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What is uranium?

Uranium is a radioactive element that occurs naturally in low concentrations (a few parts per million) in soil, rock, and surface and groundwater. It is the heaviest naturally occurring element, with an atomic number of 92. Uranium in its pure form is a silver-colored heavy metal that is nearly twice as dense as lead. In nature, uranium atoms exist as several isotopes: primarily uranium-238, uranium-235, and a very small amount of uranium-234. (Isotopes are different forms of an element that have the same number of protons in the nucleus, but a different number of neutrons.) In a typical sample of natural uranium, most of the mass (99.27%) consists of atoms of uranium-238. About 0.72% of the mass consists of atoms of uranium-235, and a very small amount (0.0055% by mass) is uranium-234.

Uranium is naturally radioactive, which means that atoms of uranium are unstable and decay by emitting particles and energy. Uranium decays very slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.5 billion years, which means it is not very radioactive. In fact, its very long half-life (and thus low radioactivity) is the reason uranium still exists on the earth. Three additional isotopes of uranium are not naturally present but can be produced by nuclear transformations. These are uranium-232, uranium-233, and uranium-236. Like the natural uranium isotopes, these three also decay by emitting an alpha particle.

For many years, uranium was used primarily as a colorant in ceramic glazes, producing colors that ranged from orange-red to lemon yellow. It was also used for tinting in early photography. Its radioactive properties were not recognized until 1896, and its potential for use as an energy source was not realized until the middle of the 20th century. Its primary use is as fuel in nuclear power reactors to generate electricity. Uranium is also used in small nuclear reactors to produce isotopes for medical and industrial purposes around the world.

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