Which statement best describes xenon-135 poisoning?

• A. It is produced only from iodine decay, not from fission.
• B. It acts as a neutron moderator in the core.
• C. It decays rapidly to a stable gas with no effect on reactivity.
• D. It is a strong neutron absorber produced in fission and from iodine decay; its buildup reduces reactivity and reactor power.

Answer: (D)

Xenon-135 poisoning arises because Xe-135 is an extremely strong neutron absorber, so even small amounts in the core can steal neutrons that would otherwise sustain fission. It is produced directly in fission and also formed from the decay of iodine-135, which itself is a fission product. This combination means Xe-135 can accumulate while the reactor is operating, and because its neutron-absorbing capability is so large, its buildup markedly reduces the neutron flux available for fission, lowering reactivity and reactor power.

Its effect is time-dependent: during operation, a balance is reached between production and removal by neutron absorption, but the net result is a reduced reactivity. If the reactor is shut down, Xe-135 decays away slowly, so reactivity can recover only gradually, which is why startups after shutdown can be delayed or require compensating adjustments. Xe-135 is not a moderator, and it does not disappear instantly or become inert; its role as a powerful neutron absorber is what makes it a dominant poison in reactor behavior.