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Physicist J. Robert Oppenheimer, head of the Manhattan Project. Courtesy of the National Archives and Records Administration.

An explosive device that releases nuclear energy (energy that comes from an atom's core). All previous explosive devices were powered by rapid burning or decomposition of a chemical compound; they only released energy from the outermost electrons of an atom. Nuclear explosives are energized by splitting an atom, a process called fission. On July 16, 1945 the first atomic bomb was detonated at 5:30 a.m. The resulting implosion initiated a chain reaction of nearly 60 fission generations in about a micro-second. It produced an intense flash of light, followed by a fireball expanding to a diameter of about 600 meters in two seconds, and then it rose to a height of more than 12 kilometers, forming its ominous mushroom shape. Forty seconds later, an air blast hit the observer bunkers, followed by a sustained and awesome roar. In August 1945, the United States dropped atomic bombs on Japan over the cities of Hiroshima and Nagasaki. The Hiroshima bomb, using the code name "Little Boy," had a force of 13,000 tons (11,791 metric tons), or 13 kilotons, of TNT. Its element base was uranium-235. The Nagasaki bomb, using the code name "Fat Man," had a force of 22,000 tons (19,954 metric tons), or 22 kilotons, of TNT. Its element base was plutonium. In 1949 the Soviet Union tested its first atomic bomb, followed by Britain (1952), France (1960), China (1964), India (1974), and Pakistan (1998).