Atomic and Nuclear PhysicsDOE-HDBK-1019/1-93NUCLEAR FISSIONNuclear Fission Summary The fission process can be explained using the liquid drop model of a nucleus.In the ground state the nucleus is nearly spherical in shape. After the absorptionof a neutron, the nucleus will be in an excited state and start to oscillate andbecome distorted. If the oscillations cause the nucleus to become shaped like adumbbell, the repulsive electrostatic forces will overcome the short-rangeattractive nuclear forces, and the nucleus will split in two. Excitation energy is the amount of energy a nucleus has above its ground state.Critical energy is the minimum excitation energy that a nucleus must have beforeit can fission.Fissile material is material for which fission is possible with neutrons that havezero kinetic energy. Fissionable material is material for which fission caused byneutron absorption is possible provided the kinetic energy added with the bindingenergy is greater than the critical energy. Fertile material is material that canundergo transmutation to become fissile material. Transmutation is the process of neutron absorption and subsequent decay, whichchanges one nuclide to another nuclide. Conversion is the process of transmutingfertile material into fissile material in a reactor, where the amount of fissilematerial produced is less than the amount of fissile material consumed. Breedingis the same as conversion, except the amount of fissile material produced is morethan the amount of fissile material consumed.The curve of binding energy per nucleon increases quickly through the lightnuclides and reaches a maximum at a mass number of about 56. The curvedecreases slowly for mass numbers greater than 60.The heaviest nuclei are easily fissionable because they require only a smalldistortion from the spherical shape to allow the coulomb forces to overcomingthe attractive nuclear force, forcing the two halves of the nucleus apart.Uranium-235 fissions with thermal neutrons because the binding energy releasedby the absorption of a neutron is greater than the critical energy for fission. Thebinding energy released by uranium-238 absorbing a neutron is less than thecritical energy, so additional kinetic energy must be possessed by the neutron forfission to be possible.Rev. 0Page 55NP-01
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