Fertile material is a material that, while not itself fissionable by thermal neutrons, can be converted into a fissile material by neutron absorption and subsequent nuclei conversions. 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. In a fast reactor, they may require fewer neutrons to achieve fission, as well as producing more neutrons when they do fission.
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. Since plutonium-238, plutonium-240 and plutonium-242 are fertile, accumulation of these and other nonfissile isotopes is less of a problem than in thermal reactors, which cannot burn them efficiently. Breeder reactors using thermal-spectrum neutrons are only practical if the thorium fuel cycle is used, as uranium-233 fissions far more reliably with thermal neutrons than plutonium-239.
Famous quotes containing the words fertile and/or material:
“Almost any biographer, if he respects facts, can give us much more than another fact to add to our collection. He can give us the creative fact; the fertile fact; the fact that suggests and engenders.”
—Virginia Woolf (1882–1941)
“And what avails it that science has come to treat space and time as simply forms of thought, and the material world as hypothetical, and withal our pretension of property and even of self-hood are fading with the rest, if, at last, even our thoughts are not finalities, but the incessant flowing and ascension reach these also, and each thought which yesterday was a finality, to-day is yielding to a larger generalization?”
—Ralph Waldo Emerson (1803–1882)