Most neutrons (99.35 % for 235U fission by thermal neutrons) are emitted immediately by a nuclear fission event. These are called “prompt neutrons.” A few neutrons are emitted a little after nuclear fission occurs and they are called “delayed neutrons.”
The delayed neutrons are primarily produced from the decay of the fission products emitting neutrons. The fission products that emit delayed neutrons are called delayed neutron precursors. There are many delayed neutron precursors and they have different half-lives. The delayed neutron precursors are treated in six groups with different half-lives for analysis of nuclear reactor kinetics.
Delayed neutron data differ from those of the fission nuclides and are between the data of thermal fission and fast fission neutrons. The data need to be used correctly depending on the type of the fuel and neutron spectrum of the reactor.
The delayed neutrons have approximately 0.4-MeV average energy, which is lower than the approximate 2-MeV average energy of the prompt neutrons. Therefore, the fraction of delayed neutrons that is leaked outside the reactor and lost disappear is slightly smaller than that of the prompt neutrons. The fraction of delayed neutrons that contributes to the fission chain reactions is slightly larger than that of the prompt neutrons.
This effect is considered in the analysis of nuclear reactor kinetics. A slightly larger delayed neutron fraction is used than the absolute value “b” depending on the reactor and the effect is shown as “b effective” If the reactor has a large core volume, neutron leakage is very small during moderation and there is almost no difference between them. The “b effective” value depends on the reactor size and neutron spectrum. Although the delayed neutron fraction is low, it slows down the transient change of the reactor and, therefore, it plays a very important role in the reactor control.