Editing Delayed neutron (section)

==Fraction definitions==

The precursor yield fraction β 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 almost the same thing, but not quite; they differ in the case a rapid (faster than the decay time of the precursor atoms) change in the number of neutrons in the reactor.

Another concept, is the ''effective fraction of delayed neutrons'' β<sub>eff</sub>, 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 [[Per cent mille|50 pcm]].<ref>{{cite journal|title=Deterministic and Monte Carlo Modeling and Analyses of Yalina-Thermal Subcritical Assembly.|first1=A.|last1=Talamo|first2=Y.|last2=Gohar|first3=Nuclear Engineering|last3=Division|date=29 July 2010|doi=10.2172/991100 |osti = 991100|url=https://digital.library.unt.edu/ark:/67531/metadc1012530/ }}</ref>