Even in a subcritical assembly such as a shut-down reactor core, any stray neutron that happens to be present in the core (for example from spontaneous fission of the fuel, from radioactive decay of fission products, or from a [[neutron source]]) will trigger an exponentially decaying chain reaction.
Including the effect of an external neutron source ("external" to the fission process, not physically external to the core), one can write a modified evolution equation: .