What is the typical consequence if the Xenon population increases after reactor restart?

When the reactor is restarted, an increase in the Xenon population typically leads to the inability of the reactor to sustain operation effectively. Xenon-135 is a fission product that is a potent neutron absorber. After a reactor shutdown, as the reactor begins to restart and heat up, the concentration of Xenon-135 can accumulate during the cooling period because its decay time is relatively long compared to the time it takes for the reactor to operate again.

When the reactor reaches certain power levels, the presence of this increased Xenon population absorbs excess neutrons that are necessary for maintaining the fission chain reaction. This results in a situation where the reactor can become sub-critical or unable to maintain the desired level of operation.

In practical terms, reactor operators must manage the Xenon population carefully, particularly in the first hours after a restart, to ensure that the reactor can gradually ramp up to its operating power. The condition described reflects a critical aspect of nuclear chemistry in maintaining reactivity and operational safety within a reactor system.