Regional Basic Professional Training Course in Korea

❙ 29 ❙
1. Nuclear Reactor Principles
for escaping. In the quantum mechanical approach, it is always possible for the system to
pass the wall according to the "Tunnel effect".
1.2.3. The fission process and fission chain reaction
The fission process concerns only heavy nuclides. It was discovered in 1938 by Hahn and
Strassman. It could be spontaneous or a result of nuclear reaction (neutron-, or other light
or heavier particles- induced). The most important for reactor applications is of course
primarily neutron-induced fission reactions and, to less extend spontaneous fission. Here
we will mainly describe neutron-induced fission process.
The following figure gives a simplified description of this process. We first see that after
neutron absorption, the compound nucleus undergoes important deformations leading to
the split in two fragments, disexciting in flight by the emission of neutrons (2.4 in
average for a low-energy neutron induced fission with 235
U) and gamma rays. After
emission of prompt neutrons, the intermediate-mass nuclei are generally above the
stability curve (the ratio of neutrons to protons for these nuclei is higher than the one of
stable nuclei in this region) and would disintegrate by the emission of beta- rays
(transformation of a neutron into a proton an electron and an anti-neutrino). Despite this
disintegration the residual nuclei are still too rich of neutrons and would lose some
neutrons (called delayed neutrons). It is important to distinguish between these two kinds
of neutrons, from the time scale point of view since it has an important effect on the
kinetic aspect of fission chain reaction. Roughly, prompt neutrons are emitted in a time
interval between 10 -21
and 4.10 -4
seconds after fission while delayed neutrons are emitted
in a time interval between 0.08 seconds and 58.2 seconds after fission.