Nuclear fission produces the atomic bomb, a weapon of mass destruction that uses power released by the splitting of atomic nuclei.

When a single free neutron strikes the nucleus of an atom of radioactive material like uranium or plutonium, it knocks two or three more neutrons free. Energy is released when those neutrons split off from the nucleus, and the newly released neutrons strike other uranium or plutonium nuclei, splitting them in the same way, releasing more energy and more neutrons. This chain reaction spreads almost instantaneously.

See www.pcf.city.hiroshima.jp for a diagram of how this works in practice.
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Atomic bombs were exploded in war in Hiroshima and Nagasaki at the end of World War II. The characteristics of the bombs are shown below.

The Hiroshima Bomb

Due to its long, thin shape, the Hiroshima bomb was called “Little Boy”. The material used was uranium 235. It is believed that the fission of slightly less than one kilogram of uranium 235 released energy equivalent to approximately 15,000 tons of TNT

The Hiroshima Bomb "Little Boy"

Hiroshima Bomb Diagram - Length: Approx. 3 meters (120 inches)
Weight: Approx. 4 tons (9,000 lbs) Diameter: Approx. 0.7 meters (28 inches)
Element: Uranium 235

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The Nagasaki Bomb

Compared to the one used on Hiroshima, the Nagasaki bomb was rounder and fatter. It was called "Fat Man." The material used was plutonium 239. The fission of slightly more than one kilogram of plutonium 239 is thought to have released destructive energy equivalent to about 21,000 tons of TNT.

The Nagasaki Bomb "Fat Man"

Nagasaki Bomb Diagram - Length: Approx. 3.2 meters (128 inches)
Weight: Approx. 4.5 tons (10,000 pounds)
Diameter: Approx. 1.5 meters (60 inches)
Element: Plutonium 239

Diagrams are from http://www.pcf.city.hiroshima.jp/peacesite/English/Stage1/S1-3E.html

For more information, please see:
http://www.pcf.city.hiroshima.jp/peacesite/English/Stage1/S1-5E.html
http://www.pcf.city.hiroshima.jp/peacesite/English/Stage1/S1-3E.html

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http://people.howstuffworks.com/nuclear-bomb5.htm

The Hydrogen Bomb >>

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