Fission product poisons affect reactivity by doing what?

• A. Increasing reactivity
• B. Decreasing reactivity
• C. Having no effect
• D. Only at startup

Answer: (B)

Fission product poisons act by absorbing neutrons, which lowers the neutron population and reduces the reactor’s reactivity. As fission occurs, certain products build up in the fuel and they are effective neutron absorbers. The most influential of these is xenon-135, which has an extremely high neutron absorption cross-section for thermal neutrons. Even small amounts can significantly cut the number of neutrons available to sustain fission, pulling down the effective multiplication factor (keff) and thus decreasing reactivity.

This effect builds up during operation as poisons accumulate, and it changes with time because some of the poisons decay away. That’s why operators must manage reactivity not just at startup but throughout operation and after shutdown, as the poison levels evolve. Because of this neutron-absorbing behavior, fission product poisons do not increase reactivity, nor do they have no effect, nor are they limited to startup conditions. They primarily cause a decrease in reactivity.