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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 overall yield of xenon-135 from fission is 6.3%, though most of this results from the radioactive decay of fission-produced tellurium-135 and iodine-135. Xe-135 exerts a significant effect on nuclear reactor operation . It is discharged to the atmosphere in small quantities by some nuclear power plants. [20]
Neutron capture (29 barns) slowly converts stable 133 Cs to 134 Cs, which itself is low-yield because beta decay stops at 134 Xe; can be further converted (140 barns) to 135 Cs. 6.3333%: Iodine, xenon: 135 I → 135 Xe: 6.57 h: Most important neutron poison; neutron capture converts 10–50% of 135 Xe to 136 Xe; remainder decays (9.14h) to 135 ...
Caesium-135 is a long-lived fission product with much weaker radioactivity. Neutron capture inside the reactor transmutes much of the xenon-135 that would otherwise decay to Cs-135. Caesium-137, with a half-life of 30 years, is the main medium-lived fission product, along with Sr-90.
The yield of 135 Xe for uranium fission is 6.3%; about 95% of 135 Xe originates from decay of 135 I. 135 Xe is the most powerful known neutron absorber , with a cross section for thermal neutrons of 2.6×10 6 barns , [ 1 ] so it acts as a " poison " that can slow or stop the chain reaction after a period of operation.
Xenon-135 is a notable neutron poison with a high fission product yield. As it is relatively short lived, it decays at the same rate it is produced during steady operation of a nuclear reactor. However, if power is reduced or the reactor is scrammed , less xenon is destroyed than is produced from the beta decay of its parent nuclides .
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Other fission products, such as xenon-135 and samarium-149, have a high neutron absorption cross section. Since a nuclear reactor must balance neutron production and absorption rates, fission products that absorb neutrons tend to "poison" or shut the reactor down; this is controlled with burnable poisons and control rods.