Fertile material

Transmutation flow between ²³⁸Pu and ²⁴⁵Cm in LWR.^[1] Speed of transmutation varies greatly by nuclide, and percentages are relative to total transmutation and decay. After removal of fuel from reactor, decay will predominate for shorter-lived isotopes such as ²³⁸Pu, ²⁴¹Pu, ^242–244Cm; but ^245–248Cm are all long-lived.

Fertile material is a term used to describe nuclides which generally themselves do not undergo induced fission (fissionable by thermal neutrons) but from which fissile 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 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.

References

1. ↑ Sasahara, Akihiro; Matsumura, Tetsuo; Nicolaou, Giorgos; Papaioannou, Dimitri (April 2004). "Neutron and Gamma Ray Source Evaluation of LWR High Burn-up UO2 and MOX Spent Fuels". Journal of NUCLEAR SCIENCE and TECHNOLOGY 41 (4): 448–456. doi:10.3327/jnst.41.448. http://www.jstage.jst.go.jp/article/jnst/41/4/448/_pdf. 

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