Atomic and Nuclear PhysicsDOE-HDBK-1019/1-93NUCLEAR FISSIONFissionableMaterialA fissionable material is composed of nuclides for which fission with neutrons is possible. Allfissile nuclides fall into this category. However, also included are those nuclides that can befissioned only with high energy neutrons. The change in binding energy that occurs as the resultof neutron absorption results in a nuclear excitation energy level that is less than the requiredcritical energy. Therefore, the additional excitation energy must be supplied by the kinetic energyof the incident neutron. The reason for this difference between fissile and fissionable materialsis the so-called odd-even effect for nuclei. It has been observed that nuclei with even numbersof neutrons and/or protons are more stable than those with odd numbers. Therefore, adding aneutron to change a nucleus with an odd number of neutrons to a nucleus with an even numberof neutrons produces an appreciably higher binding energy than adding a neutron to a nucleusalready possessing an even number of neutrons. Some examples of nuclides requiring highenergy neutrons to cause fission are thorium-232, uranium-238, and plutonium-240. Table 4indicates the critical energy (E_{crit}) and the binding energy change for an added neutron (BE_{n}) totarget nuclei of interest. For fission to be possible, the change in binding energy plus the kineticenergy must equal or exceed the critical energy (DBE + KE > E_{crit}).TABLE 4Critical Energies Compared to Binding Energy of Last NeutronTargetNucleusCritical EnergyE_{crit}Binding Energy ofLast Neutron BE_{n}BE_{n} - E_{crit}23920Th7.5 MeV5.4 MeV-2.1 MeV23982U7.0 MeV5.5 MeV-1.5 MeV23952U6.5 MeV6.8 MeV+0.3 MeV23932U6.0 MeV7.0 MeV+1.0 MeV23994Pu5.0 MeV6.6 MeV+1.6 MeVUranium-235 fissions with thermal neutrons because the binding energy released by theabsorption of a neutron is greater than the critical energy for fission; therefore uranium-235 isa fissile material. The binding energy released by uranium-238 absorbing a thermal neutron isless than the critical energy, so additional energy must be possessed by the neutron for fissionto be possible. Consequently, uranium-238 is a fissionable material.Rev. 0Page 51NP-01

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