Britannica Sources used in this answer

How does a fission chain reaction produce an atomic explosion?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

U.S. hydrogen bomb test in the Marshall Islands — nuclear weapon A test of a U.S. thermonuclear weapon (hydrogen bomb) at Enewetak atoll in the Marshall Islands, November 1, 1952.

An atomic explosion is produced by a rapidly divergent nuclear chain reaction within a fissile material, such as uranium-235 or plutonium-239.1

The process begins when a neutron strikes a fissile nucleus, causing it to undergo nuclear fission. This split releases significant energy and, on average, two to three additional neutrons. In an atomic bomb, the configuration is made "supercritical," meaning the multiplication factor (k) is greater than one. This ensures that more than one neutron from each fission event successfully induces another fission, causing the number of fissions to grow exponentially.12

To achieve an explosion, this reaction must occur extremely rapidly—within approximately $10^{- 7}$ seconds—so that a massive amount of energy is released in a brief period. By maintaining a supercritical state, the chain reaction accelerates until the material is dispersed, resulting in a devastating nuclear explosion.21