Discrete geometry model of heat in granular bentonite barriers

Abstract

The mineral-engineered barrier in the planned repository for radioactive waste in Switzerland is being designed as a granular bentonite mixture. The infiltrating pore water from the (clayey) host rock will start to resaturate the granular barrier after its emplacement. The discrete structure of the mineral barrier requires a more detailed analysis, because the porosity alone applied in the continuum approach cannot adequately describe the heat transfer. Towards this analysis, three granular size fractions have been arranged into square or hexagonal packings and considered in the heat transfer model presented in this paper. The six different granular arrangements are set either in a planar or in a cylindrical spatial system. A major feature of the approach is providing the modelled systems with an interstitial fluid of smoothly adjustable thermophysical parameters, approximating variable water contents. The effective thermal conductivity of various systems is an important output of the model. The calculation of the temperature equilibrium period directly depends on this parameter and represents a step towards a solution for the reliable determination of the resaturation timescale. The validity of the modelled effective thermal conductivities was confirmed by laboratory experiments on real granular bentonite.