Page 163 - Dynamics and Control of Nuclear Reactors

160 CHAPTER 12 Pressurized water reactors

12.11 Control of a PWR with once-through
steam generators [3]

PWRs with once-through steam generators employ steam pressure control. The pres-
sure controller uses two input signals: the measured steam flow demand indicated by
the turbine load and measured steam pressure deviation from its set point. Conse-
quently, the pressure controller is called a two-element controller. Note that a
PWR with once-through steam generators operates in the turbine-following-
reactor mode.

12.12 Turbine control
The turbine control system modulates the steam valve to achieve desired steam flow.
For PWRs with U-tube steam generators, the controller modulates the valve to admit
steam until the turbine power reaches the power set point. For PWRs with once
through steam generators the controller modulates the valve to maintain constant
steam pressure.

12.13 Summary of main PWR controllers

There are many control systems for auxiliary systems in PWRs. But there are five
main control systems. These systems and their function are summarized as follows:

Reactivity: Controller actuates control rods using set points from the steady state
program.
Primary Loop Pressure: Controller actuates heaters and spray flow in the
pressurizer based on departure from the pressure set point.
Pressurizer Level: CVCS actuates flows into and out of pressurizer based on
departure from the level set point. This maintains the primary system coolant
inventory.
Feedwater Flow:
• PWR with a UTSG uses a 3-element controller (downcomer level, steam flow
rate and feedwater flow rate).
• PWR with an OTSG uses a 2-element controller (feedwater demand and
steam pressure)
Turbine: Manually specified demand actuates steam valve or electrical frequency
measured by a governor actuates steam valve in load following PWRs.

Other controllers include – main condenser pressure, feedwater temperature, feed-
water heater water level, etc.