Atomic and Nuclear Physics DOE-HDBK-1019/1-93 NUCLEAR FISSION Fissionable  Material A fissionable material is composed of nuclides for which fission with neutrons is possible.  All fissile nuclides  fall into this  category.   However, also included are those nuclides  that can be fissioned only with high energy neutrons.  The change in binding energy that occurs as the result of neutron absorption results in a nuclear excitation energy level that is less  than the required critical energy.  Therefore, the additional excitation energy must be supplied by the kinetic energy of the incident neutron.  The reason for this difference between fissile and fissionable materials is the so-called odd-even effect for nuclei.   It has been observed that nuclei with even numbers of neutrons and/or protons are more stable than those with odd numbers.   Therefore, adding a neutron to change a nucleus with an odd number of neutrons to a nucleus with an even number of neutrons produces an appreciably higher binding energy than adding a neutron to a nucleus already  possessing  an  even  number  of  neutrons.    Some  examples  of  nuclides  requiring  high energy  neutrons  to  cause  fission  are  thorium-232,  uranium-238,  and  plutonium-240.   Table 4 indicates the critical energy (E_crit) and the binding energy change for an added neutron (BE_n) to target nuclei of interest.  For fission to be possible, the change in binding energy plus the kinetic energy must equal or exceed the critical energy (DBE + KE > E_crit). TABLE  4 Critical Energies  Compared to  Binding Energy of  Last Neutron Target Nucleus Critical Energy E_crit Binding Energy of Last Neutron BE_n BE_n - E_crit 23 9 2 0 Th 7.5 MeV 5.4 MeV -2.1 MeV 23 9 8 2 U 7.0 MeV 5.5 MeV -1.5 MeV 23 9 5 2 U 6.5 MeV 6.8 MeV +0.3 MeV 23 9 3 2 U 6.0 MeV 7.0 MeV +1.0 MeV 23 9 9 4 Pu 5.0 MeV 6.6 MeV +1.6 MeV Uranium-235  fissions   with  thermal  neutrons   because  the  binding  energy  released  by  the absorption of a neutron is greater than the critical energy for fission; therefore uranium-235 is a fissile material.   The binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so additional energy must be possessed by the neutron for fission to be possible.   Consequently, uranium-238 is a fissionable material. Rev. 0 Page 51 NP-01