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 most powerful 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 which 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.
One of the common fission products is 135Te, which undergoes beta decay with half-life of 19 seconds to 135I. 135I itself is a weak neutron absorber. It builds up in the reactor in the rate proportional to the rate of fission, which is proportional to the reactor thermal power. 135Iodine undergoes beta decay with half-life of 6.57 hours to 135Xe. The yield of 135Xe for uranium fission is 6.3%; about 95% of 135Xe originates from decay of 135I.
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