Page 159 - Dynamics and Control of Nuclear Reactors

156 CHAPTER 12 Pressurized water reactors

FIG. 12.10
Steady-state program for a PWR with once-through steam generators.

controlled separately by adjusting the steam flow rate (or feedwater flow rate).
Fig. 12.10 shows a steady-state program for a PWR with a once-through steam gen-
erator, similar to that used in a B&W reactor.
Eq. (12.11) provides the resulting steam temperature changes. Reactivity control
uses the coolant average temperature specified in the steady state program as the set
point used for control rod motion. The control action causes a sequence of events that
eventually affects the quantities of interest. The sequence of events is as follows:

– change in coolant average temperature or change in reactor power #
– deviation from set point from the steady-state program causes control rod
motion #
– control rods change reactivity #
– reactivity change causes power change #
– power change causes fuel temperature change #
– fuel temperature change causes coolant temperature change #

12.9 Control rod operating band and control rod operation
Two considerations dictate the allowable positioning of control rods:

– There must be enough out-of-core and partially-inserted rods to bring the reactor
subcritical.
– There must be enough control rods in the reactor to be able to introduce reactivity
needed in power change maneuvers.
These constraints dictate that there is an allowable band of control rod positions.