If control rods are inserted deeper into the core, what is the expected effect on reactivity?

• A. They increase reactivity when inserted.
• B. They have no effect on reactivity.
• C. They decrease reactivity when inserted significantly.
• D. They only affect coolant chemistry.

Answer: (C)

Control rods work by absorbing neutrons. When they are inserted deeper into the core, more neutrons are captured before they can induce fission, so the neutron population drops and the reactor’s reactivity decreases. The effect grows as the insertion goes further, so significantly deeper insertion pushes the system toward subcriticality. The other ideas don’t fit because inserting rods clearly changes neutron absorption and reduces reactivity, not the other way around or only affecting coolant chemistry.