Editing Delayed neutron (section)

==Importance in nuclear reactors==

If a [[nuclear reactor]] happened to be [[prompt critical]] – even very slightly – the number of neutrons would increase exponentially at a high rate, and very quickly the reactor would become uncontrollable by means of external mechanisms. The control of the power rise would then be left to its intrinsic physical stability factors, like the thermal dilatation of the core, or the increased [[resonance integral|resonance absorption]]s of neutrons, that usually tend to decrease the reactor's reactivity when temperature rises; but the reactor would run the risk of being damaged or destroyed by heat.

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. In that regime, neutron production overall still grows exponentially, but on a time scale that is governed by the delayed neutron production, which is slow enough to be controlled (just as an otherwise unstable bicycle can be balanced because human reflexes are quick enough on the time scale of its instability). Thus, by widening the margins of non-operation and supercriticality and allowing more time to regulate the reactor, the delayed neutrons are essential to [[Passive nuclear safety|inherent reactor safety]], even in reactors requiring active control.

The lower percentage<ref>{{cite web|url=https://www-nds.iaea.org/sgnucdat/a6.htm|title=Nuclear Data for Safeguards}}</ref> of delayed neutrons makes the use of large percentages of plutonium in nuclear reactors more challenging.