¯ ¯ ¯ eff REACTOR KINETICS DOE-HDBK-1019/2-93 Reactor Theory (Reactor Operations) NP-04 Rev. 0 Page 12 TABLE 1 Delayed Neutron Fractions for Various Fuels Group Half-Life (sec) Uranium-235 Uranium-238 Plutonium-239 1 55.6 0.00021 0.0002 0.00021 2 22.7 0.00141 0.0022 0.00182 3 6.22 0.00127 0.0025 0.00129 4 2.30 0.00255 0.0061 0.00199 5 0.61 0.00074 0.0035 0.00052 6 0.23 0.00027 0.0012 0.00027 TOTAL - 0.00650 0.0157 0.00200 The term     (pronounced beta-bar) is the average delayed neutron fraction.  The value of      is the weighted average of the total delayed neutron fractions of the individual types of fuel.  Each total delayed neutron fraction value for each type of fuel is weighted by the percent of total neutrons that the fuel contributes through fission.  If the percentage of fissions occurring in the different types of fuel in a reactor changes over the life of the core, the average delayed neutron fraction will also change.  For a light water reactor using low enriched fuel, the average delayed neutron  fraction  can  change  from  0.0070  to  0.0055  as  uranium-235  is  burned  out  and plutonium-239 is produced from uranium-238. Delayed neutrons do not have the same properties as prompt neutrons released directly from fission.  The average energy of prompt neutrons is about 2 MeV.  This is much greater than the average energy of delayed neutrons (about 0.5 MeV).  The fact that delayed neutrons are born at lower energies has two significant impacts on the way they proceed through the neutron life cycle.  First, delayed neutrons have a much lower probability of causing fast fissions than prompt neutrons because their average energy is less than the minimum required for fast fission to occur.  Second, delayed neutrons have a lower probability of leaking out of the core while they are at fast energies, because they are born at lower energies and subsequently travel a shorter distance as fast neutrons.  These two considerations (lower fast fission factor and higher fast non-leakage probability for delayed neutrons) are taken into account by a term called the importance factor (I).  The importance factor relates the average delayed neutron fraction to the effective delayed neutron fraction. The effective delayed neutron fraction is defined as the fraction of neutrons at thermal energies which were born delayed.  The effective delayed neutron fraction is the product of the average delayed neutron fraction and the importance factor.