The plant dynamics analysis covers the plant control system. Figure 2.38 pre­sents the plant control system of a SCWR as an example [27]. Typical control systems of LWRs are actually more complex, but they are based on the concept below. The plant control system of nuclear power plants uses proportional — integral-derivative (PID) control which is widely used in the control system of other types of practical plants as well.

Composition of the plant control system is based on similar existing plant control systems and its parameters are determined using plant dynamics anal­ysis codes using the procedure below.

© Investigate responses to external disturbances without control system, and decide what state variables (e. g., power) to control and how to control them (e. g., by control rod) according to their sensitivities.

® Design a control system.

® Confirm responses to the external disturbances under the control system.

For example, in the case of reducing feedwater flow rate, changing control rod position, inserting a reactivity, or closing main steam control valves, variations in main steam temperature, power, or pressure are investigated for each case using plant dynamics analysis codes. In Fig. 2.38, the main steam temperature is controlled by regulating feedwater flow rate, the reactor power by control rods, and the main steam pressure by main steam control valves considering highly sensitive parameters.

Figure 2.39 shows the power control system by control rods. This control system is governed by a proportional controller which decides a control rod drive speed in proportion to the deviation between actual power and power setpoint. Control rods are driven at a maximum speed for a deviation larger than a constant value b.

Figure 2.40 represents the main steam temperature control system which is driven by a proportional-integral (PI) controller after a lead compensation for time-lag of a temperature sensor. The PI control parameter values are deter­mined for stable and fast-convergence responses through fine tuning of the integral gain with a little change of the proportional gain.

и (t) =Kpe (t) +Ki fo e (t) dt

Fig. 2.40 Main steam temperature control system

Figure 2.41 shows the pressure control system which controls valve opening by converting the deviation from the pressure setpoint into a valve opening signal with lead/lag compensation. The control parameter value is determined for a stable convergence response from investigation of a time variation in the main steam pressure with a little change of the pressure gain.

Valve Opening Signal
	Valve Opening
Pressure Gain
Deviation
P-R
100
dVd
dv
[MPaJ
[MPaJ
К [MPa]
Fig. 2.41 Pressure control system
Feedwater pump model : SP=Cpump8u

Fig. 2.42 Calculation model for plant startup and stability analysis
dP dz	d, d 2_l 2f 2 —Pu+—Pu +—pu
Feedwater line model:

Valve model: AP=£

P: pressure (Pa)

AP: pressure drop (Pa)

5P: pressure change in pump (Pa)

Z: orifice or valve pressure drop coefficient p: coolant density (kg/cm3) u: fluid velocity (m/s)

8u: fluid velocity change in pump Cpump: pressure drop coefficient of pump z: position (m) t: time (s)

f: friction pressure drop coefficient D: diameter of feedwater pipe (m)

The plant startup procedure is established to satisfy constraints at startup using the calculation models above. The constraints at startup depend on reactor type: for example, maximum cladding surface temperature for SCWRs; restric­tions in heat flux, various safety parameters, and pump cavitation for LWRs; restrictions of vapor fraction in main steam for direct-cycle reactors. There are also restrictions from thermal stress to temperature rise of the reactor vessel.