Which item is a safety limit specifically used to prevent Departure from Nucleate Boiling in a Pressurized Water Reactor (PWR)?

Which item is a safety limit specifically used to prevent Departure from Nucleate Boiling in a Pressurized Water Reactor (PWR)?

• A. Departure from Nucleate Boiling Ratio (DNBR) greater than some minimum value
• B. Fuel pin centerline temperature less than maximum
• C. RCS pressure less than maximum
• D. Reactor steam dome pressure less than maximum

Answer: (A)

The essential idea is to maintain a margin against Departure from Nucleate Boiling by keeping the DNBR above a defined minimum value. In a PWR, heat is removed from fuel pins through coolant boiling; if local conditions push the heat flux to where nucleate boiling can no longer be sustained, the surface can become covered by a vapor film (DNB), drastically reducing heat transfer and risking fuel damage. The DNBR measure compares the current operating conditions to the heat flux at which DNB would occur. Keeping DNBR above a specified minimum ensures there is a buffer against entering that dangerous regime under design-basis conditions. This is why DNBR is the explicit safety limit for preventing DNB. Other limits like fuel pin centerline temperature, RCS pressure, or steam dome pressure relate to overall reactor safety and operation, but they do not directly quantify the margin to DNB. DNBR directly encapsulates the local heat transfer risk to DNB and is the targeted limit used to prevent it.

The essential idea is to maintain a margin against Departure from Nucleate Boiling by keeping the DNBR above a defined minimum value. In a PWR, heat is removed from fuel pins through coolant boiling; if local conditions push the heat flux to where nucleate boiling can no longer be sustained, the surface can become covered by a vapor film (DNB), drastically reducing heat transfer and risking fuel damage. The DNBR measure compares the current operating conditions to the heat flux at which DNB would occur. Keeping DNBR above a specified minimum ensures there is a buffer against entering that dangerous regime under design-basis conditions. This is why DNBR is the explicit safety limit for preventing DNB.

Other limits like fuel pin centerline temperature, RCS pressure, or steam dome pressure relate to overall reactor safety and operation, but they do not directly quantify the margin to DNB. DNBR directly encapsulates the local heat transfer risk to DNB and is the targeted limit used to prevent it.