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Prompt criticality
In nuclear engineering, prompt criticality is said to be reached during a nuclear fission event if one or more of the immediate or prompt neutrons released by an atom in the event causes an additional fission event resulting in a rapid, exponential increase in the number of fission events. Prompt criticality is a special case of supercriticality.
== Criticality ==
An assembly is critical if each fission event causes, on average, exactly one additional such event in a continual chain. Such a chain is a self-sustaining fission chain reaction. When a uranium-235 (U-235) atom undergoes nuclear fission, it typically releases between one and seven neutrons (with an average of 2.4). In this situation, an assembly is critical if every released neutron has a 1/2.4 = 0.42 = 42% probability of causing another fission event as opposed to either being absorbed by a non-fission capture event or escaping from the fissile core.
The average number of neutrons that cause new fission events is called the effective neutron multiplication factor, usually denoted by the symbols ''k-effective'', ''k-eff'' or ''k''. When ''k-effective'' is equal to 1, the assembly is called critical, if ''k-effective'' is less than 1 the assembly is said to be subcritical, and if ''k-effective'' is greater than 1 the assembly is called supercritical.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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