delayed criticality

delayed criticality (usually uncountable, plural delayed criticalities)

1. (nuclear physics) The point at which the prompt neutrons and delayed neutrons produced by a nuclear fission chain reaction, taken together, are just sufficient to sustain the chain reaction (the state of being delayed critical); in this state, changes in reaction power occur slowly, allowing the reaction to be controlled on human timescales, due to the relatively long time delay associated with the emission of the delayed neutrons necessary to maintain the chain reaction.
   • 2019 March 13, Carey Sublette, “4.1 Elements of Fission Weapon Design”, in Nuclear Weapon Archive‎, archived from the original on 3 March 2021, 4.1.4 History of a Fission Explosion:

     Although nearly all neutrons produced by fission are emitted as soon as the atom splits (within 10^-14 sec or so), a very small proportion of neutrons (0.65% for U-235, 0.25% for Pu-239) are emitted by fission fragments with delays of up to a few minutes. In delayed criticality these neutrons are required to maintain the chain reaction. These long delays mean that power level changes can only occur slowly. All nuclear reactors operate in a state of delayed criticality.

• prompt criticality