Characterizing and Improving the Thermal Conductivity of Engineered Clay Barriers for Sealing a Deep Geological Repository

Abstract

Engineered clay barriers are important components of Canada’s concepts for the isolation of used nuclear fuel within a deep geological repository. Different clay-based materials have been proposed for various applications within a repository. These include highly compacted bentonite (HCB), bentonite-sand buffer (BSB), light backfill (LBF), dense backfill (DBF) and gap fill (GF). Characterization of the thermal, hydraulic, mechanical, and chemical (T-H-M-C) properties of these materials is required to evaluate the long-term performance of a repository. The thermal properties of these materials are required to model the dissipation of heat from the used-fuel containers (UFCs). Thermal conductivity is an important repository design parameter since it affects the UFC spacing required to prevent the development of excessive temperatures within the repository. High thermal conductivities are desired since this condition accommodates the rapid transfer of heat from the UFCs to the surrounding rock mass, and in turn allows for a smaller repository footprint.

This paper presents some recent characterization work on the thermal conductivity of the proposed engineered clay barriers. HCB (at low moisture content) and GF pellets were identified as possible insulating layers, having lower thermal conductivity than the desired range of 0.7 W/(m·K) to 0.9 W/(m·K). Preliminary work is presented on increasing thermal conductivity using admixtures such as silica sand, copper powder and titanium dioxide. At low degrees of saturation, silica sand and copper powder increased the thermal conductivity of HCB to the desired range.