Remote monitoring of Molten Core-Concrete Interaction experiment with Optical Fibre Sensors & perspectives to improve nuclear safety – DISCOMS project

Abstract

The DISCOMS project (Distributed Sensing for Corium Monitoring and Safety) aimed at providing innovative solutions not requiring local electrical power supplies, for remote monitoring of a severe nuclear accident. The solutions are based on both long length SPNDs (Self Powered Neutron Detectors) and on distributed OFSs (Optical Fibre Sensors) capable to detect the onset of a severe accident, the corium pouring on the containment building concrete basemat, and its interaction with the concrete floor under the reactor vessel, until it spreads in the core catcher (EPR case). This paper mainly focuses on these last three detection targets achievable with distributed OFSs. It is based on the results of a Molten Core & Concrete Interaction (MCCI) experiment, namely VULCANO, held in June 2018 with a concrete crucible equipped with overall ~ 180 m long optical fibre sensing cables. This small scale experiment (50 kg of prototypical corium) has demonstrated the ability of distributed OFSs to remotely provide useful data during the MCCI run: i) temperature profiles images up to about 580°C (single wavelength Raman DTS reflectometer) until cooling down to room temperature, ii) high spatial-resolution frequency shifts profiles, due to combined (non-selective) strain and temperature influences (Rayleigh OFDR and Brillouin reflectometers), and iii) cables lengths ablated by the corium on sections weakened by the temperature (Raman DTS, Rayleigh OFDR, telecom and photon counting reflectometers).

Tests under irradiation of optical fibers and cables devoted to corium monitoring in case of severe accident in a Nuclear Power Plant

“…The behavior of fibers under ionizing radiations, especially γ rays, has been widely studied during the last decades, highlighting the various parameters which influence the RIA levels and kinetics, see [5] for a recent review. The measurement wavelength, which is a critical parameter, is somehow imposed by the available optical technologies [6]: 1550 nm being the dominating telecom wavelength, for both singlemode fibers and distributed sensing instruments.…”

“…After a first γ irradiation test with a batch of standalone optical fibers, the best 3 rad-hard fibers fulfilling these requirements have all been selected, in order to be embedded in 3 different types of sensing cables (distributed temperature and strain measurements cables, plus safety cables) [6] for complementary irradiation tests to evaluate any additional cable influence on the RIA. The irradiation tests were mainly focused on long length temperature sensing cables (a few mm diameters), requiring high volume irradiators.…”

Irradiation Tests of Optical Fibers and Cables Devoted to Corium Monitoring in Case of a Severe Accident in a Nuclear Power Plant

“…The behavior of fibers under ionizing radiations, especially γ rays, has been widely studied during the last decades, highlighting the various parameters which impact the RIA levels and kinetics, see [5] for a recent review. The measurement wavelength, which is a critical parameter, is somehow imposed by the available technologies [3]: 1550 nm being the dominating telecom wavelength, for both singlemode fibers and distributed sensing instruments.…”

“…Additionally, such sensing cables can be used as thermal fuses with telecom or photon counting Optical Time Domain Reflectometers (OTDRs) to detect corium vicinity. Detailed description of the measurements considered with these technologies can be found in [1][2][3].…”

“…It is important to notice that one task of the project was precisely to better assess these expected irradiation doses, during normal operation of the reactor [4], and also in case of severe accident. Resistance to temperature rise when the corium approaches the cable, which is of major importance for the severe accident monitoring in order to get measurements from the reactor pit as long as possible, is not in the scope of this work and some complementary information are available in [3]. Tests up to 750°C have been performed, and are discussed in this reference.…”