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Concept
Fertile material
Summary
Fertile material is a material that, although not fissile itself, can be converted into a fissile material by neutron absorption.
Naturally occurring fertile materials that can be converted into a fissile material by irradiation in a reactor include:
thorium-232 which converts into uranium-233
uranium-234 which converts into uranium-235
uranium-238 which converts into plutonium-239
Artificial isotopes formed in the reactor which can be converted into fissile material by one neutron capture include:
plutonium-238 which converts into plutonium-239
plutonium-240 which converts into plutonium-241
Some other actinides need more than one neutron capture before arriving at an isotope which is both fissile and long-lived enough to probably be able to capture another neutron and fission instead of decaying.
plutonium-242 to americium-243 to curium-244 to curium-245
uranium-236 to neptunium-237 to plutonium-238 to plutonium-239
americium-241 to curium-242 to curium-243 (or, more likely, curium-242 decays to plutonium-238, which also requires one additional neutron to reach a fissile nuclide)
Since these require a total of 3 or 4 thermal neutrons to eventually fission, and a thermal neutron fission generates only about 2 to 3 neutrons, these nuclides represent a net loss of neutrons. A subcritical reactor operating in the thermal neutron spectrum would have to adjust the strength of the external neutron source in accordance with the build-up or consumption of such materials. In a fast reactor, those nuclides may require fewer neutrons to achieve fission, as well as producing more neutrons when they do fission. However, there is also the chance of (n,2n) or even (n,3n) "knockout" reactions (an incident fast neutron hits a nucleus and more than one neutron leaves) with fast neutrons which are not possible with thermal neutrons.
A fast-neutron reactor, meaning one with little or no neutron moderator and hence utilising fast neutrons, can be configured as a breeder reactor, producing more fissile material than it consumes, using fertile material in a blanket around the core, or contained in special fuel rods.
Official source
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