Page 95 - Dynamics and Control of Nuclear Reactors

CHAPTER

8
Reactor control

8.1 Introduction
All control systems have two purposes: to hold a process at a desired condition (or set
point) when experiencing external disturbances and to move a process from one con-
dition to another desired condition.
Since the primary purpose of a power reactor is to produce power, usually elec-
tricity but sometimes process heat, the first job of the control system is to cause the
delivered power to match the desired power. The control system also causes various
plant variables (such as temperature, pressure, liquid level, etc.) to attain desired
values and to reach these values in a timely fashion. These plant controllers have
the additional function of establishing conditions that enhance economics, avoid
undesirable plant conditions, maximize durability of components, and establish ini-
tial conditions that ensure a manageable and safe response to abnormal occurrences.

8.2 Open-loop and closed-loop control systems
Fig. 8.1 shows an open-loop system, in which the system input, X(t), is not directly
affected by the system output.
A perturbation is applied to the system with the intention that the output would
achieve the desired value. The input is “calibrated” to generate an output which is
close to the desired output.
Some examples of systems with open-loop control are.
• Opening or closing a kitchen sink tap to provide desired water flow.
• Operation of a toaster.
• Hitting a golf ball towards a target.

Note that in each of the above examples, the response of the system (water flow rate,
cooking time, flight and final position of the golf ball) is dependent on a pre-set com-
mand and cannot be modified once the command is executed.
A closed-loop system is one in which the control action, X(t), depends on the
system output, Y(t). Closed-loop control systems are commonly called feedback
control systems. See Fig. 8.2. This controller/actuator module is designated by the
block, G c (s).

89
Dynamics and Control of Nuclear Reactors. https://doi.org/10.1016/B978-0-12-815261-4.00008-1
# 2019 Elsevier Inc. All rights reserved.