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How does a fission chain reaction begin?

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nuclear fission Italian-born physicist Enrico Fermi explaining a problem in physics, c. 1950. He was awarded the 1938 Nobel Prize for Physics.

A fission chain reaction begins when a heavy atomic nucleus, such as uranium-235, undergoes nuclear fission after being struck by a neutron. This splitting process releases a significant amount of energy along with several additional neutrons.12

For a chain reaction to be sustained, at least one of these newly emitted neutrons must strike another fissile nucleus, inducing it to fission and release further neutrons. This sequence continues as long as the fissile material is present in a sufficient quantity known as critical mass, which allows the reaction to become self-sustaining.34

The progression of this reaction is determined by the multiplication factor, k, which represents the ratio of fissions in one generation to the previous one. If k is less than one, the reaction dies out; if k equals one, it remains steady; and if k is greater than one, the reaction diverges, leading to a rapid increase in fissions.3