Main Steam Temperature Control System

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. 

T2 is set to 20 s based on a lag time for general thermometers. Ti is also set to 20 s in order to avoid instability. 

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. 

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

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 pressure control system and the power control system designed in Chap. 4 are used. However, the main steam temperature control system cannot be used because the core outlet temperature is the saturation temperature. Therefore, a feedwater controller for subcritical pressure operating conditions is needed. Dining subcritical pressure operation, the feedwater flow rate is regulated in order to keep the water level in the steam water separator, instead of regulating the main steam temperature. A combined proportional and derivative controller (PD controller) is found to be suitable for that purpose [12]. 

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]. 

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. 

Control rod withdrawal at normal operation is analyzed. The reactivity worth of the withdrawn CR cluster is conservatively assumed as 1.3%dk/k. The same withdrawal speed as of PWRs (114 cm/min) is taken. The CR cluster is withdrawn until the reactor power reaches the scram setpoint (120% of rated power). The inserted reactivity is 0.69. The calculation results are shown in Fig. 6.32. The power increasing rate is small due to the reactivity feedbacks from the water density and fuel temperature. The cladding temperature increases by only 10°C because the main coolant flow rate is increased by the control system so as to keep the main steam temperature. If the control system is not considered, the increase in the temperature is about 110°C. The influence of the CR worth is small because the inserted reactivity before the reactor scram is almost the same [5]. 

In the control systems of the Super LWR designed in Chap. 4, the main steam temperature is simply controlled by the feedwater pumps. Since the moderator in the large water rods mitigates the change in the coolant temperature in the Super LWR, the change in the main steam temperature is not very large (within 8°C). The... 

The analyses in the previous section show that the Super FR qualitatively has the same basic plant dynamics as the Super LWR although the change in the main steam temperature is larger. Thus, the same plant control system as that of the Super LWR is designed and tuned here as the basis of improvements. 

The gain AT in the pressure control system, described as (4.9) and (4.10), is tuned as 0.396 so that the overshoot is minimized against the 1 % stepwise increase in the pressure setpoint. The main steam temperature is controlled by regulating the feedwater flow rate. The equation for the feedwater controller is written again because it is the basis for the improvement in the next section. 

In order to clarify the characteristics of the reference control system, the plant dynamics is analyzed with the designed control system against the 10% stepwise decrease in the setpoint of the reactor power. The results are shown in Fig. 7.71 [31]. The pressure control system and the power control system work well. However, the change in the main steam temperature is still considerable. 

The same event is analyzed in Sects. 7.9.3 and 7.9.4. The results are shown in Fig. 7.76 [31]. The reactor power settles to the new setpoint at around 100 s with all the control systems. Since the feedwater flow rate follows the reactor power more closely with the improved control systems than the reference case, the changes in the main steam temperature are kept smaller. 

The results are shown in Fig. 7.77 [31]. With all the control systems, the pressure quickly settles to the new setpoint by regulating the turbine control valves. Since the plant response to the valve action is too fast to be affected by the feedwater controller, the peak value of the main steam temperature is almost the same for all the cases. 

The second term of (7.28) in Control system (A) tries to keep the enthalpy rise in the core equal to that in the initial condition. Since the core outlet temperature increases with the pressure when the outlet coolant enthalpy is kept constant, the main steam temperature settles to a slightly higher value than the initial one in this event. However, the difference is 0.7°C, and it does not seem to be a problem in practice. 

The results are shown in Fig. 7.78 [31]. In order to increase the main steam temperature, the power to flow rate ratio needs to be increased by decreasing the feedwater flow rate. The main steam temperature does not reach the new setpoint with control system (A) because the second term of (7.28) tries to make the flow rate follow the power. With other control systems, the main steam temperature settles to the new setpoint... 

Since the reactor power temporarily decreases due to the coolant density feedback in this event the change in the feedwater flow rate is slowed down by the second term of (7.29), so that the main steam temperature reaches the new setpoint without overshoot with Control system (B). On the other hand, the change in the feedwater flow rate is accelerated by the second term of (7.30), so that the settling time is shorter with Craitrol system (C) than that with the reference control system. 

With Control system (A), the recovery of the flow rate is faster, and hence the change in the main steam temperature is smaller than the reference case. This is because the power to flow rate ratio itself is fed back to the feedwater controller. 

The reactor power is not sensitive to the flow rate because the Super FR is a fast reactor with small reactivity feedback from coolant density. The reactor power is mainly regulated by the CRs. Therefore, the responses of the reactor power do not significantly differ with the four control systems including the reference one. Since the responses of the core and main steam pressures are very fast and determined by only the turbine control valves, they are almost the same with the four control systems. The changes in the main steam temperature obtained by the plant stability analyses are summarized in Table 7.37 [31]. The advantages and the issues of each control system are discussed below. 

Control system (B), where the reactor power is fed back to the feedwater controller, keeps the main steam temperature more stable than the reference control system when the power level is changed by the CRs as in Fig. 7.76 [31]. However, this control system gives similar plant dynamics to those with the reference control system unless the reactor power is significantly changed by the CRs. This is because the reactor power is less sensitive to the flow rate and mainly influenced by CRs. 

The feedwater controller of Control system (C) also makes the flow rate follow the reactor power. When the change in the power is caused by CRs, this control system works better than the reference control system as evinced in Fig. 7.76 [31]. When it is caused by the reactivity feedback from the coolant density, the change in the main steam temperature is larger than the reference cases as in Figs. 7.79 [31] and 7.80 [31]. 

Capacity Control The simplest way to regulate the capacity of most steam vacuum refrigeration systems is to furnish several primary boosters in parallel and operate only those required to handle the heat load. It is not uncommon to have as many as four main boosters on larger units for capacity variation. A simple automatic on-off type of control may be used for this purpose. By sensing the chilled-water temperature leaving the flash tank, a controller can turn steam on and off to each ejector as required. 

Also shown in Figure 23.17 are let-down stations between the steam mains to control the mains pressures via a pressure control system. The let-down stations in Figure 23.17 also have de-superheaters. When steam is let down from a high to a low pressure under adiabatic conditions, the amount of superheat increases. Desuperheating is achieved by the injection of boiler feed-water under temperature control, which evaporates and reduces the superheat. There are two important factors determining the desirable amount of superheat in the steam mains. 

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