Fat Man Mark the Third

"Fat Man" was the design of the nuclear weapon the United States used for seven of the first eight nuclear weapons ever detonated in history. It is also the most powerful design to ever be used in warfare. A Fat Man device was detonated over the Japanese city of Nagasaki on 9 August 1945. Wikipedia

Plutonium is the element with the highest atomic number known to occur in nature. In nuclear engineering, fissile material, like plutonium, is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal chain reaction can only be achieved with fissile material.