Uranium is a silvery-gray metallic chemical element in the actinide series of the periodic table. It has the chemical symbol U and atomic number (number of protons) 92. It is weakly radioactive. It occurs naturally in low concentrations in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite.
In nature, uranium atoms exist as uranium-238, uranium-235, and a very small amount of uranium-234. Uranium decays slowly by emitting an alpha particle.
It is used for its nuclear properties, including nuclear power generation and nuclear weapons, as well as the mechanical characteristics of depleted uranium, which is all 238U. Depleted uranium is extremely dense, and has a number of applications as mechanical ballast. Its combined density and pyrophoric property make it effective as antitank ammunition.
U.S. civilian power plants typically use 3 to 5 percent uranium-235. Weapons use "highly enriched uranium" (HEU) with over 90 percent uranium-235. During the Manhattan Project, enriched uranium was given the codename oralloy, a shortened version of Oak Ridge alloy, after the location of the plants where the uranium was enriched.