The iodine pit, also called the iodine hole or xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short-lived nuclear poisons in the reactor core. The main isotope responsible is 135Xe, mainly produced by natural decay of 135I. 135I is a weak neutron absorber, while 135Xe is the strongest known neutron absorber. When 135Xe 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 135I and 135Xe 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. Buildup of stable or long-lived neutron poisons is called reactor slagging.