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(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. Studies show that the percentage weight of 235U
in natural uranium varies by as much as 0.1%, depending on the source. The US DOE has
adopted the value of 0.711 as being their official percentage of 235U
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.
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
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
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
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
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.
Sources: CRC Handbook of Chemistry
and Physics and the American Chemical Society.