Nuclear reactor control – role of control rods In thermal nuclear reactors, the primary function of the control rods in the control system is to:

Introduction / Context:
Reactor power depends on the effective neutron multiplication factor. Control systems maintain this near unity during steady operation and adjust it during transients. Control rods are central to this function in many reactor types.

Given Data / Assumptions:

• Thermal reactor using a moderator to slow neutrons.
• Control rods made of materials with high neutron absorption cross-section (e.g., boron, cadmium, hafnium).
• Objective: regulate reactivity safely.

Concept / Approach:
Control rods are inserted into or withdrawn from the core to change the net neutron population by absorbing neutrons. Insertion increases absorption, reduces reactivity, and lowers power; withdrawal does the opposite. While temperature and fuel consumption are indirectly affected, the direct function is neutron absorption.

Step-by-Step Solution:
Recognize control variable: reactivity via neutron economy.Identify actuator: control rods with strong absorbers.Insert rods → absorb more neutrons → reduce k_eff → power falls.Withdraw rods → absorb fewer neutrons → increase k_eff → power rises.

Verification / Alternative check:
Startup, shutdown, and scram procedures explicitly position control rods to achieve desired reactivity; in a scram, rods are rapidly inserted to terminate the chain reaction.

Why Other Options Are Wrong:

• Fuel consumption is not directly metered by rods; it follows from power level.
• Coolant flow or temperature control is a separate system.
• Control rods do not chemically enhance moderation.
• Hence, “all of these” is incorrect.

Common Pitfalls:
Assuming rods are for temperature control; temperature changes result from reactivity changes but are not directly controlled by heat removal via rods.

Final Answer:
absorb excess neutrons to regulate the chain reaction