For glass coloring.
| Atomic Number: | 92 | Atomic Symbol: | U |
| Atomic Weight: | 238.029 | Electron Configuration: | 2-8-18-32-21-9-2 |
| Shells: | 2,8,18,32,21,9,2 | Filling Orbital: | 5f3 |
| Melting Point: | 1132oC | Boiling Point: | 3818oC |
| Description: | Silvery-white radioactive metal. |
| Uses: | For many centuries it was used as a pigment for glass.
Now it is
used as a fuel in nuclear reactors and in bombs. |
History
(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.
Sources
Uranium, not as rare as once thought, is now considered to be
more plentiful than mercury, antimonry, 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.
Properties
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.
Isotopes
Uranium
has sixteen isotopes, all of which are radioactive. Naturally occuring 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 sufficently radioative 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.
238U with a half-life of 4.51 x 10^9 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.
Uses
Uranium is of great importance as a nuclear fuel. 238U 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.
235U 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.
235U can be concentrated by gaseous diffusion and other physical proceses, 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%.
Uranion 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 procution 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.
Hazards
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 0.2 mg/m^3 (8-hour, time-weighted average
-- 40-hour week).
The maximum permissible total body burden of natural uranium (based on
radiotoxicity) is 0.2 micro-curie for soluble compounds.
Recently, the natural presence of uranium in many soils has become of concern
to homeowners because of the generation of radon and its daughters.