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

In a high-level waste repository, a carbon steel overpack will corrode after the repository is closed. 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 plutonium. The apparent diffusion coefficients of plutonium were measured in compacted bentonites (Kunigel V1 and Kunipia F, Japan) in contact with carbon steel and its corrosion products under a reducing condition and, for comparison, without carbon steel under an oxidizing condition. Gas bubbles were observed in some bentonite specimens which had low dry densities after contact with carbon steel for approximately two years. This observation suggests hydrogen generation during corrosion of the carbon steel. The apparent diffusion coefficients measured were approximately 10^-14 m²/s under the reducing condition and less than 10^-15 m²/s under the oxidizing condition. There was a significant effect of redox conditions on the apparent diffusion coefficients. The effects of dry density (0.8 to 2.0 Mg/m³) and montmorillonite contents (50% for Kunigel V1 or 100% for Kunipia F), however, were not observed clearly. The chemical species of plutonium were expected to be PuOH²⁺ for the reducing condition and Pu(OH)₄ for the oxidizing condition, respectively.