Influence of thermal gradient and canister corrosion on the hydration of compacted Barmer bentonite

Abstract

Compacted bentonite is being used as a buffer material for the safe dumping of high-level radioactive wastes (HLW) in the deep geological repository (DGR) due to its favorable physicochemical properties. However, the temperature variation, probably because of continuous heat emitting canister (initially −150 °C to 250 °C) and geothermal gradient, affect its performance. There would be bentonite iron interface reactions due to the corrosion of steel canister, which might cause reactive transport through the compacted bentonite buffer. Therefore, it is essential to know the influence of a combination of thermal as well as a chemical gradient on the reactive transport through the compacted bentonite buffer in terms of its physicochemical, thermomechanical, mineralogical stability. The purpose of this particular study is to establish the field conditions in the lab by fabricating a small scale set up which would create a thermal gradient on the compacted Barmer bentonite (B1 and B2) samples (compacted at 1.5 Mg/m3, 1.75 Mg/m3 and 2 Mg/m3 densities) by heating its one side at 60 °C, 110 °C, 150 °C and 200 °C temperatures for 28 days each and the system was hydrated by distilled water from the other side and was kept in touch with the metallic iron powder. It was observed that the degree of saturation of both bentonites decreased with an increase in temperature as well as density. However, the cation exchange capacity at lower density reduced more than that of higher density. The microstructural observation revealed the presence of impurities between the lamella of bentonite after 28 days that might be hematite.