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

G. Cheymol (1) , Laurent Maurin (2) , L. Remy (1) , V. Arounassalame (1) , H. Maskrot (1) , Stéphane Rougeault (2) , V. Dauvois (3) , P. Le Tutour (3) , N. Huot (4) , Y. Ouerdane (5) , Pierre Ferdinand (1)

1 SEARS - Service d'études analytiques et de réactivité des surfaces
2 LCFO (CEA, LIST) - Laboratoire Capteurs Fibres Optiques (CEA, LIST)
3 SECR - Service d'Etudes du Comportement des Radionucléides
4 SEROS - SEROS
5 LHC - Laboratoire Hubert Curien

The DISCOMS project, which stands for “DIstributed Sensing for COrium Monitoring and Safety”, considers the potential of distributed sensing technologies, based on remote instrumentations and Optical Fiber Sensing cables embedded into the concrete floor under the reactor vessel, to monitor the status of this third barrier of confinement. This paper focuses on the selection and testing of singlemode (SM) optical fibers with limited RIA (Radiation Induced Attenuation) to be compliant with remote distributed instruments optical budgets, the ionizing radiation doses to sustain, and their reduction provided by the concrete basemat shielding. The tests aimed at exposing these fibers and the corresponding sensitive optical cables, to the irradiation doses expected during the normal operation of the reactor (up to 60 years for the European Pressurized Reactor), followed by a severe accident. Several gamma and mixed (neutron-gamma) irradiations were performed at CEA Saclay facilities: POSÉÏDON irradiator and ISIS reactor, up to a gamma cumulated dose of about 2 MGy and fast neutron fluence (E > 1 MeV) of 6 x 10$^{15}$ n/cm$^2$ . The first gamma test permitted to assess the RIA at various optical wavelengths, and to select three radiation tolerant singlemode fibers (RIA < 5 dB/100 m, at 1550 nm operating wavelength). The second one was performed on voluminous strands of sensitive cables encapsulating the selected optical fibers, up to approximately the same accumulated dose, at two temperatures: 30°C and 80°C. A significant increase of the RIA, without any saturation tendency, appeared for fibers inserted into cables, correlated with the increase of the hydroxyl attenuation peak at 1380 nm. Molecular hydrogen generated by the radiolysis of compounds of the cable is at the origin of this phenomenon. A third gamma irradiation run permitted to measure the radiolytic hydrogen production yield of some compounds of a dedicated temperature cable sample. The efficiency of a carbon coating layer over the silica cladding, acting as a barrier against hydrogen diffusion, was also successfully confirmed. Finally, the efficiency of this carbon coating layer has also been tested under neutron irradiation, then qualified as a protection barrier against hydrogen diffusion in the optical fiber cores.

Mots clés: instrumentation; nuclear instrumentation; sensor; Optical Fiber Sensor; Fiber Bragg Grating; Distributed and Quasi-distributed sensing; corium monitoring; nuclear power plant; cable; Radiation Induced Attenuation; ionizing radiation; Radioactivity; carbon coating layer; distributed measurement; hydrogen diffusion; gamma-rays; neutrons; rad-hard optical fiber; radiation effects; radiolysis

Domaines: Instrumentations et Détecteurs [physics.ins-det]; Optique / photonique

G. Cheymol, Laurent Maurin, L. Remy, V. Arounassalame, H. Maskrot, et al.. Tests under irradiation of optical fibers and cables devoted to corium monitoring in case of severe accident in a Nuclear Power Plant. 6th International Conference on Advancements in Nuclear Instrumentation, Measurement Methods and their Applications (ANIMMA 2019), Jun 2019, Portoroz, Slovenia. 08006 (6 p.), ⟨10.1051/epjconf/202022508006⟩. ⟨cea-03910238⟩