Main steam temperature

At the present time efforts to analyse an additional measures to reduce the capital cost are continued. A successful operation of BN-600 NPP testifies to the fact that the parameters and steam cycle efficiency of large reactor could be improved in comparison with BN-800 NPP. From the time the basic decision on this plant was taken, more than 20 years have passed. During this period, the steam parameters in fossil fuelled plants were increased (24 MPa, 560°C) and industry has started turning out turbines and generators from 500-800 to 1000-1200 MW(e). The main steam temperatures are 540 to 560°C is achieved. The decision on the use of such steam cycle and standard turbines is studied. 

Main steam temperature at steam generator outlet 440 C... 

HP cylinder inlet steam temperature - the main steam temperature before the turbine stop valves (or in the main steam line) at the rated turbine power. The actual value should be... 

According to the calculated step responses, the pressure is sensitive to the turbine control valve opening and the feedwater flow rate. The main steam temperature is sensitive to the control rod position and the feedwater flow rate. Therefore the turbine inlet pressure is controlled by the turbine control valves. The main steam temperature is controlled by the feedwater pumps. The core power is controlled by the control rods. 

The plant and safety systems of the Super FR are the same as that of the Super LWR. The safety and stability analyses of the Super FR have been reported [97-100]. Improvement of the plant control system was studied for the Super FR. The power to flow rate ratio was taken for the control parameter of the feedwater pumps in order to suppress a fluctuation of the main steam temperature. This is the same as in supercritical FPPs. It showed better convergence than taking only the feedwater flow rate as the control parameter [101]. 

A positive reactivity of 0.1 is inserted stepwise as a reactivity perturbation. The feedwater flow rate and the turbine control valve opening are kept constant. The results are shown in Figs. 4.9 and 4.10. The power quickly increases to 111% of the initial value. It is consistent with the analytical solution of prompt jump. Then, the power decreases due to reactivity feedbacks from Doppler and coolant density. The main steam temperature changes by following the power. The main steam pressure and the core pressure increase due to increases in the temperature and hence the volume flow rate of the main steam. The fuel channel inlet flow rate changes with the core pressure due to the relation between the feedwater flow rate and the core pressure shown in Fig. 4.4. The plant almost reaches a new steady state in 40 s. 

Since the Super LWR does not use saturated steam, the main steam temperature changes with the power to flow rate ratio in the core. It needs to be kept constant in order to avoid too much thermal stress or thermal fatigue on the structures. Since the Super LWR has no superheaters that are utilized to control the main steam temperature as in FPPs, another method is needed. The analysis results described in Sect. 4.3.2 show that the main steam temperature is sensitive to the feedwater flow rate. Thus, the main steam temperature is controlled by regulating the feedwater flow rate. It is also suitable from the viewpoint of the safety principle of the Super LWR, i.e., keeping the core coolant flow rate (described in Sect. 6.2) because the feedwater flow rate indirectly follows the reactor power in this control method. The plant control system employed for the Super LWR is shown in Fig. 4.16. The plant control strategies of the Super LWR, PWRs, BWRs, and FPPs are compared in Table 4.3. 

The main steam temperature is kept constant by regulating the feedwater flow rate. The logic is shown in Fig. 4.19. A PI controller is used. The feedwater flow rate is calculated based on the following equations. 

Sensitivity analysis is carried out with various Kp and Kj when the setpoint of the main steam temperature increases stepwise by 4" C. Operation of the pressure control system tuned in the previous section is considered. The criteria for selecting these values are as follows. 

The influence of Kp without using the integral controller is shown in Fig. 4.20. From these results, 0.5 is selected as Kp. The influence of Kj with fixed Kp of 0.5 is shown in Fig. 4.21. From these results, it is seen that the integral controller makes the Super LWR less stable. Thus, only the proportional controller with the gain of 0.5 is selected for the main steam temperature control system. 

Fig. 4.20 Calculation results for tuning proportional gain in main steam temperature control system...

Fluctuation of main steam temperature is within 5% of that which has been achieved in FPPs. 

Settling time is the shortest. It is defined as the time by which the change of the power from its initial condition settles into the range of 95-105% of the setpoint change and also the change of the main steam temperature from initial condition is within 0.2°C. 

Stepwise increase in the setpoint of the main steam temperature by 4°C. 

The setpoint of the main steam temperature increases stepwise from 500 to 504°C The results are shown in Figs. 4.27 and 4.28. The feedwater flow rate is decreased so as to increase the main steam temperature. Although the power decreases by the coolant density feedback, it is only about 2%, and then, the power returns to the... 

The power setpoint decreases stepwise from 100 to 90%. The results are shown in Figs. 4.29 and 4.30. The control rods are inserted so as to decrease the power. The power reaches the new setpoint without oscillation. The main steam temperature decreases with the power. The feedwater flow rate is gradually decreased to 90% of the initial value so as to keep the main steam temperature 500°C. The main steam pressure is kept crmstant by the turbine control valves. The pressure loss in the main steam lines decreases because of the decrease in the main steam flow rate. As a result, the core pressure decreases by about 0.1 MPa. After 80 s, the plant is settled at a new steady state. The variation of the main steam temperature is around yC. 

The feedwater flow rate drops stepwise from 100 to 95%. The results are shown in Figs. 4.31 and 4.32. The main steam temperature increases and then returns to the initial value as the feedwater flow rate is recovered by the main steam temperature control system. Although the main steam temperature oscillates, the decay ratio is... 

The feedwater temperature decreases stepwise from 280 to 270°C. The results are shown in Figs. 4.33 and 4.34. At the beginning, the volume flow rate at the reactor vessel inlet decreases because the density of feedwater increases. It temporarily decreases the flow rate at the fuel channel inlet, and hence the main steam temperature increases and the power decreases. This behavior is one of the characteristics of the Super LWR with the once-through coolant cycle which differs... 

The controllers of the feedwater pumps and the main steam temperature have longer time constants than the mrbine control valves have. Thus, the power and the main steam temperature settle more slowly than the pressure at all the perturbations. 

The core outlet coolant enthalpy must be greater than that of the saturated steam from the flash tank to prevent the main steam temperature from decreasing through the line switching. The relation between the required core outlet temperature and the flash tank pressure is shown in Fig. 5.4 [3]. If the flash tank pressure is taken to be 6.9 MPa (the same as that of supercritical FPPs), the coolant temperature at the core outlet must be greater than 420°C. This core outlet temperature is readily achievable in the present design (1,000 MWe class) of the Super LWR. 

---