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)
“We soon saw, as he saw, that he was not to be pardoned or rescued by men. That would have been to disarm him, to restore to him a material weapon, a Sharp’s rifle, when he had taken up the sword of the spirit,—the sword with which he has really won his greatest and most memorable victories. Now he has not laid aside the sword of the spirit, for he is pure spirit himself, and his sword is pure spirit also.”
—Henry David Thoreau (1817–1862)