Which regime consists of steam with entrained moisture droplets and is typically observed only under LOCA conditions in PWRs?

• A. Mist Flow
• B. Single Phase
• C. Bubble Flow
• D. Annular Flow

Answer: (A)

Entrained droplets riding in the steam is a two-phase flow regime where the gas (steam) is the dominant phase and carries small liquid droplets with it. This situation occurs when liquid water is flashed to steam and becomes dispersed as fine droplets that are entrained by the moving vapor, so you have steam with moisture droplets throughout. In LOCA events in PWRs, rapid depressurization and flashing promote this exact condition, so you observe steam that contains suspended droplets rather than a smooth gas or a continuous liquid film. This is distinct from other regimes: bubble flow has gas bubbles dispersed in a liquid, annular flow has a liquid film along the walls with a gas core, and single-phase flow has no dispersed droplets or secondary phase. Thus, the regime described is mist flow.