Reactor Theory (Nuclear Parameters)
Xenon-135 Response to Reactor Power Changes
During periods of steady state operation, at a constant neutron flux level, the xenon-135
concentration builds up to its equilibrium value for that reactor power in about 40 to 50 hours.
Figure 6 illustrates a typical xenon transient that occurs as a result of a change in reactor power
level. At time zero, reactor power is raised from 50% power to 100% power. When the
reactor power is increased, xenon concentration initially decreases because the burnup is
increased at the new higher power level. Because 95% of the xenon production is from
iodine-135 decay, which has a 6 to 7 hour half-life, the production of xenon remains constant
for several hours. After a few hours (roughly 4 to 6 hours depending on power levels) the rate
of production of xenon from iodine and fission equals the rate of removal of xenon by burnup
and decay. At this point, the xenon concentration reaches a minimum. The xenon concentration
then increases to the new equilibrium level for the new power level in roughly 40 to 50 hours.
It should be noted that the magnitude and the rate of change of xenon concentration during the
initial 4 to 6 hours following the power change is dependent upon the initial power level and on
the amount of change in power level. The xenon concentration change is greater for a larger
change in power level.
Figure 6 Xenon-135 Variations During Power Changes