2011 Volume 15 Issue 2 Pages 89-102
In the Japanese high-level waste (HLW) repository design, compacted bentonite and carbon steel overpack are the main components of the engineering barrier system. The possibility that the chemical and physical properties of the compacted bentonite might change after interaction with corrosion products of the carbon steel has been noted. Experimental validation of the interaction of smectite in compacted bentonite as simulated in the HLW repository system is still very limited. This study focuses on smectite and corrosion product interaction and Fe migration behavior in compacted bentonite systems. In this study, the alteration of smectite that was in contact with carbon steel for 10 years was investigated by X-ray diffraction, and Fe migration behavior was observed by electron probe micro-analysis (EPMA). Smectite interstratification was observed only in an experiment using synthetic sea water at 80℃; in this experiment, cation exchange in which the original Na^+ changed to Fe^<2+> was also observed. The apparent diffusion coefficients were calculated from the Fe profile measured by EPMA. The values were smaller than those reported in the literature, indicating not only a pure diffusion process but also contributions from precipitation and porosity reduction. The experimental results were also modeled using the calculated apparent diffusion coefficients to investigate Fe migration properties and the stability of minerals in compacted bentonite. The calculated and observed smectite stability and precipitation behavior of mineral phases were in agreement. However, the total number of cation exchange sites and the amount of Fe distribution were not well described and were significantly affected by a transient state early in the experiment.