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.
Highlights
Distributed temperature sensing (DTS) is a technology for measuring temperature along a fiber-optic (FO) cable, which acts as a distributed temperature sensor
The results indicated that the heat from the FO cable would propagate through the bentonite and reach the boundary walls roughly after one hour
In test 2 where the granulated bentonite mixtures (GBM) in boxes [5,6,7,8] were slightly compacted, the mean temperature in these boxes still showed more increase than that in boxes [1,2,3,4] but less than those in test 1. This was expected as the compaction progresses, the dry density and therewith thermal conductivity increases leading to a less temperature increase
Summary
Distributed temperature sensing (DTS) is a technology for measuring temperature along a fiber-optic (FO) cable, which acts as a distributed temperature sensor. FO sensing technologies have been developed in the recent years. The DTS technology is widely used in various industrial applications focusing often on fire and leakage detection of assets and infrastructure objects as well as dam and levee monitoring.[1] In addition, in recent years DTS found its way in several hydrological *. In most DTS applications, the temperature profile is measured in a passive manner. Active heating of a FO cable combined with DTS technology was successfully used for investigating soil moisture (e.g.4–8 ) water leakage in an embankment (e.g.9 ), or hydraulic conductivity in aquifers (e.g.10 ). Throughout this paper, in order to distinguish between the newer method and conventional passive temperature monitoring, the method that involves heating of FO cables is referred to as ‘‘active DTS’’
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