Thermal conductivity of bentonite-graphite mixture and its prediction for high-level radioactive waste repository

Abstract

Thermal conductivity of buffer material is crucial for the design and performance assessment of high-level radioactive waste (HLW) repository. Large amount of decay heat will increase the maximum temperature in bentonite buffer, which threatens the safety of HLW repository. One possible method to relieve such problem is adding graphite to bentonite for increasing its thermal conductivity. Therefore, the thermal conductivities of a bentonite with different graphite contents were measured by the heat flow meter method and the results showed that the thermal conductivity was a function of graphite content, water content and void ratio, while the temperature had ignorable influence for dry bentonite or bentonite-graphite mixture. The volume fraction of air and degree of saturation were strongly linear with the thermal conductivity. Based on experimental data, the Cote-Konrad model, geometric mean model and two Maxwell-like models were introduced and improved, and the suggested predictive methods proved to be effective by literature data.