Heated fibre-optic cable installed in compacted bentonite layers: numerical evaluation of influences of the compaction-induced variability on thermal conductivity estimation

Abstract

Active heating of fibre-optic (FO) cable combined with distributed temperature sensing has been applied as a quality control tool for emplacing granulated bentonite buffer. For calibrating the tool, the FO cable is installed in specimens with known dry densities. Layer-by-layer compaction is often employed to prepare the specimens. Although the average dry density is guaranteed, some studies report possible variations in the dry density within a layer. In this study, heat transfer from a heated FO cable installed in granulated bentonite with different compaction-induced variabilities was numerically simulated. The reference case used homogeneous bentonite with an average dry density of 1.5 g cm −3 . For three other cases the granulated bentonite was filled in two, three and four layers, respectively, each of which had a distinct dry density gradient ranging from 1.3 to 1.7 g cm −3 . For each case, the FO cable placed at the centre of the container was numerically heated, and the thermal conductivity was calculated using temperature changes. In the cases where the FO cable was placed on the layer interface at which the dry density was discontinuous, the calculated apparent thermal conductivity was found to have been underestimated with an error of as much as 4%. In the other two cases, the thermal conductivities obtained were nearly identical to those corresponding to the average dry density. The influence of the compaction-induced variability in the dry density was found to be insignificant and the estimated thermal conductivity was essentially that of the bentonite represented by its average dry density. Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive