Negative reactivity addition means which?

• A. Any change that decreases the number of neutrons.
• B. Any change that increases the number of neutrons.
• C. Any change that increases power rapidly.
• D. Any change that causes immediate shutdown.

Answer: (A)

Negative reactivity addition is any change that reduces the neutron population. Reactivity is a measure of how far the reactor is from criticality, so inserting negative reactivity moves the system toward subcritical, causing the neutron population to fall and the power to decrease. In other words, the defining effect is a decrease in the number of neutrons. By contrast, increasing the number of neutrons corresponds to positive reactivity, and statements about rapid power rise or an immediate shutdown describe outcomes that can result from reactivity changes but don’t define the sign of the reactivity insertion itself.