Page 154 - Dynamics and Control of Nuclear Reactors

12.8 Steady-state programs for PWRs 151

would respond to a change in power demand with no control action. For example,
run a simulation with opening or closing the main steam valve and observe what
happens. Then the designer could undertake design of control strategies that
exploit desirable features of the inherent response or overcome undesirable
features.
First, consider the inherent (no control action) response for a PWR. The scenario
following opening of the steam valve is as follows:

– steam valve opening "
– steam flow to turbine "
– turbine power "
– steam pressure #
– steam production in steam generator "
– temperature on secondary side of steam generator #
– heat transfer rate from primary to secondary "
– primary side fluid temperature #
– reactor inlet moderator/coolant temperature #.

Reactor with negative moderator/ Reactor with positive moderator/
coolant temperature coefficient of coolant temperature coefficient of
reactivity reactivity

reactivity " reactivity #
reactor power " reactor power #
fuel temperature #
reactivity "
reactor power "

This scenario shows that reactors with negative coolant and fuel temperature coef-
ficients inherently respond by increasing reactor power. Also, reactors with positive
coolant temperature coefficients but negative fuel temperature coefficients in which
the feedback from fuel temperature is stronger, inherently respond by increasing
reactor power. The net result in both cases is an inherent reactor power increase
in response to a steam flow increase. It can be shown that the ultimate reactor power
change exactly matches the turbine power change. Therefore, the control engineer’s
job is to enhance this performance to achieve desired values for other process vari-
ables besides power.

12.8 Steady-state programs for PWRs

The next step after understanding a power reactor response with no control action is to
develop a control strategy that ensures that all process variables achieve desired values.
This involves establishing set points and identification of control actions that will drive