Which isotopes are the primary concerns due to high neutron absorption cross sections?

• A. Xenon-135 and Samarium-149
• B. Uranium-235 and Plutonium-239
• C. Cesium-137 and Iodine-131
• D. Helium-4 and Hydrogen-3

Answer: (A)

When thinking about neutron economy, materials with very high neutron absorption cross sections act as neutron poisons, soaking up neutrons and reducing reactivity. Xenon-135 and Samarium-149 are the classic examples in reactor operation. Xenon-135 forms during fission and has an enormous thermal neutron capture cross section, so it rapidly traps neutrons and causes a drop in the neutron population, a phenomenon known as xenon poisoning that can affect power changes and shutdowns. Samarium-149 also has a very large neutron absorption cross section and persists in the core, contributing to longer-term reactivity loss as it accumulates during irradiation. The other isotopes listed either contribute neutrons through fission or have much smaller absorption cross sections, so they don’t dominate neutron absorption in the same way.