Inserting more control rods into the core results in what?

• A. Additional positive reactivity
• B. Negative reactivity
• C. No change
• D. Increased fuel burnup

Answer: (B)

Inserting more control rods increases neutron absorption in the reactor. That extra absorption lowers the effective neutron multiplication factor, moving the system toward subcritical. Reactivity represents how far the reactor is from criticality; when the multiplication factor is reduced below 1, reactivity becomes negative. So adding rods makes the reactor less reactive, which is why the result is negative reactivity.

Materials used in control rods are chosen for their high neutron absorption, so they actively remove neutrons that would otherwise sustain fission. This is different from increasing fuel burnup, which is a long-term effect of how much energy has already been produced, not an immediate sign change caused by inserting absorbers. It’s also not a situation where there’s no change, since the extra absorbers alter the neutron economy and power level.