A radioactive, silvery-white, metallic element. Symbol, U. Occurs in numerous minerals such as pitchblende, uraninite, carnotite, autunite, uranophane, davidite, and tobernite. It is also found in phosphate rock, lignite, and monazite sands. Uranium and its compounds are highly toxic, both chemically and radiologically. Uranium is of great importance as a nuclear fuel; it is used as ballast for missile reentry vehicles, as a shielding material, and for production of high-energy X-rays.