Temperature monitoring of nuclear reactor cores with multiplexed fiber Bragg grating sensors

In-core temperature measurement is a critical issue for the safe operation of nuclear reactors. Classical thermocouples require shielded connections and are known to drift under high neutron fluence. As an alternative, we propose to take advantage of the multiplexing capabilities of fiber Bragg grating (FBG) temperature sensors. Our experiments show that sensitivity to radiation depends on both the radiation field and the grating characteristics. For some FBGs installed in an aircooled graphite-moderated nuclear reactor the difference between the measurements and the readings of calibrated backup thermocouples was within the measurement uncertainty. In the worst case, the difference saturated after 30 h of reactor operation at about 5C. To reach megagray per hour level gamma-dose rates and 10 fluences, we irradiated multiplexed FBG sensors in a material testing nuclear reactor. At room temperature, FBG temperature sensors can survive in such radiation conditions, but at 90C a severe degradation is observed. We evidence the possibility to use FBG sensing technology for in-core monitoring of nuclear reactors with specific care under wellspecified conditions. 2002 Society of Photo-Optical Instrumentation Engineers.