Uranium is a weakly radioactive, silvery-grey metallic chemical element with a density that is about 70 percent greater than lead and slightly lower than gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rocks and water, and is extracted commercially from uranium-bearing minerals such as uraninite.
Nuclear power plants and nuclear weapons make use of uranium's unique nuclear properties. Uranium-235 is the only naturally occurring isotope of an element that is fissile, that is, whose atoms can be split readily and yield a large amount of energy under certain conditions.