Editing Prompt criticality (section)

== 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 [[nuclear chain reaction|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 <sup>1</sup>/<sub>2.4</sub> = 0.42 = 42&nbsp;% probability of causing another fission event as opposed to either being absorbed by a non-fission [[Neutron capture|capture event]] or escaping from the fissile core.

The average number of neutrons that cause new fission events is called the [[nuclear chain reaction|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.