(Nucl Phys; Symb: Pu) A heavy, radioactive, man-made, silvery metallic element with atomic number 94, mp 639. 5ºC. Note: Its most important isotope is fissionable plutonium-239, produced by neutron irradiation of uranium-238, which is used as reactor fuel and in weapons. In finely divided form it can ignite spontaneously in moist air. Plutonium is a radiological poison, being specifically absorbed by the bone marrow