Editing Fast-neutron reactor (section)

==Coolant==

All nuclear reactors produce heat which must be removed from the reactor core. [[Water]], the most common [[nuclear reactor coolant|coolant]] in [[thermal reactor]]s, is generally not feasible for a fast reactor, because it acts as an effective [[neutron moderator]].<ref name=":0">{{Cite web |title=How do fast breeder reactors differ from regular nuclear power plants? |url=https://www.scientificamerican.com/article/how-do-fast-breeder-react/ |access-date=2023-12-05 |website=Scientific American |language=en}}</ref>

All operating fast reactors are [[liquid metal cooled reactor]]s, which use sodium, lead, or [[lead-bismuth eutectic]] as coolants.<ref>{{Cite web |title=Fast Neutron Reactors |url=https://world-nuclear.org/information-library/current-and-future-generation/fast-neutron-reactors.aspx#:~:text=Fast%20neutron%20reactors%20have%20a,at%20or%20near%20atmospheric%20pressure. |access-date=2023-12-05 |website=world-nuclear.org}}</ref> The early [[Clementine (nuclear reactor)|Clementine reactor]] used [[mercury (element)|mercury]] coolant and [[plutonium]] metal fuel. In addition to its toxicity to humans, mercury has a high capture cross section (thus, it readily absorbs the neutrons, which causes nuclear reactions) for the (n,gamma) reaction, causing activation in the coolant and losing neutrons that could otherwise be absorbed in the fuel, which is why it is no longer considered useful as a coolant.

Russia has developed reactors that use molten [[lead]] and [[lead]]-[[bismuth]] [[eutectic]] alloys, which have been used on a larger scale in naval propulsion units, particularly the Soviet [[Alfa-class submarine]], as well as some prototype reactors. 
[[Sodium-potassium alloy]] (NaK) is popular in test reactors due to its low [[melting point]].

Another proposed fast reactor is a [[molten salt reactor]], in which the salt's moderating properties are insignificant. The particular salt formula used is crucial as some formulas are effective moderators while others are not.<ref>{{cite web|url=http://www.moltexenergy.com/|title=Moltex Energy {{!}} Safer Cheaper Cleaner Nuclear {{!}} Stable Salt Reactors {{!}} SSR|website=www.moltexenergy.com|access-date=2016-10-20}}</ref>

[[Gas-cooled fast reactor]]s have been the subject of research commonly using helium, which has small absorption and scattering cross sections, thus preserving the fast neutron spectrum without significant neutron absorption in the coolant. Purified nitrogen-15 has also been proposed as a coolant gas because it is more common than helium and also has a very low neutron absorption cross section.<ref>{{cite journal | doi=10.1155/2009/965757 | doi-access=free | title=Gas-Cooled Fast Reactor: A Historical Overview and Future Outlook | year=2009 | last1=Van Rooijen | first1=W. F. G. | journal=Science and Technology of Nuclear Installations | volume=2009 | pages=1–11 }}</ref><ref>{{cite web | url=https://atomicinsights.com/will-heavy-nitrogen-become-a-widely-used-fission-reactor-coolant/ | title=Will heavy nitrogen become a widely used fission reactor coolant? - Atomic Insights | date=17 November 2020 }}</ref>

However, all large-scale fast reactors have used molten metal coolant. Advantages of molten metals are low cost, the small activation potential and the large liquid ranges. The latter means that the material has a low melting point, and a high boiling point. Examples of these reactors include [[Sodium cooled fast reactor]], which are still being pursued worldwide. Russia currently operates two such reactors on a commercial scale. 
Additionally, Russia has around eighty reactor years of experience with the [[Lead-cooled fast reactor]] which is rapidly gaining interest.