Tunnel structural health monitoring in radioactive environment based on special distributed optical fibre strain sensing cables

In the context of Cigeo, the planned underground repository for long-lived radioactive waste, the repository cells must be monitored over the operation period to help guaranteeing the reversibility of the project. Being horizontal and loaded by 500 m of rock, the reduction of the cell, i.e. the convergence, must be followed to guarantee the possibility to retrieve the waste. This is possible with distributed optical fiber strain sensors, which are an adequate feasible sensor for the application. The research focuses on distributed strain sensing based on Brillouin and Rayleigh scatterings. As they will work in harsh environment, first of all we quantify the coupled temperature and γ-ray influence on the Pulse Pre Pump Brillouin Optical Time Domain Analysis (PPP-BOTDA) and Tunable Wavelength Coherent Optical Time Domain Reflectometry (TW-COTDR) response. Ageing tests of these distributed strain measuring systems are performed on-line, up to 1 MGy, at room temperature, 80 deg. C, 100 deg. C and 120 deg. C. Brillouin and Rayleigh measurement errors remain identical regardless of the temperature, while the higher temperature helps increasing the measurement distance range. The performed tests helped also to explain the origin of the Brillouin frequency shift under γ-rays, with an acoustic velocity variation of about 1 m/s in 1 MGy irradiated samples. In the application optical fibers will be protected into strain sensing cables, so it is important to assess the impact of a harsh environment on the sensor external sheath too. Considering a newly developed strain sensing cable for the specific environment, the analysis focus mainly (but not only) on (i) the impact of the different constituent layers on the behaviour of a strain sensing cable and its strain and temperature sensitivities and (ii) the radiation influence on the optical fiber strain sensing cable response (500 kGy of γ-rays). Radiation impact on strain sensitivity is negligible for practical application, i.e., the coefficient changes by 4% at the max. The influence of the composition of the cable is also assessed: the sensitivity differences remain under 15%. Its elasto-plastic behaviour is also evaluated, highlighting residual strain (about 1600 με after imposing 10000 με) of the cable (especially for metallic parts). Finally, starting from ortho-radial strain measurements around the section of a tunnel-like structure, an inverse-analysis finite-element method is developed and used to transform strain into convergence values. Its validation on a mock-up of the high-level waste repository cell is reported, using the same cable type analysed earlier. Brillouin and Rayleigh scattering are exploited, as well as different cable's anchoring methods on the structure's circumference. The proposed innovative solution does not restrict the tunnel practicable section, withstands harsh environment and performs distributed measurement which can provide more information than standard methods. Its performances are assessed by identifying which are the influencing parameters and how their uncertainty plays on the inverse analysis outcome. The validation proved the ability to determine geophysical convergence with 1 mm resolution, consistently with expected performances, not only in a controlled environment but also in Andra's underground laboratory. (author)