In natural circulation, the RCS cold leg temperature is at the saturation temperature.

• A. True
• B. False
• C. Not applicable
• D. Depends on design

Answer: (A)

In natural circulation, the primary coolant loop operates at a fixed high pressure, so there is a defined saturation temperature for the system. The loop often contains both liquid and vapor as heat is exchanged between the core, hot leg, and the steam generators. In the cold leg, any vapor that reaches it condenses back into liquid, and the liquid present is at the same temperature as the vapor–liquid interface for that pressure—that is, at the saturation temperature. Because the phase boundary enforces this equilibrium temperature, the cold leg temperature tends to be the saturation temperature corresponding to the system pressure. This is why the statement is true: the cold leg ends up at Tsat for the RCS pressure during natural circulation.