Editing Fast-neutron reactor (section)

{{Short description|Nuclear reactor where fast neutrons maintain a fission chain reaction}}
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| image1            = Superphénix 5.jpg
| image2            = Clementine reactor cross section.jpg
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# [[Superphenix]], the largest fast reactor ever
# Cross-section of [[Clementine (nuclear reactor)|Clementine]], the first fast reactor
# [[EBR-I]], the first [[fast breeder reactor]]
# [[RORSAT]] Soviet space probe, extensively using the [[BES-5]] reactor 
# [[BN-800]], the largest operating fast reactor
# [[Fast Flux Test Facility]], a large research fast reactor at the [[Hanford Site]]
}}
A '''fast-neutron reactor''' ('''FNR''') or '''fast-spectrum reactor''' or simply a '''fast reactor''' is a category of [[nuclear reactor]] in which the fission [[nuclear chain reaction|chain reaction]] is sustained by [[fast neutron]]s (carrying energies above 1 [[Electronvolt|MeV]], on average), as opposed to slow [[thermal neutron]]s used in [[thermal-neutron reactor]]s. 
Such a fast reactor needs no [[neutron moderator]], but requires [[nuclear fuel|fuel]] that is comparatively rich in [[fissile material]].

The fast spectrum is key to [[breeder reactors]], which convert highly abundant [[uranium-238]] into fissile [[plutonium-239]], without requiring [[Uranium enrichment|enrichment]]. It also leads to high [[Burnupena rotunda|burnup]]: many [[transuranic]] [[isotopes]], such as of [[americium]] and [[curium]], accumulate in thermal reactor [[spent fuel]]; in fast reactors they undergo [[fast fission]], reducing total [[nuclear waste]]. As a strong fast-spectrum [[neutron source]], they can also be used to [[Nuclear transmutation|transmute]] existing nuclear waste into manageable or non-radioactive isotopes.

These characteristics also cause fast reactors to be judged a higher [[nuclear proliferation]] risk, especially as breeder reactors require [[nuclear reprocessing]], which can be redirected to produce [[Weapons-grade nuclear material|weapons-grade plutonium]].

{{As of|2025}}, every fast reactor has used a [[Liquid metal cooled reactor|liquid metal coolant]], typically [[Sodium-cooled fast reactor|sodium-cooled]] or [[Lead-cooled fast reactor|lead-cooled]].{{Efn|With the exception of small fast [[critical assembly|critical assemblies]]}} This allows high [[thermal efficiency]], without [[pressurization]] systems, however it also contributes to historical high costs and operational difficulties.

In total, 13 fast breeder reactors have been constructed for commercial [[nuclear power]],<ref name="h429">{{cite journal |last=Agency |first=International Atomic Energy |date=2021 |title=Nuclear Power Reactors in the World |url=https://www.iaea.org/publications/14989/nuclear-power-reactors-in-the-world |journal=Nuclear Power Reactors in the World |pages=1–84 |access-date=2025-05-24}}</ref> alongside 65 fast-spectrum [[research reactors]] of various configurations.<ref>{{cite web |title=RRDB Search |url=https://nucleus.iaea.org/RRDB/RR/ReactorSearch.aspx?rf=1 |url-status=live |archive-url=https://web.archive.org/web/20100918002503/https://nucleus.iaea.org/RRDB/RR/ReactorSearch.aspx?rf=1 |archive-date=18 September 2010 |access-date=6 January 2019 |website=nucleus.iaea.org}}</ref> The first fast reactor was [[Los Alamos Laboratory]]'s [[Clementine]], operated from 1946. The largest was [[Superphénix]], in France, designed to deliver 1,242 MWe. In the [[GEN IV initiative]], about two thirds of the proposed reactors for the future use a fast spectrum.<ref>{{cite web |date=13 April 2016 |title=Fast-neutron reactors, Fast nuclear reactors |url=https://www.iaea.org/topics/fast-reactors |accessdate=2022-04-13 |publisher=IAEA}}</ref>