Reactive Diffusion of Uranium in Compacted Clay: Evaluation of Diffusion Coefficients by a Kinetic Approach

Abstract

The effect of compaction of bentonite on the diffusion behavior of uranium was studied for the safety assessment of radioactive waste (storage and disposal practices). Since the permeability of the compacted clay is very low, the main mechanism for radionuclide transport is governed by the diffusion phenomenon. The diffusion process of uranium in compacted clay as a porous medium has been modeled by Fick's second law taking into account the effect of sorption and considering the non-steady state. The diffusion coefficients and profiles concentration values were calculated by a computational method using a numerical program based on the Newton-Raphson algorithm. In this simulation, the experimental values were used to determine the uranium concentration profiles versus depth of the clay pellet. It was concluded that the dry density of the compacted clay and the aqueous solution properties (pH and ionic strength of background electrolyte) played an important role in the uranium transport through compacted clays.