Regional Basic Professional Training Course in Korea

TABLE 1.3. Slowing down efficiency of some nuclides
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1. Nuclear Reactor Principles
Nuclide Average number of collisions required to slow-down a neutron
1H 14.5
2H 20.0
12C 92.0
238U 1716.9
Table 1.3 shows that the lighter the nuclide is the most efficient is the slowing down.
Hydrogen is then, according to the pure slowing-down process the best moderator. But
once neutrons are in thermal range, the moderator shouldn’t contribute too much to
neutron capture. According to the table above of average cross-section, hydrogen is,
among the reported light nuclides the one who has the biggest capture cross-section.
Fission chain reaction with natural uranium is not possible with water-moderated systems
but possible with heavy-water-moderated or carbon-based moderators. For
water-moderated systems, it is necessary to increase 235
U proportion in the fuel. In
low-enriched uranium fuel (natural or slightly enriched) it is also important to physically
separate the fuel and the moderator. Neutrons born at high energy (about 2 MeV) leave
the fuel quite quickly and undergo several scattering reactions in the moderator before
their energy becomes reasonably low. As cross-sections in the low energy range are very
large, neutrons are easily absorbed when they return back into the fuel rodsafter the
moderation. In all thermal systems, moderator contributes to two competing phenomena:
slowing down efficiency (or non-absorption in the intermediate energy resonance) on the
one hand and low-energy moderator capture on the other hand. Figure 1.23 shows the
variation of multiplication factor with the ratio of hydrogen to uranium number of nuclei.
The example considered here consists of a homogeneous mixture of uranium and water
with 235
U enrichment equal to 5%.