPAC's Isotopic Compositions of the Elements 1997 (K.J.R. Rosman and P.D.P. Taylor, Pure Appl. Chem. 70 (1): 217-236, 1998 (IUPAC reference). NIST lists the isotopic abundance as 0.7200 (51). The variation in the NIST listed abundance occurs in part from the measurements of some commercial samples which may include some non-natural sources. See (Atomic Weights and Isotopic Compositions). The atomic weights data were published by T.B. Coplen (1) in Atomic Weights of the Elements 1999, (and include changes reported from the 2001 review in Chem. Int., 23, 179 (2001) and the isotopic compositions data were published by K.J.R. Rosman (2) and P.D.P. Taylor (3) in Isotopic Compositions of the Elements 1997. The relative atomic masses of the isotopes data were published by G. Audi (4) and A. H. Wapstra (5) in The 1995 Update To The Atomic Mass Evaluation.
- T.B. Coplen : U.S. Geological Survey, Reston, Virginia, USA
- K. J. R. Rosman : Department of Applied Physics, Curtin University of Technology, Australia
- P. D. P. Taylor : Institute for Reference Materials and Measurements, European Commission, Belgium
- G. Audi : Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, Orsay Campus, France
- A. H. Wapstra : National Institute of Nuclear Physics and High-Energy Physics, Amsterdam, The Netherlands
Uranuim-238 with a half-life of 4.51 x 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. However it may be presumed that uranium is a decay product of elements with higher atomic weight, which may have once been present on earth or elsewhere in the universe. These original elements may have been formed as a result of a primordial creation, known as the big bang, in a supernova, or in some other stellar processes.
Uranium is of great importance as a nuclear fuel. Uranium-238 can be converted into fissionable plutonium by the following reactions: 238U(n, gamma) --> 239U --(beta)--> 239Np --(beta)--> 239Pu. 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.
Uranium-235 is of even greater importance because it is the key to utilizing uranium. 235U, while occuring 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 in a reactor constructed from natural uranium and a suitable moderator, such as heavy water or graphite, alone.
Uranium-235 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 235U by a small percentage, is used to fuel nuclear power reactors to generate electricity. Natural thorium can be irradiated with neutrons as follows to produce the important isotope 233U: 232Th(n, gamma)--> 233Th --(beta)--> 233Pa --(beta)--> 233U. While thorium itself is not fissionable, 233U 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. The estimated world-wide capacity of the 429 nuclear power reactors in operation in January 1990 amounted to about 311,000 megawatts.
Uranium in the U.S.A. is controlled by the U.S. Nuclear Regulatory Commission. New uses are being found for depleted uranium, ie., uranium with the percentage of 235U lowered to about 0.2%.
Uranium is used in inertial guidance devices, in gyro compasses, as 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 a 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.
Working with uranium requires the knowledge of the maximum allowable concentrations that may be inhaled or ingested.
Recently, the natural presence of uranium in many soils has become of concern to homeowners because of the generation of radon and its daughters.