Editing Void coefficient (section)

{{Short description|The change in nuclear fission rate caused by voids within a reactor}}
In [[nuclear engineering]], the '''void coefficient''' (more properly called '''void coefficient of reactivity''') is a number that can be used to estimate how much the [[Nuclear chain reaction|reactivity]] of a [[nuclear reactor]] changes as voids (typically [[steam]] bubbles) form in the reactor [[Neutron moderator|moderator]] or [[coolant]]. Net reactivity in a reactor depends on several factors, one of which is the void coefficient. Reactors in which either the moderator or the coolant is a liquid will typically have a void coefficient which is either negative (if the reactor is under-moderated) or positive (if the reactor is over-moderated). Reactors in which neither the moderator nor the coolant is a liquid (e.g., a graphite-moderated, [[gas-cooled reactor]]) will have a zero void coefficient. It is unclear how the definition of "void" coefficient applies to reactors in which the moderator/coolant is neither liquid nor gas ([[supercritical water reactor]]).