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Xenon-135 (135 Xe) is an unstable isotope of xenon with a half-life of about 9.2 hours. 135 Xe is a fission product of uranium and it is the most powerful known neutron-absorbing nuclear poison (2 million barns; [1] up to 3 million barns [1] under reactor conditions [2]), with a significant effect on nuclear reactor operation.
The main isotope responsible is 135 Xe, mainly produced by natural decay of 135 I. 135 I is a weak neutron absorber, while 135 Xe is the strongest known neutron absorber. When 135 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 artificial isotope 135 Xe is of considerable significance in the operation of nuclear fission reactors. 135 Xe has a huge cross section for thermal neutrons, 2.65×10 6 barns, so it acts as a neutron absorber or "poison" that can slow or stop the chain reaction after a period of operation.
Because 135 Xe is a potent nuclear poison with the largest cross section for thermal neutron absorption, the buildup of 135 Xe in the fuel inside a power reactor can lower the reactivity greatly. If a power reactor is shut down or left running at a low power level, then large amounts of 135 Xe can build up through decay of 135 I.
135 Xe reactor poisoning was a major factor in the Chernobyl disaster. [91] A shutdown or decrease of power of a reactor can result in buildup of 135 Xe, with reactor operation going into a condition known as the iodine pit.
Xenon-135 in particular tremendously affects the operation of a nuclear reactor because it is the most powerful known neutron poison. The inability of a reactor to be restarted due to the buildup of xenon-135 (reaches a maximum after about 10 hours) is sometimes referred to as xenon precluded start-up.
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In solid-fuel reactors, on restart the 135 Xe in the fuel absorbs neutrons, followed by a sudden jump in reactivity as the 135 Xe is burned out. Conventional reactors may have to wait hours until xenon-135 decays after shutting down and not immediately restarting (so-called iodine pit).