Four factor formula

The four-factor formula, also known as Fermi's four factor formula is used in nuclear engineering to determine the multiplication of a nuclear chain reaction in an infinite medium.
The six factor formula defines each of these terms in much more detail.
Multiplication.
The multiplication factor, , is defined as (see Nuclear chain reaction): 
In an infinite medium, neutrons cannot leak out of the system and the multiplication factor becomes the infinite multiplication factor, formula_2, which is approximated by the four-factor formula.