Which statement best describes reactivity control in US light-water reactors?

• A. PWRs use boric acid addition or dilution; BWRs use control rods and recirculation flow control.
• B. PWRs rely on control rods only; BWRs rely on boric acid dosing.
• C. PWRs use mechanical moderator changes; BWRs use heavy water moderation.
• D. PWRs have no control rods; BWRs use boron injection.

Answer: (A)

In US light-water reactors, the main way to tune reactivity in a PWR is through soluble boron in the coolant. Adding boron-rich solution (boric acid) absorbs neutrons and lowers reactivity, while diluting the boron increases reactivity. This provides a flexible, global reactivity control since the coolant loop is a single-phase system and boron concentration can be adjusted over a wide range as fuel burns and reactivity changes.

In BWRs, reactivity is controlled primarily by moving control rods to absorb neutrons and by adjusting the core flow through recirculation pumping. Changing the recirculation flow also affects the void fraction and neutron moderation, giving another handle on reactivity in the two-phase, boiling core. Boron dosing is not used for routine reactivity control in the typical BWR design.

So the best description is that PWRs use boric acid addition or dilution, while BWRs rely on control rods and recirculation flow control.