Facts About Uranium

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Atomic Number: 92
Atomic Symbol: U
Atomic Weight: 238.02891
Melting Point: 2,075 F (1,135 C)
Boiling Point: 7,468 F (4,131 C)

Word Origin: Uranium is named after the planet Uranus, which was newly discovered when the element was identified in 1789. The planet is named after the Greek god of the sky or heaven.

Discovery: Marin H. Klaproth is generally credited with identifying uranium as an element while experimenting with pitchblende in 1789. Humans had, however, been aware of uranium for millennia, using its natural oxide as a yellow coloring agent for ceramic glazes as early as A.D. 79. Klaproth did not identify the pure element, but uranium oxide. The element was first isolated by Eugène-Melchior Péligot in 1841.

Properties of uranium

Pure uranium is a silvery white metal. It is weakly radioactive. Pure uranium metal is malleable, ductile, slightly paramagnetic, and strongly electropositive. It is a poor electrical conductor. It is harder than most other elements, though a little softer than steel, and has a very high density — about 70 percent denser than lead and slightly less dense than gold. Uranium is the heaviest naturally occurring element available in large quantities. Uranium metal has three crystallographic modifications: alpha, beta and gamma. [See Periodic Table of the Elements]

Hydrochloric and nitric acids dissolve uranium metal, and non-oxidizing acids attack it slowly. It is unaffected by alkalis. Uranium reacts with almost all nonmetallic elements in their compounds, and reactivity increases with temperature. When finely divided, uranium is pyrophoric and can react with cold water.

Uranium metal oxidizes in air. It becomes coated with a dark layer of uranium oxide. Its oxidation states form a variety of alloys and compounds, the most important being uranium (IV) and uranium (VI), with their corresponding oxides of uranium dioxide and uranium trioxide. In addition to their oxides, other significant uranium compounds include fluorides, chlorides, bromides, iodides, hydrides, carbonates, barbides, nitrides and phosphates.

Uranium exists in aqueous solutions at 3, +4, +5, and +6 oxidation states. Oxidation state +6 as the UO22+ ion (which is yellow in color) is the most stable state in solution.

Uranium and its compounds are highly toxic, both from a chemical and radiological standpoint. Finely divided uranium metal, being pyrophoric, presents a fire hazard.

Sources of uranium

Though naturally occurring uranium exists in low concentrations (a few parts per million) in soil, rocks, and water, it is not as rare as once thought. It is now considered to be about as abundant as molybdenum and arsenic, and more plentiful than mercury, silver, cadmium, and antimony. Uranium occurs naturally in numerous minerals including pitchblende, carnotite, uranophane, autunite and tobernite. It is also found in phosphate rocks, monazite sands, and lignite. It is commercially recovered from all of these sources. The United States Department of Energy purchases uranium in the form of acceptable U3O8 concentrates. This purchasing has proved to be a great incentive for uranium recovery and has led to more known uranium reserves.

In nature, U(VI) forms highly soluble carbonate complexes at alkaline pH. It often forms at nuclear waste repositories, and the solubility of the carbonate complexes means that uranium is more mobile. This increases the presence of uranium in groundwater and soil around nuclear waste repositories, which can cause health hazards.

Uranium metal 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 salt mixture of CaCl2 and NaCl. High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament.

Uses of uranium

People have been using uranium for millennia. In ancient Rome and during the Middle Ages, it was used as a coloring agent in ceramic glazes and glass. It produced hues from orange-red to lemon-yellow. In modern times, it was used as an orange glaze in the Fiestaware brand of dishware but was later discontinued for health reasons.

Today, uranium is mostly used for its unique nuclear properties. When in sufficient concentration, uranium’s many fissile isotopes can cause a nuclear chain reaction that generates heat in nuclear power reactors and produces the fissile material for nuclear weapons. One pound of completely fissioned uranium has the fuel value of over 1,500 tons of coal. The nuclear conversion for the fissile materials can also be made in breeder reactors, which can make more fissile material than the chain reaction. The atomic bomb dropped on Hiroshima, Japan, in 1945 had a uranium core.

U-235 is hugely important. It is considered the key to maximizing uranium. Though it occurs in natural uranium in small amounts, 235U 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. Concentrated 235U and natural uranium infused slightly with 235U can also be used directly as nuclear fuel to generate electricity. Additionally, 235U can be used as an explosive.

Uranium has other uses besides nuclear power and weapons. Uranium 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 salts have also been used for producing yellow "Vaseline" glass and glazes.238U that has been depleted of 235U is used in ballistic armor penetration and as armor plating. Uranium metal is used for X-ray targets for production of high-energy X-rays. Previously, the metal was used as photographic toner and the acetate was used in analytical chemistry.

Isotopes of uranium

There are 27 known isotopes of uranium and all are unstable. Their half-lives range from a few nanoseconds to billions of years. The half-life of 238U is about 4.47 billion years and that of 235U is 704 million years, making them useful in dating the age of the Earth. That also suggests that half of the uranium that existed from the formation of the Earth has decayed to other radioactive elements and eventually to stable elements. Much of the Earth’s internal heat is attributed to the decay of uranium and thorium radioisotopes.

Naturally occurring uranium consists of three major isotopes: 238U (99.28 percent abundance), 235U (0.71 percent) and 234U (0.0054 percent). All three isotopes are radioactive, with small probabilities of undergoing spontaneous fission but preferentially decaying by alpha emission. 235U has is the only naturally occurring fissile isotope. This means it can be split into two or three fragments (fission products) by thermal neutrons. 238U is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile 239Pu in a nuclear reactor. 239Pu was used as fissile material in the first atomic bomb detonated in the "Trinity test" on July 15, 1945 in New Mexico.

Another fissile isotope, 233U, can be produced from natural thoriumand is also important in nuclear technology. It has a higher fission cross-section for slow neutrons than other uranium isotopes.

(Source: Los Alamos National Laboratory)

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