A radioactive element which occurs only in trace amounts in nature, with atomic number 94 and symbol Pu. As produced by irradiating uranium fuels, plutonium contains varying percentages of the isotopes 238, 239, 240, 241 and 242. Plutonium containing any ²³⁹Pu is considered a special fissionable material and, except for plutonium containing 80% or more of ²³⁸Pu, a direct use material