Uranium, a radioactive, white, metallic element. It is the chief source of nuclear energy in nuclear power plants. It is also used in making nuclear weapons. Uranium compounds have been used to color glass, to stain wood and leather, and as toners in photography and mordants (fixers) in dyeing silk and wool. Uranium was discovered by Martin Klaproth of Germany in 1789. Henri Becquerel of France discovered radioactivity while working with uranium in 1896.
Uranium is hard, but it may be hammered into shape. It is extremely heavy, 18.7 times as dense as water. By radioactive decay, it eventually becomes lead. Natural uranium is made up of three isotopes (forms of the same element having different atomic weights). They are U-238, U-235, and U-234. More than 99 per cent of natural uranium is U-238. Other isotopes are produced in nuclear reactors.
When U-235 is bombarded with neutrons, some of its atoms split into two smaller atoms, releasing energy and one or more neutrons. This process is called fission. When a quantity of U-235 is arranged in a certain way, neutrons released by a fission reaction can be made to split yet other U-235 atoms, producing a chain reaction in which a large amount of energy is released. Although U-238 is not fissionable, it can be used to produce plutonium 239, which is fissionable.
Uranium is present to some extent in nearly all of the rocks in the earth's crust. It is more abundant than gold and silver, but less common than nickel, copper, or lead. Uranium is always found in chemical combination with other elements, with which it forms more than 100 known minerals. The most common uranium minerals are uraninite—especially the variety known as pitchblende—and carnotite. The largest deposits of these minerals are in Australia, South Africa, Canada, Brazil, Niger, and the United States.
Symbol: U. Atomic number: 92. Atomic weight (of the most stable isotope): 238. Specific gravity: 18.9. Melting point: 2,069.4 F. (1,132 C.). Boiling point: 6,904 F. (3,818 C.). Half life: U-238, 4,510,000,000 years; U-235, 710,000,000 years; U-234, 250,000 years. Uranium belongs to the actinide series of the Periodic Table and may have a valence of + 3, 4, 5, or 6.