Xenon135

Xenon-135 is a radioactive isotope of xenon with mass number 135. It decays by beta emission to cesium-135 and has a half-life of about 9.14 hours. As a noble gas, it is volatile and can migrate through reactor environments, contributing to neutron-absorbing phenomena in nuclear systems.

Production and decay pathways are central to its significance in nuclear science. Xe-135 is produced in nuclear reactors as a fission product of uranium-235 and plutonium-239. A substantial portion of its presence also arises from the decay of iodine-135, which is generated in fission and has a half-life of 6.57 hours; I-135 decays to Xe-135, contributing to the buildup of xenon-135 after reactor startup or following shutdown.

A defining property of Xe-135 is its extremely large thermal neutron capture cross-section, on the order of 2.6 million barns. This makes Xe-135 one of the most effective neutron poisons known, capable of significantly reducing reactor reactivity. In reactor physics, the buildup of xenon-135 leads to xenon poisoning (also linked with the broader iodine–xenon feedback) and can cause delayed changes in reactivity after power changes. Because Xe-135 is a gas, its distribution within a reactor can affect local neutron flux and complicate control.

In practical terms, Xe-135 concentration is a key factor in reactor control and safety analyses. Operators monitor and model its production and decay to manage startup, load changes, and post-shutdown reactivity. While Xe-135 itself is not used for diagnostic or therapeutic medical purposes, its behavior is central to understanding reactor kinetics and neutron economy.