Four factor formula

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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]

$k_{\infty} = \eta f p \epsilon$

Symbol	Name	Meaning	Formula
$η$	Eta	The number of fission neutrons produced per absorption in the fuel.	$η$ = νσ_f/σ_a
$f$	The thermal utilization factor	Probability that a neutron that gets absorbed does so in the fuel material.	$f$ = Σ_aF/Σ_a
$p$	The resonance escape probability	Fraction of fission neutrons that manage to slow down from fission to thermal energies without being absorbed.
$ε$	The fast fission factor	$\frac{\mbox{total number of fission neutrons}}{\mbox{number of fission neutrons from just thermal fissions}}$

[edit] Multiplication

The multiplication factor, k, is defined as

$k = \frac{\mbox{number of neutrons in one generation}}{\mbox{number of neutrons in preceding generation}}$

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, $k = k_{\infty}$ , which is approximated by the four-factor formula.