Positive reactivity addition means which of the following?

• A. Any change that increases the number of neutrons.
• B. Any change that decreases the number of neutrons.
• C. Any change that does not affect neutron population.
• D. Any change that reduces reactor power to zero.

Answer: (A)

Positive reactivity addition means increasing the neutron population in the reactor. Reactivity measures how the neutron population changes from one generation to the next. A positive change means more neutrons are produced than are lost, pushing the reactor toward supercritical (k_eff > 1) and causing the power to rise. So any change that increases the number of neutrons is a positive reactivity addition. Conversely, removing neutrons corresponds to negative reactivity, no change in neutron population is zero reactivity, and halting power requires a large negative reactivity insertion.