Question

The main function of cadmium rods used in a nuclear reactor is to:

• A. Remove the heat produced at the core of the reactor
• B. Slow down the fast-moving secondary neutrons produced during nuclear fission
• C. Capture slow neutrons to control the chain reaction rate
• D. Give energy to the secondary neutrons produced in nuclear fission

Hint:

Think of control rods as the brakes of a nuclear reactor. Heavy water moderates (slows down) the speed of particles, but cadmium controls (stops) the chain reaction by absorbing them entirely.

Answer 1

The Correct Option is C

Understanding the Concept: In a nuclear fission reactor, a self-sustaining chain reaction occurs when neutrons produced by fission go on to split more uranium nuclei. To prevent this reaction from growing exponentially out of control, the population of active thermal neutrons must be regulated. Materials like Cadmium ($Cd$) or Boron ($B$) have an exceptionally high neutron absorption cross-section, enabling them to capture incoming neutrons without undergoing fission themselves.
Step 1: Differentiate between reactor component functions.

Moderators (like heavy water or graphite) are used to slow down fast neutrons.
Coolants (like liquid sodium or water) carry away thermal heat.
Control Rods (made of cadmium) are inserted or withdrawn to absorb neutrons directly, regulating or safely stopping the core's fission rate.
Thus, the primary function of cadmium rods is the capture of excess slow neutrons.