Loss of turbine load

These are sized using a conservative Design Basis Event, namely a loss of turbine load with a delayed reactor trip and in accordance with the ASME B PV code to limit the pressure boundary design pressure see CESSAR-DC, Section 5.2.2, Appendix 5A. 

Abnormal transients Decrease in core coolant flow rate Partial loss of reactor coolant flow Loss of offsite power Abnormality in reactor pressure Loss of turbine load Isolation of main steam line Pressure control system failure Abnormality in reactivity Loss of feedwater heating Inadvertent startup of AFS Reactor coolant flow control system failure Uncontrolled CR withdrawal at normal operation Uncontrolled CR withdrawal at startup Accidents... 

The results of the total loss of reactor coolant flow accident are explained in Fig. 1.40. The heat conduction to the water rods increases and the water rods serve as a heat sink . This heat conduction also thermally expands the water in the water rods and temporarily supplies water to the fuel channels. Thus, water rods serve as a water source also and enable the backup pmnps (AFSs) to have a realistic delay time. The results of loss of turbine load without turbine bypass transient are shown in Fig. 1.41. This is a type of pressurization event and an important one for... 

Fig. 1.41 Calculated results for loss of turbine load without turbine bypass ...

Loss of turbine load without turbine bypass... 

Loss of the supply of steam to the turbine-driven RCPs would also cause a trip of the RCPs. The loss of turbine load transient and the isolation of main steam Une transient are followed by this situation. The RCPs are assumed not to stop for some period because there is residual steam in the main steam lines and turbines, similar to BWRs. In the safety analyses, the trip of the RCPs is assumed to occur 10 s after the initiation of the transients. A reactor scram by the signal of turbine control valves quickly closed or MSIV closure is credited before the trip of the RCPs. 

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 loss of turbine load without bypass is a typical pressurization event over a short duration. It is also a typical loss of flow event over a long time scale because both of the turbine-driven RCPs are assumed to trip due to the shutdown of the steam supply to the turbines. Since the difference of the analysis sequences between this event and the loss of offsite power is only the success or failure of the turbine bypass, the long-term reactor behavior is similar to the behavior shown in Fig. 6.51. The behavior in the isolation of main steam line is almost the same as that in the loss of turbine load without bypass. At uncontrolled CR withdrawal at startup, ... 

Abnormality type Typical ATWS event Density coefficient (dk/k/(g/cm )) Loss of flow Loss of offsite power AMCST (°C) Pressurization Loss of turbine load without bypass " Peak pressure (MPa)/peak power (%) Reactivity insertion Uncontrolled CR withdrawal Peak fuel enthalpy (cal/g) Flow increase Main coolant flow control system failure Peak power (%)... 

Loss of External Load Turbine Trip Loss of Condenser Vacuum Closure of Main Steam... 

Steam Generator Pressure Complete Loss of Turbine Generator Load... 

SKiKK UJ COMPLETE LOSS OF TURBINE GENERATOR LOAD 5A-2... 

Increase in reactor heat removal inadvertent opening of steam relief valves secondary pressure control malfunctions leading to an increase in steam flow rate feedwater system malfunctions leading to an increase in the heat removal rate. —Decrease in reactor heat removal feedwater pump trips reduction in the steam flow rate for various reasons (control malfunctions, main steam valve closure, turbine trip, loss of external load, loss of power, loss of condenser vacuum). 

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. 

A loss of electrical load transient could occur from a generator trip, a turbine trip, or a loss of main condenser vacuum. Generally, the most severe transient would be caused by the loss of condenser vacuum. The main feedwater pumps in many plants are steam turbine-driven and exhaust to the main condenser. Thus, loss of condenser vacuum also could cause a loss of the main feedwater pumps. In this case the sequence of events would be similar to the loss of feedwater transient. The most severe effect of the transient, the peak pressure in the primary system, would be of about the same magnitude as in the loss of feedwater flow transient. 

Figure 9 shows the temperatures through the Brayton loop are affected by tlie loss of electrical load. Since more energy is used by the turbine to operate at higher speed, the turbine outlet and reactor inlet temperatures gets cooler. 

Answer This problem is related to the risk from the Loss of Off-Site Power (LOOP) initiator. It is necessary to determine the risk from LOOP, and the frequency of the LOOP initiator. Correct the LOOP initiator for the frequency that the plant could not have continued operation if it could have been connected to a load. Then a systems analysis of the dummy load systems must be performed to determine its reliability which is used to correct the LOOP initiator. Any risk associated with the dummy load must be added to the LOOP risk. These corrections give the risk change due to not having to shut the plant down when it loses its connection to the power line. Another way to avoid shutting down when the load is lost is with a steam bypass around the turbine so the turbine-generator only supplies enough power for the hotel load. 

The selection is dictated by economics governing the initial plant cost versus higher turbine output. Usually, the turbine exhaust steam is designed to be slightly superheated, which is desirable, as it allows for heat loss from the steam with minimum condensate losses. At low loads from the turbine, the degree of superheat can rise sharply, well in excess of the normal design conditions, and for this purpose, desuperheaters are often employed to trim the steam temperature at exhaust. 

The turbogenerator speed must be maintained within narrow limits if it is to generate power acceptably and the control system must be capable of preventing over-speed upon sudden loss of load. For this latter requirement, fastacting valves are necessary with full modulation within 0.5 s. The security of the turbine is also dependent upon the lubrication of its bearings, and it will be found that the control systems are closely linked with the various lubricating systems (turbine, gearbox and alternator). 

Emergency loads Loads that are only applied on loss of normal supplies. These are usually supplied only for a fixed period of time (e.g. turbine emergency mndown lube oil pump). 

The gas that accumulates inside the surface condenser is called the noncondensable load to the steam jets. Some of the noncondensable load consists of C02 accidentally produced when the boiler feedwater is vaporized into steam. Air leaks through piping flanges and valves are other sources of noncondensable vapors. But the largest source of noncondensable vapors is often air drawn into the turbine case, through the shaft s mechanical seals. To minimize this source of leaks, 2 or 3 psig of steam pressure is ordinarily maintained around the seals. However, as the turbine s shaft seals deteriorate, air in-leakage problems can overwhelm the jet capacity. This will cause a loss of vacuum in the surface condenser. 

To avoid shaft over speed on loss of load, the bypass valve is opened to raise the outlet pressure of the turbine so that it is closer to the inlet pressure thereby reducing the shaft work performed by the turbine. A bypass line containing the bypass valve serves to divert high pressure compressor outlet coolant around the reactor and turbine. The bypass line connects the compressor outlet to the recuperator hot side inlet as shown in Figure 12. 

The catalyst should also be resistant to thermal shock, that is, a sudden increase or decrease in temperature. Rapid temperature changes occur during start-up or shut-down of the turbine. The most serious thermal shocks occur upon sudden loss of the turbine load. If the turbine load is lost (by opening a circuit breaker, for example) the fuel must be shut off immediately to prevent overspeeding and destruction of the turbine. The air continues to flow, however, so the temperatme of the catalyst drops very rapidly. Under these conditions the catalyst temperature can fall 1000°C in 100ms, which poses severe problems for ceramic materials. Most... 

---