Radiation vulnerability of optical fiber cables for underground nuclear waste monitoring

This work presents our evaluation of the radiation vulnerability of optical fiber cables candidate to monitor temperature and strain in nuclear waste repositories. For this, the cables have been both exposed to γ-1 MGy Total Ionizing Dose (TID) and to mixed-field neutron-γ-rays up to 150 Gy TID and neutron fluence of 2.8 × 10¹³ n/cm². The effect of hydrogen-rich atmospheres is also investigated as this constraint is associated with the targeted environments. The evolution of the properties of seven optical fiber cables, differing in their compositions and structures, were evaluated during these two irradiation campaigns. Radiation-Induced Attenuation (RIA), Brillouin Frequency Shift (BFS), and Rayleigh Frequency Shift (RFS) were measured online at room temperature. Additional post-irradiation assessments have been performed: thermo-mechanical properties of both Brillouin and Rayleigh scatterings were evaluated pre- and post-irradiation, along with carbon coating H₂-blocking capabilities and radiation-induced degradation. Results demonstrated that radiation induces hydrogen diffusion from some of the cable structures, particularly from gel components. This, in addition to RIA, significantly increases the optical loss levels at infrared wavelengths under γ-rays (>1200 dB/km, at 1550 nm). The study explores the origins of these losses and the radiation-induced BFS and RFS levels. Post-mortem analyses, including spectral assessments, macroscopic cable degradation observations, and hydrogenation, provided further insights into cable behavior in such conditions. A PEEK-structured cable showed notable resilience, with minimal changes in its Brillouin/Rayleigh sensitivity, no visible degradation, and the lowest H₂-blocking capability degradation under irradiation. This work offers a comprehensive qualification process for evaluating optical fiber cable performance for nuclear waste monitoring, and the findings exhibit broader implications for various nuclear industry applications.