Neutron Generation Time

The neutron generation time is the time required for neutrons from one generation to cause the fissions that produce the next generation of neutrons. The generation time for prompt neutrons (* - pronounced "ell-star") is the total time from birth to rebirth. Three time intervals are involved: (a) the time it takes a fast neutron to slow down to thermal energy, (b) the time the now thermal neutron exists prior to absorption in fuel, and (c) the time required for a fissionable nucleus to emit a fast neutron after neutron absorption.
Fast neutrons slow to thermal energies or leak out of the reactor in 10^-4 seconds to 10^-6 seconds, depending on the moderator. In water moderated reactors, thermal neutrons tend to exist for about 10^-4 seconds before they are absorbed. Fission and fast neutron production following neutron absorption in a fissionable nucleus occurs in about 10^-13 seconds. Thus, fast reactors have an of about 10-6 seconds, while thermal reactors have an of about 10^-6 seconds + 10^-4 seconds, which is about 10^-4 seconds to 10^-5 seconds.

On the other hand, the average generation time for the six delayed neutron groups is the total time from the birth of the fast neutron to the emission of the delayed neutron. Again, three time intervals are involved: (a) the time it takes a fast neutron to slow down to thermal energy, (b) the time the thermal neutron exists prior to absorption, and (c) the average time from neutron absorption to neutron emission by the six precursor groups. The average time for decay of precursors from uranium-235 is 12.5 seconds. The other terms in the delayed neutron generation time are insignificant when compared to this value, and the average delayed neutron generation time becomes ∼ 12.5 seconds.
A neutron generation time in the range of 10^-4 seconds to 10^-5 seconds or faster could result in very rapid power excursions, and control would not be possible without the dependence upon delayed neutrons to slow down the rate of the reaction. The average generation time, and hence the rate that power can rise, is determined largely by the delayed neutron generation time. The following equation shows this mathematically.

(1-1)