Page 221 - Dynamics and Control of Nuclear Reactors

220 CHAPTER 16 Nuclear plant instrumentation

– Instrumentation measures the open-end temperature using a different type of
sensor (usually a thermistor or integrated circuit sensor operating at the
instrumentation which operates at ambient temperature).
– Instrumentation calculates the voltage that would occur for a thermocouple with a
junction at the ambient temperature measured at the instrumentation and the open
end at 0°C.
– Instrumentation adds the measured thermocouple junction-to-open end voltage
and the calculated ambient-to 0°C voltage.
– Instrumentation uses the summed voltages to calculate junction temperature
using standard calibration data for a thermocouple with the open end at 0°C.

Thermocouples may be used for in-core coolant temperature measurements. Type-K
thermocouples (chromel - alumel) or type-N (nicrosil - nisil) are suitable. Type-N is
usually preferred for high temperature measurements because of a decalibration ten-
dency in Type-K. Thermocouples may have the junction insulated from the sheath or
have the thermocouple wires attached to the sheath (a grounded-junction thermocou-
ple). Grounded-junction thermocouples have faster time response than insulated
junction thermocouples.

16.2.2.3 Thermowells and bypass installation
It is necessary to be able to replace temperature sensors if they malfunction, decali-
brate, or develop unsatisfactory time response. Therefore, coolant temperature sen-
sors are placed in thermowells (see Fig. 16.9) or in bypass lines that can be valved out
of the primary system.
Thermowells and sheathed sensors are sometimes modified to reduce the heat
transfer resistance and increase the speed of response. One approach is to taper
the end of the sensor and taper the bore of the thermowell with a matching taper near
the end. This provides a snug fit in the region containing the sensing element. But
caution is required. If foreign material collects at the tip of the thermowell or if
the sensor is not fully inserted, then an air gap between the sensor and the thermowell
occurs and the speed of response decreases. Another approach is to use a silver bush-
ing around the sensor sheath in the section where the sensing element is located. The
silver is threaded to provide contact points between the sensor and the thermowell.
The silver is soft and will conform to make contact with the thermowell. But caution
is required here too. If the sensor is withdrawn and reinserted, some of the silver will
be rubbed off and the enhanced contact with the thermowell will diminish.

FIG. 16.9
A thermowell.