What best describes the function of control rods in a reactor?

• A. A rod made up of a substance that controls the neutron population inside a reactor and provides rapid shutdown capability
• B. A device used to heat the fuel by radiative emission
• C. A component that expands steam to drive turbines
• D. A layer of shielding around the core to absorb gamma rays

Answer: (A)

Control rods regulate the reactor’s neutron population by absorbing neutrons. They’re made from materials with high neutron absorption (like boron, cadmium, or hafnium) and can be inserted into or withdrawn from the core. When inserted, they soak up neutrons and slow or stop the fission chain reaction, providing precise reactivity control. In an emergency, they can be driven in rapidly to shut down the reactor quickly—a scram. This combination of moderating the neutron population and enabling a rapid shutdown is exactly what the function of control rods is. Heating the fuel by radiative emission, expanding steam to drive turbines, or shielding gamma rays describe other parts of the system, not the rods’ role.