Abstract
We present an innovative architecture of a Rayleigh-based optical fibre sensor for the monitoring of water level and temperature inside storage nuclear fuel pools. This sensor, able to withstand the harsh constraints encountered under accidental conditions such as those pointed-out during the Fukushima-Daiichi event (temperature up to 100 °C and radiation dose level up to ~20 kGy), exploits the Optical Frequency Domain Reflectometry technique to remotely monitor a radiation resistant silica-based optical fibre i.e. its sensing probe. We validate the efficiency and the robustness of water level measurements, which are extrapolated from the temperature profile along the fibre length, in a dedicated test bench allowing the simulation of the environmental operating and accidental conditions. The conceived prototype ensures an easy, practical and no invasive integration into existing nuclear facilities. The obtained results represent a significant breakthrough and comfort the ability of the developed system to overcome both operating and accidental constraints providing the distributed profiles of the water level (0–to–5 m) and temperature (20–to–100 °C) with a resolution that in accidental condition is better than 3 cm and of ~0.5 °C respectively. These new sensors will be able, as safeguards, to contribute and reinforce the safety in existing and future nuclear power plants.
Highlights
Fukushima-Daiichi event on March 11th, 2011, signed a turning point in nuclear industry by highlighting several weaknesses in the control of the critical systems ensuring the safety in nuclear power plant (NPP), in particular when they are exposed to harsh constraints associated to extreme and accidental conditions
This work has dealt with the conception and validation of an Optical Fibre Sensor (OFS) prototype for distributed temperature measurements and water level (WL) monitoring inside SFPs
We demonstrated that WL measurements exploiting the Optical Frequency Domain Reflectometry (OFDR) technology can be achieved
Summary
Fukushima-Daiichi event on March 11th, 2011, signed a turning point in nuclear industry by highlighting several weaknesses in the control of the critical systems ensuring the safety in nuclear power plant (NPP), in particular when they are exposed to harsh constraints associated to extreme and accidental conditions. In this paper we tested and validated, for the first time to our knowledge, a radiation resistant OFS prototype able to measure both the water level and the temperature inside the SFPs of a NPP To be implemented this environment such a sensor have to possess a good spatial resolution for the water level measurement (of the order of the cm) and it has to be able to withstand radiation doses in the MGy(SiO2) range. Concerning the OTDR sensors, their spatial resolution being 20 cm for the best ones, they are not suitable candidates for high spatial resolution measurements With both SPR and OTDR sensors the simultaneous temperature and water level measurement with one sensing fibre is not allowed, making OFDR sensors the best candidates for SFP environments. This ensures that radiations are kept below an acceptable level with regards to radioprotection policy, in any operational conditions, without any specific additional shielding[23, 24]
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