Page 219 - Dynamics and Control of Nuclear Reactors

218 CHAPTER 16 Nuclear plant instrumentation

16.2.2 Temperature sensors
Temperature sensors in a presently-operating reactor are either resistance tempera-
ture detectors (RTDs) or thermocouples inside a stainless-steel sheath.

16.2.2.1 Resistance thermometers
RTDs have sensing elements made of metal, typically platinum. The platinum metal
in some reactor RTDs is in the form of a wire wrapped around a mandrel (typically
magnesium oxide) inside a stainless-steel tube with magnesium oxide insulator
between the mandrel and the inner wall of the sheath. See Fig. 16.6.
Another RTD design uses a platinum wire coil cemented to the inside wall of a
hollow section of a metallic tube. This approach provides a very fast-responding tem-
perature measurement because the heat transfer resistance between the coil and the
sheath is small. See Fig. 16.7.
Platinum has a well-defined temperature resistance relationship. Instrumentation
measures the resistance and converts it to a temperature measurement using temper-
ature vs. resistance calibration data. The resistance increases with temperature and
the temperature-resistance relation is almost linear. But the readout instrumentation
accounts for the small non-linearity.

FIG. 16.6
A resistance temperature detector.