What is a chain reaction in nuclear fission?

A chain reaction in nuclear fission is a self-sustaining process where the splitting of one atomic nucleus triggers further fissions.

Here’s how it works:

1. Fission event
   • A neutron strikes a heavy nucleus (like uranium-235 or plutonium-239).
   • The nucleus splits into two smaller nuclei (fission fragments), releasing energy and more neutrons (usually 2–3).
2. Released neutrons
   • These neutrons can collide with other nearby nuclei.
3. More fissions
   • If each fission causes (on average) at least one more fission, the reaction continues.
4. Outcomes
   • Controlled chain reaction: In a nuclear reactor, control rods and moderators keep the reaction steady, producing heat for electricity.
   • Uncontrolled chain reaction: In a nuclear bomb, the reaction grows explosively, releasing enormous energy in a fraction of a second.

The key factor is the neutron economy:

• If each fission produces fewer than one further fission → reaction dies out.
• If exactly one → steady, controlled reaction.
• If more than one → reaction grows rapidly (exponential increase).