¯¯¯effREACTOR KINETICSDOE-HDBK-1019/2-93Reactor Theory (Reactor Operations)NP-04Rev. 0Page 12TABLE 1Delayed Neutron Fractions for Various FuelsGroupHalf-Life (sec)Uranium-235Uranium-238Plutonium-239155.60.000210.00020.00021222.70.001410.00220.0018236.220.001270.00250.0012942.300.002550.00610.0019950.610.000740.00350.0005260.230.000270.00120.00027TOTAL-0.006500.01570.00200The term (pronounced beta-bar) is the average delayed neutron fraction. The value of isthe weighted average of the total delayed neutron fractions of the individual types of fuel. Eachtotal delayed neutron fraction value for each type of fuel is weighted by the percent of totalneutrons that the fuel contributes through fission. If the percentage of fissions occurring in thedifferent types of fuel in a reactor changes over the life of the core, the average delayed neutronfraction will also change. For a light water reactor using low enriched fuel, the average delayedneutron fraction can change from 0.0070 to 0.0055 as uranium-235 is burned out andplutonium-239 is produced from uranium-238.Delayed neutrons do not have the same properties as prompt neutrons released directly fromfission. The average energy of prompt neutrons is about 2 MeV. This is much greater than theaverage energy of delayed neutrons (about 0.5 MeV). The fact that delayed neutrons are bornat lower energies has two significant impacts on the way they proceed through the neutron lifecycle. First, delayed neutrons have a much lower probability of causing fast fissions thanprompt neutrons because their average energy is less than the minimum required for fast fissionto occur. Second, delayed neutrons have a lower probability of leaking out of the core whilethey are at fast energies, because they are born at lower energies and subsequently travel ashorter distance as fast neutrons. These two considerations (lower fast fission factor and higherfast non-leakage probability for delayed neutrons) are taken into account by a term called theimportance factor(I). The importance factor relates the average delayed neutron fraction to theeffective delayed neutron fraction.The effective delayed neutron fractionis defined as the fraction of neutrons at thermalenergies which were born delayed. The effective delayed neutron fraction is the product of theaverage delayed neutron fraction and the importance factor.

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