Influence of graphite and quartz addition on the thermo–physical properties of bentonite for sealing heat-generating radioactive waste

Bentonite is one of the most favored buffer materials for the deep geological disposal of waste in clay and granite formations all over the world. Buffer material, used to isolate heat emitting waste canisters, has to take up a strong heat load. This paper presents results of investigations on enhancing the heat conduction within the bentonite sealing. Admixtures of quartz and graphite accelerate the heat transfer into the host rock. Test samples consisting of different bentonite–quartz and bentonite–graphite mixtures were prepared. The thermal conductivity was determined as a function of admixture content, temperature, water content, and sample density within 35° to 140 °C at a uniaxial pressure of 2 MPa. The necessary conductivity could not be achieved with quartz, but the addition of graphite led to a suitable thermal conductivity. A set of equations was developed for the calculation of the thermal conductivity and the design of an engineered geotechnical barrier with heat conduction properties similar to those of the particular host rock.