Surrogate Model for Multi-Component Diffusion of Uranium through Opalinus Clay on the Host Rock Scale

Abstract

Multi-component (MC) diffusion simulations enable a process based and more precise approach to calculate transport and sorption compared to the commonly used single-component (SC) models following Fick’s law. The MC approach takes into account the interaction of chemical species in the porewater with the diffuse double layer (DDL) adhering clay mineral surfaces. We studied the shaly, sandy and carbonate-rich facies of the Opalinus Clay. High clay contents dominate diffusion and sorption of uranium. The MC simulations show shorter diffusion lengths than the SC models due to anion exclusion from the DDL. This hampers diffusion of the predominant species CaUO2(CO3)32−. On the one side, species concentrations and ionic strengths of the porewater and on the other side surface charge of the clay minerals control the composition and behaviour of the DDL. For some instances, it amplifies the diffusion of uranium. We developed a workflow to transfer computationally intensive MC simulations to SC models via calibrated effective diffusion and distribution coefficients. Simulations for one million years depict maximum uranium diffusion lengths between 10 m and 35 m. With respect to the minimum requirement of a thickness of 100 m, the Opalinus Clay seems to be a suitable host rock for nuclear waste repositories.

Potential Uranium Migration within the Geochemical Gradient of the Opalinus Clay System at the Mont Terri

“…The underlying thermodynamic data are based on the PSI/Nagra database version 12/07 [18] with the extensions and updates described in Hennig et al [16] and Hennig and Kühn [8]. Details are given in the corresponding Supplementary Materials.…”

“…Calcium, magnesium, pCO 2 and pH are the governing parameters for uranium speciation and hence sorption and diffusion [8,16]. Therefore, only these profiles are exemplary shown for the other pore water components (Figure 3).…”

“…The retardation of radionuclide migration has been demonstrated for claystones, like the Swiss Opalinus Clay, in many laboratories as well as in-situ experiments, usually applying constant and homogeneous (geochemical) experimental conditions [2][3][4][5]. However, for many radionuclides it has been shown that sorption and migration are affected by geochemistry and can deviate by one or more orders of magnitude [6][7][8]. Hence, the transport parameters determined in experiments may no longer be applicable in the case of changing geochemical conditions.…”

“…In the present study, we assess the influence of measured geochemical gradients on potential radionuclide migration of uranium as one of the main components in spent fuel [15] in the hydro-geological system of the Opalinus Clay formation in the Mont Terri region. From previous numerical studies we know [8,16], that uranium speciation is controlled by the pore water composition and that it is mainly present in anionic and neutral complexes with calcium, magnesium and carbonate, e.g., Ca 2 UO 2 (CO 3 ) 3 , CaUO 2 (CO 3 ) 2− 3 , MgUO 2 (CO 3 ) 2− 3 . Moreover, sorption as well as diffusion processes of uranium in the geochemical system are governed by the partial pressure of carbon dioxide pCO 2 , Ca 2+ concentration, pH, pe, the hydro-physical parameters and the clay mineral quantity.…”

Quantification of Lithological Heterogeneity Within Opalinus Clay: Toward a Uniform Subfacies Classification Scheme Using a Novel Automated Core Image Recognition Tool

“…Many studies at Mont Terri have emphasized the tight relations between various rock properties, lithology and texture. Typical examples include the small-scale heterogeneity of geomechanical properties (Klinkenberg et al, 2009;Kaufhold et al, 2013;Siegesmund et al, 2014), the distribution of micro-porosity (Houben et al, 2013(Houben et al, , 2014Keller et al, 2013;Philipp et al, 2017), sorption processes and radionuclides diffusive transport (Hennig et al, 2020;Hennig and Kühn, 2021), the responses of seismic P-wave velocity (Kneuker et al, 2017;Schuster et al, 2017;Lauper et al, 2018), and more generally, the overall (geochemical, petrophysical and hydro-mechanical) anisotropic behavior of the OPA (Van Loon et al, 2004;Wenk et al, 2008;Favero et al, 2018). Jaeggi and Bossart (2014) provide an overview of the main rock properties (petrophysical, hydrogeological, rock mechanical, thermic, geochemical/pore water composition, and gas entry pressure parameters) of the OPA at Mont Terri and their relationship with lithology.…”