Diffusion of Uranium in Compacted Bentonites in the Reducing Condition with Corrosion Products of Iron

Abstract

In high-level waste repository, a carbon steel overpack will be corroded after closure. This will create a reducing environment in the vicinity of the repository. Reducing conditions are expected to retard the migration of redox-sensitive radionuclides such as uranium.

The apparent diffusion coefficients of uranium were measured in compacted bentonites (Kunigel VI® 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 10⁻¹² to 10⁻¹⁴ m²/s under oxidizing conditions and 10⁻¹³ to 10⁻¹⁴ m²/s under reducing conditions. There were significant effects of redox conditions, dry density (0.8 to 2.0 g/cm³) and montmorillonite contents (60% for Kunigel VI or 100% for Kunipia F) on the apparent diffusion coefficients. The sorption of uranium on corrosion products of iron was smaller than that on montmorillonite. Montmorillonite density could be a good index to explain density dependence of the diffusion coefficients under the reducing conditions. Uranium would diffuse in free pore water with diffusion coefficients greater than 10⁻¹³ m²/s in low density bentonites under both redox conditions because the diffusion coefficients depended on sorption coefficients. Since diffusion coefficients were independent of sorption coefficients in high density bentonites where free pore water is scarce, surface diffusion might occur with coefficients were of about 10⁻¹³ m²/s.