The hot‑leg and cold‑leg piping in a PWR form the primary thermal‑hydraulic loop that transports heat from the reactor core to the steam generators. Their temperature, flow characteristics, and geometry define the reactor’s overall heat transfer performance and transient response.
Hot‑Leg Characteristics
High Temperature: Typically 315–330°C, carrying coolant directly from the core outlet.
Large Diameter Piping: Minimizes pressure drop and supports high flow rates.
Thermal Stratification: Can occur during low‑flow or shutdown conditions.
Flow Mixing: Influences temperature uniformity entering the steam generator.
Cold‑Leg Characteristics
Lower Temperature: Typically 280–295°C after heat removal in the steam generator.
Reactor Coolant Pump Suction: Cold‑leg piping feeds the RCPs.
Injection Points: Safety injection and CVCS makeup enter through the cold leg.
Thermal Shock Considerations: Rapid temperature changes can stress vessel nozzles.
System‑Level Behavior
Loop ΔT: Temperature difference between hot and cold legs defines reactor power.
Natural Circulation: Hot‑leg buoyancy and cold‑leg density drive passive flow during low‑power conditions.
Transient Response: Pump trips, load rejections, and SI actuation all manifest in hot‑leg/cold‑leg temperature shifts.
Mixing & Stratification: Key factors in thermal fatigue and nozzle integrity.
Why It Matters
Defines core outlet and inlet temperature boundaries.
Influences RCP performance and steam generator heat transfer.
Critical for LOCA, natural circulation, and thermal fatigue analyses.