BWRs use which gas in their dry wells to prevent a flammable mixture of O2 and H2?

• A. Argon
• B. Helium
• C. Neon
• D. Nitrogen

Answer: (D)

In BWRs, the drywell is kept filled with an inert gas to prevent any hydrogen that can be produced during operation from finding enough oxygen to ignite. Hydrogen can form from hydrogen generation processes inside the containment, and if air (which contains oxygen) were present, a flammable mix could develop. Using a non-reactive, inert gas dilutes any oxygen that might enter and does not support combustion, so the risk of an explosion is greatly reduced.

Nitrogen is the gas chosen because it makes up most of the air, is inexpensive, and is readily obtained from air separation processes. It provides effective inerting without adding significant cost or complexity to the system. Other inert gases like argon, helium, or neon are far less practical due to higher cost or handling considerations, so nitrogen is preferred for large-volume inerting in the drywell.