Iodine pit

The iodine pit, also called the iodine hole or xenon pit, is a temporary disabling of a nuclear reactor due to the buildup of short-livedneutron poisons in the reactor core. The main isotope responsible is Xe, mainly produced by beta decay of I. I is a weak neutron absorber, while Xe is the strongest known neutron absorber. When Xe builds up in the fuel rods of a reactor, it significantly lowers their reactivity, by absorbing a significant amount of the neutrons that provide the nuclear reaction.

The presence of I and Xe in the reactor is one of the main reasons for its power fluctuations in reaction to change of control rod positions.

The buildup of short-lived fission products acting as nuclear poisons is called reactor poisoning, or xenon poisoning. The buildup of stable or long-lived neutron poisons is called reactor slagging.