Page 214 - Dynamics and Control of Nuclear Reactors

CHAPTER

Nuclear plant

instrumentation 16

16.1 Introduction
Nuclear power plants require several thousand measurements during operation.
These instrumentation systems supply the information needed by safety systems
(protection systems), control systems, and plant monitoring systems.
Reactor instrumentation includes the following devices:
• Ex-vessel neutron detectors, gamma ray detectors, resistance temperature
detectors (RTDs) and thermocouples to measure primary and secondary fluid
temperatures; pressure transducers for primary and secondary side pressures;
flow meters for primary and secondary flow measurements; and liquid level
detectors.
• Some PWRs have in-core neutron detectors. All BWRs have in-core neutron
detector strings.
• Important safety and control system instrumentation have redundant sensors in
order to maintain the reliability of measurements, and to minimize forced
plant shutdowns caused by a few malfunctioning instrument channels.
Measurements based on multiple redundant sensors use a sort of voting system.
Consider a measurement that uses three redundant sensors. If all three sensors
provide the same output (within tolerances) the sensors are judged to be
functioning properly and the measurement is accepted. If one sensor output
disagrees with the other two, it is judged to have malfunctioned. The disagreeing
sensor is ignored and the two agreeing sensors are used. This is called two out of
three logic. If less than two sensors agree, the measurement is judged to be
unreliable and appropriate action is undertaken, including shutdown.
• A large nuclear power plant uses as many as 10,000 sensors and detectors in the
control, safety, and monitoring instrumentation systems.

Refs. [1, 2] provide a comprehensive description of nuclear power plant instrumen-
tation. These handbooks were written many years ago, but most of the information is
still pertinent. Refs. [3, 4] provide information on state-of-the-art nuclear plant
instrumentation and their performance.

213
Dynamics and Control of Nuclear Reactors. https://doi.org/10.1016/B978-0-12-815261-4.00016-0
# 2019 Elsevier Inc. All rights reserved.