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Fertile material is a term used to describe nuclides which generally themselves do not undergo induced fission
Nuclear fission

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the atomic nucleus of an atom splits into smaller parts, often producing free neutrons and lighter atomic nucleus, which may eventually produce photons ....
 (fissionable by thermal neutrons) but from which fissile
Fissile

In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission.All known fissile materials are capable of sustaining a chain reaction in which either thermal or slow neutrons or fast neutrons predominate....
 material is generated by neutron absorption and subsequent nuclei conversions. Fertile materials that occur naturally which can be converted into a fissile material by irradiation in a reactor include: Artificial isotopes formed in the reactor which can be converted into fissile material by one neutron capture include: 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. 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.






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Fertile material is a term used to describe nuclides which generally themselves do not undergo induced fission
Nuclear fission

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the atomic nucleus of an atom splits into smaller parts, often producing free neutrons and lighter atomic nucleus, which may eventually produce photons ....
 (fissionable by thermal neutrons) but from which fissile
Fissile

In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission.All known fissile materials are capable of sustaining a chain reaction in which either thermal or slow neutrons or fast neutrons predominate....
 material is generated by neutron absorption and subsequent nuclei conversions. Fertile materials that occur naturally which can be converted into a fissile material by irradiation in a reactor include:
  • thorium-232
    Thorium-232

    Thorium-232 is the primary abundant nuclide of thorium. It is a slightly unstable radionuclide that is found in the earth's crust. It is a fertile material able to absorb a neutron and undergo Nuclear transmutation into a nuclide that is able to support nuclear fission ....
     which converts into uranium-233
    Uranium-233

    Uranium-233 is a fissile artificial Isotopes of uranium , which has been used in a few nuclear reactors and has been proposed for much wider use as a nuclear fuel....
  • uranium-234
    Uranium-234

    Uranium-234 is an Isotopes of uranium. In natural uranium and uranium ore, 234U occurs as an indirect decay product of Uranium-238, but it makes up only 0.0055% of the raw uranium because its half-life of just 246,000 years is only about 1/18,000 as long as the half-life of 238U....
     which converts into uranium-235
    Uranium-235

    Uranium-235 is an Isotopes of uranium that differs from the element's other common isotope, uranium-238, by its ability to cause a rapidly expanding nuclear fission chain reaction, i.e., it is fissile....
  • uranium-238
    Uranium-238

    Uranium-238 , is the most common Isotopes of uranium of uranium found in nature. When hit by a neutron, it becomes uranium-239 , an unstable isotope which radioactive decay into neptunium-239 , which then itself decays, with a half-life of 2.355 days, into plutonium-239 ....
     which converts into plutonium-239
    Plutonium-239

    Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope....
Artificial isotopes formed in the reactor which can be converted into fissile material by one neutron capture include:
  • plutonium-238
    Plutonium-238

    Plutonium-238, is a radioactive isotope of plutonium with a half-life of 87.7 years and is a very powerful alpha emitter. Because of its high level of alpha activity, it is used for radioisotope thermoelectric generators and radioisotope heater units....
     which converts into plutonium-239
    Plutonium-239

    Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope....
  • plutonium-240
    Plutonium-240

    Plutonium-240 is an isotope of the metal plutonium formed when plutonium-239 captures a neutron. About 62% to 73% of the time when Pu-239 captures a neutron it undergoes nuclear fission; the rest of the time it forms Pu-240....
     which converts into plutonium-241
    Plutonium-241

    Plutonium-241 is an isotope of plutonium formed when plutonium-240 captures a neutron. Unlike 240Pu, 241Pu is fissionable, with a neutron absorption cross section about 1/3 greater than Pu-239, and a similar chance of fissioning on neutron absorption, around 73%....
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
    Americium

    Americium is a synthetic element that has the symbol Am and atomic number 95. A radioactive decay metallic element, americium is an actinide that was obtained in 1944 by Glenn T....
    -243 to curium
    Curium

    Curium is a synthetic element with the symbol Cm and atomic number 96. A Radioactive decay metallic transuranic element of the actinide series, curium is produced by bombarding plutonium with alpha particles and was named for Maria Sklodowska-Curie and her husband Pierre Curie....
    -244 to curium
    Curium

    Curium is a synthetic element with the symbol Cm and atomic number 96. A Radioactive decay metallic transuranic element of the actinide series, curium is produced by bombarding plutonium with alpha particles and was named for Maria Sklodowska-Curie and her husband Pierre Curie....
    -245
  • uranium-236
    Uranium-236

    Uranium-236 is an Isotopes of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste....
     to neptunium
    Neptunium

    Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactivity metallic element, neptunium is the first transuranic element and belongs to the actinide series....
    -237 to plutonium-238
    Plutonium-238

    Plutonium-238, is a radioactive isotope of plutonium with a half-life of 87.7 years and is a very powerful alpha emitter. Because of its high level of alpha activity, it is used for radioisotope thermoelectric generators and radioisotope heater units....
     to plutonium-239
    Plutonium-239

    Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope....
  • americium
    Americium

    Americium is a synthetic element that has the symbol Am and atomic number 95. A radioactive decay metallic element, americium is an actinide that was obtained in 1944 by Glenn T....
    -241 to curium
    Curium

    Curium is a synthetic element with the symbol Cm and atomic number 96. A Radioactive decay metallic transuranic element of the actinide series, curium is produced by bombarding plutonium with alpha particles and was named for Maria Sklodowska-Curie and her husband Pierre Curie....
    -242 to curium
    Curium

    Curium is a synthetic element with the symbol Cm and atomic number 96. A Radioactive decay metallic transuranic element of the actinide series, curium is produced by bombarding plutonium with alpha particles and was named for Maria Sklodowska-Curie and her husband Pierre Curie....
    -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. In a fast reactor, they may require fewer neutrons to achieve fission, as well as producing more neutrons when they do fission.

A fast breeder reactor, a reactor with little or no neutron moderator
Neutron moderator

In nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
 and hence utilising fast neutrons, can be configured to produce more fissile material than it consumes, using fertile material in a blanket around the core, or contained in special fuel rods. Since plutonium-238
Plutonium-238

Plutonium-238, is a radioactive isotope of plutonium with a half-life of 87.7 years and is a very powerful alpha emitter. Because of its high level of alpha activity, it is used for radioisotope thermoelectric generators and radioisotope heater units....
, plutonium-240
Plutonium-240

Plutonium-240 is an isotope of the metal plutonium formed when plutonium-239 captures a neutron. About 62% to 73% of the time when Pu-239 captures a neutron it undergoes nuclear fission; the rest of the time it forms Pu-240....
 and plutonium-242 are fertile, accumulation of these and other nonfissile isotopes is less of a problem than in thermal reactor
Thermal reactor

A thermal reactor uses slow or thermal neutrons. Most power reactors are of this type. These type of reactors use a neutron moderator to slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, to reduce the speed of the neutrons to low velocity thermal neutrons....
s, which cannot burn them efficiently.