Iodine pit

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.