Long length SPNDs and Distributed Optical Fiber Sensors for Severe Accident remote monitoring & their contribution to Nuclear Safety in the post-Fukushima context

International audienceThe Fukushima-Daiichi nuclear accident of March 2011, and the subsequent loss of internal power supplies after the NPP (Nuclear Power Plant) water flooding caused by the tsunami, leaving the operator TEPCO with almost no information from the reactor pits, demonstrates that safety must always prevail.Accordingly, the French public authorities initiated the RSNR research program, to stimulate and fund new R&D projects to improve the safety of nuclear reactors in service and those of future NPPs.The DISCOMS project (Distributed Sensing for Corium Monitoring and Safety) aimed at developing and testing innovative and passive sensors dedicated to Nuclear Safety, namely an instrumented pole equipped with long length SPNDs (Self-Powered Neutron Detectors)-Thermocouple poles, and Distributed Optical Fiber Sensors, to be installed ex-core in both the reactor pit and concrete floor. The sensors, remotely operated from a safe place, will not only provide additional information during the Severe Accident, but also in post-accidental situation, even in case of loss of all power supplies.The modelling of a 60 year normal operation followed by a Severe Accident for two generations of reactors (Gen II, Gen III) permitted to demonstrate that ex-core long length SPNDs can identify different scenarios: reactor shut down, Normal Operation, Severe Accident without corium relocation, and Severe Accident with corium pouring on the concrete floor.Long length SPNDs were designed and manufactured, along with their electronics, to measure low currents ranging from 1 pA to 100 nA collected under radiations, and qualified in a research reactor with fluxes compliant with modelled scenarios.Optical Fibers Sensor cables are devoted to monitor the Molten Core - Concrete Interaction (MCCI): temperature and strain profiles can be provided in the concrete depth by embedded cables, as a result of using the Raman DTS, Brillouin and Rayleigh OFDR reflectometry techniques, based on the analysis of the backscattered light in single-mode optical fibers, for distributed measurements potentially up to 1000°C with Brillouin instrumentations. Additionally, such sensor cables can be used as fuses with telecom or photon counting OTDRs to detect corium vicinity.Sensor cables and radiation resistant optical fibers have been selected and tested to comply with the radiation conditions in the reactor pits as depicted by the modelling.A final MCCI experiment with prototypical corium, performed at the VULCANO CEA facility, involving also two instrumented SPNDs-Thermocouple poles, has demonstrated the ability of both kinds of sensors and corresponding instrumentations to deliver useful information about the corium status and its progression through the concrete