This paper presents the state of the art distributed sensing systems, based on optical fibres, developed and qualified for the French Cigéo project, the underground repository for high level and intermediate level long-lived radioactive wastes. Four main parameters, namely strain, temperature, radiation and hydrogen concentration are currently investigated by optical fibre sensors, as well as the tolerances of selected technologies to the unique constraints of the Cigéo’s severe environment. Using fluorine-doped silica optical fibre surrounded by a carbon layer and polyimide coating, it is possible to exploit its Raman, Brillouin and Rayleigh scattering signatures to achieve the distributed sensing of the temperature and the strain inside the repository cells of radioactive wastes. Regarding the dose measurement, promising solutions are proposed based on Radiation Induced Attenuation (RIA) responses of sensitive fibres such as the P-doped ones. While for hydrogen measurements, the potential of specialty optical fibres with Pd particles embedded in their silica matrix is currently studied for this gas monitoring through its impact on the fibre Brillouin signature evolution.

We present an innovative electrochemical probe for the monitoring of pH, redox potential and conductivity in near-field rocks of deep geological radioactive waste repositories. The probe is composed of a monocrystalline antimony electrode for pH sensing, four AgCl/Ag-based reference or Cl⁻ selective electrodes, one Ag₂S/Ag-based reference or S²⁻ selective electrode, as well as four platinum electrodes, a gold electrode and a glassy-carbon electrode for redox potential measurements. Galvanostatic electrochemistry impedance spectroscopy using AgCl/Ag-based and platinum electrodes measure conductivity. The use of such a multi-parameter probe provides redundant information, based as it is on the simultaneous behaviour under identical conditions of different electrodes of the same material, as well as on that of electrodes made of different materials. This identifies the changes in physical and chemical parameters in a solution, as well as the redox reactions controlling the measured potential, both in the solution and/or at the electrode/solution interface. Understanding the electrochemical behaviour of selected materials thus is a key point of our research, as provides the basis for constructing the abacuses needed for developing robust and reliable field sensors.

Near-neutral pH and a low redox potential are considered favorable conditions for immobilizing radionuclides in deep repository systems within clay formations. Cigeo is the future French Industrial Center for Geological Disposal for high- and intermediate-level long-lived radioactive waste, to be built at a depth of 500 m within the Callovian–Oxfordian clay. In-depth knowledge of the mechanisms and kinetics of corrosion occurring on the surface of API 5L X65 (X65) carbon steel tubing is essential for the reversible nuclear waste management of the Cigeo site. By using all-solid and robust handmade electrodes in addition to electrochemical and gravimetric techniques, we determined the corrosion phenomenology and kinetics of X65 in contact with natural Cox pore water in equilibrium with its rock gases, flowing continuously through a multi-parameter probe device and placed at a depth of 500 m at the Bure Underground Research Laboratory, for over 180 days. Two iron oxidants were encountered, namely, depleted dioxygen (O₂) and proton H^(I), accompanied by hydrogen sulfide. Corrosion mechanisms and kinetics were well established for the two X65 electrodes, whether electrochemically perturbed or not. The corrosion thickness loss rates, determined by both electrochemical and gravimetric techniques, were between 0.016 and 0.032 mm/year. This study demonstrates, on site, the reliability of a developed methodology for continuous monitoring of the corrosion kinetics of the API 5L X65 carbon steel at the same time as the temporal variation of the key geochemical parameters of the fluid was assessed.

The combined effects of temperature (from −80 °C to +80 °C) and 100 kV X-ray exposure (up to 108 kGy(SiO₂)) on the physical properties of Brillouin scattering and losses in three differently doped silica-based optical fibers, with varying dopant type and concentration (4 wt%(Ge), 10 wt%(Ge) and 1 wt%(F)), are experimentally studied in this work. The dependencies of Brillouin Frequency Shifts (BFS), Radiation-Induced Attenuation (RIA) levels, Brillouin gain attenuation, Brillouin frequency temperature (C_T) and strain (C_ε) sensitivity coefficients are studied under X-rays in a wide temperature range [−80 °C; +80 °C]. Brillouin sensing capabilities are investigated using a Brillouin Optical Time Domain Analyzer (BOTDA), and several properties are reported: (i) similar behavior of the Brillouin gain amplitude decrease with the increase in the RIA; (ii) the F-doped and heavily Ge-doped fibers do not exhibit a temperature dependence under radiation for their responses in Brillouin gain losses. Increasing Ge dopant concentration also reduces the irradiation temperature effect on RIA. In addition, Radiation-Induced Brillouin Frequency Shift (RI-BFS) manifests a slightly different behavior for lower temperatures than RIA, presenting an opportunity for a comprehensive understanding of RI-BFS origins. Related temperature and strain sensors are designed for harsh environments over an extended irradiation temperature range, which is useful for a wide range of applications.

Measuring pH with an optical sensor requires the immobilization of a chemical recognition phase on a solid surface. Neutral red (NR), an acid base indicator was used to develop two different optical probe configurations. The chemistry of aryl diazonium salts was chosen for the elaboration of this chemical phase, as it enables strong covalent bonds to be established on the surface of metallized glass or metallic surfaces. It also allows the formation of a thick film required to obtain an exploitable spectral response. The surfaces of interest (metallized optical fiber and 316 L stainless-steel mirror) are modelized by flat surfaces (metallized glass plates and 316 L stainless-steel plates). The analytical characterizations carried out (IR, XPS, UV-Visible, and profilometry) show that NR was covalently grafted onto the model surfaces as well as on the surfaces of interest. The supports grafted with NR to develop optical pH probes exhibit spectral changes, particularly the values of pKa, the pH range, and the isosbestic point wavelength. The experimental results show that the optical probe can be used for pH measurements between 4 and 8.

In the context of the deep geological disposal of high-level and intermediate-level long-lived radioactive waste in France, the Callovian–Oxfordian (Cox) clay formation has been selected as a natural barrier. Thus, understanding the corrosion phenomena between the carbon steel used (API 5L X65) for the waste lining tubes and the Cox pore water, as well as its possible future evolutions, is of great importance. A controlled laboratory experiment was conducted using robust handmade API 5L X65 carbon steel electrodes in synthetic Cox pore water under equilibrium with three distinct gas atmospheres, simulating oxic, anoxic, and sulfide-rich environments at 25 °C and 80 °C, in a batch-type electrochemical cell. The experimental methodology involved linear polarization resistance (LPR) cycles, electrochemical impedance spectroscopy (EIS), and Tafel extrapolation at regular intervals over a period of 70 to 100 h to elucidate corrosion mechanisms and obtain corrosion current densities. At the same time, the fluid’s key geochemical parameters (temperature, pH, and redox potential) were monitored for temporal variation. This study, with results showing high corrosion rates under the three conditions investigated at two temperatures, underscores the importance of controlling the immediate environment of the containment materials to prevent exposure to variable conditions and to ensure that corrosion remains controlled over the long term.