In a typical nuclear reactor fueled with uranium-235, the presence of 135Xe as a fission product presents designers and operators with problems due to its large neutron cross section for absorption.

Enrico Fermi suspected that 135Xe would act as a powerful neutron poison and followed the advice of Emilio Segrè by contacting his student Chien-Shiung Wu.

Wu's unpublished paper on 135Xe verified Fermi's guess that it absorbed neutrons and was the cause of the disruptions to the B Reactor then in use at Hanford, Washington to breed plutonium for the American implosion bomb.

[3][4] During periods of steady state operation at a constant neutron flux level, the 135Xe concentration builds up to its equilibrium value for that reactor power in about 40 to 50 hours.

If sufficient reactivity control authority is available, the reactor can be restarted, but the xenon burn-out transient must be carefully managed.

A flaw in the SCRAM system inserted positive reactivity, causing a thermal transient and a steam explosion that tore the reactor apart.

[11] For instance, in a (somewhat high) neutron flux of 1014 n·cm−2·s−1, the xenon cross section of σ = 2.65×10−18 cm2 (2.65×106 barn) would lead to a capture probability of 2.65×10−4 s−1, which corresponds to a half-life of about one hour.

Compared to the 9.17 hour half-life of 135Xe, this nearly ten-to-one ratio means that under such conditions, essentially all 135Xe would capture a neutron before decay.

Large thermal reactors with low flux coupling between regions may experience spatial power oscillations[12] because of the non-uniform presence of xenon-135.

The combination of delayed generation and high neutron-capture cross section produces a diversity of impacts on nuclear reactor operation.

This oscillation may go unnoticed and reach dangerous local flux levels if only the total power of the core is monitored.

Therefore, most PWRs use tandem power range excore neutron detectors to monitor upper and lower halves of the core separately.