Atomic and Nuclear Physics
DOE-HDBK-1019/1-93
NUCLEAR FISSION
Nuclear 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 absorption
of a neutron, the nucleus will be in an excited state and start to oscillate and
become distorted. If the oscillations cause the nucleus to become shaped like a
dumbbell, the repulsive electrostatic forces will overcome the short-range
attractive 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 before
it can fission.
Fissile material is material for which fission is possible with neutrons that have
zero kinetic energy. Fissionable material is material for which fission caused by
neutron absorption is possible provided the kinetic energy added with the binding
energy is greater than the critical energy. Fertile material is material that can
undergo transmutation to become fissile material.
Transmutation is the process of neutron absorption and subsequent decay, which
changes one nuclide to another nuclide. Conversion is the process of transmuting
fertile material into fissile material in a reactor, where the amount of fissile
material produced is less than the amount of fissile material consumed. Breeding
is the same as conversion, except the amount of fissile material produced is more
than the amount of fissile material consumed.
The curve of binding energy per nucleon increases quickly through the light
nuclides and reaches a maximum at a mass number of about 56. The curve
decreases slowly for mass numbers greater than 60.
The heaviest nuclei are easily fissionable because they require only a small
distortion from the spherical shape to allow the coulomb forces to overcoming
the attractive nuclear force, forcing the two halves of the nucleus apart.
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. The
binding energy released by uranium-238 absorbing a neutron is less than the
critical energy, so additional kinetic energy must be possessed by the neutron for
fission to be possible.
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