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Fertile material is a material that, although notfissile itself, can be converted into afissile material byneutron absorption.
Naturally occurring fertile materials that can be converted into a fissile material by irradiation in a reactor include:
Artificial isotopes formed in the reactor which can be converted intofissile material by one neutron capture include:
Some otheractinides 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.
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. Asubcritical reactor operating in the thermal neutron spectrum would have to adjust the strength of the externalneutron source in accordance with the build-up or consumption of such materials. In afast 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.
Afast-neutron reactor, meaning one with little or noneutron moderator and hence utilisingfast neutrons, can be configured as abreeder reactor, producing more fissile material than it consumes, using fertile material in a blanket around the core, or contained in specialfuel rods. Sinceplutonium-238,plutonium-240 andplutonium-242 are fertile, accumulation of these and other nonfissile isotopes is less of a problem than inthermal reactors, which cannot burn them efficiently. Breeder reactors using thermal-spectrum neutrons are only practical if thethorium fuel cycle is used, asuranium-233 fissions far more reliably with thermal neutrons than plutonium-239. Asubcritical reactor —regardless of neutron spectrum— can also "breed" fissile nuclides from fertile material, allowing in principle the consumption of very low grade actinides (e.g. SpentMOX fuel whose plutonium-240 content is too high for use in current critical thermal reactors) without the need for highly enriched material as used in afast breeder reactor.
Proposed applications for fertile material includes a space-based facility for the manufacture of fissile material forspacecraftnuclear propulsion. The facility would notionally transport fertile materials from Earth, safely through theatmosphere, and locate them at a space facility at theEarth–Moon L1 Lagrangian point where manufacture of fissile material would occur, eliminating the safety risk of transport of fissile materials from Earth.[2] While uranium and thorium are present on the moon, they seem to be in more limited supply than on earth, especially near the surface. Ifin situ resource utilization is desired to fuel nuclear power plants on the moon, converting fertile material to fissile material could be a way to make the resources last longer and to reduce the need foruranium enrichment which requires the chemically aggressive volatilefluorine to prepareuranium hexafluoride as used in the current enrichment technology.
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