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Multiplexed fiber Bragg grating sensors for in-core thermometry in nuclear reactors

In-core temperature measurment 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 FThre Bragg Grating (FBG) temperature sensors to perform the in-core temperature measurements. We first report on our irradiation experiments on multiplexed FBGs, written in different fibres, employed to measure the in-pile temperature of an air-cooled graphite-moderated nuclear reactor. For some FBGs 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 hours of reactor operation at about 5°C. Afterwards, we irradiated multiplexed FBG sensors in our material testing nuclear reactor, evidencing the possibility to use FBG sensing technology for in-core monitoring of nuclear reactors with specific care and in well-specified conditions.