Plant Dynamic and Control System Design of The Super FR

It is necessary to analyze stepwise perturbation responses of the plant for the design of control system before performing the safety analysis. The analysis is to understand the general characteristics of the plant in responding an abnormality. Firstly the stepwise response without control system is carried out. Afterwards a control system is designed based on the stepwise response characteristics. Then the safety analysis is carried out by using the designed control system.

Types of the perturbations are taken from the parameters used for the control system design. Since a Super FR is a plant with a direct-steam cycle like BWRs, the same components of control system as in BWRs are used in the Super FR (Oka et al., 2010;

Ishiwatari et al., 2010). Components which are selected for control system design of the Super FR are control rods, feedwater pumps, and turbine control valves (Ishiwatari et al., 2010).

These control system components are used in the single-pass Super FR. The same major perturbations are applied as follows:

1. Reactivity increase by 0.1 dollars resulting from withdrawal of a control rod cluster.

2. Feedwater flow rate decrease by 5%.

3. Main steam flow rate decrease by 5% resulting from closure of the turbine control valves.

Core design of the single-flow pass core Super FR is shown in Fig. 1-6(c) and the core characteristics are shown in Table 1-2 (Liu and Oka, 2013). Coolant with temperature of 280

oC flows from the cold-leg to the bottom dome through the down comer. Then the coolant is distributed into seed (92.3%) and blanket (7.7%) assemblies. The flow distribution is carried out by adjusting orifices at both fuel assemblies. The coolant is heated in the fuel assemblies with power distribution as shown in Fig. 1-9. From the fuel assemblies, the coolant mixes in the upper plenum and flows out through hot-leg with the outlet temperature of 500^oC.

In the singe-flow pass core design, coolant density changes largely along the fuel assemblies. Calculation results of the coolant density change at normal operation for both beginning of cycle (BOC) and end of cycle (EOC) are shown in Fig. 4-1 while Fig. 4-2 shows the comparison of the coolant density change between the single-flow pass core and the

two-flow pass core. Larger density change of the single-flow pass core leads to a higher power peaking which will influence the safety performance at abnormal conditions.

Fig. 4-1 Coolant density change in the single-pass Super FR

Fig. 4-2 Comparison of coolant density change between the single-pass and two-pass cores Calculation model of the safety analysis as shown in Fig. 2-3 considers both average and hot channel calculations. The hot channel is used to evaluate the MCST during abnormal events. Initial or normal operation conditions are determined before the abnormality analysis.

In average channel, orifices are adjusted to satisfy the flow distribution between seed and blanket assemblies. In hot channel, orifices are adjusted to satisfy the MCST. At BOC the MCSTs are 646 ^oC and 617^oC for seed and blanket assemblies respectively, while at EOC the

MCSTs are 647^oC for both seed and blanket assemblies. The calculation results are shown in Fig. 4-3. The flow ratios of hot-average channels are 1.7 for seed and 1.4 for blanket. In the plant dynamic analysis, average channel calculations are presented. The safety analysis will also show the results of hot channel calculation.

Fig. 4-3 Avarage-hot channel at normal operation of BOC 4.1.1 Plant dynamic without control system

The plant dynamic analysis is carried out at BOC and EOC conditions. The plant responses are only considering the reactivity feedback during perturbations.

4.1.1.1 Withdrawal of a control rod cluster

A positive reactivity of 0.1 dollars is inserted. The feedwater flow rate and the turbine control valve opening are kept constant. The calculation results are shown in Fig. 4-4. The power increases rapidly to 111% of the initial value due to the control rod withdrawal. It agrees with the analytical OKHQPEKJ KB[LNKILPFQIL\ =O IAJPEKJA@ EJ PDA LNAREKQO OPQ@U (Ishiwatari et al., 2010). The reactivity is decreased due to the reactivity feedbacks of Doppler and density which the Doppler reactivity feedback is more dominant due to high power density of the Super FR. The power finally is decreased due to the reactivity feedback. The core pressure is increased due to coolant heat-up which in turn decreases the flow rate. The

outlet temperature is increased to be about 530 ^oC due to high power to flow ratio. High increase of the outlet temperature is due to high power density in the Super FR. All the parameters finally achieve stable conditions at about 20 s due to the feedbacks. Both BOC and EOC conditions have similar responses.

Fig. 4-4 Response of the Super FR to stepwise reactivity insertion

Fig. 4-5 Response of the Super FR to stepwise decrease in feedwater flow rate

4.1.1.2 Decrease in feedwater flow rate

The feedwater flow rate decreases stepwise to 95% from the initial value. The control rod position and the turbine control valve opening are kept constant. The calculation results are shown in Fig. 4-5. The decreased feedwater flow rate leads firstly the pressure to decrease and the outlet temperature to increase due to high power to flow ratio. Then the core pressure is increased slightly due to coolant heat-up. The reactivity is decreased due to density reactivity feedback leading to a decrease of the power. All parameters become stable at about 10 s due to the feedbacks. Both BOC and EOC conditions have similar responses.

4.1.1.3 Decrease in turbine control valve opening

The turbine control valve opening decreases stepwise to 95% from the initial value. The control rod position and feedwater flow rate are kept constant. The calculation results are shown in Fig. 4-6. Closure of the valve by 5% leads the core pressure to increase and the flow rate to decrease. As the results the outlet temperature is increased and the power is decreased due to density reactivity feedback. At 10 s all the parameters become stable. Both BOC and EOC conditions also have similar responses.

Fig. 4-6 Response of the Super FR to stepwise decrease in turbine control valve opening 4.1.2 Control system design

There are two types of plant control strategies for steam cycle power stations. The first

is the reactor-following-turbine control and the second is turbine-following-reactor control. In a Super FR the second strategy is used referring to plant control strategy of BWRs (Ishiwatari et al., 2010). The single-pass Super FR control system refers to that study. As mentioned before that the control system consist of main steam temperature control system, core pressure control system, and power control system. The scheme of control system for the single-pass Super FR is shown in Fig. 4-7 which is the same as the control system of Super LWR (Ishiwatari et al., 2003; Oka, et al., 2010). The main steam temperature is controlled by adjusting the feedwater flow rate. The core pressure is controlled by adjusting the turbine control valve opening and the power is controlled by adjusting the position of the control rods.

Fig. 4-7 Scheme of control system for the Super FR (Ishiwatari et al., 2003)

4.1.2.1 Pressure control system

The way to control the pressure by turbine control valves is the same as that of Super LWR which the scheme of pressure control system is shown in Fig. 4-8. The turbine control valve opening ratio is calculated by the equation as follows:

Vs(t) : Signal of turbine control valve opening (%) V(t) : Turbine control valve opening (%)

P(t) : Main steam pressure (MPa)

Pset : Setpoint of main steam pressure (MPa) T1 : Lead time (s)

T2 : Lag time (s)

K : Gain of conversion from pressure to valve opening

Fig. 4-8 Pressure control system of the Super FR (Oka et al., 2010)

Fig. 4-9 Calculation results for tuning gain in the pressure control system of the Super FR The values ofT1 andT2 are the same as that of Super LWR which are 2 and 5 s respectively.

Sensitivity calculation is carried out by changing the setpoint from 24.5 MPa to 24.8 MPa.

The pressure control system performance is adjusted by changing the value of K. The calculation results are shown in Fig. 4-9. The gain values of 0.01 and 0.03 give overshoots of main steam pressure when the set point is changed to 24.8 MPa, while the gain value of 0.09 gives an offset error which the main steam pressure is higher than the setpoint. For the safety

analysis, the gain value of 0.06 is chosen as the pressure control parameter due to no overshoot and small offset pressure.

4.1.2.2 Main steam temperature control system

The main steam temperature control system uses proportional-integral (PI) controller which is the same as that of Super LWR. The scheme of the control system is shown in Fig.

4-10. The feedwater flow rate is calculated based on the following equations.

Fig. 4-10 Scheme of main steam temperature control system (Oka et al., 2010)

Fig. 4-11 Calculation results for tuning proportional gain in main steam control system of the Super FR

Fig. 4-12 Calculation results for tuning integral gain in main steam temperature control system 4.1.2.3 Reactor power control system

The scheme of power control system which is shown in Fig. 4-13 uses the same as that

of Super LWR which is based on widely used in nuclear reactors (Oka et al., 2010). The speed of the control rod drive is calculated by using the following equation.

power within 5% of the set point change, (b) fluctuation of the main steam temperature within 5% of that which has been achieved in fossil fired power plants, (c) Settling time is the shortest. The calculation results of the sensitivity analysis are shown in Fig. 4-14. At low value of b, the power change will be sensitive to the action of the power control system. The power follows the set point rapidly but results a peak of undershoot which exceeds the criterion. The main steam temperature also experiences a peak which is more sensitive to low value of b. Increasing the value ofb will damp the peaks but the settling time will be longer.

By considering the criteria, the b value of 30% is selected as the power control parameter for safety analysis.