Xe 135Edit

Xenon-135 (Xe-135) is a radioactive isotope of the noble gas xenon that emerges prominently from nuclear fission in reactors and, on rare occasions, in historical tests of nuclear devices. Its most consequential feature is not its radioactivity alone but its extraordinarily large neutron absorption cross-section, which makes Xe-135 a powerful neutron poison in reactor cores. Because of this, Xe-135 plays a central role in reactor dynamics, safety, and the practical management of reactor power.

Xe-135 is produced as part of the fission product inventory that results when heavy nuclei such as Uranium-235 or Plutonium-239 undergo fission Fission. A significant portion of fission events leads to precursors that eventually yield Xe-135, most notably via the beta decay of iodine-135 (Iodine-135 → Xe-135). Xe-135 itself decays by beta emission to cesium-135 (Cesium-135), with a half-life of about 9.14 hours. The chain of production and decay—I-135 decaying to Xe-135, followed by Xe-135 decaying to Cs-135—establishes a dynamic balance in operating reactors that affects neutron economy over time. For the physics involved, see discussions of a Half-life and Beta decay.

A defining property of Xe-135 is its thermal neutron capture cross-section, which is among the largest of all isotopes. At thermal energies, Xe-135 has a cross-section on the order of a few million barns, meaning it readily absorbs neutrons that otherwise would sustain fission in the fuel. This makes Xe-135 an especially potent neutron absorber or “poison” in reactor terms. Operators must account for this effect because it shifts reactivity, especially during power changes and after shutdown as the Xe-135 concentration evolves. See Neutron capture cross-section for the underlying physics and see Nuclear reactor for the operational context.

In practical reactor operation, Xe-135 is a primary factor in what engineers describe as xenon poisoning. When a reactor runs at steady power, Xe-135 builds up from its precursors and begins to absorb neutrons, reducing reactivity. If the reactor power is rapidly increased, Xe-135 initially snags neutrons and tempers the response, requiring adjustments with control systems or chemical shim (where applicable) to maintain the desired power level. Conversely, when power is reduced or the reactor is shut down, Xe-135 persists while iodine-135 decays away, and reactivity can lag behind operator intents. This interplay complicates startup, reboot after outages, and load-following behavior, and it is a standard consideration in reactor physics and operation in Boiling water reactors and Pressurized water reactors. For more detail on the chemical and physical behavior of this class of isotopes, see Fission products and Xenon poisoning.

Xe-135 is chemically a noble gas, which means it does not readily bind with materials in the reactor plumbing or fuel, but instead exists in the gas phase within the reactor containment or the cover gas system. Its gaseous nature makes it possible—under accident conditions or during venting—for xenon to be released from containment, though modern designs and safety systems are intended to limit environmental releases. The isotope itself is a reminder of how a single fission product can influence the broader safety case for nuclear power, even though the underlying physics is well understood and managed through design, operation, and regulatory regimes. See Noble gas for background on the chemistry, and Xenon poisoning for a focused treatment of the operational implications.

The presence of Xe-135 also interacts with other fission products in the reactor neutron economy. Its enormous neutron absorption competes with fuel fission during operation, while its decay product Cs-135 remains long-lived, contributing to the long-term isotopic inventory of the reactor and to downstream considerations in waste management. The broader topic of fission products and their collective effects can be explored in articles on Fission products and Isotope basics.

See also - Xenon - Isotope - Nuclear reactor - Fission product - Uranium-235 - Plutonium-239 - Iodine-135 - Cesium-135 - Iodine-131 - Beta decay - Half-life - Neutron capture cross-section - Noble gas - Boiling water reactor - Pressurized water reactor - Xenon poisoning