The four-factor formula is used in nuclear engineering to determine the multiplication of a nuclear chain reaction in an infinite medium. The formula is[1]
| Symbol | Name | Meaning | Formula |
|---|---|---|---|
| Reproduction Factor (Eta) | The number of fission neutrons produced per absorption in the fuel. | = νσf/σa | |
| The thermal utilization factor | Probability that a neutron that gets absorbed does so in the fuel material. | = ΣaF/Σa | |
| The resonance escape probability | Fraction of fission neutrons that manage to slow down from fission to thermal energies without being absorbed. | ||
| The fast fission factor | |
The multiplication factor, k, is defined as
If k is greater than 1, the chain reaction is supercritical, and the neutron population will grow exponentially.
If k is less than 1, the chain reaction is subcritical, and the neutron population will exponentially decay.
If k = 1, the chain reaction is critical and the neutron population will remain constant.
In an infinite medium, neutrons cannot leak out of the system and the multiplication factor becomes the infinite multiplication factor, , which is approximated by the four-factor formula.
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