Long-term diffusion of neptunium and other elements in compacted bentonite reveals filtration

The flow-through method and the profile method are used for determining diffusion coefficients. If the diffusion coefficient is small, only the profile method is suitable. In the profile method, the apparent diffusion coefficient can be obtained by setting an appropriate diffusion period. However, if an experiment is conducted with a longer diffusion period, the apparent diffusion coefficient may appear small. This phenomenon is observed in the diffusion of neptunium in compacted bentonite in an atmosphere with high carbonate ion concentrations. One interpretation is that the neptunyl carbonate complex can only pass through a limited size of pores in the compacted bentonite because the complex is so large and negatively charged. In this work, a long-term diffusion experiment was conducted over 3 years, and the diffusion profile of neptunium in compacted bentonite was exponential owing to filtration and did not change over time. Exponential profiles were also observed for large oxoanions, such as molybdate ions, and for plutonium in a 20-year iron corrosion environment. Graphical abstract: [Figure not available: see fulltext.]