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Nuclear chain reaction
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===Prompt neutron lifetime=== The '''prompt neutron lifetime''', <math>l</math>, is the average time between the emission of a neutron and either its absorption or escape from the system.<ref name=Lamarsh /> The neutrons that occur directly from fission are called ''[[prompt neutron|prompt]]'' neutrons, and the ones that are a result of radioactive decay of fission fragments are called ''[[delayed neutron|delayed]]'' neutrons. The term ''lifetime'' is used because the emission of a neutron is often considered its ''birth'', and its subsequent absorption or escape from the core is considered its ''death''. For "thermal" (slow-neutron) fission reactors, the typical prompt neutron lifetime is on the order of 10<sup>β4</sup> seconds, and for fast fission reactors, the prompt neutron lifetime is on the order of 10<sup>β7</sup> seconds.<ref name="Duderstadt" /> These extremely short lifetimes mean that in 1 second, 10,000 to 10,000,000 neutron lifetimes can pass. The ''average'' (also referred to as the ''adjoint unweighted'') prompt neutron lifetime takes into account all prompt neutrons regardless of their importance in the [[Nuclear reactor core|reactor core]]; the ''effective'' prompt neutron lifetime (referred to as the ''adjoint weighted'' over space, energy, and angle) refers to a neutron with average importance.<ref>{{cite tech report | title = Deterministic and Monte Carlo Analyses of YALINA Thermal Subcritical Assembly |publisher=Argonne National Laboratory | year = 2010| doi = 10.2172/991100 |doi-access=free| last1 = Talamo| first1 = A.| last2 = Gohar| first2 = Y. |osti=991100 |osti-access=free}}</ref>
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