Editing Nuclear chain reaction (section)

==== Criticality ====
Because the value of <math>k_{\mathrm{eff}}</math> is directly related to the time rate of change of the neutron population in a system, it is convenient to classify the state of the nuclear system with regards to the critical value of the neutron population equation. The point at which the behavior of a nuclear system shifts is when <math>k_{\mathrm{eff}}</math> is exactly equal to 1. This point is called "criticality," and describes a system in which the production rate and loss rate of neutrons is exactly equal.

When <math>k_{\mathrm{eff}}</math> is less than or greater than one, the terms subcriticality and supercriticality are used respectively to describe the system:
* <math>k_{\mathrm{eff}} < 1</math> ([[subcriticality]]): The neutron population in the system is decreasing exponentially. 
* <math>k_{\mathrm{eff}} = 1</math>  ([[critical mass|criticality]]): The neutron population is maintaining a constant value.
* '''<math>k_{\mathrm{eff}} > 1</math>'''  ([[supercriticality]]): The neutron population in the system is increasing exponentially.
In a practical nuclear system, like a fission reactor, if criticality is intended it is likely that <math>k_{\mathrm{eff}}</math> will actually oscillate from slightly less than 1 to slightly more than 1, primarily due to thermal feedback effects. The neutron population, when averaged over time, appears constant, leaving the average value of <math>k_{\mathrm{eff}}</math> at around 1 during a constant power run. Both delayed neutrons and the transient fission product "[[burnable poison]]s" play an important role in the timing of these oscillations.