Editing Nuclear reactor (section)

===Reactivity control===
{{Main|Nuclear reactor physics|Passive nuclear safety|Delayed neutron|Iodine pit|SCRAM|Decay heat}}
The rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events. Nuclear reactors typically employ several methods of neutron control to adjust the reactor's power output. Some of these methods arise naturally from the physics of radioactive decay and are simply accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose.

The fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the [[control rod]]s. Control rods are made of so-called [[neutron poison]]s and therefore absorb neutrons. When a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces – often the moderator. This action results in fewer neutrons available to cause fission and reduces the reactor's power output. Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power.

The physics of radioactive decay also affects neutron populations in a reactor. One such process is [[delayed neutron]] emission by a number of neutron-rich fission isotopes. These delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder (termed "[[prompt neutron]]s") released immediately upon fission. The fission products which produce delayed neutrons have [[Half-life|half-lives]] for their [[Radioactive decay|decay]] by [[neutron emission]] that range from milliseconds to as long as several minutes, and so considerable time is required to determine exactly when a reactor reaches the [[critical mass (nuclear)|critical]] point. Keeping the reactor in the zone of chain reactivity where delayed neutrons are ''necessary'' to achieve a [[critical mass]] state allows mechanical devices or human operators to control a chain reaction in "real time"; otherwise the time between achievement of criticality and [[nuclear meltdown]] as a result of an exponential power surge from the normal nuclear chain reaction, would be too short to allow for intervention. This last stage, where delayed neutrons are no longer required to maintain criticality, is known as the [[prompt critical]] point. There is a scale for describing criticality in numerical form, in which bare criticality is known as ''zero [[dollar (reactivity)|dollars]]'' and the prompt critical point is ''one dollar'', and other points in the process interpolated in cents.

In some reactors, the [[coolant]] also acts as a [[neutron moderator]]. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. [[Thermal neutron]]s are more likely than [[fast neutron]]s to cause fission. If the coolant is a moderator, then temperature changes can affect the density of the coolant/moderator and therefore change power output. A higher temperature coolant would be less dense, and therefore a less effective moderator.

In other reactors, the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. In these reactors, power output can be increased by heating the coolant, which makes it a less dense poison. Nuclear reactors generally have automatic and manual systems to [[scram]] the reactor in an emergency shut down. These systems insert large amounts of poison (often [[boron]] in the form of [[boric acid]]) into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated.<ref name="TOURISTRP">{{cite web |title=Reactor Protection & Engineered Safety Feature Systems |work=The Nuclear Tourist |url=http://www.nucleartourist.com/systems/rp.htm |access-date=25 September 2008 |archive-date=22 August 2018 |archive-url=https://web.archive.org/web/20180822051052/http://www.nucleartourist.com/systems/rp.htm |url-status=live }}</ref>

Most types of reactors are sensitive to a process variously known as xenon poisoning, or the [[iodine pit]]. The common [[fission product]] [[Xenon-135]] produced in the fission process acts as a neutron poison that absorbs neutrons and therefore tends to shut the reactor down. Xenon-135 accumulation can be controlled by keeping power levels high enough to destroy it by neutron absorption as fast as it is produced. Fission also produces [[iodine-135]], which in turn decays (with a half-life of 6.57 hours) to new xenon-135. When the reactor is shut down, iodine-135 continues to decay to xenon-135, making restarting the reactor more difficult for a day or two, as the xenon-135 decays into cesium-135, which is not nearly as poisonous as xenon-135, with a half-life of 9.2 hours. This temporary state is the "iodine pit." If the reactor has sufficient extra reactivity capacity, it can be restarted. As the extra xenon-135 is transmuted to xenon-136, which is much less a neutron poison, within a few hours the reactor experiences a "xenon burnoff (power) transient". Control rods must be further inserted to replace the neutron absorption of the lost xenon-135. Failure to properly follow such a procedure was a key step in the [[Chernobyl disaster]].<ref>{{cite web|url=http://www.eepublishers.co.za/images/upload/Meyer%20Chernobyl%205.pdf |title=Chernobyl: what happened and why? by CM Meyer, technical journalist. |url-status=dead |archive-url=https://web.archive.org/web/20131211073343/http://www.eepublishers.co.za/images/upload/Meyer%20Chernobyl%205.pdf |archive-date=11 December 2013 }}</ref>

Reactors used in [[nuclear marine propulsion]] (especially [[nuclear submarine]]s) often cannot be run at continuous power around the clock in the same way that land-based power reactors are normally run, and in addition often need to have a very long core life without [[Reactor refueling|refueling]]. For this reason many designs use highly enriched uranium but incorporate burnable neutron poison in the fuel rods.<ref>{{cite book|last1=Tsetkov|first1=Pavel|last2=Usman|first2=Shoaib|editor=Krivit, Steven|title=Nuclear Energy Encyclopedia: Science, Technology, and Applications|year=2011|publisher=Wiley|location=Hoboken, NJ|isbn=978-0-470-89439-2|pages=48; 85}}</ref> This allows the reactor to be constructed with an excess of fissionable material, which is nevertheless made relatively safe early in the reactor's fuel burn cycle by the presence of the neutron-absorbing material which is later replaced by normally produced long-lived neutron poisons (far longer-lived than xenon-135) which gradually accumulate over the fuel load's operating life.