neutron it will also undergo fission at a low rate with the remainder of captures being "radiative" and entering the decay chain to Plutonium-239. Crucially, when a reactor runs on fast neutrons, the 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 upon absorbing a fast neutron fissioning 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% , 53% , 25% , 15% , 5% and traces of ), 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 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 and particularly 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% . Test runs at various facilities have also been done using and . The natural uranium (mostly ) will be turned into , while in the case of , is the result. As new fuel is created during the operation, this process is called breeding. 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 or which captures excess neutrons, the extra neutrons breed more or respectively. The blanket material can then be processed to extract the new fissile material, which