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Safety and operational monitoring of nuclear waste repositories with fiber optic sensing systems


Radiation sensing is performed through special doped fibres which show a first order (or even linear) increase of the radiation-induced optical attenuation around 1300 to 1500 nm when placed in a radiation field. The temperature dependence of the response is negligible in the envisaged operating range (20 to 90 °C), while the dose-rate dependence was demonstrated to below in a region between 1 and 100 Gy/h. In order to obtain distributed radiation sensing, the use of a high-resolution Optical Time Domain Reflectometer (OTDR) was attempted. Although the specifications, such as a high spatial resolution, high sensitivity and dynamic range seemed to be ideal for distributed measurements, results obtained were disappointing. Finally, distributed radiation sensing was abandoned for this project.
As part of the demonstration aspects of the SOMOS project, temperature and radiation sensing were planned using dedicated stainless steel tubes engineered and emplaced within the CORALUS tests in the HADES underground research laboratory in Belgium. Though the placement of optical fibres in capillary tubes is a mature technology in telecommunication systems, the particular set-up used is far more demanding. Mainly the fact that the capillary tubes were placed with several loops around the CORALUS test tubes imposed a lot more friction during emplacement. The (fragile) optical fibres used for radiation sensing proved to be impossible to inject fully given the local geometry of the CORALUS experiment. However, the carbon-reinforced Tower grating fibres for temperature sensing were successfully placed and benchmarked against the available classical temperature sensors.