Turbine Bypass System

A Turbine Bypass System (TBS) is provided which passes steam directly to the main condenser under the control of the pressure regulator. The TBS has the capability to shed 40% of the turbine generator rated load without reactor trip or operation of a SRV. The TBS does not serve or support any safety-related function and has no safety design. 

The main condense is designed to cond ise and deaerate the exhaust steam from the main turbine and provide a heat sink for the Turbine Bypass System (TBS). 

The main conventional system is composed of main steam system, turbine bypass system, main feedwater system, and other auxiliary systems. 

The turbine bypass system enable the NSSS to following large turbine load reductions, bypassing the turbine and discharging steam directly to the condensers. It is also used for temperature control of the RCS during hot shutdown and cooldown until start up of the reactor residual heat removal system. 

Following a reactor scram during normal plant operation, steam generation continues at a reduced rate due to the core fission producf decay heat. The turbine bypass system directs the steam to the main condenser, and the feedwater system provides makeup water required to maintain the reactor vessel inventory. 

A pressure relief function is used to control large pressure transients. This system will operate safety/relief valves following closure of the main steam isolation valves or the sudden closure of the turbine admission or stop valves and failure of the turbine bypass system to relieve the excess pressure. For this fimction, the safety/relief valves discharge steam from the steam lines inside the drywell to the suppression chamber. Each safety/ relief valve is operated from its own overpressure signal for the relief fimction, and by direct spring action for the safety function. 

The main steam system is required during start-up and operation at power, to remove the heat intentionally produced by the reactor core and also in the initial phase of shutdown operation before the normal residual heat removal system can be connected, to remove the fission product heat from the reactor core. The heat sinks are provided either by venting steam to atmosphere or by the turbine bypass system dumping steam to the condenser. 

For small power reductions (less than a 10% change in load), the turbine bypass system is not actuated. Instead, it is accommodated by the reactor power control, the pressuriser level control, the pressuriser pressure control and the steam generator level control systems. 

For medium load rejections (greater than 10% but less than 50%, or a turbine trip from 50% power or less), the turbine bypass system operates in conjunction with the same control systems used for the small power reductions. 

For large load rejections (greater than 50% power), the rapid power reduction system operates in conjunction not only with the previously mentioned control systems but also with the rapid power reduction system (Section 7.7.1.10 ofReference 6.1), which is designed to rapidly reduce the reactor power to a value that can be handled by the turbine bypass system. Upon the detection of a large and rapid turbine power reduction, a preselected number of control rods are dropped into the reactor core, causing the reactor power to reduce to approximately 50%. 

The following design requirements for the turbine bypass system support safe operation of the plant under normal eonditions ... 

The turbine bypass system and condenser, in conjunction with the power cutback system, can accommodate a 100% load rejection without a reactor trip and without lifting either primary or secondary safety valves. 

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