The Fiso Fabry-Perot fiber-optic temperature sensor was selected for performance evaluation and for potential application in nuclear power plants because of its unique interferometric sensing mechanism and data-processing technique, and its commercial availability. It employs a Fizeau interferometer and a charge-coupled device array to locate the position of the maximum interference fringe intensity, which is directly related to the environmental temperature. Consequently, the basic sensing mechanism is independent of the absolute transmitted light intensity, which is the most likely parameter to be affected by external harsh environments such as nuclear irradiation, high pressure/temperature, and cyclical vibration.
This paper reports research on the performance of two Fiso Fabry-Perot temperature sensors in environmental conditions expected in nuclear power plants during both normal and abnormal (i.e., accident) conditions. The environmental conditions simulated in this paper include gamma-only (60Co) irradiation, pressure/temperature environmental transient, and mixed neutron/gamma field, respectively.
The first sensor exhibited no failure or degradation in performance during and following gamma-only irradiation in which a total dose of 15 kGy was delivered at a dose rate of 2.5 kGy/h. Following gamma irradiation, this sensor was then tested for 10.75 days in a thermohydraulic environment prescribed by the Institute of Electrical and Electronics Engineers IEEE323-1983. Intermittent behavior was observed throughout the latter portions of this test, and degradation in performance occurred after the test. Visual evaluation after opening the sensor head indicated that the internal welding methodology was the primary contributor to the observed behavior during this test. Further consultation with the vendor shows that the robustness and reliability of Fiso sensors can be substantially improved by modifying the internal welding methods.
The second Fiso temperature sensor was tested in a mixed neutron/gamma environment, in which the total neutron fluence was 2.6 × 1016 neutrons/cm2 and the total gamma dose was 1.09 × 108 rads. During the initial calibration following completion of the test, the sensor exhibited a fixed offset of 18.9°C (34°F offset) but responded linearly to change in temperature. An annealing phenomenon was observed as the temperature increased, which reduced the offset by ~63%. This observed phenomenon will be very valuable in the model building for on-line maintenance and calibration of the Fiso temperature sensor in a mixed neutron/gamma irradiation field. Fiso Technologies in Canada has suggested that the temperature offset may also be reduced by improved welding methods. If these shifts can be compensated in the on-line sensing or calibration model of the Fiso sensor, then this type of fiber optic sensor will be competitive in nuclear power plant applications.
