Uranium is an element.
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes . Uranium is a chemical element with symbol U and atomic number 92 . Uranium has the highest atomic weight of the primordially occurring elements . It occurs naturally in low concentrations of a few parts per million in soil , rock and water , and is commercially extracted from uranium-bearing minerals such as uraninite . An ensuing arms race during the Cold War between the United States and the Soviet Union produced tens of thousands of nuclear weapons that used uranium metal and uranium-derived plutonium-239 .