Editing Fast-neutron reactor (section)

===Fast fission and breeding===

Although {{chem|235|U}} and {{chem|239|Pu}} have a lower capture cross section with higher-energy neutrons, they still remain reactive well into the MeV range. If the density of {{chem|235|U}} or {{chem|239|Pu}} is sufficient, a threshold will be reached where there are enough fissile atoms in the fuel to maintain a chain reaction with fast neutrons. In fact, in the fast spectrum, when {{chem|238|U}} captures a fast neutron it will also undergo fission around 11% of the time with the remainder of captures being "radiative" and entering the decay chain to plutonium-239.

Crucially, when a reactor runs on fast neutrons, the {{chem|239|Pu}} isotope is likely to fission 74% of the time instead of the 62% of fissions when it captures a thermal neutron. In addition the probability of a {{chem|240|Pu}} atom fissioning upon absorbing a fast neutron  is 70% while for a thermal neutron it is less than 20%. Fast neutrons have a smaller chance of being captured by the uranium and plutonium, but when they are captured, have a significantly higher probability of causing a fission. The inventory of spent fast reactor fuel therefore contains virtually no [[actinides]] except for uranium and plutonium, which can be effectively recycled. Even when the core is initially loaded with 20% mass [[reactor-grade plutonium]] (containing on average 2% {{chem|238|Pu}}, 53% {{chem|239|Pu}}, 25% {{chem|240|Pu}}, 15% {{chem|241|Pu}}, 5% {{chem|242|Pu}} and traces of {{chem|244|Pu}}), the fast spectrum neutrons are capable of causing each of these to fission at significant rates. By the end of a fuel cycle of some 24 months, these ratios will have shifted with an increase of {{chem|239|Pu}} to over 80% while all the other plutonium isotopes will have decreased in proportion.

By removing the moderator, the size of the reactor core volume can be greatly reduced, and to some extent the complexity. As {{chem|239|Pu}} and particularly {{chem|240|Pu}} are far more likely to fission when they capture a fast neutron, it is possible to fuel such reactors with a mixture of plutonium and natural uranium, or with enriched material, containing around 20% {{chem|235|U}}. Test runs at various facilities have also been done using {{chem|233|U}} and {{chem|232|Th}}. The natural uranium (mostly {{chem|238|U}}) will be turned into {{chem|239|Pu}}, while in the case of {{chem|232|Th}}, {{chem|233|U}} is the result. As new fuel is created during the operation, this process is called breeding.<ref name=":0" /> All fast reactors can be used for breeding, or by carefully selecting the materials in the core and eliminating the blanket they can be operated to maintain the same level of fissionable material without creating any excess material. This is a process called Conversion because it transmutes fertile materials into fissile fuels on a 1:1 basis. By surrounding the reactor core with a blanket of {{chem|238|U}} or {{chem|232|Th}} which captures excess neutrons, the extra neutrons breed more {{chem|239|Pu}} or {{chem|233|U}} respectively.

The blanket material can then be processed to extract the new fissile material, which can then be mixed with depleted uranium to produce [[MOX fuel]], mixed with lightly enriched uranium fuel to form [[Remix Fuel|REMIX]] fuel, both for conventional slow-neutron reactors. Alternatively it can be mixed as in greater percentage of 17%-19.75% fissile fuel for fast reactor cores. A single fast reactor can thereby supply its own fuel indefinitely as well as feed several thermal ones, greatly increasing the amount of energy extracted from the natural uranium. The most effective breeder configuration theoretically is able to produce 14 {{chem|239|Pu}} nuclei for every 10 (14:10) actinide nuclei consumed, however real world fast reactors have so far achieved a ratio of 12:10 ending the fuel cycle with 20% more fissile material than they held at the start of the cycle.<ref>{{Cite web|url=https://www.nuclear-power.com/nuclear-power-plant/nuclear-fuel/conversion-factor-breeding-ratio/|title=Conversion Factor - Breeding Ratio|website=Nuclear Power}}</ref> Less than 1% of the total uranium mined is consumed in a thermal [[Once-through nuclear fuel cycle|once-through cycle]], while up to 60% of the natural uranium is fissioned in the best existing fast reactor cycles.

Given the current inventory of spent nuclear fuel (which contains reactor grade plutonium), it is possible to process this spent fuel material and reuse the actinide isotopes as fuel in a large number of fast reactors. This effectively consumes the {{chem|237|Np}}, [[reactor-grade plutonium]], {{chem|241|Am}}, and {{chem|244|Cm}}. Enormous amounts of energy are still present in the spent reactor fuel inventories; if fast reactor types were to be employed to use this material, that energy can be extracted for useful purposes.