Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
At the equilibrium, the concentration of Sm is given by: S eq = YΣf / σ which doesn’t
depend on the flux level. The poisoning effect is given by Π eq = YΣf /Σa. In a PWR
reactor, this poisoning is about 700 pcm. Now, after the reactor shutdown (null flux in the
above equations), this poisoning increases since, as Sm is a stable nuclide the only
remaining process is its accumulation from Pm decay. The asymptotic poisoning is given
by Π as = Π eq (1+ σ Φ/λ) which depends on the reactor’s flux before shutdown. In a PWR
reactor, the increase is not important (about 300 pcm) but in high flux reactors, this could
be huge. In such case, it is very important to operate with significantly lower flux level
before shutdown in order to avoid excessive poisoning, which could disable the restart of
the reactor after shutdown.
The poisoning effect discussed until now supposes that the flux in the reactor is the same
everywhere, which is in fact an approximation. If we suppose different variations of the
flux in say, two region of the reactor the poisoning will have a spatial effect. For instance,
if the flux slightly increases in one region and slightly decreases in the other region, then,
the poisoning in these two regions will be different. In fact, the poisoning will increase in
the region where the flux decreases, resulting to a further decrease of reactivity…and the
opposite phenomenon will happen in the other region. In such way, the no-equilibrium
between the two regions will be much more accentuated. This is why the monitoring of
only the global power in the reactor is not sufficient. It is important to control the relative
power in different regions. For that, we use for instance the Axial Offset, which measures
the non-equilibrium between the top and the bottom of the reactor using different
detectors located in the two regions (signals ST and SB) an defined as: AO = (ST - SB)/
(ST + SB) to monitor these instabilities.
To close the discussion about fuel burn up, we mention the decrease of multiplication
factor with burn up. In fact, with the decrease of the number of fissile nuclei, the
apparition of non-fissile actinides and of fission products the effective multiplication
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