Boron dilution in a PWR refers to which situation?

• A. Water with low boron concentration injected into the primary loop
• B. Water with high boron concentration injected into the primary loop
• C. Boron-free water used for refueling
• D. Boron is removed from the primary loop during operation

Answer: (A)

Boron dilution works by lowering the amount of neutron-absorbing boron in the primary coolant, which raises reactivity. In a PWR, soluble boron (boric acid) helps control reactivity because boron-10 captures neutrons. When you inject water with a lower boron concentration into the primary loop, you dilute the boron in the coolant. The overall neutron absorption decreases, so the reactor becomes more reactive. This approach is used during startup, refueling, or maneuvering to bring power to the desired level while keeping shutdown margins under control, and it requires careful monitoring of boron concentration, temperature, and core reactivity.

The situation described—introducing water with a low boron concentration into the primary loop—fits this dilution process exactly, which is why it’s the correct interpretation.

Injecting water with high boron would reduce reactivity rather than increase it. Boron-free water for refueling isn’t the standard mechanism of dilution, since it implies removing boron but not the deliberate, controlled lowering of boron concentration in the loop. Removing boron from the loop during operation isn’t a typical defined dilution scenario.