Editing Void coefficient (section)

== Explanation ==
[[Nuclear fission]] reactors run on nuclear [[chain reaction]]s, in which each [[atomic nucleus|nucleus]] that undergoes fission releases heat and neutrons.  Each [[neutron]] may impact another nucleus and cause it to undergo fission.  The speed of this neutron affects its probability of causing additional fission, as does the presence of neutron-absorbing material.  On the one hand, [[thermal neutron]]s are more easily absorbed by fissile nuclei than [[fast neutron]]s, so a [[neutron moderator]] that slows neutrons will increase the reactivity of a nuclear reactor.  On the other hand, a neutron absorber will decrease the reactivity of a nuclear reactor.  These two mechanisms are used to control the thermal power output of a nuclear reactor.

In order to extract useful power from a nuclear reactor, and (for most reactor designs) to keep it intact and functioning, a cooling system must be used.  Some reactors circulate pressurized water; some use liquid [[metal]], such as [[sodium]], [[NaK]], [[lead]], or [[Mercury (element)|mercury]]; others use gases (see [[advanced gas-cooled reactor]]).  If the coolant is a liquid, it may boil if the temperature inside the reactor rises.  This boiling leads to ''voids'' inside the reactor.  Voids may also form if coolant is lost from the reactor in some sort of accident (called a [[loss of coolant]] accident, which has other dangers). Some reactors operate with the coolant in a constant state of boiling, using the generated vapor to turn turbines.

The coolant liquid may act as a neutron absorber, as a neutron moderator, usually as both but with one or other role as the most influential.  In either case, the amount of void inside the reactor can affect the reactivity of the reactor.  The change in reactivity caused by a change of voids inside the reactor is directly proportional to the ''void coefficient''. 

A positive void coefficient means that the reactivity increases as the void content inside the reactor increases due to increased boiling or loss of coolant; for example, if the coolant acts predominantly as neutron absorber. This positive void coefficient causes a [[positive feedback]] loop, starting with the first occurrence of steam bubbles. This can quickly boil all the coolant in the reactor, if not countered by an (automatic) control mechanism, or if said mechanism's response time is too slow.  This happened in the [[RBMK]] reactor that was destroyed in the [[Chernobyl disaster]] due to positive void coefficient and fast power coefficients present at lower reactor power levels, the EPS scram accelerating rather than dampening reactivity resulting in the explosion.

A negative void coefficient means that the reactivity decreases as the void content inside the reactor increases—but it also means that the reactivity increases if the void content inside the reactor is reduced.  In boiling-water reactors with large negative void coefficients, a sudden pressure rise (caused, for example, by unplanned closure of a streamline valve) will result in a sudden decrease in void content: the increased pressure will cause some of the steam bubbles to condense ("collapse"); and the thermal output will possibly increase until it is terminated by safety systems, by increased void formation due to the higher power, or, possibly, by system or component failures that relieve pressure, causing void content to increase and power to decrease.  Boiling water reactors are all designed (and required) to handle this type of transient.  On the other hand, if a reactor is designed to operate with no voids at all, a large negative void coefficient may serve as a safety system.  A loss of coolant in such a reactor decreases the thermal output, but of course heat that is generated is no longer removed, so the temperature could rise (if all other safety systems simultaneously failed).

Thus, a large void coefficient, whether positive or negative, can be either a design issue (requiring more careful, faster-acting control systems) or a desired quality depending on reactor design.  Gas-cooled reactors do not have issues with voids forming.