Regional Basic Professional Training Course in Korea

❙ 45 ❙
1. Nuclear Reactor Principles
Once emitted by fission, these fragments emit prompt neutrons, decay by beta mode and,
since they are produced in a reactor, absorb neutrons and continue their transformations.
There are different routes of production or destruction of a particular nuclide; the decay
of the nuclide, the transmutation by neutron absorption, the production from fission, the
production by decay of precursors, the production by neutron reactions on other nuclides.
This can be written in the general form as:
Where λi is the decay constant of nuclide i, and σiis its absorption cross-section. The first
summation (index k) is done over all fissile isotope k with the corresponding fission
cross-section σ k
f and Y k
i the direct fission yield of nuclide i from fission of isotope k. The
second sum is done over all fission products leading to nuclide i either by decay
(corresponding constant λ ji ) or absorption (cross section σ ji ).
There are a huge number of fission products. The most famous ones are 135
Xe and 149
Sm
because they have very large absorption cross-sections in the thermal range (about 3
million barns for 135
Xe and forty thousand barns for 149
Sm). Once created, these fission
products contribute to neutron absorption in the fuel and thus have direct implications in
reactor operations. This is the reason why we usually speak about fission products as
poisoning nuclides.
The following diagram (all data taken from JEF-PC version 2) gives the production and
the destruction of 135
Xe. Horizontal arrows show beta decay with the corresponding
half-life and vertical arrows show direct production by fission with the corresponding
fission yield for a thermal neutron-induced fission of 235
U. Although not represented,
each one of these nuclides may be destructed by capture. Nevertheless, except for 135
Xe,