Editing Prompt neutron (section)

==Fraction definitions==

The factor <math>\beta</math> is defined as:

:<math>
\beta = \frac{\mbox{precursor atoms}}
             {\mbox{prompt neutrons}+\mbox{precursor atoms}}.
</math>

and it is equal to 0.0064 for [[U-235]].

The delayed neutron fraction (DNF) is defined as:

:<math>
DNF = \frac{\mbox{delayed neutrons}}
           {\mbox{prompt neutrons}+\mbox{delayed neutrons}}.
</math>

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).<ref>[http://www.osti.gov/bridge/product.biblio.jsp?query_id=1&page=0&osti_id=991100 Deterministic and Monte Carlo Analyses of YALINA Thermal Subcritical Assembly]</ref>