Diffusion of Uranium in Compacted Bentonite in the Presence of Carbon Steel

Abstract

ABSTRACTIn a high-level waste repository, a carbon steel overpack will be corroded by consuming oxygen trapped in the repository after closure. This will create a reducing environment in the vicinity of repository. Reducing conditions are expected to retard the migration of redox-sensitive radionuclides such as uranium.The apparent diffusivities of uranium were measured in compacted bentonite (Kunigel VI®, Japan) in contact with carbon steel under reducing conditions or without carbon steel under oxidizing conditions for comparison. The apparent diffusivities of uranium were 3.5 × 10-14 to 1.1 × 10-13 m2/s under reducing conditions and 9.0 × 10-13 to 1.4 × 10-12 m2/s under oxidizing conditions. There was no significant effect of dry density (1.6 to 1.8 g/cm3) and silica sand (0 or 40%) on the apparent diffusivities.Since the bentonite pore water would be buffered at a pH between 8 and 9, uranium in the bentonite pore water would probably exist as a neutral hydroxide complex under reducing conditions and as an anioníc carbonate or hydroxide complex under oxidizing conditions. The anion exclusion theory cannot explain the difference of diffusivities between the two conditions. The uranium concentrations in bentonite under oxidizing conditions were one order of magnitude higher than those under the reducing conditions. The uranium concentration in the bentonite pore water under the reducing condition is estimated to be two orders of magnitude lower than that under the oxidizing conditions under the assumption of diffusion in porous media.