What Long-Term Diffusion Experiments in Compacted Bentonite Reveal

Methods for determining diffusion coefficients include the flow-through method and the profile one. If the diffusion coefficient is small, only the profile method is applicable. 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 fact, long term diffusion experiment as long as 3 years, the diffusion profile of neptunium in compacted bentonite was exponential, as filtered, and did not change over time. Such exponential profiles have also been observed for relatively large oxo-anions, e.g., molybdate ions. An exponential diffusion profile has also been observed for plutonium in a 20-year iron corrosion environment. With additional information, little plutonium was observed in the iron corrosion products.