Editing Integral fast reactor (section)

===Passive safety===
[[File:Ifr concept.jpg|thumb|upright=1.5|IFR concept (color); an animation of the pyroprocessing cycle is also available.<ref>{{cite web |url=https://www.youtube.com/watch?v=cBThTwFhRlA |archive-url=https://ghostarchive.org/varchive/youtube/20211212/cBThTwFhRlA| archive-date=2021-12-12 |url-status=live|title=Historical video about the Integral Fast Reactor (IFR) concept. Uploaded by – Nuclear Engineering at Argonne|website=[[YouTube]] |date=3 March 2014 }}{{cbignore}}</ref>]]
[[File:IFR concept.png|thumb|upright=1.5|IFR concept (black and white with clearer text)]]

The IFR uses metal alloy fuel (uranium, plutonium, and/or zirconium), which is a good conductor of heat, unlike the [[uranium oxide]] used by LWRs (and even some fast breeder reactors), which is a poor conductor of heat and reaches high temperatures at the center of fuel pellets. The IFR also has a smaller volume of fuel, since the fissile material is diluted with fertile material by a ratio of 5 or less, compared to about 30 for LWR fuel. The IFR core requires more heat removal per core volume during operation than the LWR core; but on the other hand, after a shutdown, there is far less trapped heat that is still diffusing out and needs to be removed. However, [[decay heat]] generation from short-lived fission products and actinides is comparable in both cases, starting at a high level and decreasing with time elapsed after shutdown. The high volume of liquid sodium primary coolant in the pool configuration is designed to absorb decay heat without reaching fuel melting temperature. The primary sodium pumps are designed with [[flywheel]]s so they will coast down slowly (90 seconds) if power is removed. This coast-down further aids core cooling upon shutdown. If the primary cooling loop were to be somehow suddenly stopped, or if the control rods were suddenly removed, the metal fuel can melt, as accidentally demonstrated in EBR-I; however, the melting fuel is then extruded up the steel fuel cladding tubes and out of the active core region leading to permanent reactor shutdown and no further fission heat generation or fuel melting.<ref name=TillAndYang>{{cite book|last=Till and Chang|first=Charles E. and Yoon Il|title=Plentiful Energy: The Story of the Integral Fast Reactor|year=2011|publisher=CreateSpace|isbn=978-1466384606|pages=157–158|url=http://www.sustainablenuclear.org/PADs/pad0509till.html|access-date=2011-06-23|archive-url=https://web.archive.org/web/20110605030654/http://www.sustainablenuclear.org/PADs/pad0509till.html|archive-date=2011-06-05|url-status=dead}}</ref> With metal fuel, the cladding is not breached and no radioactivity is released even in extreme overpower transients.

Self-regulation of the IFR's power level depends mainly on thermal expansion of the fuel, which allows more neutrons to escape, damping the [[chain reaction]]. LWRs have less effect from thermal expansion of fuel (since much of the core is the [[neutron moderator]]) but have strong [[negative feedback]] from [[Doppler broadening]] (which acts on thermal and epithermal neutrons, not fast neutrons) and negative [[void coefficient]] from boiling of the water moderator/coolant; the less dense steam returns fewer and less-thermalized neutrons to the fuel, which are more likely to be captured by U-238 than induce fissions. However, the IFR's positive void coefficient could be reduced to an acceptable level by adding technetium to the core, helping destroy the [[long-lived fission product]] named [[technetium-99]] by [[nuclear transmutation]] in the process.<ref name="osti.gov"/>

IFRs are able to withstand both a loss of flow without [[SCRAM]] and loss of heat sink without SCRAM. In addition to the passive shutdown of the reactor, the convection current generated in the primary coolant system will prevent fuel damage (core meltdown). These capabilities were demonstrated in the [[EBR-II]].<ref name="ANL" /> The ultimate goal is that no radioactivity is released under any circumstance.

The flammability of sodium is a risk to operators. Sodium burns easily in air and will ignite spontaneously on contact with water. The use of an intermediate coolant loop between the reactor and the turbines minimizes the risk of a sodium fire in the reactor core.

Under neutron bombardment, [[sodium-24]] is produced. This is highly radioactive, emitting an energetic [[gamma ray]] of 2.7 [[Electronvolt|MeV]] followed by a [[beta decay]] to form [[magnesium-24]]. Half-life is only 15 hours, so this isotope is not a long-term hazard. Nevertheless, the presence of sodium-24 further necessitates the use of the intermediate coolant loop between the reactor and the turbines.