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What makes a chain reaction self-sustaining?
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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 nuclear chain reaction becomes self-sustaining when, on average, at least one neutron released from a fission event successfully induces fission in another fissile nucleus.12
This condition is measured by the multiplication factor, k, defined as the ratio of the number of fissions in one generation to the number of fissions in the preceding generation. The reaction dynamics are categorized as follows:1
- Subcritical (k < 1): The neutron population decreases over time, and the reaction cannot be sustained.13
- Critical (k = 1): The neutron population remains constant, allowing for a steady-state chain reaction, as seen in nuclear reactors.13
- Supercritical (k > 1): The number of fissions increases at each step, leading to a divergent chain reaction, which is the principle behind nuclear explosions.12
To maintain this balance, the system must account for neutrons lost through leakage or non-fission capture. In reactors, operators use control rods made of neutron-absorbing materials to precisely adjust k and ensure the reaction remains stable.2413