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Four factor formula
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  • The way in which neutrons are increased and absorbed can be described by a combination of factors in the 'four factor formula'
  • Neutrons are increased in two ways
    • Thermal fission factor h
    • Fast fission factor e
  • Neutrons are absorbed in two ways
    • Resonance escape probability p
    • Thermal utlisation factor f

h = the average number of neutrons produced per thermal neutron absorbed in the fuel.

e = the number of fission neutrons produced by fast neutrons before they are moderated. This will be largely in the 238U.

f = the fraction of thermal neutrons absorbed in the reactor that are absorbed in the fuel.

p = the fraction of neutrons that are in the course of thermalisation escape being captured by the resonance structure of 238U.

L = combined leakage factor L < 1

k = hepf  

keff = hepfL


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