What is the purpose of including burnable poison in some fuel assemblies after refueling?

• A. To compensate for the high initial positive reactivity of fresh fuel
• B. To increase reactivity
• C. To decrease fuel temperature
• D. To reduce neutron absorption

Answer: (A)

Burnable poison is used to offset the large amount of reactivity found in fresh fuel. When new fuel is loaded, there’s excess fissile material and not enough buildup of neutron-absorbing fission products, so the core tends to be too reactive. Adding a small amount of burnable absorber in some fuel rods provides negative reactivity early in the cycle by absorbing neutrons. As the reactor runs, the poison is burned away, so its absorption effect diminishes, letting reactivity rise back toward the planned level without needing large changes in control rods or soluble boron. This helps flatten the reactivity swing and allows a longer, more stable fuel cycle.