Editing Critical mass (section)

==Prompt criticality==
{{main|Prompt criticality}}
The event of fission must release, on the average, more than one free neutron of the desired energy level in order to sustain a chain reaction, and each must find other nuclei and cause them to fission.  Most of the neutrons released from a fission event come immediately from that event, but a fraction of them come later, when the fission products decay, which may be on the average from microseconds to minutes later. This is fortunate for atomic power generation, for without this delay "going critical" would be an immediately catastrophic event, as it is in a nuclear bomb where upwards of 80 generations of chain reaction occur in less than a microsecond, far too fast for a human, or even a machine, to react.  Physicists recognize two points in the gradual increase of neutron flux which are significant: critical, where the chain reaction becomes self-sustaining thanks to the contributions of both kinds of neutron generation,<ref>{{Cite Q | Q105755363 | last1 = Rhodes | first1 = Richard | author-link1 = Richard Rhodes | df = dmy-all | via = [[Internet Archive]] }}In the description of the Soviet equivalent of the CP1 startup at the University of Chicago in 1942, the long waits for those tardy neutrons is described in detail</ref> and [[prompt critical]], where the immediate "prompt" neutrons alone will sustain the reaction without need for the decay neutrons.  Nuclear power plants operate between these two points of [[Nuclear chain reaction|reactivity]], while above the prompt critical point is the domain of nuclear weapons, pulsed reactors designs such as [[TRIGA]] research reactors and the [[pulsed nuclear thermal rocket]], and some nuclear power accidents, such as the 1961 US [[SL-1|SL-1 accident]] and 1986 Soviet [[Chernobyl disaster]].