Page 217 - Dynamics and Control of Nuclear Reactors

216 CHAPTER 16 Nuclear plant instrumentation

FIG. 16.3
A self-powered neutron detector.

Since the appearance of the beta particle in a self-powered detector must wait for
radioactive decay of the emitter, the measurement is not instantaneous. So, the mea-
surement experiences a time lag that depends on the half-life of the emitter. Also, the
sensor calibration changes as the absorber is consumed by neutron absorptions.

16.2.1.4 Scintillation detectors
Scintillation detectors contain material that emits light upon absorbing a neutron (see
Fig. 16.4). The sensor monitors the light intensity and converts that signal into
neutron flux.

16.2.1.5 Gamma thermometers
Fig. 16.5 shows the general layout of a gamma thermometer.
The sensor contains a differential thermocouple that measures the temperature
difference between two locations inside the sensor. One location is insulated and
the other is un-insulated. Heat deposited (mainly by gamma rays) inside the sensor
causes the temperatures at the two locations to be different because of different heat
transfer resistances. Following equations describe the principle of a gamma
thermometer.

FIG. 16.4
A scintillation detector.