In nuclear engineering, an assembly is prompt critical if for each nuclear fission event, one or more of the immediate or prompt neutrons released causes an additional fission event. This causes a rapidly exponential increase in the number of fission events, and an explosion.

An assembly can be supercritical without being prompt critical.

Prompt criticality must be avoided in the operation of a nuclear reactor, and reactors are designed to make it as unlikely as possible that it will occur. Only two reactor accidents are suspected of having achieved prompt criticality, those of Chernobyl #4 and SL-1, and in both cases there is doubt, although in both the uncontrolled surge in power was sufficient to cause an explosion that destroyed the reactor. Note that in either accident even if prompt criticality did occur, the explosive force was not directly due to prompt criticality. Rather, the near instantaneous heat generated by prompt criticality would cause the sudden flashing of coolant water to steam. Although this results in an enormous force, it is not a nuclear explosion but rather a steam explosion.

Many reactor designs do succeed in making prompt criticality practically impossible. A PWR, for example, does not contain enough fuel of high enough enrichment to make a prompt critical assembly with the materials in the core however they are reconfigured.

In the design of nuclear weapons, on the other hand, achieving prompt criticality is essential. Indeed, one of the problems to be overcome in constructing a plutonium fueled bomb is to achieve prompt criticality before the energy released by the reaction in an assembly that is merely supercritical destroys the bomb. This is also the reason that high-grade plutonium is used; Lower grades make the timely assembly of a prompt critical configuration even more difficult.