The second nuclear weapon used in warfare was dropped on Nagasaki, Japan, on August 9, 1945 (the first, "Little Boy," had been dropped on Hiroshima just three days before). "Fat Boy" was dropped from a B-29 piloted by Major Charles W. Sweeney. When the bomb reached 1,850 feet, control plugs and a radar antenna detonated an explosive charge which crushed a hollow sphere of plutonium into a beryllium-polonium core, creating an explosive force equal to 20 kilotons of TNT. At least 40,000 people were killed in the blast.

How It Worked

"Fat Man" was 7 feet 8 inches long, 5 feet in diameter, and weighed 10,200 pounds. It was an implosion-type weapon with a plutonium core. A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosive. Numerous detonators located on the surface of the high explosive were fired simultaneously to produce a powerful inward pressure on the core, squeezing it and increasing its density, resulting in a supercritical condition and a nuclear explosion.

The implosion method used for "Fat Man" is not only more efficient than the gun-type method used for "Little Boy," it is also far safer. Because a perfect synchronization of the explosive charges is required for the core to properly detonate, the chances of an accidental nuclear detonation due to a plane crash or fire are practically nil.

"Little Boy"
B-29

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This page was last updated on 01/06/2013.