- Fissile materials are a subset of fissionable materials.
- Fissionable material consist 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 consist of fissionable isotopes that are capable of undergoing nuclear fission only after capturing a thermal neutron. 238U is not fissile isotope, because 238U cannot be fissioned by thermal neutron. 238U does not meet also alternative requirement to fissile materials. 238U is not capable of sustaining a nuclear fission chain reaction, because neutrons produced by fission of 238U have lower energies than original neutron (usually below the threshold energy of 1 MeV). Typical fissile materials: 235U, 233U, 239Pu, 241Pu.
- Fertile material consist 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
Source: JANIS (Java-based nuclear information software) http://www.oecd-nea.org/janis/