What are typical fission product poisons that accumulate during operation?

• A. Xenon-135 and Samarium-149 are notable absorbers that reduce reactivity during operation.
• B. Strontium-90 and Cesium-137 are inert to neutron absorption.
• C. Helium-4 and Neon-20 are strong absorbers in the core.
• D. Uranium-238 and Plutonium-239 are primary poisons.

Answer: (A)

Fission product poisons accumulate in the reactor and act as neutron absorbers, reducing reactivity. The most notable examples are xenon-135 and samarium-149. Xenon-135 forms from decay of fission fragments and has an exceptionally large neutron absorption cross-section for thermal neutrons. As the reactor runs, xenon-135 builds up and steadily absorbs neutrons, causing a drop in reactivity known as xenon poisoning; its concentration evolves with time and can complicate power changes, then decays away after shutdown. Samarium-149 is produced directly in fission and also has a very high neutron capture cross-section, so it accumulates and continues to absorb neutrons, contributing to reactivity loss over the fuel cycle.

Other options don’t fit because those isotopes either do not absorb neutrons significantly or are not fission-product poisons. Strontium-90 and cesium-137 are fission products but their neutron absorption is weak and their impact is mainly radiological and thermal rather to neutron poisoning. Helium-4 and neon-20 are noble gases or inert species with negligible neutron absorption. Uranium-238 and plutonium-239 are fuel and fertile materials, not fission-product poisons that accumulate during operation.