Page 199 - Dynamics and Control of Nuclear Reactors

14.9 Reactor dynamics 197

A mixture of around 10% steam and 90% liquid water emerges from the heated sec-
tion. Steam separators separate the steam from the water. The steam, containing less
than 0.1% of water flows to the turbine. The separated liquid water passes to the
downcomer region where it mixes with feedwater before it passes into the riser
region.
PHWRs use three-element feedwater control during power operation. The three-
element controller involves the measurement of steam generator level, steam flow
rate, and feedwater flow rate. Level set point mismatch error and flow mismatch
error are used in the controller to adjust the feed flow rate to minimize the combined
errors. As in U.S. reactors, this approach overcomes problems with shrink and swell
(see Chapter 10).

14.7.5 Steam generator pressure control
Steam pressure is held constant during power operation. There are two modes of
steam generator pressure control.
In the “NORMAL” mode, pressure is controlled by changing reactor power.
In the “ALTERNATE” mode, pressure is controlled by changing steam flow to
the turbine.

14.8 Maneuvering
PHWR computers enable flexibility in maneuvering strategies. Operators can choose
to operate in a reactor-follow-turbine mode or a turbine-follow-reactor mode.
In the reactor-following or “NORMAL” mode, the steam flow to the turbine
responds first after a change in power demand. This causes a change in steam pres-
sure that leads to calculation of a change in the reactor power set point. The reactor
regulating system then changes reactivity with the reactivity control mechanisms
until the delivered reactor power equals to power set point.
In the turbine-following or “ALTERNATE” mode, the reactor power changes
first. The new power demand is used as the reactor power set point in the reactor
regulating system. Reactivity control mechanisms are activated to change reactivity
until reactor power equals the set point. The change in reactor power causes changes
in heat transfer in the steam generators, and, consequently, changes in steam pres-
sure. The turbine controller will then adjust steam flow until steam pressure returns
to its set point. Fig. 14.3 is a block diagram of the CANDU reactor regulation system
(RRS) [5].

14.9 Reactor dynamics

An example of a CANDU reactor modeling and dynamic simulation is presented in
this section. For details of modeling see Refs. [5, 6].