A pellet of plutonium, illuminated by the glow of its own radioactivity. Plutonium (Atomic number 94, symbol Pu) does not occur in nature except in minute quantities from the decay of Uranium-238, the heaviest naturally- occuring element. All of plutonium's isotopes are radioactive; Plutonium- 239 being of major importance as an explosive in atomic bombs and a fuel in types of nuclear reactors. Plutonium is a highly toxic and carcinogenic (cancer forming) substance and has been found to be highly dangerous even when present in minute concentrations.