CSE Community Seminar

February 28, 2025, 12-1PM

Modeling Diffusive Ion Transport in Clay Rocks

Dauren Sarsenbayev
PhD Student, Department of Nuclear Science and Engineering, MIT

Abstract:

Accurately predicting the behavior of radionuclides in geological formations is a fundamental aspect of high-level nuclear waste management. Reactive transport models have greatly aided our understanding of these processes, but many models often overlook important characteristics of charged porous media. This omission is particularly significant in compacted clay environments with low permeability, where ion movement is influenced not only by concentration gradients but also by electrostatic interactions. Such interactions can cause phenomena like anion exclusion, which cannot be adequately described by Fick’s law alone. Due to isomorphic substitution occurring in clay minerals, the surfaces of clays become negatively charged. This negative charge repels anions from the diffuse double layer that forms around the clay particles, leading to the exclusion of anions from these regions. As a result, anions may be hindered from entering the pore network of the clay. Our study aims to develop a predictive understanding of the coupled processes governing the evolution of the Cl-D (Cement-Clay Interaction Diffusion) experiment at Mont Terri—an international research initiative focused on geochemical studies within a clay formation in northwestern Switzerland. This project spans the entire thickness of the Opalinus Clay, an argillaceous formation recognized as a potential host rock for nuclear waste disposal due to its low permeability and natural barrier properties. We investigate the interaction between an Ordinary Portland Cement (OPC) plug and the surrounding anisotropic Opalinus Clay, which presents complex geometrical challenges. In May 2019, tritiated water (HTO) and chloride anions (Cl-36) were introduced into the system from a localized point source within a concrete-filled borehole. This experimental setup allows us to study the diffusion of these species through the clay formation under real-world conditions. In addition, the Cl-D experiment replicates the generic engineered barrier system (EBS) used in nuclear waste disposal, as interfaces between cement and clay are common components of EBS designs. In this study, we employed CrunchClay, an evolving branch of the CrunchFlow reactive transport code. CrunchClay enables the incorporation of electrostatic effects on ion transport, such as anion exclusion due to the electric double layer (EDL), which is crucial for accurately modeling the system’s reactivity.

February 28, 2025, CSE Community Seminar
Dauren Sarsenbayev
PhD Student
Department of Nuclear Science and Engineering, MIT