Page 106 - Dynamics and Control of Nuclear Reactors

100 CHAPTER 8 Reactor control

this to happen with proportional control, there must be a non-zero value of (P Set - P).
For changes in reactor power set point, there is no such occurrence.

8.5 Control options in power reactors
In a system such as a power reactor, the control engineer must begin by asking,
“What do I need to control?” The main possibilities include:

• Reactor power (total and local)
• Power mismatch between reactor and turbine
• Temperatures at various points in the system
• Pressures (primary pressure in systems with steam generators and steam pressure
in BWRs or in steam generators)
• Water level in BWRs and in U-tube steam generators
• Feedwater flow rate
• Frequency of generator electrical output.
These are the main possibilities, but, of course ancillary equipment such as feedwater
heaters, steam reheaters, and condensers also must be monitored and controlled.
Then the control engineer asks, “What control actions are available?” Here are
the main ones:
• Control rod position (full length, full strength; full length, part strength; part
length (gray))
• Dissolved neutron absorber (slow)
• Burnable poisons (very slow and the effect is pre-determined)
• Water in in-core liquid chambers (used in CANDU reactors)
• Main steam valve
• Heater and spray in the pressurizer
• Feedwater flow rate to steam generators
• Recirculation flow rate in BWRs.
• Primary and secondary liquid sodium flow rates in sodium fast reactors.
This list encompasses the main choices available to the control engineer.

8.6 Effect of inherent feedbacks on control options

The usual form of a control system appears in Fig. 8.2 Because of the strong inherent
feedback effects in power reactors, the control system might be viewed as operating
in parallel with inherent feedbacks. But in power reactors, the strategy is often to
control variables (such as flow rates or pressures) that affect feedbacks. For example,
reactivity might be adjusted to achieve a coolant temperature set point. In this case,
reactivity change causes a power change and this power change is driven to the level
that causes coolant temperature to reach a new set point.