Page 198 - Dynamics and Control of Nuclear Reactors

196 CHAPTER 14 Pressurized heavy water reactors

14.7 Control systems
The control systems for PHWR are more complex than LWR control systems, but
also very flexible in providing multiple options for reactivity control. PHWR reac-
tors employ computer control. All nuclear station control and monitoring functions
are performed by redundant digital computer systems [4]. The digital control system
performs the following functions [4]:

• Maneuvering during normal and abnormal plant conditions
• Automatic control of normal reactor operation during startup and at any
power level
• Automatic reactor shut down if a reactor safety is issue is encountered
• Fault tolerant to instrumentation failures and continued safe operation.
There are five main control systems, described below.

14.7.1 Unit power regulator
The Unit Power Regulator modulates steam flow to the turbine in order to cause the
electrical power to equal set point power.

14.7.2 Reactor regulating system
The computer uses an array of neutronic and thermal measurements to determine the
reactor power. The difference between the measured power and the power set point
causes the computer to introduce reactivity changes by the reactivity control devices.
Note that there is no direct control of coolant temperature as in a PWR.

14.7.3 Pressure and inventory control

A PHWR pressurizer performs control of primary pressure and the inventory of
primary D 2 O coolant.
PHWR pressurizers use electrical heaters that can be modulated to increase or
decrease pressure. In a PHWR pressurizer, pressure reduction can be initiated by
bleeding steam to a condenser.
The D 2 O level in the pressurizer is monitored. Differences between the measured
level and the level set point causes D 2 O addition or withdrawal by a feed and
bleed system.

14.7.4 Steam generator level control
PHWRs use U-tube steam generators to transfer heat from the heavy water primary
coolant inside tubes to secondary light water outside the tubes. Integral preheaters
inside the steam generator brings the feedwater to saturation, then boiling ensues
as the feedwater continues its upward path. The heated section is called the riser.