What is the effect of Xenon-135 buildup on reactor operation and how is this managed?

Xenon-135 is a fission product that strongly absorbs neutrons and its buildup in a reactor core significantly affects reactor operation by reducing reactivity. Reactivity is a measure of the reactor's ability to sustain a chain reaction. Xenon-135 is produced directly from fission and also from the decay of Iodine-135. At full power, Xenon-135 is also destroyed by neutron absorption, reaching an equilibrium concentration where its production rate equals its destruction rate. When the reactor power is decreased or shut down, the destruction rate of Xenon-135 decreases, but its production from Iodine-135 decay continues. This leads to a buildup of Xenon-135, which can significantly reduce reactivity, potentially making it difficult or impossible to restart the reactor for a period of time, a phenomenon known as "xenon poisoning". The magnitude and duration of the xenon poisoning effect depend on the reactor's power history and design. To manage xenon buildup, reactor operators use several strategies. One strategy is to maintain a sufficiently high power level to burn out Xenon-135 as it is produced. Another strategy is to use control rods or boron injection to compensate for the negative reactivity caused by xenon buildup, allowing the reactor to be restarted sooner. Waiting for the Xenon-135 to decay naturally is another option, but this can take several hours or even days. Understanding and managing Xenon-135 is crucial for ensuring stable and predictable reactor operation, especially during power transients and shutdowns.