Editing Fast-neutron reactor (section)

==Fuel==
In practice, sustaining a fission [[nuclear chain reaction|chain reaction]] with [[fast neutron]]s means using relatively [[enriched uranium]] or [[plutonium]]. The reason for this is that fissile reactions are favored at thermal energies, since the ratio between the {{chem|239|Pu}} [[nuclear cross section|fission cross section]] and {{chem|238|U}} [[absorption cross section]] is ~100 in a thermal spectrum and 8 in a fast spectrum. Fission and absorption cross sections are low for both {{chem|239|Pu}} and {{chem|238|U}} at high (fast) energies, which means that fast neutrons are likelier to pass through fuel without interacting than thermal neutrons; thus, more fissile material is needed. Therefore, a fast reactor cannot run on [[natural uranium]] fuel. However, it is possible to build a fast reactor that [[breeder reactor|breed]]s fuel by producing more than it consumes. After the initial fuel charge such a reactor can be refueled by [[nuclear reprocessing|reprocessing]]. [[Nuclear fission product|Fission products]] can be replaced by adding natural or even depleted uranium without further enrichment. This is the concept of the [[fast breeder reactor]] or FBR.

So far, most fast-neutron reactors have used either [[MOX fuel|MOX]] (mixed oxide) or [[Metal fuel|metal alloy]] fuel. Soviet fast-neutron reactors used (highly {{Chem|235|U}} enriched) uranium fuel initially, then in 2022 switched to using MOX.<ref>{{cite web | url=https://www.world-nuclear-news.org/Articles/Beloyarsk-BN-800-fast-reactor-running-on-MOX | title=Beloyarsk BN-800 fast reactor running on MOX : Uranium & Fuel - World Nuclear News }}</ref> The Indian prototype reactor uses uranium-carbide fuel.

While criticality at fast energies may be achieved with uranium enriched to 5.5 (weight) percent {{Chem|235|U}}, fast reactor designs have been proposed with enrichment in the range of 20 percent for reasons including core lifetime: if a fast reactor were loaded with the minimal critical mass, then the reactor would become subcritical after the first fission. Rather, an excess of fuel is inserted with reactivity control mechanisms, such that the reactivity control is inserted fully at the beginning of life to bring the reactor from supercritical to critical; as the fuel is depleted, the reactivity control is withdrawn to support continuing fission. In a [[fast breeder reactor]], the above applies, though the reactivity from fuel depletion is also compensated by breeding either {{Chem|link=|233|U}} or {{Chem|239|Pu}} and {{Chem|241|Pu}} from {{Chem|232|Th}} or {{Chem|238|U}}, respectively. Some designs use burnable poisons also known as burnable absorbers which contain isotopes with high neutron capture cross sections. Concentrated {{Chem|10|[[Boron]]}} or {{Chem|155|[[Gadolinium]]}} & {{Chem|157|[[Gadolinium]]}} in natural gadolinium are typically used for this purpose. As these isotopes absorb excess neutrons they are transmuted into isotopes with low absorption cross sections so that over the life of the fuel cycle they are eliminated as more fission products with high capture cross section are generated. This makes it easier to maintain control of the reactivity rate in the core at start up with fresh fuel.<ref>{{Cite web|url=https://www.nuclear-power.com/nuclear-power-plant/nuclear-fuel/burnable-absorbers-burnable-poisons/|title=Burnable Absorbers - Burnable Poisons |website=Nuclear Power}}</ref>