For heavy nuclides with atomic number of higher than 90, most of fissile isotopes meet the fissile rule:
Fissile isotopes have 2 x Z – N = 43 ± 2 (example for 235U: 2 x 92 – 143 = 41)
where Z is number of protons and N is number of neutrons.
Distinction between Fissionable, Fissile and Fertile
Fissile materials undergoes fission reaction after absorption of the binding energy of thermal neutron. They do not require additional kinetic energy for fission. If the neutron has higher kinetic energy, this energy will be transformed into additional excitation energy of the compound nucleus. On the other hand, the binding energy released by compound nucleus of (238U + n) after absorption of 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 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