Development of a Temperature Distributed Monitoring System Based On Raman Scattering in Harsh Environment
Résumé
Raman Distributed Temperature Sensors (RDTSs) offer exceptional advantages to monitor the envisioned French deep geological repository for nuclear wastes, called Cigéo. Both gamma-radiation and hydrogen release from nuclear wastes can strongly affect the temperature measurements made with RDTS. We present experimental studies on how the performances of RDTS evolve in harsh environments like those associated with -rays or combined radiations and H2 release. The response of two standard and one radiation tolerant multimode fibers (MMFs) were investigated. In all fibers the differential induced attenuation between Stokes and anti-Stokes signal, (α_AS-α_S) , causes a temperature error, up to 30°C in standard multimode fibers irradiated at 10 MGy that is more detrimental than the radiation induced attenuation (RIA) limiting only the sensing range. The attenuation in the spectral range 800-1600 nm at room temperature was explored for the three fibers gamma-irradiated and/or hydrogen loaded to understand the origin of this differential RIA. We show that by adapting the characteristics of the used fiber for the sensing, we could limit its degradation but that additional hardening by system procedure is necessary to correct the T error in view of the integration of RDTS in Cigéo. The current version of our correction technique allows today to limit the temperature error to ~2°C for 10 MGy irradiated samples.