The DNBR margin used to maintain the PWR thermal limit is which value?

The DNBR margin used to maintain the PWR thermal limit is which value?

• A. 1.3
• B. 1.0
• C. 1.5
• D. 2.0

Answer: (A)

DNBR margin is the safety cushion that keeps the local heat transfer well above the point where Departure from Nucleate Boiling would begin. In a PWR, operators set this margin at a fixed, moderate level to cover uncertainties in heat transfer correlations, fuel geometry, power peaking, flow distribution, instrumentation accuracy, and potential transients. The idea is to ensure DNBR stays comfortably above the threshold (unity) so that nucleate boiling remains stable and the thermal limit is not approached. This chosen cushion is a balance: large enough to guard against worst‑case deviations, but not so large that it unduly limits plant performance.

DNBR margin is the safety cushion that keeps the local heat transfer well above the point where Departure from Nucleate Boiling would begin. In a PWR, operators set this margin at a fixed, moderate level to cover uncertainties in heat transfer correlations, fuel geometry, power peaking, flow distribution, instrumentation accuracy, and potential transients. The idea is to ensure DNBR stays comfortably above the threshold (unity) so that nucleate boiling remains stable and the thermal limit is not approached. This chosen cushion is a balance: large enough to guard against worst‑case deviations, but not so large that it unduly limits plant performance.