Main steam pressure

The water leak rate increases as the tubes break, but is limited by the feedwater pump capacity. After the water leak rate reaches the peak value, the main steam pressure decreases because the turbine trips due to the reduction of the main steam flow rate, and the feed water pump trips due to the reduction of its rotational firequency. Due to these quasipassive features, the water leak rate decreases and adverse consequences can be avoided by providing two pressure relief pipings with rupture disks at Ae bottom of the steam generator which operate at a pressure of 11 kg/cm. ... 

Main steam pressure at steam generator outlet 4.0 MPa... 

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

Figure A1.21 T—s diagram for typical ABWR NPP turbine cycle (reheat pressure was assumed to be one-quarter of the main steam pressure).

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. 

Figure 4.15 shows that the main steam pressure is sensitive to the turbine control valve opening. Thus, the main steam pressure is regulated by the turbine control valves like in BWRs. The reactor power is controlled by the control rods. Even though Fig. 4.12 shows that the reactor power is also sensitive to the flow rate, control of the reactor power by regulating the core coolant flow rate as is done in BWRs is not suitable for the Super LWR because ... 

Sensitivity analysis is carried out with various K to minimize overshoot of the pressure when the setpoint of the main steam pressure increases stepwise by approximately 1% (from 24.5 to 24.75 MPa). The results are shown in Fig. 4.18 and K is determined to be 0.25 MPa. 

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 plant transient analysis code for the Super FR is called SPRAT-F. It is based on the 1-D node junction model with radial heat transfer and point kinetics models such as SPRAT-DOWN for the Super LWR (see Chaps. 4 and 6). The nodalization is shown in Fig. 7.67. The models used in SPRAT-F are the same as those in SPRAT-DOWN. The turbine control valve regulates the main steam pressure by changing the main steam flow rate as in BWRs and the Super LWR. The relation between its stroke and the steam flow rate is the same as that of the Super LWR (Fig. 4.4). The relation between the core pressure and the feedwater flow rate (with constant pump speed) is also the same as that in the Super LWR (Fig. 4.5). 

The results are shown in Fig. 7.69 [31]. The main steam pressure increases. The core coolant flow 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 12°C. 

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. 

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