Fraction Definitions
The factor β is defined as:
and it is equal to 0.0064 for U-235.
The delayed neutron fraction (DNF) is defined as:
These two factors, β and DNF, are not the same thing in case of a rapid change in the number of neutrons in the reactor.
Another concept, is the effective fraction of delayed neutrons, which is the fraction of delayed neutrons weighted (over space, energy, and angle) on the adjoint neutron flux. This concept arises because delayed neutrons are emitted with an energy spectrum more thermalized relative to prompt neutrons. For low enriched uranium fuel working on a thermal neutron spectrum, the difference between the average and effective delayed neutron fractions can reach 50 pcm (1 pcm = 1e-5).
Read more about this topic: Prompt Neutron
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