Fertile materials are not capable of undergoing fission reaction after absorbing thermal (slow or low energy) neutrons, and these materials are not capable of sustaining a nuclear fission chain reaction. There are two basic fertile materials: 238U and 232Th.
239Pu and 241Pu are products of the transmutation of the fertile isotope 238U, while 233U is the product of the transmutation of the fertile isotope 232Th. These two transmutations and decay chains are shown below:
232Th is the predominant isotope of natural thorium. If this fertile material is loaded in the nuclear reactor, the nuclei of 232Th absorb a neutron and become nuclei of 233Th. The half-life of 233Th is approximately 21.8 minutes. 233Th decays (negative beta decay) to 233Pa (protactinium), whose half-life is 26.97 days. 233Pa decays (negative beta decay) to 233U, which is a very good fissile material. On the other hand, proposed reactor designs must physically isolate the protactinium from further neutron capture before beta decay occurs.
Neutron capture may also be used to create fissile 239Pu from 238U, the dominant constituent of naturally occurring uranium (99.28%). Absorption of a neutron in the 238U nucleus yields 239U. The half-life of 239U is approximately 23.5 minutes. 239U decays (negative beta decay) to 239Np (neptunium), whose half-life is 2.36 days. 239Np decays (negative beta decay) to 239Pu.
All commercial light water reactors contain both fissile and fertile materials. For example, most PWRs use low enriched uranium fuel with enrichment of 235U up to 5%. Therefore more than 95% of the content of fresh fuel is fertile isotope 238U. During fuel burnup, the fertile materials (conversion of 238U to fissile 239Pu known as fuel breeding) partially replace fissile 235U, thus permitting the power reactor to operate longer before the amount of fissile material decreases to the point where reactor criticality is no longer manageable.
The fuel breeding in the fuel cycle of all commercial light water reactors plays a significant role. In recent years, the commercial power industry has been emphasizing high-burnup fuels (up to 60 – 70 GWd/tU), typically enriched to higher percentages of 235U (up to 5%). As burnup increases, a higher percentage of the total power produced in a reactor is due to the fuel bred inside the reactor.
At a burnup of 30 GWd/tU (gigawatt-days per metric ton of uranium), about 30% of the total energy released comes from bred plutonium. At 40 GWd/tU, that percentage increases to about forty percent. This corresponds to a breeding ratio for these reactors of about 0.4 to 0.5. That means about half of the fissile fuel in these reactors is bred there. This effect extends the cycle length for such fuels to sometimes nearly twice what it would be otherwise. MOX fuel has a smaller breeding effect than 235U fuel and is thus more challenging and slightly less economical to use due to a quicker drop-off in reactivity through cycle life.
The distinction between Fissionable, Fissile, and Fertile
Fissile materials undergo fission reaction after absorption of the binding energy of the thermal neutron. They do not require additional kinetic energy for fission. If the neutron has higher kinetic energy, this energy will be transformed into the additional excitation energy of the compound nucleus. On the other hand, the binding energy released by the compound nucleus of (238U + n) after absorption of the thermal neutron is less than the critical energy, so the fission reaction cannot occur. The distinction is described in the following points.
- Fissile materials are a subset of fissionable materials.
- Fissionable material consists of isotopes that are capable of undergoing nuclear fission after capturing either fast neutron (high energy neutron – let say >1 MeV) or thermal neutron (low energy neutron – let say 0.025 eV). Typical fissionable materials: 238U, 240Pu, but also 235U, 233U, 239Pu, 241Pu
- Fissile material consists of fissionable isotopes capable of undergoing nuclear fission only after capturing a thermal neutron. 238U is not a fissile isotope because a thermal neutron cannot fission 238U. 238U does not also meet the alternative requirement to fissile materials. 238U cannot sustain a nuclear fission chain reaction because neutrons produced by the fission of 238U have lower energies than the original neutron (usually below the threshold energy of 1 MeV). Typical fissile materials: 235U, 233U, 239Pu, 241Pu.
- Fertile material consists of isotopes that are not fissionable by thermal neutrons but can be converted into fissile isotopes (after neutron absorption and subsequent nuclear decay). Typical fertile materials: 238U, 232Th.
See also: Neutron cross-section.
Comparison of cross-sections
Fissile / Fertile Material Cross-sections. Comparison of total fission cross-sections.
Uranium 238. Comparison of total fission cross-section and cross-section for radiative capture.
Thorium 232. Comparison of total fission cross-section and cross-section for radiative capture.
Uranium 235. Typical fissile isotope. Comparison of total fission cross-section and cross-section for radiative capture.