Modeling of colloidal uranium transport in a fractured medium

Abstract The present study offers a contribution to the understanding of the problem of migration of radioactive elements from a deep disposal zone to Man's environment after a repository failure. More precisely, it proposes to shed light on the possible involvement of colloidal matter in the transport velocity and residence time of radionuclides. Interaction mechanisms between the different components of the modeled system, i.e. water, solutes, colloids and solid media, describe the retention capacity of the geological medium, the final barrier between pollutant and environment. In a first attempt to model migration of colloids through a fractured medium, we used simple models describing the interaction between the colloids, solute and rock matrix: a linear adsorption isotherm with or without a maximum adsorption capacity and the Langmuir isotherm. Colloids may be generated when ionic metal species hydrolyse and precipitate, forming so-called ‘true’ or ‘intrinsic’ colloids. Redissolution of the colloids is envisaged when the solute fraction drops below its solubility limit. Simulations with a numerical transport model demonstrate the effects of uranium leakage in a fractured granitic rock when the generation of colloidal uranium is considered. It is shown how, in a given flow velocity field, the residence time of the total uranium mass depends highly on the retention capacity of the medium. The maximum adsorbed fraction appears to be a key parameter and different hypothetical values are used in order to evaluate the quantitative effects. The diffusion of soluble uranium into the micro-fissures of the rock matrix also appears as an important process, even for transport of the colloidal phase.