Regional Basic Professional Training Course in Korea

❙ 23 ❙
1. Nuclear Reactor Principles
The combination of light nuclides results in an intermediate nuclei with larger binding
energy and thus with higher stability. From the energy balance point of view, this zone is
characterized by the fact that the sum of light nuclei masses is higher than the one of the
compound nuclei. Thus, the combination of two light nuclei will provide energy excess.
This is the zone of Exoenergetic Nuclear Fusion Reactions. An example is 3
H + 2
H →
4
He + 1
n.
At the opposite, if we split a heavy nucleus in two, the resulting two nuclei will have
higher unitary binding energy. From the energy balance point of view, the mass of a
heavy nucleus is higher than the sum of two intermediate nuclei masses. In this mass
zone, Exoenergetic Nuclear Fission reactions are possible.
Although theoretically possible, energy production by the means of fusion reactions
encounters technological difficulties. In fact, it is required to maintain during a certain
period of time the two light nuclei in a sufficiently close position despite the electrostatic
repulsion (caused by the fact that all nucleiare positively charged). However, Fission
reaction has proven simpler feasibility and a sustainable fission reaction has been
possible, few years only after the discovery of fission reaction (Chicago pile by Fermi).
1.2.2. Neutron-induced nuclear reactions cross-sections
1.2.2.1. Definitions
Let us consider a beam of I particles per cm 2 knocking a very thin target containing N
nuclei per cm 3 (see figure below). The number R of nuclear reactions in the target per cm 3
is obviously proportional to I and to N. The proportionality factor σt = R/NI is called the
microscopic cross-section. According to its definition, the dimension of [σ t ]= cm 2
usually expressed in barns; 1 barn =10 −24
. It is
cm 2
and interpreted as the geometrical