Uranium, chemical element, symbol U; for physical constants see Periodic Table. Discovered by Martin H. Klaproth in 1789, it occurs in the minerals pitchblende, uraninite, carnotite, autunite, and uranophane, among others. It is prepared by reduction of the tetrafluoride in a Thermit type of reaction. Uranium is a silver-white, lustrous, radioactive, reactive metal, a member of the actinide series. It is pyrophoric. Uranium-235 is fissionable and is used as fuel in nuclear reactors, and in atomic and hydrogen bombs. One pound of uranium has the fuel value of 1,500 tons of coal. Uranium-238 is fertile and used to produce plutonium-239, which is fissionable in breeder reactors. Uranium and its salts are highly toxic.