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Uranium is a heavy, naturally radioactive (it doesn't have stable isotopes), metallic element, in the periodic table uranium has the symbol U and atomic number 92. Its melting point is 1132°C, its density 19050 kg/m3.

Uranium metal has three allotropic forms:

  • alpha (orthorhombic) stable up to 667.7°C
  • beta (tetragonal) stable from 667.7 C to 774.8°C
  • gamma (body-centered cubic) from 774.8°C to melting point - this is the most malleable and ductile state.

Its two principally occurring isotopes are 235U and 238U. The isotope 235U is indispensable for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile, that is, fissionable by thermal neutrons. The isotope 238U is also important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope 239Pu (plutonium), which also is fissile.

The artificial 233U isotope is also fissile and is made from 232thorium by neutron bombardment.

Uranium was the first element that was found to be fissile, i.e. upon bombardment with slow neutrons, its 235U isotope becomes the very short lived 236U, that immediately divides into two smaller nuclei, liberating energy and more neutrons. If these neutron are absorbed by other 235U nuclei, a nuclear chain reaction occurs, and if there isn't anything to absorb some neutrons and slow the reaction, it is explosive. The first atomic bomb worked with by this principle (nuclear fission). A more accurate name for both this and the hydrogen bomb (nuclear fusion) would be "nuclear weapon", because only the nuclei participate.

Some Important Materials Containing Uranium

Uraninite is the most common ore of uranium.

Yellowcake is uranium concentrate. It takes its name from the color and texture of the concentrates produced by early mining operations, despite the fact that modern mills using higher calcining temperatures produce "yellowcake" that is dull green to almost black. Yellowcake typically contains 70 to 90 percent uranium oxide (U3O8) by weight.

Ammonium diuranate is an intermediate product in the production of yellowcake, and is bright yellow in colour. It is sometimes confusingly called "yellowcake" but this is not a standard name.

Uranium ore is rock containing uranium mineralization in concentrations that can be mined economically, typically 1 to 4 pounds of uranium oxide per ton or 0.05 to 0.20 percent uranium oxide.

Uranium tetrafluoride (UF4) is known as "green salt" and is an intermediate product in the production of uranium hexaflouride.

Uranium hexafluoride (UF6) is a white solid which forms a vapor at temperatures above 56 degrees Centigrade. UF6 is the compound of uranium used for the two most common enrichment processes, gaseous diffusion[?] enrichment and centrifuge enrichment. It is simply called "hex" in the industry.

Enriched uranium is uranium in which the 235U isotope concentration has been increased to greater than the 0.711 percent 235U (by weight) currently present in natural uranium.

Depleted uranium or "DU" is uranium in which the 235U isotope concentration has been decreased to less than 0.711 percent. It is a waste product from the enrichment process, and is used in aircraft counterweights and in munitions[?].

Historical Background (at least the US version)

Uranium was discovered in 1789 by the German chemist Martin Heinrich Klaproth[?] as part of the mineral called pitchblende. It was named after the planet Uranus, which had been discovered eight years earlier.

In the Manhattan Project the names tuballoy and oralloy were used to refer to natural uranium and enriched uranium respectively. These names are still used occasionally to refer to natural or enriched uranium.

The exploration and mining of radioactive ores in the United States began around the turn of the 20th century. Sources for radium (contained in uranium ore) were sought for use as luminous paint for watch dials and other instruments. Uranium became important for defense purposes during World War II. In 1943, the Union Mines Development Corporation[?] operated mills in Colorado to process uranium ore for the Manhattan Project, which applied atomic power to military use. To ensure adequate supplies of uranium for national defense, Congress passed the U.S. Atomic Energy Act of 1946[?], creating the Atomic Energy Commission. Military requirements declined in the 1960s, and the Government completed its uranium procurement program by the end of 1970. Simultaneously, a new market emerged - commercial nuclear power plants.


Owners and operators of U.S. civilian nuclear power reactors purchased from U.S. and foreign suppliers a total of 21,300 tons of uranium deliveries during 2001. The average price paid was $26.39 per kilogram of uranium, a decrease of 16 percent compared with the 1998 price.

In year 2001, the U.S. produced 1,018 tons of uranium from 7 mining operations, all of which are west of the Mississippi River.

Uranium is distributed worldwide. Generally, large countries produce more uranium than smaller ones because the worldwide distribution or uranium is very roughly uniform. Australia has extensive uranium deposits making up approximately 30% of the world's known uranium reserves.

Uranium contamination in food and soil is a problem in some areas. Uranium is not particularily rare; it is more common than tungsten, beryllium, or gold. This same problem also exists with thorium, which is much more abundant.

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See also: nuclear physics, nuclear weapon, nuclear reactor, nuclear engineering, Depleted uranium

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