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What does the multiplication factor k measure in a fission chain reaction?
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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.
In the context of nuclear fission, the multiplication factor, denoted as k, is a critical parameter that measures the ratio of the number of fissions produced in one generation of a chain reaction to the number of fissions in the preceding generation.1
The value of k determines the state of the fission process:
- Subcritical (k < 1): The chain reaction cannot be sustained, and the fission rate will eventually drop to zero.12
- Critical (k = 1): A steady-state chain reaction is maintained, where the fission rate remains constant.12
- Supercritical (k > 1): The number of fissions increases at each step, leading to a divergent chain reaction.12
In a nuclear reactor, k is typically managed using control rods to maintain steady-state operation, while in a fission bomb, the goal is to make k as large as possible to generate a rapid, massive release of energy.1