Editing Nuclear reactor (section)

===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>