Which component provides rapid shutdown capability in a reactor?

• A. Control Rods
• B. Moderator
• C. Coolant
• D. Turbine

Answer: (A)

Rapid shutdown is achieved by inserting neutron-absorbing material into the core to rapidly remove neutrons from the fission process. The component designed for this purpose is the control rod, made from materials like boron, cadmium, or hafnium that have high neutron absorption. When a scram or safety signal is issued, these rods are driven into the core quickly, increasing neutron capture and driving the reactor to a subcritical state in a very short time. This drop in reactivity, with the effective multiplication factor falling below one, stops fission almost immediately.

The moderator slows neutrons to sustain the chain reaction, so changing its state can affect reactivity but isn’t used for rapid shutdown. The coolant helps manage heat removal and can influence reactor stability over time, but it doesn’t instantly terminate the chain reaction. The turbine is part of the power conversion system and has no direct role in stopping nuclear fission.