Given a total interaction barns ), the mean free path of a prompt neutron is \ ell ^ {-1 } = n \ sigma where " n " is the nuclear number density.
22.
The "'prompt neutron lifetime "', " l ", is the average time between the emission of neutrons and either their absorption in the system or their escape from the system.
23.
Up to about 10 kilotons in yield, all nuclear weapons have prompt neutron radiation Enhanced radiation weapons also fall into this same yield range and simply enhance the intensity and range of the neutron dose for a given yield.
24.
These neutrons are sometimes emitted with a delay, giving them the term delayed neutrons, but the actual delay in their production is a delay waiting for the beta decay of fission products to produce the excited-state nuclear precursors that immediately undergo prompt neutron emission.
25.
Fermi argued that by using the delayed neutrons, and by carefully controlling the reaction rates as the power is ramped up, a pile can reach criticality at fission rates slightly below that of a chain reaction relying solely on the prompt neutrons from the fission reactions.
26.
In this case the time between successive generations of the reaction, " T ", is only limited by the fission rate from the prompt neutrons, and the increase in the reaction will be extremely rapid, causing a rapid release of energy within a few milliseconds.
27.
However, thanks to the delayed neutrons, it is possible to leave the reactor in a subcritical state as far as only prompt neutrons are concerned : the delayed neutrons come a moment later, just in time to sustain the chain reaction when it is going to die out.
28.
Most nuclear reactors are hence operated in a " prompt subcritical ", " delayed critical " condition : the prompt neutrons alone are not sufficient to sustain a chain reaction, but the delayed neutrons make up the small difference required to keep the reaction going.
29.
These are called prompt neutrons, and strike other nuclei and cause additional fissions within nanoseconds ( an average time interval used by scientists in the Manhattan Project was one half-lives, and nuclear reactions among them release additional neutrons after a long delay of up to several minutes after the initial fission event.
30.
When at least one dollar of reactivity is added above the exact critical point ( the point where neutrons produced is balanced by neutrons lost per generation ) then the chain reaction does not rely on delayed neutrons, and the rate of change of neutron population increases exponentially as the time constant is the prompt neutron lifetime.
