Prompt Criticality
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Prompt neutrons are those produced directly from fission; delayed neutrons are those produced from other sources, such as decay of fission products, etc.
Prompt neutrons, as the name implies, are produced milliseconds after fission and normally make up a fraction of the total neutron population available to cause more fissions. Delayed neutrons are produced later and make up the vast majority of the neutron population. Under normal circumstances, the reactor does not have enough neutrons to sustain a nuclear chain reaction without delayed neutrons. It is this delay that allows the fission process to be controllable.
When a huge amount of positive reactivity is injected into the core suddenly, such as a control rod ejection, the number of prompt neutrons produced may be enough for the core to be critical on prompt neutrons alone (“prompt criticality”). This leads to an exponential rise in reactor power in milliseconds.
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