Nuclear Fission

Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei, accompanied by the release of a large amount of energy. It is the process that powers nuclear power stations and was used in the first atomic weapons. Understanding fission requires bringing together everything we have learned about binding energy, the strong nuclear force, and mass-energy equivalence.

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Induced Fission

Fission can occur spontaneously in some very heavy isotopes, but the fission used in nuclear reactors is induced fission — triggered by a neutron striking a heavy nucleus.

The most important fission fuel is uranium-235. When a U-235 nucleus absorbs a slow-moving (thermal) neutron, it briefly becomes uranium-236 in a highly excited state. This compound nucleus is so unstable that it immediately splits into two medium-mass fission fragments plus two or three additional neutrons:

²³⁵₉₂U + ¹₀n → ²³⁶₉₂U → fission fragments + neutrons + energy*

A typical fission reaction:

²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3¹₀n + ~200 MeV