Editing Nuclear reactor (section)

==Operation==
{{Main|Nuclear reactor physics}}

[[File:Nuclear fission.svg|upright=1.15|thumb|An example of an induced nuclear fission event. A neutron is absorbed by the nucleus of a uranium-235 atom, which in turn splits into fast-moving lighter elements (fission products) and free neutrons. Though both reactors and [[nuclear weapons]] rely on nuclear chain reactions, the rate of reactions in a reactor is much slower than in a bomb.]]

Just as conventional [[thermal power station]]s generate electricity by harnessing the [[thermal energy]] released from burning [[fossil fuels]], nuclear reactors convert the energy released by controlled [[nuclear fission]] into thermal energy for further conversion to mechanical or electrical forms.

===Fission===
{{Main|Nuclear fission}}

When a large [[fissile]] [[atomic nucleus]] such as [[uranium-235]], [[uranium-233]], or [[plutonium-239]] absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, (the [[Nuclear fission product|fission products]]), releasing [[kinetic energy]], [[gamma rays|gamma radiation]],  and [[neutron|free neutron]]s. A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a [[nuclear chain reaction]].

To control such a nuclear chain reaction, [[control rod]]s containing [[neutron poison]]s and [[neutron moderators]] are able to change the portion of neutrons that will go on to cause more fission.<ref name="DOE HAND">
{{cite web|url=http://www.hss.energy.gov/NuclearSafety/techstds/standard/hdbk1019/h1019v2.pdf|title=DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory|publisher=US Department of Energy|archive-url=https://web.archive.org/web/20080423194722/http://www.hss.energy.gov/NuclearSafety/techstds/standard/hdbk1019/h1019v2.pdf|archive-date=23 April 2008|url-status=dead|access-date=24 September 2008}}
</ref> Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if monitoring or instrumentation detects unsafe conditions.<ref>{{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>

===Heat generation===
The reactor core generates heat in a number of ways:
* The [[kinetic energy]] of fission products is converted to [[thermal energy]] when these nuclei collide with nearby atoms.
* The reactor absorbs some of the [[gamma rays]] produced during fission and converts their energy into heat.
* Heat is produced by the [[radioactive decay]] of fission products and materials that have been activated by [[neutron absorption]]. This decay heat source will remain for some time even after the reactor is shut down.

A kilogram of [[uranium-235]] (U-235) converted via nuclear processes releases approximately three million times more energy than a kilogram of coal burned conventionally (7.2&nbsp;×&nbsp;10<sup>13</sup> [[joules]] per kilogram of uranium-235 versus 2.4&nbsp;×&nbsp;10<sup>7</sup> joules per kilogram of coal).<ref>{{cite web|url=http://bioenergy.ornl.gov/papers/misc/energy_conv.html |title=Bioenergy Conversion Factors |publisher=Bioenergy.ornl.gov |access-date=18 March 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110927181836/http://bioenergy.ornl.gov/papers/misc/energy_conv.html |archive-date=27 September 2011 }}</ref><ref>{{cite book |url=https://archive.org/details/nuclearweaponswh0000bern/page/312 |title=Nuclear Weapons: What You Need to Know |author=Bernstein, Jeremy |year=2008 |page=[https://archive.org/details/nuclearweaponswh0000bern/page/312 312] |isbn=978-0-521-88408-2 |publisher=[[Cambridge University Press]] |access-date=17 March 2011 }}</ref>{{Original research inline|date=March 2012}}<!--see this is more than [[WP:CALC]]. See [[Talk:Nuclear reactor#Energy efficiency?]]-->

The fission of one kilogram of [[uranium-235]] releases about 19 billion [[kilocalories]], so the energy released by 1&nbsp;kg of uranium-235 corresponds to that released by burning 2.7 million kg of coal.

===Cooling===
A [[nuclear reactor coolant]] – usually water but sometimes a gas or a liquid metal (like liquid sodium or lead) or [[molten salt]] – is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the [[turbines]], like the [[pressurized water reactor]]. However, in some reactors the water for the steam turbines is boiled directly by the [[reactor core]]; for example the [[boiling water reactor]].<ref name="HSWCOOLANT">{{cite web |title=How nuclear power works |date=9 October 2000 |publisher=HowStuffWorks.com |url=http://science.howstuffworks.com/nuclear-power3.htm |access-date=25 September 2008 |archive-date=22 October 2019 |archive-url=https://web.archive.org/web/20191022013236/https://science.howstuffworks.com/nuclear-power3.htm |url-status=live }}</ref>

===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.

===Electrical power generation===
The energy released in the fission process generates heat, some of which can be converted into usable energy. A common method of harnessing this [[thermal energy]] is to use it to boil water to produce pressurized steam which will then drive a [[steam turbine]] that turns an [[alternator]] and generates electricity.<ref name="TOURISTRP"/>

=== Life-times ===
Modern nuclear power plants are typically designed for a lifetime of 60 years, while older reactors were built with a planned typical lifetime of 30–40 years, though many of those have received renovations and life extensions of 15–20 years.<ref>{{cite web |title=PRIS – Miscellaneous reports – Operational by Age |url=https://pris.iaea.org/PRIS/WorldStatistics/OperationalByAge.aspx |access-date=12 July 2024 |website=IAEA Power Reactor Information System – operational by age}}</ref> Some believe nuclear power plants can operate for as long as 80 years or longer with proper maintenance and management. While most components of a nuclear power plant, such as steam generators, are replaced when they reach the end of their useful lifetime, the overall lifetime of the power plant is limited by the life of components that cannot be replaced when aged by wear and [[neutron embrittlement]], such as the reactor pressure vessel.<ref name="dismantling_sci-am-2009">[https://www.scientificamerican.com/article/nuclear-power-plant-aging-reactor-replacement-/ ''How Long Can a Nuclear Reactor Last?''] {{Webarchive|url=https://web.archive.org/web/20170202073144/http://www.scientificamerican.com/article/nuclear-power-plant-aging-reactor-replacement-/ |date=2 February 2017 }} Paul Voosen, Scientific American, 20 Nov 2009</ref> At the end of their planned life span, plants may get an extension of the operating license for some 20 years and in the US even a "subsequent license renewal" (SLR) for an additional 20 years.<ref>[https://www.nrc.gov/reactors/operating/licensing/renewal/subsequent-license-renewal.html ''Status of Subsequent License Renewal Applications.''] {{Webarchive|url=https://web.archive.org/web/20180121051705/https://www.nrc.gov/reactors/operating/licensing/renewal/subsequent-license-renewal.html |date=21 January 2018 }} NRC, 24 Feb 2022</ref><ref>[https://www.energy.gov/ne/articles/whats-lifespan-nuclear-reactor-much-longer-you-might-think ''What's the Lifespan for a Nuclear Reactor? Much Longer Than You Might Think''] {{Webarchive|url=https://web.archive.org/web/20200609230342/https://www.energy.gov/ne/articles/whats-lifespan-nuclear-reactor-much-longer-you-might-think |date=9 June 2020 }}. Office of Nuclear Energy, 16 Apr 2020</ref>

Even when a license is extended, it does not guarantee the reactor will continue to operate, particularly in the face of safety concerns or incident.<ref>{{Cite news |date=2006-08-05 |title=Swedish nuclear reactors shut down over safety concerns |url=https://en.wikinews.org/wiki/Swedish_nuclear_reactors_shut_down_over_safety_concerns |newspaper=Wikinews |access-date=16 May 2023 |archive-date=16 May 2023 |archive-url=https://web.archive.org/web/20230516123219/https://en.wikinews.org/wiki/Swedish_nuclear_reactors_shut_down_over_safety_concerns |url-status=live }}</ref> Many reactors are closed long before their license or design life expired and are [[Nuclear decommissioning|decommissioned]]. The costs for replacements or improvements required for continued safe operation may be so high that they are not cost-effective. Or they may be shut down due to technical failure.<ref name="sapl-2017">[https://saplnh.org/about-nuclear/nuclear-plant-lifespans/ ''The True Lifespan of a Nuclear Power Plant''] {{Webarchive|url=https://web.archive.org/web/20230219095448/https://saplnh.org/about-nuclear/nuclear-plant-lifespans/ |date=19 February 2023 }}. Seacoast Anti-Pollution League (SAPL), 2017</ref> Other ones have been shut down because the area was contaminated, like Fukushima, Three Mile Island, Sellafield, and Chernobyl.<ref>{{Cite book |last=IAEA |title=Cleanup of Large Areas Contaminated as a Result of a Nuclear Accident}}</ref> The British branch of the French concern [[EDF Energy]], for example, extended the operating lives of its [[Advanced Gas-cooled Reactor]]s (AGR) with only between 3 and 10 years.<ref name="edf-lifetime">[https://www.edfenergy.com/energy/nuclear-lifetime-management ''Extending the operating lives of Advanced Gas-cooled Reactors''] {{Webarchive|url=https://web.archive.org/web/20230219093947/https://www.edfenergy.com/energy/nuclear-lifetime-management |date=19 February 2023 }}. EDF Energy</ref> All seven AGR plants were expected to be shut down in 2022 and in decommissioning by 2028.<ref>[https://www.edfenergy.com/about/nuclear/decommissioning ''Nuclear decommissioning''] {{Webarchive|url=https://web.archive.org/web/20230219093959/https://www.edfenergy.com/about/nuclear/decommissioning |date=19 February 2023 }}. EDF (accessed Feb 2023)</ref> [[Hinkley Point B nuclear power station|Hinkley Point B]] was extended from 40 to 46 years, and closed. The same happened with [[Hunterston B nuclear power station|Hunterston B]], also after 46 years.

An increasing number of reactors is reaching or crossing their design lifetimes of 30 or 40 years. In 2014, [[Greenpeace]] warned that the lifetime extension of ageing nuclear power plants amounts to entering a new era of risk. It estimated the current European nuclear liability coverage in average to be too low by a factor of between 100 and 1,000 to cover the likely costs, while at the same time, the likelihood of a serious accident happening in Europe continues to increase as the reactor fleet grows older.<ref name="greenpeace-2014">[https://www.greenpeace.org/static/planet4-netherlands-stateless/2018/06/Briefing-Lifetime-extension-of-ageing-nuclear-power-plants.pdf ''Lifetime extension of ageing nuclear power plants: Entering a new era of risk.''] {{Webarchive|url=https://web.archive.org/web/20230315082620/https://www.greenpeace.org/static/planet4-netherlands-stateless/2018/06/Briefing-Lifetime-extension-of-ageing-nuclear-power-plants.pdf |date=15 March 2023 }} Greenpeace, March, 2014 (2.6&nbsp;MB). [https://inis.iaea.org/search/search.aspx?orig_q=RN:46030160 In German]</ref>