Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
The required energy to extract a neutron depends on whether the nuclei have an odd or an
even number of neutrons. In fact, when neutrons are inpair, an additional energy is
required to break-up the external pair. The binding energy is called "pairing bonus". As a
consequence, when a neutron is absorbed by a nucleus and the compound nucleus has an
even number of neutrons, extra excitation energy is delivered which could be sufficient to
overcome the fission barrier. This is why odd neutron nuclei ( 235
U, 239
Pu, 241
Pu…) do no
require additional kinetic energy to undergo fission after absorbing a neutron. We call
them fissile nuclides. At the opposite, fission reaction is a threshold process for even
nuclei ( 238
U, 240
Pu.). Depending on the initial neutron energy, the fission may or not be
probable.
FIG. 1.12. Fission cross-sections of fissile ( 235 U) and non-fissile isotopes ( 238 U).
Nevertheless, we see from the graph above that the probability of fission for 238
U is not
null for energies below the threshold. In fact, another quantum mechanical effect, called
the "Tunnel effect" explains this. In fact, in the frame work of classical mechanics, if a
particle is in a potential well with insufficient excitation energy, then there is no chance
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