Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
neutrons with kinetic energy in this domain will be referred to as intermediate-energy
E < few eV: This energy is close to the one of atoms’ motion in the matter due to thermal
agitation; neutrons with kinetic energy in this domain will be referred to as thermal
neutrons.
Because of the huge capture resonance in intermediate energies, it seems difficult to
maintain fission chain reactions with neutrons in this domain. Fast or thermal neutrons
are then the most natural possibilities. Table 1.2 shows average cross sections for thermal
(Maxwellian spectrum with a temperature of 0.0253 eV) and fast (Maxwellian spectrum
with a temperature of 1.35 MeV) neutrons.
TABLE 1.2. Average cross sections for thermal and fast neutrons
Nuclide Thermal neutrons cross-sections* Fast neutrons cross-sections
Fission (barn) Capture(barn) Fission(barn) Capture(barn)
235U 504.40 86.32 1.22 0.09
238U 2.41 - 0.30 0.07
239Pu 698.00 274.50 1.80 0.65
1H 0.29 - -
2H - 0.0004 - -
12C - 0.003 - -
In Table 1.2, we see large fission cross-sections in the thermal and comparably rather
small capture cross-sections of non-fissile nuclides. But as fission neutrons are emitted at
MeV energies, we must find a means to decrease their energies so that they can make
fissions in the thermal range. This process of neutrons slowing-down is achieved by
"moderators". Table 1.3 compares different nuclides from the point of view of slowing
down efficiency.
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