Reactor Theory (Nuclear Parameters)
Negative xenon reactivity, also called xenon poisoning, may provide sufficient negative reactivity
to make the reactor inoperable because there is insufficient positive reactivity available from
control rod removal or chemical shim dilution (if used) to counteract it. The inability of the
reactor to be started due to the effects of xenon is sometimes referred to as a xenon precluded
startup. The period of time where the reactor is unable to "override" the effects of xenon is
called xenon dead time. Because the amount of excess core reactivity available to override the
negative reactivity of the xenon is usually less than 10% Dk/k, thermal power reactors are
normally limited to flux levels of about 5 x 1013 neutrons/cm2-sec so that timely restart can be
ensured after shutdown. For reactors with very low thermal flux levels (~5 x 1012 neutrons/cm2-sec
or less), most xenon is removed by decay as opposed to neutron absorption. For these cases,
reactor shutdown does not cause any xenon-135 peaking effect.
Following the peak in xenon-135 concentration about 10 hours after shutdown, the xenon-135
concentration will decrease at a rate controlled by the decay of iodine-135 into xenon-135 and
the decay rate of xenon-135. For some reactors, the xenon-135 concentration about 20 hours
after shutdown from full power will be the same as the equilibrium xenon-135 concentration at
full power. About 3 days after shutdown, the xenon-135 concentration will have decreased to
a small percentage of its pre-shutdown level, and the reactor can be assumed to be xenon free
without a significant error introduced into reactivity calculations.
Large thermal reactors with little flux coupling between regions may experience spatial power
oscillations because of the non-uniform presence of xenon-135. The mechanism is described in
the following four steps.
An initial lack of symmetry in the core power distribution (for example, individual control
rod movement or misalignment) causes an imbalance in fission rates within the reactor
core, and therefore, in the iodine-135 buildup and the xenon-135 absorption.
In the high-flux region, xenon-135 burnout allows the flux to increase further, while in
the low-flux region, the increase in xenon-135 causes a further reduction in flux. The
iodine concentration increases where the flux is high and decreases where the flux is low.
As soon as the iodine-135 levels build up sufficiently, decay to xenon reverses the initial
situation. Flux decreases in this area, and the former low-flux region increases in power.
Repetition of these patterns can lead to xenon oscillations moving about the core with
periods on the order of about 15 hours.
With little change in overall power level, these oscillations can change the local power levels by
a factor of three or more. In a reactor system with strongly negative temperature coefficients,
the xenon-135 oscillations are damped quite readily. This is one reason for designing reactors
to have negative moderator-temperature coefficients.