How does a nuclear reaction become a chain reaction?

The idea of a chain reaction is that the neutrons released by fission go on to cause fission in other nearby atoms of the same element or others For example: An atom of Uranium-235 decays, sending out 2 or 3 neutrons. If one of these neutrons is absorbed by another U-235 atom, that U-235 atom becomes unstable, and quickly splits, again sending out 2 or 3 of its own neutrons (plus a relatively large amount of energy). If there are enough uranium atoms close enough to continue the process (a critical mass), the reaction escalates exponentially and becomes self-sustaining.

If the release of neutrons is just enough to maintain fissioning, you have a nuclear reactor. If there are many neutrons released all at once, you have a nuclear bomb.

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