Reactor Theory (Nuclear Parameters)
When reactor power is decreased from 100% to 50% power (t = 55 hours), the process is
reversed. There is an immediate decrease in xenon burnup, which results in an increase in
xenon-135 concentration. The iodine-135 concentration is still at the higher equilibrium level
for 100% power and is therefore still producing xenon-135 at the higher rate. The xenon-135
concentration continues to rise until the rate of production of xenon-135 becomes equal to the
rate of removal (roughly 7 to 8 hours after the initial reduction in power level). The xenon-135
concentration then gradually decreases to the new equilibrium level in about 50 to 60 hours. The
magnitude of the xenon peak is greatest if the initial power level is very high.
Maximum peak xenon occurs when a reactor that is operating at 100% equilibrium xenon
concentration is suddenly shut down. The most rapid possible burnout of xenon occurs when
a reactor is started up and operated at full power while this maximum peak xenon condition
The important information in this chapter is summarized below.
Xenon-135 is produced directly as a fission product and by the decay of iodine-135
during reactor operation. Xenon-135 is removed from the core by radioactive
decay and by neutron absorption during reactor operation.
The equilibrium concentration for xenon-135 is determined by the following
g Xe S
f lI NI
g Xe g I S
The xenon-135 concentration increases with increasing power level in a non-linear
manner. Equilibrium xenon-135 concentration reaches a maximum at a flux of
about 1015 neutrons/cm2-sec.
After a power increase, xenon-135 concentration will initially decrease due to the
increased removal by burnout. Xenon-135 will reach a minimum about 5 hours
after the power increase and then increase to a new, higher equilibrium value as the
production from iodine decay increases.