In-situ Cross Calibration of In-Core Thermocouples in Fast Breeder Test Reactor

Core temperature monitoring system of Fast Breeder Test Reactor uses ANSI type-K thermocouples to monitor the exit coolant temperature of fuel sub-assemblies. These thermocouples play an important role in the safety of the core. These in-core thermocouples are subject to harsh radiation environment and may undergo calibration drift due to the combined effect of radiation and aging. Since these thermocouples are not amenable for off-site calibration, in-situ and online calibration of these thermocouples can give valuable insight about their performance, and any degradation in their characteristics can be determined early well before affecting the measurement process. Cross-calibration technique, used for calibrating redundant sensors measuring the same process parameter, has evolved over the years and has been successfully used to evaluate the performance of the in-core thermocouples of FBTR. Results have shown that all the thermocouples have maintained their measurement accuracy in radiation environment with time.