Claim: Plutonium reacts with silicon.

Plutonium is a transuranic radioactive chemical element with symbol Pu and atomic number 94 . It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air , and forms a dull coating when oxidized . The element normally exhibits six allotropes and four oxidation states . It reacts with carbon , halogens , nitrogen , silicon and hydrogen . When exposed to moist air , it forms oxides and hydrides that can expand the sample up to 70 % in volume , which in turn flake off as a powder that is pyrophoric . It is radioactive and can accumulate in bones , which makes the handling of plutonium dangerous .   Plutonium was first produced and isolated on December 14 , 1940 by Dr. Glenn T. Seaborg , Joseph W. Kennedy , Edwin M. McMillan , and Arthur C. Wahl by deuteron bombardment of uranium-238 in the 60-inch cyclotron at the University of California , Berkeley . They first synthesized neptunium-238 -LRB- half-life 2.1 days -RRB- which subsequently beta-decayed to form a new element with atomic number 94 and atomic weight 238 -LRB- half-life 87.7 years -RRB- . Uranium had been named after the planet Uranus and neptunium after the planet Neptune , and so element 94 was named after Pluto , which at the time was considered to be a planet as well . Wartime secrecy prevented them from announcing the discovery until 1948 . Plutonium is the element with the highest atomic number to occur in nature , as trace quantities similarly arise from natural uranium-238 capturing neutrons . Plutonium is much more common on Earth since 1945 as a product of neutron capture and beta decay , where some of the neutrons released by the fission process convert uranium-238 nuclei into plutonium-239 .   Both plutonium-239 and plutonium-241 are fissile , meaning that they can sustain a nuclear chain reaction , leading to applications in nuclear weapons and nuclear reactors . Plutonium-240 exhibits a high rate of spontaneous fission , raising the neutron flux of any sample containing it . The presence of plutonium-240 limits a plutonium sample 's usability for weapons or its quality as reactor fuel , and the percentage of plutonium-240 determines its grade -LRB- weapons-grade , fuel-grade , or reactor-grade -RRB- . Plutonium-238 has a half-life of 88 years and emits alpha particles . It is a heat source in radioisotope thermoelectric generators , which are used to power some spacecraft . Plutonium isotopes are expensive and inconvenient to separate , so particular isotopes are usually manufactured in specialized reactors .   Producing plutonium in useful quantities for the first time was a major part of the Manhattan Project during World War II that developed the first atomic bombs . The Fat Man bombs used in the Trinity nuclear test in July 1945 , and in the bombing of Nagasaki in August 1945 , had plutonium cores . Human radiation experiments studying plutonium were conducted without informed consent , and several criticality accidents , some lethal , occurred after the war . Disposal of plutonium waste from nuclear power plants and dismantled nuclear weapons built during the Cold War is a nuclear-proliferation and environmental concern . Other sources of plutonium in the environment are fallout from numerous above-ground nuclear tests , now banned .