High Precision Sensing of TRIGA Operational Characteristics Using Optical Fiber-Based Gamma Thermometers

Abstract

An Optical Fiber Based Gamma Thermometer (OFBGT) has been developed and tested at the Texas A&M Training, Research, Isotopes, General Atomics (TRIGA) reactor as well as the Ohio State Research Reactor. This sensor technology provides temperature measurement of a thermal mass undergoing gamma heating, from which the gamma flux along the length of the sensor may be calculated. The gamma flux data from an array of OFBGTs could allow the determination of local power density within a nuclear reactor core in three dimensions. A fully developed version of this system could replace the complex Traversing In-core Probe (TIP) system used to calibrate local power-range monitors in Boiling Water Reactors (BWR). The design of the sensor makes use of optical fiber as a truly distributed temperature sensor. When interrogated with a Luna Technologies Optical Backscatter Reflectometer (OBR) it is possible to obtain temperature data with a spatial resolution less than 1mm and temperature accuracy of ±1°C. The testing program involved placing the two sensors in the TRIGA reactor core and taking measurements at 400kW and 900kW over the course of several days with one sensor moving to a new location each day while the other remained stationary as a control. The system provided the expected temperature data for each location tested. The sensors failed during one experiment, necessitating redesign and repairs. Various design problems were solved over the course of three experiments. Overall, this sensor technology shows significant promise for current Light Water Reactors (LWR) as well as advanced nuclear reactors as it provides an unparalleled level of precision in reactor power measurement. Work remains to improve and commercialize the sensor technology.