Fabrication and Electric Calibration of Prototypic PWR Gamma Thermometers

The gamma thermometer depends upon the heat produced in the metallic instrument structure by gamma rays and high-energy neutrons. The heat so produced is primarily generated by gamma rays. The resulting temperature differential is proportional to the specific power of nearby nuclear fuel rods. Currently, several U. S. and foreign utilities have research and development programs underway to evaluate gamma thermometers as power monitors in large commercial reactor units. A program to fabricate and calibrate seven prototypic gamma thermometers (7.5 mm in diameter) has been completed by the Technology for Energy Corporation (TEC). Two different designs of the gamma thermometer for PWR applications were constructed. Basically, both designs consisted of two concentric 316 L stainless steel tubes swaged onto a bundle of sheathed thermocouple cables placed circumferentially around a sheathed nichrome-wire heater. The monolithic stainless steel thermocouple assembly was calibrated using electric resistance heating. Electric resistance heating results are presented, which show the output EMF for selected difference and double difference thermocouples versus specific power for these prototypic gamma thermometers. The results of the calibration program indicated that gamma thermometers show promise as in-core local power detectors for PWR applications due to their strong signals, highly linear sensitivities and relatively fast time responses. These prototypical instruments are now being irradiated in the Oak Ridge Research Reactor at Oak Ridge National Laboratory in an effort to establish the effect of neutron damage on the instrument calibration.