Uranium [U]

Characteristics

An: 92 N: 146

Am: 238.02891 (3) g/mol

Group Name: Actinoid

Block: f-block Period: 7 (actinoid)

State: solid at 298 K

Colour: metallic grey Classification: Metallic

Boiling Point: 4200K (3927^oC)

Melting Point: 1405.3K (1132.2^oC)

Superconducting temperature: 0.2K (-272.9^oC)

Density: 19.1g/cm³

Discovery Information

Who: Martin Klaproth

When: 1789

Where: Germany

Name Origin

From planet Uranus. "Uranium" in different languages.

Sources

Occurs in many rocks, but in large amounts only in such minerals as pitchblende and carnotite (K₂(UO₂)₂(VO₄)₂ - 3H₂O). Annual production is around 35 thousand tons.

Abundance

Universe: 0.0002 ppm (by weight)

Sun: 0.001 ppm (by weight)

Carbonaceous meteorite: 0.010 ppm

Earth’s Crust: 1.8 ppm

Seawater: 3.13 x 10^-3 ppm

Human: 1 ppb by weight; 0.03 ppb by atoms

Uses

For many centuries it was used as a pigment for glass. Now it is used as a fuel in nuclear reactors and in nuclear bombs. Depleted Uranium (238U) is used in casings of armour piercing artillery shells, armour plating on tanks and as ballast in the wings of some large aircraft.

History

The use of uranium in its natural oxide form dates back to at least the year AD79, when it was used to add a yellow colour to ceramic glazes. Yellow glass with 1% uranium oxide was found in a Roman villa on Cape Posillipo in the Bay of Naples, Italy by R. T. Gunther of the University of Oxford in 1912. Starting in the late Middle Ages, pitchblende was extracted from the Habsburg silver mines in Joachimsthal, Bohemia (now Jachymov in the Czech Republic) and was used as a colouring agent in the local glassmaking industry. In the early 19th century, the world’s only known source of uranium ores were these old mines.

The discovery of the element is credited to the German chemist Martin Heinrich Klaproth. While he was working in his experimental laboratory in Berlin in 1789, Klaproth was able to precipitate a yellow compound (likely sodium diuranate) by dissolving pitchblende in nitric acid and neutralizing the solution with sodium hydroxide (NaOH). Klaproth mistakenly assumed the yellow substance was the oxide of a yet-undiscovered element and heated it with charcoal to obtain a black powder, which he thought was the newly discovered metal itself (in fact, that powder was an oxide of uranium). He named the newly discovered element after the planet Uranus, which had been discovered eight years earlier by William Herschel.

In 1841, Eugene-Melchior Peligot, who was Professor of Analytical Chemistry at the Conservatoire des arts et metiers (Central School of Arts and Manufactures) in Paris, isolated the first sample of uranium metal by heating uranium tetrachloride with potassium. Uranium was not seen as being particularly dangerous during much of the 19th century, leading to the development of various uses for the element. One such use for the oxide was the aforementioned but no longer secret colouring of pottery and glass.

Antoine Becquerel discovered radioactivity by using uranium in 1896. Becquerel made the discovery in Paris by leaving a sample of uranium on top of an unexposed photographic plate in a drawer and noting that the plate had become ’fogged’. He determined that a form of invisible light or rays emitted by uranium had exposed the plate.

Notes

Uranium metal has very high density, 65% more dense than lead, but slightly less dense than gold.

70% of the world’s known Uranium is located in Australia. The Australian government is currently advocating an expansion of uranium mining, although issues with state governments and indigenous interests complicate the issue.

Hazards

Potential occupational carcinogen (lung cancer). All isotopes and compounds of uranium are very toxic, teratogenic and radioactive. Finely-divided uranium metal presents a fire hazard because uranium is pyrophoric, so small grains will ignite spontaneously in air at room temperature.

Uranium Compounds

Uranyl acetate (UO₂(CH₃COO)₂.2H₂O) : Highly Toxic :

It is used as a negative stain in electron microscopy, in fact, most procedures in electron microscopy for biology require the use of uranyl acetate. 1% and 2% uranyl acetate solutions are used as an indicator, and a titrant in stronger concentrations in analytical chemistry, as it forms an insoluble salt with sodium.

Reactions of Uranium

Reactions with air

The most common forms of uranium oxide are triuranium octaoxide (U₃O₈) and UO₂. Both oxide forms are solids that have low solubility in water and are relatively stable over a wide range of environmental conditions.

Triuranium octaoxide is (depending on conditions) the most stable compound of uranium and is the form most commonly found in nature. Uranium dioxide is the form in which uranium is most commonly used as a nuclear reactor fuel. At ambient temperatures, UO₂ will gradually convert to U₃O₈. Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal.

Reactions with halogens

All uranium fluorides are created using uranium tetrafluoride (UF₄); UF₄ itself is prepared by hydrofluorination of uranium dioxide. Reduction of UF₄ with hydrogen at 1000^oC produces uranium trifluoride (UF₃). Under the right conditions of temperature and pressure, the reaction of solid UF₄ with gaseous uranium hexafluoride (UF₆) can form the intermediate fluorides of U₂F₉, U₄F₁₇, and UF₅.

One method of preparing uranium tetrachloride (UCl₄) is to directly combine chlorine with either uranium metal or uranium hydride. The reduction of UCl₄ by hydrogen produces uranium trichloride (UCl₃) while the higher chlorides of uranium are prepared by reaction with additional chlorine. All uranium chlorides react with water and air.

Bromides and iodides of uranium are formed by direct reaction of, respectively, bromine and iodine with uranium or by adding UH₃ to those element’s acids. Known examples include: UBr₃, UBr₄, UI₃, and UI₄.

Occurrence and Production of Uranium

Occurrence

Uranium is a naturally occurring element that can be found in low levels within all rock, soil, and water. Uranium is also the highest-numbered element to be found naturally in significant quantities on earth and is always found combined with other elements. Along with all elements having atomic weights higher than that of iron, it is only naturally formed in supernova explosions. The decay of uranium, thorium and potassium-40 in the Earth’s mantle is thought to be the main source of heat that keeps the outer core liquid and drives mantle convection, which in turn drives plate tectonics.

Its average concentration in the Earth’s crust is (depending on the reference) 2 to 4 parts per million, or about 40 times as abundant as silver. The Earth’s crust from the surface to 25 km (15 mi) down is calculated to contain 1017 kg of uranium while the oceans may contain 1013 kg. The concentration of uranium in soil ranges from 0.7 to 11 parts per million (up to 15 parts per million in farmland soil due to use of phosphate fertilizers), and 3 parts per billion of sea water is composed of the element.

It is more plentiful than antimony, tin, cadmium, mercury, or silver, and it is about as abundant as arsenic or molybdenum. It is found in hundreds of minerals including uraninite (the 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 with as little as 0.1% uranium).

Citrobacter species can have concentrations of uranium in their bodies 300 times higher than in the surrounding environment.Some microorganisms, such as the lichen Trapelia involuta or the bacterium Citrobacter, can absorb concentrations of uranium that are up to 300 times higher than their environment. Citrobactor species absorb uranyl ions when given glycerol phosphate (or other similar organic phosphates). After one day, one gram of bacteria will encrust themselves with nine grams of uranyl phosphate crystals; this creates the possibility that these organisms could be used to decontaminate uranium-polluted water.

Plants absorb some uranium from the soil they are rooted in. Dry weight concentrations of uranium in plants range from 5 to 60 parts per billion, and ash from burnt wood can have concentrations up to 4 parts per million. Dry weight concentrations of uranium in food plants are typically lower with one to two micrograms per day ingested through the food people eat.

Production and Mining

Uranium ore is mined in several ways: by open pit, underground, or by in-situ leaching (see uranium mining). Low-grade uranium ore typically contains 0.1 to 0.25% of actual uranium oxides, so extensive measures must be employed to extract the metal from its ore. High-grade ores found in Athabasca Basin deposits in Saskatchewan, Canada can contain up to 70% uranium oxides, and therefore must be diluted with waste rock prior to milling. Uranium ore is crushed and rendered into a fine powder and then leached with either an acid or alkali. The leachate is then subjected to one of several sequences of precipitation, solvent extraction, and ion exchange. The resulting mixture, called yellowcake, contains at least 75% uranium oxides. Yellowcake is then calcined to remove impurities from the milling process prior to refining and conversion.

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 calcium chloride (CaCl2) and sodium chloride (NaCl) solution. Very pure uranium can be produced through the thermal decomposition of uranium halides on a hot filament.

Availability

It is estimated that there is 4.7 million tonnes of uranium ore reserves (economically mineable) known to exist, while 35 million tonnes are classed as mineral resources (reasonable prospects for eventual economic extraction). An additional 4.6 billion tonnes of uranium are estimated to be in sea water (Japanese scientists in the 1980s proved that extraction of uranium from sea water using ion exchangers was feasible).

Exploration for uranium is continuing to increase with US$200 million being spent world wide in 2005, a 54% increase on the previous year.

Australia has 38% of the world’s uranium ore resources - the most of any country. In fact, the world’s largest single uranium deposit is located at the Olympic Dam Mine in South Australia. Almost all the uranium is exported, under strict International Atomic Energy Agency safeguards to satisfy the Australian people and government that none of the uranium is used in nuclear weapons. As of 2006, the Australian government was advocating an expansion of uranium mining, although issues with state governments and indigenous interests complicate the issue.

The largest single source of uranium ore in the United States was the Colorado Plateau located in Colorado, Utah, New Mexico, and Arizona. The U.S. federal government paid discovery bonuses and guaranteed purchase prices to anyone who found and delivered uranium ore, and was the sole legal purchaser of the uranium. The economic incentives resulted in a frenzy of exploration and mining activity throughout the Colorado Plateau from 1947 through 1959 that left thousands of miles of crudely graded roads spider-webbing the remote deserts of the Colorado Plateau, and thousands of abandoned uranium mines, exploratory shafts, and tailings piles. The frenzy ended as suddenly as it had begun, when the U.S. government stopped purchasing the uranium.

In 2005, seventeen countries produced concentrated uranium oxides, with Canada (27.9% of world production) and Australia (22.8%) being the largest producers and Kazakhstan (10.5%), Russia (8.0%), Namibia (7.5%), Niger (7.4%), Uzbekistan (5.5%), the United States (2.5%), Ukraine (1.9%) and China (1.7%) also producing significant amounts. The ultimate supply of uranium is believed to very large and sufficient for at least the next 85 years although some studies indicate underinvestment in the late twentieth century may produce supply problems in the 21st century. It is estimated that for a ten times increase in price, the supply of uranium that can be economically mined is increased 300 times.

Isotopes of Uranium

²³²U [140 neutrons]

Abundance: synthetic

Half life: 68.9 years

Decay Energy: 5.414 MeV

Decays to ²²⁸Th.

²³³U [141 neutrons]

Abundance: synthetic

Half life: 159200 years

Decay Energy: 4.909 MeV

Decays to ²²⁹Th.

²³⁴U [142 neutrons]

Abundance: 0.006%

Half life: 245500 years

Decay Energy: 4.859 MeV

Decays to ²³⁰Th.

²³⁵U [143 neutrons]

Abundance: 0.72%

Half life: 7.038 x 10⁸ years

Decay Energy: 4.679 MeV

Decays to ²³¹Th.

²³⁵U is unique in its ability to cause a rapidly expanding fission chain reaction, i.e., it is fissile. In fact, U-235 is the only fissile isotope found in nature. It was discovered in 1935 by Arthur Jeffrey Dempster. A uranium nucleus that absorbs a neutron splits into two lighter nuclei; this is called nuclear fission. It releases either two or three neutrons which continue the reaction. In nuclear reactors, the reaction is slowed down by the addition of control rods which are made of elements such as boron, cadmium, and hafnium which can absorb a large number of neutrons. In nuclear bombs, the reaction is uncontrolled and the large amount of energy released creates a nuclear explosion.

²³⁶U [144 neutrons]

Abundance: synthetic

Half life: 2.342 x 10⁷ years

Decay Energy: 4.572 MeV

Decays to ²³²Th.

²³⁸U [146 neutrons]

Abundance: 99.275%

Half life: 4.468 x 10⁹ years

Decay Energy: 4.260 MeV

Decays to ²³⁴Th.

Uranium-238 is important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which is fissile, that is, can be broken apart by thermal neutrons.

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