Interactive nuclear fission chain reaction simulator showing how neutron multiplication leads to exponential energy release
One atom splits, three neutrons escape—each finds another, and suddenly you're producing millions
Nuclear fission doesn't scale linearly—it multiplies. A single U-235 nucleus absorbing a neutron releases 2–3 more neutrons, each capable of triggering another split, and k counts how many go on to cause another. Without control, this chain reaction grows exponentially. Push the control rods in, pull them out, and watch what the chain does.
Chain reaction simulator0 Fissions
Move the control rods. Watch the neutron count and k.
Subcritical — 0 Active Neutrons
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Neutron multiplication k
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In an uncontrolled chain reaction, every neutron released has the potential to trigger another fission, causing the number of reactions to double with each generation. This exponential growth is what makes nuclear weapons so destructive—within about a microsecond, roughly 10²³ nuclei (a hundred thousand billion billion) split for every kilotonne of TNT the blast releases. The critical mass is the minimum amount of fissile material needed for this self-sustaining reaction to occur; below it, too many neutrons escape without causing fissions. In practice, weapons use sophisticated compression techniques to achieve supercriticality, while a reactor core is held exactly critical (k = 1) while it operates and is made subcritical to shut it down.
Know This
Fission of U-235 releases approximately 200 MeV per nucleus, a million times more than chemical reactions—critical mass is the minimum amount needed for a self-sustaining chain reaction, while control rods absorb excess neutrons in reactors to maintain steady reaction rates.