Effects of heat source conditions on the early temperature rise in heated fiber-optic cable emplaced in granulated bentonite mixture

Abstract The heated fiber-optic method is receiving much attention as a quality control tool for buffer emplacement in the nuclear waste repository. By heating a fiber-optic cable installed in the buffer material, the thermal conductivity profile along the cable is obtained from which physical parameters such as the dry density at the time of the buffer emplacement and water content afterward can be inferred if carefully calibrated. When the fiber-optic cable is heated, it is known that the initial temperature changes exhibit a time-lapse largely controlled by the properties of the cable. The temperature changes after this initial portion reflect the thermal conductivity of the medium surrounding the cable. In this study, in order to identify how the heat source properties influence the thermal response, heating was numerically simulated for a simplified heat source and a realistic heatable fiber-optic cable. The simulated results showed that the radius and heat capacity of the heat source were the factors that control the time-lapse in the initial temperature rise. For tests with a heating time of 30 or 60 min, it was shown that excluding the initial temperature data from t = 0 to about 3–5 min would effectively reduce error in thermal conductivity estimation.