Power turbine bypass valve

This is the typical transient where both RCPs trip. However, its sequence is different from a total loss of reactor coolant flow accident as described in Sect. 6.4. In the loss of offsite power, the motor-driven condensate pumps are assumed to trip instantaneously. The turbine control valves are quickly closed due to a turbine trip. The turbine bypass valves open immediately after that. A scram signal and AFS signal are released by detecting the loss of offsite power or turbine control valves quickly closed or condensate pump trip. Both RCPs are assumed to trip at 10 s... 

Turbine trip from turbine design power, failure of direct scram on turbine stop valve closure, failure of the steam bypass system, and reactor scrams from an indirect scram... 

The second function of the bypass valve is to control reactor pressure during startup of the turbine. This allows the reactor power level to be held constant while the turbine steam flow is varied as the turbine is brought up to speed under the control of its speed governor. 

For normal plant shutdown, reactor power and plant output are reduced by manual insertion of control rods. After turbine load is reduced to a minimum value, steam flow is maintained through the bypass valve and the generator is disconnected from the system. 

Performance characteristics of part-load operations of a solid oxide fuel cell/gas turbine hybrid system using air-bypass valves. /. Power Sources, 175, 296-302. 

The turbine bypass pipe work and valves provide the capability to dissipate heat directly to the condenser during plant start-up, and during the cool down of the reactor coolant system up to the point where the normal residual heat removal system can be placed in service (Section 10.4.4 of Reference 6.1). They also reduce the challenges to the main steam power-operated relief valves, the main steam safety valves, the steam generator level control and the pressuriser safety valves following a reactor trip, rapid load reductions during normal operation and turbine trips without a reactor trip. 

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. 

An "anticipated transient" is an event that is expected to occur one or more times during the life of a nuclear power plant. There are a number of anticipated transients, some quite trivial and others that are more significant in terms of the demands imposed on plant equipment. Anticipated transients include such events as a loss of electrical load that leads to closing of the turbine stop valves, a load increase such as opening of a condenser bypass valve, a loss of feedwater flow, and a loss of reactor coolant flow. 

All of the MSIVs are assumed to be closed with the characteristics previously shown in Fig. 6.8. The calculation results before the trip of the RCPs are shown in Fig. 6.28. The reactor behavior is similar to that of loss of turbine load without turbine bypass . Since the closure of the MSIVs is much slower than that of the turbine control valves at the turbine trip, the increases in the pressure and cladding temperature are slightly smaller than those at loss of turbine load without turbine bypass . The reactor power does not increase from the initial value. 

The loss of turbine load is a t5 ical pressurization event The turbine bypass is not credited. The ADS is initiated at 5 s by the ATWS signal of the turbine cmitrol valve quickly closed and reactor power ATWS permissive for 5 s. The calculation results are shown in Fig. 6.49. The pressure increases due to the closure of the turbine control valves. As described in Sect. 6.7.1.3, the inherent characteristics of the Super LWR design make the reactivity insertion and the power increase very small. The peak power is only 104% of the initial value. When the SRVs open, the pressure begins to decrease. After initiating the ADS as the alternative action, the pressure, power, and cladding temperature decrease. The increase in the cladding temperature is about 50°C and the peak pressure is about 26.8 MPa. They are exactly the same as those obtained in the abnormal transient analysis with a reactor scram (see Sect. 6.7.1.3). 

The saturated steam from the reactor vessel is conveyed to the admission valves of the high pressure cylinder via the four steam lines. After expansion through the HP unit, the steam passes through a steam moisture separator unit and a steam reheater, on its way to the admission valves of the three (or four) low pressure turbine cylinders. A special "steam bypass" line, from the steam lines to an intermediate extraction point, provides a means for temporary increases of the steam flow to the turbine cylinders - for power control purposes. 

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