In PWRs, boron concentration is used as soluble poison; as fuel depletes, what do operators do with boron concentration?

• A. Increase
• B. Decrease
• C. Maintain
• D. Replace with a different poison

Answer: (B)

Boron in the coolant acts as a neutron absorber, providing negative reactivity to help control the reactor. As fuel burns up, the inherent reactivity of the core tends to fall, so to keep the reactor at the desired power level, operators progressively lower the boron concentration. Reducing the soluble boron lessens the absorbed neutron flux, allowing the core to stay closer to critical without needing more fuel or mechanical changes. Increasing boron would push reactivity down further, which isn’t desired; maintaining boron wouldn’t offset the burnup-driven reactivity loss; and replacing boron with a different poison isn’t the standard cycle adjustment. Therefore, the appropriate action is to decrease the boron concentration.