Diffusion of technetium in compacted bentonites in the reducing condition with corrosion products of iron

In the vicinity of a high-level waste repository, corrosion of carbon steel overpacks will create a reducing environment. Reducing conditions are expected to retard the migration of redox-sensitive radionuclides such as technetium. The apparent diffusion coefficients of technetium were measured in compacted bentonites (Kunigel V1 and Kunipia F, JAPAN) in contact with carbon steel and its corrosion products under reducing conditions or without carbon steel under oxidizing conditions for comparison. The apparent diffusion coefficients measured were 10^-10 to 10^-11 m²/s under oxidizing conditions and 10^-12 to 10^-13 m²/s under reducing conditions. There were significant effects of redox condition, dry density (0.2 to 2.3 g/cm³) and montmorillonite content (50% for Kunigel V1 or 100% for Kunipia F) on the apparent diffusion coefficients. Montmorillonite density could be a good index to explain density dependence of the diffusion coefficients under both reducing and oxidizing conditions.