Role of clay microstructure in expandable buffer clay

Smectite-rich clay is a major engineered barrier (buffer) in repositories for heat-producing highly radioactive waste. It has the form of densely compacted blocks of clay granules and is only partly water saturated from start. The initial porewater is redistributed by the thermal gradient acting in the buffer and additional water is taken up to give ultimately complete saturation. The compaction of blocks gives a stochastic distribution of dense and less dense clay grains of different size and the initially heterogeneous microstructure is partly preserved in the fully hydrated buffer. Thus, local softer zones combine to create channel flow under hydraulic gradients and they can be permanent by precipitation of cementing agents caused by salt in the porewater and by partial dissolution of the smectite particles. A fundamental question is how one can characterize the channels and quantify their hydraulic performance. This has been answered by using different techniques in the present study, including optical and electron-optical microscopy and gas adsorption. Interpretation of oedometer tests with freshly prepared and hydrothermally treated smectite samples has revealed that permeation of dense buffer clay primarily takes place through a network of channels that represent up a few percent of the entire cross section. Hydrothermal treatment in the early period of maturation of the buffer widens flow paths and increase the bulk conductivity, and precipitation of cementing silicious compounds causes stiffening and prevents expanded voids to close. Journal of Purity, Utility Reaction and Environment Vol.1 No.6, August 2012, 267-292 268