NUCLEAR FISSIONDOE-HDBK-1019/1-93Atomic and Nuclear PhysicsFigure 18 Liquid Drop Model of Fission CriticalEnergyThe measure of how far the energy level of a nucleus is above its ground state is called theexcitation energy(Eexc). For fission to occur, the excitation energy must be above a particularvalue for that nuclide. The critical energy (Ecrit) is the minimum excitation energy required forfission to occur.FissileMaterialA fissile material is composed of nuclides for which fission is possible with neutrons of anyenergy level. What is especially significant about these nuclides is their ability to be fissionedwith zero kinetic energy neutrons (thermal neutrons). Thermal neutrons have very low kineticenergy levels (essentially zero) because they are roughly in equilibrium with the thermal motionof surrounding materials. Therefore, in order to be classified as fissile, a material must becapable of fissioning after absorbing a thermal neutron. Consequently, they impart essentiallyno kinetic energy to the reaction. Fission is possible in these materials with thermal neutrons,since the change in binding energy supplied by the neutron addition alone is high enough toexceed the critical energy. Some examples of fissile nuclides are uranium-235, uranium-233, andplutonium-239.NP-01Page 50Rev. 0
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