Heated fiber-optic cables for distributed dry density measurements of granulated bentonite mixtures: Feasibility experiments

Abstract

In the Swiss concept for the disposal of spent fuel and vitrified high-level radioactive waste, the buffer material of the engineered barrier system in the disposal tunnels consists of granulated bentonite mixtures (GBM) and blocks of highly compacted bentonite. For the GBM to perform sufficiently as a barrier, the dry density of the emplaced GBM is an essential parameter as well as a quality control indicator at the time of tunnel backfilling. In this study, an actively heated fiber-optic cable combined with distributed temperature sensing was applied with the aim of estimating the in-situ dry density distribution of the GBM. A set of experiments was performed using GBM specimens with controlled dry density conditions at the Grimsel Test Site in Switzerland to examine how the thermal responses vary with the dry density of the GBM material. The results indicated that the thermal responses were sufficiently sensitive to allow distributed temperature sensing in combination with an actively heated fiber-optic cable to be used as a reliable tool for estimating the dry density profile in the tunnels backfilled and sealed with the GBM along the cable with a high resolution.