Decrease in Feedwater Flow Rate

The feedwater flow rate decreases stepwise to 95% of the initial value. The control rod position and the turbine control valve opening are kept constant. The results are shown in Figs. 4.11 and 4.12. Due to the once-through coolant cycle, a decrease in the feedwater flow rate directly leads to a decrease in the core coolant flow rate. The main steam temperature increases. The core and main steam pressures 

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Fig. 4.11 Response to stepwise decrease in feedwater flow rate (1)...

Fig. 4.32 Response to impulsive decrease in feedwater flow rate with control system (2)...

Fig. 7.70 Response of the Super ER to stepwise decrease in feedwater flow rate. (Taken from [31] and used with permission from Atomic Energy Society of Japan)...

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... 

With Control system (C), the feedwater flow rate decreases further at the beginning and hence the change in the main steam temperature is larger than the reference case. This is because the derivative control term of (7.30) decreases the feedwater flow rate by detecting the initial decrease in the reactor power. 

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). 

In supercritical FPPs, high thermal efficiency is favorable because the fuel cost occupies a large fraction in the total power cost. Fuel cost fraction, however, is a relatively small part of the NPP cost. Lowering the feedwater temperature reduces the number of feedwater heaters. This decreases the size of the BOP. The decrease in the coolant flow rate per electric output will be more effective in decreasing the capital cost than increasing the thermal efficiency in the Super LWR. The flow rate of the Super LWR with the outlet temperature around 500°C decreases by 31% when the feedwater temperature is 210 0 and by 35% when it is 150°C, compared with that of the ABWR [7]. The size of the turbines and the capacities of the pumps will decrease with the feedwater flow rate. 

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. 

The setpoint of the main steam pressure increases stepwise from 24.5 to 24.75 MPa. The results are shown in Figs. 4.25 and 4.26. The turbine control valves are rapidly closed by the pressure control system. At the beginning, the feedwater flow rate decreases because of the increase in the core pressure. Thus, the main steam... 

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 coolant density coefficient depends on the core design, while the Doppler coefficient is almost constant as long as low-enriched UO2 fuel is used. Sensitivity analysis is carried out in order to look at the robustness of the plant dynamics of the Super LWR plant against the density coefficient. The impulsive decrease in the feedwater flow rate analyzed in Sect. 4.5.4 is selected as the perturbation. The... 

The power to flow rate ratio must be low enough to keep MCST below the criterion and to prevent boiling in the water rods. The maximum allowable powers are calculated with various feedwater flow rates and various feedwater temperatures. Figs. 5.12 and 5.13 [3] show the results at two pressures of 10 and 20 MPa, respectively. The maximum allowable power increases with increasing flow rate, or decreasing feedwater temperature. Figure 5.14 [3] shows the maximum allowable powers as a function of the pressure. The maximum allowable powers for various flow rates during pressurization from 8 to 25 MPa with constant feedwater temperature of 280°C are shown in Fig. 5.15. 

Unlike FPPs, the Super LWR has no superheater. Thus, the main steam conditions during startup of the Super LWR need to be adjusted so that they are suitable for steam turbines. The enthalpy of the core outlet coolant must be high enough to provide the required turbine inlet steam enthalpy. Herein, it is assumed that 5% of the rated power is necessary for turbine startup. The minimum required powers are calculated with various feedwater flow rates and feedwater temperatures. The calculated results at 10 MPa are shown in Fig. 5.16 and those at 20 MPa are shown in Fig. 5.17. It is found that the minimum required power decreases with decreasing flow rate or increasing the feedwater temperature. Figure 5.18 [3] shows the minimum required powers as function of the pressure. 

The feedwater flow rate decreases by 5%. The CR position and the turbine cmitrol valve stroke are kept constant. The results are shown in Fig. 7.70 [31 ]. The main steam pressure increases. The core coolantflow rate decreases with the main steam flow rate, which increases the main steam temperature. The increase in the main steam temperature is nearly 20°C while that in the Super LWR is below 5°C. 

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. 

In this event, the feedwater flow rate decreases stepwise by 5% and then is recovered by the feedwater controller. The results are shown in Fig. 7.79 [31]. 

Consider the simple drum boiler shown in Figure PII. 10. If the flow rate of the cold feedwater is increased by a step, the total volume of the boiling water and consequently the liquid level will be decreased for a short period and then it will start increasing, as shown by the response in Figure 12.4b. Such behavior is the net result of two opposing effects and can be explained as follows ... 

The rate of feedwater flow reduction is different in all these cases. For analysis it is practical (even if unrealistic) to consider the flow to decrease to zero so as to produce an enveloping scenario for the transient. 

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 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 core inlet temperature affects the temperature and velocity distributions in the core. When it is lower, the coolant temperatures and density ratio between the core inlet and outlet become smaller which has a stabilizing effect. On the other hand, it also leads to lower coolant flow velocity and core pressure drop, resulting in a longer time delay which destabilizes the system. The net effect depends on the individual operating conditions. Here, the core power and flow rate are kept constant and the effect of inlet feedwater temperature on stability is investigated. As shown in Fig. 5.42 [11], under the operating conditions of the present parametric study, decreasing the inlet temperature leads to lower oscillation frequency and decay ratio which stabilizes the reactor. 

The net effect on the stabihty depends on the balance between the above two effects. In the present analysis, the neutronic feedback effect is found to be dominant over the hydraulic feedback effect. Here, the core power and flow rate are kept constant and the decay ratios are calculated with various feedwater temperatures. The decay ratio is found to decrease when the core inlet temperature decreases as shown in Fig. 5.61. 

Loss of one stage of the feedwater heating will cause a 35°C drop of the feedwater temperature. In the safety analysis, it is conservatively assumed as 55°C as is done in the safety analysis of ABWRs. The result is shown in Fig. 6.29. At the beginning of the transient, the fuel channel inlet flow rate decreases because the coolant density increases, and hence the volume flow rate decreases upstream from the fuel channel. This is one of the characteristics of the once-through coolant cycle without recirculation. The cladding temperature increases and the reactor power... 

Three units of the AFS are assumed to start. The AFS flow (12% of rated value, 30°C) is added stepwise to the main coolant flow at 0 s. The results are shown in Fig. 6.30. The main coolant flow rate and the fuel channel inlet flow rate increase due to the AFS startup. At the beginning, the fuel channel inlet flow rate is lower than the main coolant flow rate because the feedwater temperature, which is the same as the loss of feedwater heating transient described above, decreases. The... 

A positive reactivity ( 0.1) is inserted stepwise by withdrawing the CRs. The feedwater pump speed and the turbine control valve stroke are kept constant. The results are shown in Fig. 7.68 [31]. The reactor power increases about 10% almost stepwise due to the prompt jump and then gradually decreases due to the reactivity feedbacks from the fuel temperature and coolant density. This behavior implies that the Super FR also has inherent self controllability of the reactor power despite the much smaller density reactivity coefficient compared to that of the Super LWR. The main steam temperature increases, which leads to an increase in the main steam and core pressures because the specific volume of the main steam increases. The increase in the core pressure leads to a decrease in the feedwater and core flow rates, which increases the main steam temperature further. As a result, the maximum increase in the main steam temperature is nearly 40°C while that in the Super LWR is only 9°C (see Fig. 4.10). 

The improved control system with the feedwater controller described as (7.28) is designated Control system (A). The gain A p2a is tuned by analyzing the plant dynamics against the 10% stepwise decrease in the setpoint of the reactor power. The results are shown in Fig. 7.73 [31]. The changes in the power to flow rate ratio and the main steam temperature can be decreased from the case with the reference control system. 0.4 is chosen as Kp2 so that these changes are minimized. 

When the reactor power is higher than the rated value, the CRs are inserted and then the power will decrease. The opposite case is also expected. The second idea is that the deviation of the power is fed back to the feedwater controller too in order to make the flow rate follow the power. 

Pressure Control It is the intention to operate the reactor with a constant steam pressure of 600 psi. If the pressure should have a tendency to rise gradually, this can be taken care of automatically as shown on the flow diagram. The increased pressure will open the feedwater by-pass. Less water will then enter the injector nozzle and the circulation will decrease, resulting in a reduced power output such as described above. This will tend to re-establish the correct pressure. To obtain the correct rate of influence, the valve opening and spring can be adjusted during initial operations or at any other time. 

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