Main Turbine Bypass System

Main Turbine Bypass System (TBS) aka steam dump system discharges main steam from the steam generators (SG) directly to the main condenser in a controlled manner, bypassing the turbine.

This process minimizes transient effects on the reactor coolant system (RCS) during plant startup, hot shutdown and cooldown; and step load reductions in generator load.

Turbine Trip scenario

When the Turbine shuts down the Main Turbine Bypass valves open to divert steam to the condenser. Otherwise a pressure transient would occur that would challenge equipment. It is important to continue to remove energy from the reactor to reduce boiling and therefore control reactivity smoothly. As a backup, the safety relief valves are available to remove steam to the suppression pool however it is generally preferred to not use safety safety systems for normal operations (these scenarios are considered "complicated" scrams).

Low power and startup

Steam is diverted to the condenser when at low power because the steam quality (temperature/saturation aka "dry steam") is not ready for turning the turbine yet. The need for Main Turbine Bypass System is to let the pressure and energy release; otherwise the vessel is prone to pressure transients especially if you factor a new accident scenario in this period.