prompt criticality

noun1

1. •
   The point at which the prompt neutrons produced by a nuclear fission chain reaction are just sufficient to sustain the chain reaction on their own (the state of being prompt critical); past this point, changes in reaction power are controlled by changes in the prompt neutron flux, and occur on a timeframe of milliseconds, too rapidly to be controlled by mechanical means.

   • The system is thought to have approached prompt criticality, at which time the rate of power increase would have been determined by the neutron lifetime (on the order of milliseconds). — 2000