Uranium

Uranium is a chemical element in the periodic table that has the symbol U and atomic number 92. A heavy, silvery-white, toxic, metallic , and naturally-radioactive element, uranium belongs to the actinide series and its isotope uranium-235 is used as the fuel for nuclear reactors and nuclear weapons. Uranium is commonly found in very small amounts in rocks, soil, water, plants, and animals (including humans).

Notable characteristics

When refined, uranium is a silvery white, weakly radioactive metal, which is slightly softer than steel. It is malleable, ductile, and slightly paramagnetic. Uranium metal has very high density, 65% more dense than lead. When finely divided, it can react with cold water; in air, uranium metal becomes coated with uranium oxide. Uranium in ores can be extracted and chemically converted into uranium dioxide or other chemical forms usable in industry.

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 principal isotopes are ²³⁵U and ²³⁸U (see Uranium-235 and Uranium-238). Naturally-occurring Uranium also contains a small amount of the ²³⁴U isotope, which is a decay product of ²³⁸U. The isotope ²³⁵U is important 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 neutrons are absorbed by other ²³⁵U nuclei, a nuclear chain reaction occurs, and if there is nothing 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.

Applications

Uranium metal is very dense and heavy. Depleted uranium (almost pure U-238 with less than 0.2% U-235) is used by some militaries as shielding to protect tanks, and also in parts of bullets and missiles. The military also uses enriched uranium (more than natural levels of U-235) to power nuclear propelled navy ships and submarines, and in nuclear weapons. Fuel used for United States Navy reactors is typically highly enriched in U-235 (the exact values are classified information). In nuclear weapons uranium is also highly enriched, usually over 90% (again, the exact values are classified information).

The main use of uranium in the civilian sector is to fuel commercial nuclear power plants, where fuel is typically enriched in U-235 to 2-3%. However, the Canadian Candu reactors use natural unenriched uranium as fuel. Depleted uranium is used in helicopters and airplanes as counterweights on certain wing parts. Other uses include;

• Ceramic glazes where small amounts of natural uranium (that is, not having gone through the enrichment process) may be added for color.
• The long half-life of the isotope uranium-238 (4.51 × 10⁹) make it well-suited for use in estimating the age of the earliest igneous rocks.
• U-238 is converted into plutonium in breeder reactors. Plutonium in turn in used in hydrogen bombs.
• Uranyl acetate, UO₂(CH₃COO)₂ is used in analytical chemistry. It forms an insoluble salt with Sodium.
• Some lighting fixtures utilize uranium, as do some photographic chemicals (esp. uranium nitrate).
• Phosphate fertilizers often contain high amounts of natural uranium, because the mineral material from which they are made is typically high in uranium.
• Uranium metal is used for X-ray targets in making of high-energy X-rays.
• The element has found use in inertial guidance devices and in gyroscopic compasses.

History

The use of uranium, in its natural oxide form, dates back to at least 79 AD, when it was used to add a yellow color to ceramic glazes (yellow glass with 1% uranium oxide was found near Naples, Italy).

The discovery of the element is credited to the German chemist Martin Heinrich Klaproth who in 1789 found uranium as part of the mineral called pitchblende. It was named after the planet Uranus, which had been discovered eight years earlier. It was first isolated as a metal in 1841 by Eugene-Melchior Peligot. Uranium was found to be radioactive by French physicist Henri Becquerel in 1896, who first discovered the process of radioactivity with uranium minerals.

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 which had the power to withdraw prospective uranium mining land from public purchase, and also to manipulate the price of uranium to meet national needs (leading to a uranium "boom" in the early 1950s). 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.

Compounds

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

Uranium hexafluoride (UF₆) is a white solid which forms a vapor at temperatures above 56 degrees Celsius. 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.

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" as well, but this is not a standard name.

Occurrence

Uranium is a naturally-occurring element found at low levels in virtually all rock, soil, and water. It is considered to be more plentiful than antimony, beryllium, cadmium, gold, mercury, silver, or tungsten and is about as abundant as arsenic or molybdenum. It is found in many minerals including pitchblende, uraninite (most common uranium ore), autunite, uranophane, tobernite, and coffinite. Significant concentrations of uranium occur in some substances such as phosphate rock deposits, and minerals such as lignite, and monazite sands in uranium-rich ores (it is recovered commercially from these sources). Because uranium has such a long radioactive half-life (4.47x10⁹ years for U-238), the total amount of it on Earth stays almost the same.

The decay of uranium and its nuclear reactions with thorium in the Earth's core is thought to be the source for much of the heat that keeps the outer core liquid, which in turn drives plate tectonics.

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.

Production and distribution

Commercial-grade uranium can be produced through the reduction of uranium halides with alkali or alkaline earth metals. Uranium metal can also be made through electrolysis of KUF₅ or UF₄, dissolved in a molten CaCl₂ and NaCl. Very pure uranium can be produced through the thermal decomposition of uranium halides on a hot filament.

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, especially by the French. Generally, large countries produce more uranium than smaller ones because the worldwide distribution of uranium is very roughly uniform. Canada is the world's largest producer of uranium, with the world's richest deposits in Saskatchewan. Saskatchewan through three large mines produces over a quarter of the world's uranium. Because of this production, extra capacity, and the close government control of the industry the provincial government plays a central role in setting international uranium prices. Australia also has extensive uranium deposits making up approximately 30% of the world's known uranium reserves.

Isotopes

Naturally occurring uranium is composed of 3 major isotopes, U-238, U-235, and U-234, with U-238 being the most abundant (99.3% natural abundance). These 3 isotopes are radioactive, creating radioisotopes, with the most abundant and stable being U-238 with a half-life of 4.5 × 10⁹ years, U-235 with a half-life of 7 × 10⁸ years, and U-234 with a half-life of 2.5 × 10⁵ years.

Uranium isotopes can be separated to increase the concentration of one isotope relative to another. This process is called "enrichment" (see enriched uranium). To be considered to be 'enriched' the U-235 fraction has to increased to greater than the 0.711% (by weight). Uranium-235 is better for nuclear power reactors, and for making nuclear weapons. The process produces huge quantities of uranium that are depleted in U-235, but are almost pure U-238, called depleted uranium or "DU". To be considered to be 'depleted', the U-235 isotope concentration has to have been decreased to less than 0.711% (by weight).

Precautions

All isotopes and compounds of uranium are toxic and radioactive. Toxicity can be lethal. In less than lethal doses toxicity is limited primarliy to recoverable kidney damage. Radiological effects are systemic. Uranium compounds in general are poorly absorbed by the lining in the lungs and may remain a radiological hazard indefinitely. Finely-divided uranium metal presents a fire hazard.

A person can be exposed to uranium by inhaling dust in air, or ingesting water and food. The general population is exposed to uranium primarily through food and water. The average daily intake of uranium from food ranges from 0.07 to 1.1 micrograms per day. The amount of uranium in air is usually very small. People who live near government facilities that made or tested nuclear weapons, or facilities that mine or process uranium ore or enrich uranium for reactor fuel, may have increased exposure to uranium.

Uranium can enter the body when it is inhaled or swallowed, or under rare circumstances it may enter through cuts in the skin. Uranium does not absorb through the skin, and alpha particles released by uranium cannot penetrate the skin, so uranium that is outside the body is much less harmful than it would be if it were inhaled or swallowed. When uranium enters the body it can lead to cancer or kidney damage.

See also

• nuclear physics
• nuclear weapon
• nuclear reactor
• nuclear engineering
• Depleted uranium

References

• Los Alamos National Laboratory's Chemistry Division: Periodic Table - Uranium
• U.S. EPA: Radiation Information for Uranium (some adapted public domain text)

External links

• WebElements.com - Uranium (also used as a reference)
• EnvironmentalChemistry.com - Uranium (also used as a reference)
• It's Elemental - Uranium
• The US government provides lots of statistics and information relevant to the energy industry at
• The Uranium Information Centre also has lots of information on uranium
• Uranium Proliferation Nice Joke