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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 235U 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. 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.

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

The main use of uranium in the civilian sector is to fuel commercial nuclear power plants, where fuel is typically enriched in 235U 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.
- Addition of uranium makes fluorescent yellow or green colored glass.
- The long half-life of the isotope 238U (4.51 × 109 years) make it well-suited for use in estimating the age of the earliest igneous rocks and for other types of radiometric dating (including uranium-thorium dating and uranium-lead dating).
- 238U is converted into plutonium in breeder reactors. Plutonium can be used in reactors, or in nuclear weapons.
- 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. In 1850 the first commercial use of Uranium in glass was developed by Lloyd & Summerfield of Birmingham England. Uranium was found to be radioactive by French physicist Henri Becquerel in 1896, who first discovered the process of radioactivity with uranium minerals.

Military applications

During the Manhattan Project, the wartime Allied program to develop the first atomic bombs during World War II, uranium gained new importance on the world political scene. Before the discovery of plutonium, only uranium was considered for the development of an atomic bomb, though the process of enriching it to applicable levels required gargantuan facilities (see Oak Ridge National Laboratory). Eventually enough uranium was enriched for one atomic bomb, which was dropped on Hiroshima, Japan in 1945. The other nuclear weapons developed during the war used plutonium as their fissionable material, which itself requires uranium to produce. Initially it was believed that uranium was relatively rare, though within a decade large deposits of it were discovered in many places around the world.

Uranium exploration and mining

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, as well as for health-related applications (some of which in retrospect were incredibly unhealthy). Because of the need for the element during World War II, the Manhattan Project contracted with numerous vanadium mining companies in the American Southwest, and also purchased uranium ore from the Belgian Congo, through the Union Minière du Haut Katanga, and in Canada from the Eldorado Mining and Refining Limited company. American uranium ores mined in Colorado were primarily mixes of vanadium and uranium, but because of wartime secrecy the Manhattan Project would only publicly admit to purchasing the vanadium, and did not pay the uranium miners for the uranium ore (in a much later lawsuit, many miners were able to reclaim lost profits from the U.S. government). American uranium ores did not have nearly as high uranium concentrations as the ore from the Belgian Congo, but they were pursued vigorously to ensure nuclear self-sufficiency. Similar efforts were undertaken in the Soviet Union, which did not have native stocks of uranium when it started developing its own weapons program.

Rise and subsequent stagnation of uranium mining

In the beginning of the Cold War, to ensure adequate supplies of uranium for national defense, the United States Congress passed the U.S. Atomic Energy Act of 1946, creating the Atomic Energy Commission (AEC) 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. By setting a high price for uranium ore, the AEC created a uranium “boom” in the early 1950s, which attracted many prospectors to the four corners region of the country. Moab, Utah became known as the Uranium-capital of the world, when geologist Charles Steen discovered such an ore in 1952, even though American ore sources were considerably less potent than those in the Belgian Congo or South Africa.

At the height of the nuclear energy euphoria in the 1950 methods for extracting diluted uranium and thorium found in abundance in granite or seawater was pursued. ORNL Review (http://www.ornl.gov/info/ornlreview/rev25-34/chapter4.shtm) Scientists promised that used in a breeder reactor these materials would potentially provide limitless source of energy.

American 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. However, in the U.S. this market virtually collapsed by the end of the 1970s as a result of industrial strains caused by the energy crisis, popular opposition, and finally the Three Mile Island nuclear accident in 1979, all of which led to a de facto moratorium on the development of new nuclear reactor power stations.

In Europe a mixed situation exists. Considerable nuclear power capacities have been developed, notably in France, Germany, Spain, Sweden, Switzerland and the UK. In many countries development of nuclear power has been stopped by legal actions. In Italy the use of nuclear power has been barred by a referendum in 1987.

In France and Switzerland the use of nuclear power continues, but there is little new demand that would stimulate the market for uranium.

Since 1981 uranium prices and quantities in the US are reported by the Department of Energy [1] (http://www.eia.doe.gov/emeu/aer/pdf/pages/sec9.pdf). Import price dropped from 32.90 US$/lb U3O8 in 1981 down to 12.55 in 1990 and to below 10 US$/lb U3O8 in the year 2000. Prices paid for uranium during the 1970s were higher, 43 US$/lb U3O8 is reported as the selling price for Australian uranium in 1978 by the Nuclear Information Centre (http://www.ccsa.asn.au/nic/Uranium/UMarket.htm).

Risks of uranium mining

Because uranium ores emit radon gas, and their harmful and highly radioactive daughter products, uranium mining is significantly more dangerous than other (already dangerous) hard rock mining, requiring adequate ventilation systems if the mines are not open pit. During the 1950s, a significant amount of American uranium miners were Navajo Indians, as many uranium deposits were discovered on Navajo reservations. An unusually high number of these miners later developed lung cancer. Some survivors and their descendants received compensation under the Radiation Exposure Compensation Act in 1990.

Codenames tuballoy and oralloy

During the Manhattan Project, the names tuballoy and oralloy were used to refer to natural uranium and enriched uranium respectively, originally for purposes of secrecy. These names are still used occasionally to refer to natural or enriched uranium.

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 gas 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. (Other uranium oxides, such as UO2 and UO3, exist; the most stable oxide, U3O8, is actually considered to be a 2:3 molar mixture of these.)

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.

Uranyl nitrate (UO2(NO3)2) is an extraordinarily toxic, soluble uranium salt.

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 uraninite (most common uranium ore), autunite, uranophane, torbernite, 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).

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. 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. The world’s largest single uranium deposit is located at the Olympic Dam Mine in South Australia. [2] (http://www.uraniumsa.org/processing/processing.htm) [3] (http://money.cnn.com/services/tickerheadlines/for5/200411231804DOWJONESDJONLINE000797_FORTUNE5.htm)

The ultimate supply of uranium is very large. It is estimated that for every doubling of price, that the supply of uranium will be multiplied 2.5 times. A ten fold increase in price will result in an increase of supply of 300 times.

Isotopes

Naturally occurring uranium is composed of 3 major isotopes, 238U, 235U, and 234U, with 238U being the most abundant (99.3% natural abundance). These 3 isotopes are radioactive, creating radioisotopes, with the most abundant and stable being 238U with a half-life of 4.5 × 109 years, 235U with a half-life of 7 × 108 years, and 234U with a half-life of 2.5 × 105 years. 238U is an α emmiter, decaying into Lead-206.

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 235U fraction has to be increased to significantly greater than the 0.711% (by weight) (eg typically to levels from 3% to 7%). 235U is typically the main fissile material for nuclear power reactors. Either 235U or 239Pu are used for making nuclear weapons. The process produces huge quantities of uranium that is depleted of 235U and with a correspondingly increased fraction of 238U, called depleted uranium or “DU”. To be considered to be ‘depleted’, the 235U isotope concentration has to have been decreased to significantly less than 0.711% (by weight). Typically the amount of 235U left in depleted uranium is 0.2% to 0.3%. This represents anywhere from 28% to 42% of the original fraction of 235U.

Given that the half life of 235U is considerably shorter than 238U, the “depleted” uranium is still significantly radioactive as is the natural uranium after refining.

Another way to look at this is as follows: CANDU reactors use natural uranium (0.71% fissile material). From Pressurized water reactors (PWRs) of typical design (most USA reactors are PWR) we note the fuel goes in with about 4% 235U and 96% 238U and comes out with about 1% 235U, 1% 239Pu and 95% 238U. If the 239Pu were removed (fuel reprocessing is not allowed in the USA) and this were added to the “depleted uranium” then we would have 1.2% fissile material in the reprocessed “depleted uranium” and at the same time have 1% fissile material in the left over “spent” fuel. Both of these would be considered “enriched” fuels for a CANDU style reactor.

Precautions

All isotopes and compounds of uranium are toxic and radioactive. Toxicity can be lethal. In less than lethal doses toxicity is limited primarily 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; however, 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. Houses or structures which are over uranium deposits (either natural or man-made slag deposits) may have an increased incidence of exposure to radon gas, a radioactive carcinogen.

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 kidney damage. Uranium itself is not a chemical carcinogen.

Uranium mining carries the danger of airborne radioactive dust and the release of radioactive radon gas and its daughter products (an added danger to the already dangerous activity of all hard rock mining). As a result, without proper ventilation, uranium miners have a dramatically increased risk of later development of lung cancer and other pulmonary diseases. There is also the possible danger of groundwater contamination with the toxic chemicals used in the separation of the uranium ore.

Uranium


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