Worker handling plutonium in a glove box at a processing plant at Los Alamos National Laboratory, New Mexico, USA. Plutonium (atomic number 94, symbol Pu) is almost exlusively a man- made substance, although a minute quantity of its major isotope, Pu-239, is formed by the decay of uranium, the heaviest naturally-occurring element. Pu-239 is capable of undergoing nuclear fission (splitting) into smaller nuclei, a process that is accompanied by a vast release of energy. Hence its use as an explosive in atomic bombs and as a fuel in nuclear reactors. Plutonium is highly toxic and a potent carcinogen (cancer-forming agent), even in minute quantities.