Control and safety in reactor types: which reactor type, if any, may operate without control rods for reactivity management under normal practice?

Introduction:
Reactivity control is essential in all power and research reactors to ensure safe startup, power maneuvering, and shutdown. Control rods or functionally equivalent systems (e.g., soluble boron, burnable absorbers) are standard.

Given Data / Assumptions:

• Normal reactor operation, not emergency-only systems.
• “Control rod” includes equivalent negative reactivity devices even if named differently.
• Reactor types listed: fast breeder, CANDU, liquid-metal-cooled, and others.

Concept / Approach:
Regardless of neutron spectrum or coolant, each reactor design provides controllable negative reactivity insertion to track fuel burnup, temperature feedbacks, and load changes. Fast reactors lack moderators but still rely on control rod banks and sometimes burnable poisons. CANDU reactors use adjustable control/shutoff rods and sometimes liquid zone control. Liquid-metal-cooled systems similarly use control rod mechanisms compatible with the coolant environment.

Step-by-Step Solution:
Identify requirement: all reactor types must manage reactivity over time.Check each listed type: all employ control devices for startup, regulation, and scram.Conclusion: none operate without reactivity control systems.

Verification / Alternative check:
Safety standards and design documents universally include a rapid shutdown (scram) system and routine control mechanisms irrespective of reactor class.

Why Other Options Are Wrong:

• Fast breeder/CANDU/liquid-metal-cooled: each uses control rods or equivalent; saying they may not need them is unsafe and incorrect.
• Gas-cooled graphite-moderated reactors also require control systems.

Common Pitfalls:
Confusing the presence or absence of a moderator with the need for control rods. Moderator choice affects spectrum, not the necessity of reactivity control.

Final Answer:
None of these (all require control rods or equivalent reactivity control)