Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
among the most abundant fission products. Most of the Xe‐135 is formed from decay of
I‐135 in the chain:
During operation, Xe‐135 will build up to an equilibrium level, with its poisoning effect
overridden by control rod withdrawal. After a power reduction, however, there will be a
transient increase in the xenon concentration due to reduced removal by absorption but
sustained production in the above decay chain. Xenon transients and potential instabilities
can be an important consideration in control system design and reactor operation.
Absorption of neutrons in fission products is also important in the overall long‐term
neutron balance of the reactor. Eventually, the combination of fissile material depletion
and fission product poisoning will limit the lifetime of the fuel.
3.4.2.2. Other radioactive materials
Neutron activation of structural materials
Most commonly used structural materials will absorb neutrons to some extent, and will,
therefore, become radioactive. Materials used in the reactor core itself are selected to
have as low an absorption level as possible in the interest of good neutron economy.
However, some activation will occur, even in low absorption materials such as Zircaloy
used for fuel pin cladding. In addition, leakage neutrons will activate material in the
reactor vessel and in‐vessel structures. For example, neutron absorption in nickel‐bearing
alloys will produce cobalt‐60, which decays with a high‐energy gamma, and is, therefore,
a shielding problem. Corrosion products of activated structural material, carried through
the coolant system, also contribute to the need for shielding of the coolant system to
protect plant workers.
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