Editing Critical mass (section)

== Point of criticality==
{{See also|Dollar (reactivity)}}
When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a ''critical'' state in which there is no increase or decrease in power, temperature, or [[neutron]] population.

A numerical measure of a critical mass depends on the [[Nuclear chain reaction#Effective neutron multiplication factor|effective neutron multiplication factor]] {{math|<var>k</var>}}, the average number of neutrons released per fission event that go on to cause another fission event rather than being absorbed or leaving the material. 
  
A ''subcritical'' mass is a mass that does not have the ability to sustain a fission chain reaction. A population of neutrons introduced to a subcritical assembly will exponentially decrease. In this case, known as {{em|subcriticality}}, {{nowrap|{{em|k}} < 1}}.

A ''critical'' mass is a mass of fissile material that self-sustains a fission chain reaction. In this case, known as {{em|criticality}}, {{nowrap|{{em|k}} {{=}} 1}}. A steady rate of [[spontaneous fission]] causes a proportionally steady level of neutron activity.

A ''supercritical'' mass is a mass which, once fission has started, will proceed at an increasing rate.<ref>{{Cite book |last=Hewitt |first=Paul G. |title=Conceptual Physics |publisher=[[Pearson PLC|Pearson]] |year=2015 |isbn=978-1-292-05713-2 |edition=12th |location=300 Beach Drive NE, 1103, St. Petersburg |pages=666}}</ref> In this case, known as {{em|supercriticality}}, {{nowrap|{{em|k}} > 1}}. The constant of proportionality increases as {{math|<var>k</var>}} increases. The material may settle into equilibrium (''i.e.'' become critical again) at an elevated temperature/power level or destroy itself.

Due to [[spontaneous fission]] a supercritical mass will undergo a chain reaction. For example, a spherical critical mass of pure [[uranium-235]] (<sup>235</sup>U) with a mass of about {{convert|52|kg}} would experience around 15&nbsp;spontaneous fission events per second.{{Citation needed|reason=Citation needed for spontaneous fission rate of Uranium-235|date=June 2018}} The probability that one such event will cause a chain reaction depends on how much the mass exceeds the critical mass. If there is [[uranium-238]] (<sup>238</sup>U) present, the rate of spontaneous fission will be much higher.{{cn|date=April 2024}} Fission can also be initiated by neutrons produced by [[cosmic rays]].