How does a boron dilution accident affect reactivity and what is a typical mitigation?

How does a boron dilution accident affect reactivity and what is a typical mitigation?

• A. Dilution increases neutron absorption, decreasing reactivity
• B. Dilution decreases neutron absorption, increasing reactivity
• C. Dilution has no effect on reactivity
• D. Dilution removes boron from the system

Answer: (B)

Boron in the coolant acts as a neutron absorber to keep reactivity under control. When boron is diluted, its concentration drops, so fewer neutrons are captured by boron. That leaves more neutrons available to sustain fission, causing reactivity to rise. A typical way to mitigate this is to restore boron concentration by injecting borated water into the system or otherwise adding boron so the neutron poison level returns to the desired value. Operators may also use control rods to help counteract the reactivity rise if needed. The idea is to re-establish adequate neutron absorption, not to remove boron or to have no effect.

Boron in the coolant acts as a neutron absorber to keep reactivity under control. When boron is diluted, its concentration drops, so fewer neutrons are captured by boron. That leaves more neutrons available to sustain fission, causing reactivity to rise.

A typical way to mitigate this is to restore boron concentration by injecting borated water into the system or otherwise adding boron so the neutron poison level returns to the desired value. Operators may also use control rods to help counteract the reactivity rise if needed. The idea is to re-establish adequate neutron absorption, not to remove boron or to have no effect.