What is the effect of xenon-135 on reactor reactivity and how does it arise?

• A. Xenon-135 is produced in fission and from iodine decay; its buildup strongly absorbs neutrons and reduces reactivity, lowering reactor power until it decays away.
• B. Xenon-135 acts as a moderator that slows neutrons, increasing reactivity.
• C. Xenon-135 is stable and has no impact on reactivity.
• D. Xenon-135 immediately decays into harmless gas with no effect.

Answer: (A)

Xenon-135 is a very strong neutron absorber at thermal energies, so its presence directly reduces reactor reactivity by removing neutrons from the chain reaction. It is created in fission and also produced from the decay of iodine-135, another fission product. As irradiation continues, Xe-135 builds up and its neutron absorption depresses the neutron flux, causing the reactor power to fall. When irradiation stops, production drops but the existing Xe-135 decays away over a timescale of several hours, so reactivity gradually recovers. This is why xenon poisoning is a classic dynamic in reactor control: the fuel can be at high power, then a buildup of Xe-135 reduces reactivity, impairing power; later, decay allows reactivity to return. Xenon-135 is not a moderator, it is not stable, and it does not vanish instantly.