What is the shutdown margin and why is it required?

• A. The minimum excess subcritical reactivity maintained under postulated conditions to ensure safety even with certain failures.
• B. The maximum reactive power allowed during startup.
• C. The temperature at which the reactor must be shut down after a loss of coolant.
• D. The time allowed to insert control rods in an emergency.

Answer: (A)

Shutdown margin is the amount of negative reactivity that must remain in the reactor core when it is shut down, so that the system stays subcritical even under worst‑case postulated conditions. This safety buffer accounts for uncertainties and possible failures, such as incomplete control rod insertion, changes in soluble boron concentration, or other reactivity feedback effects. By guaranteeing a defined minimum margin, the plant cannot accidentally become critical after a shutdown command, ensuring a robust and reliable shutdown path. It is a design and licensing requirement to maintain safe operation, rather than a limit on startup power, a shutdown temperature, or a scrambling of emergency insertion times.