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

Abstract

Transport properties of potential host rocks for nuclear waste disposal are typically determined in laboratory or in-situ experiments under geochemically controlled and constant conditions. Such a homogeneous assumption is no longer applicable on the host rock scale as can be seen from the pore water profiles of the potential host rock Opalinus Clay at Mont Terri (Switzerland). The embedding aquifers are the hydro-geological boundaries, that established gradients in the 210 m thick low permeable section through diffusive exchange over millions of years. Present-day pore water profiles were confirmed by a data-driven as well as by a conceptual scenario. Based on the modelled profiles, the influence of the geochemical gradient on uranium migration was quantified by comparing the distances after one million years with results of common homogeneous models. Considering the heterogeneous system, uranium migrated up to 24 m farther through the formation depending on the source term position within the gradient and on the partial pressure of carbon dioxide pCO2 of the system. Migration lengths were almost equal for single- and multicomponent diffusion. Differences can predominantly be attributed to changes in the sorption capacity, whereby pCO2 governs how strong uranium migration is affected by the geochemical gradient. Thus, the governing parameters for uranium migration in the Opalinus Clay can be ordered in descending priority: pCO2, geochemical gradients, mineralogical heterogeneity.

Reactive transport simulations of uranium migration in the Opalinus Clay depend on ion speciation governed by underlying thermodynamic data

“…Aqueous uranium speciation in the modelled system is dominated by ternary complexes with carbonate and calcium (Hennig et al, 2020;Hennig and Kühn, 2021a; that are not considered for sorption (Joseph et al, 2013a). Diffusion and sorption experiments of uranium on clay minerals could be modelled without the requirement to include uranium surface complexes with carbonate as they do not sorb or only weakly (Joseph et al, 2013a;Hennig et al, 2020;Hennig and Kühn, 2021b;Tournassat et al, 2018). As ionic strengths in the hydrogeological system of the Opalinus Clay at Mont Terri do not exceed 0.5 mol L −1 (Pearson et al, 2003;Hennig and Kühn, 2021b), the Davies approach (Davies and Shedlovsky, 1964) is used for the database to represent ion-ion interactions (Stockmann et al, 2017;Noseck et al, 2018).…”

“…Sorption must be quantified as a function of the geochemical conditions. Further, the results showed that present-day pore water profiles can be explained by diffusive exchange with the embedding aquifers (Mazurek et al, 2011;Hennig and Kühn, 2021b). Previous simulations were conducted using the PSI/Nagra thermodynamic database (Thoenen et al, 2014) including the Nuclear Energy Agency (NEA) data for uranium published by Guillaumont et al (2003).…”

“…Migration of uranium, the main component of spent fuel, in the potential host rock Opalinus Clay has been quantified as a function of mineralogical heterogeneities between three litho-facies (shaly, sandy and carbonate-rich, Fig. 1) and for varying partial pressures of carbon dioxide (pCO 2 , Hennig et al, 2020;Hennig and Kühn, 2021a) within the hydrogeological system at Mont Terri (Hennig and Kühn, 2021b). It has been shown that the application of the process-based multi-component diffusion approach (Appelo and Wersin, 2007) provided no additional profit on the host rock scale.…”

“…For instance, a difference of 1.33 log units in the value for the stability constant of the neutral complex Ca 2 UO 2 (CO 3 ) 3 changes the migration lengths by 5 to 7 m for the sandy and shaly facies, respectively. The next step, presented here, is the investigation what such a change in underlying thermodynamic data mean in metres difference on the host rock scale after one million years quantified with one-dimensional reactive transport models based on simulations of the hydrogeological system of the Opalinus Clay at Mont Terri (Hennig and Kühn, 2021b). For this, resulting migration lengths are compared between simulations with varying thermodynamic data sets of the NEA for uranium (Guillaumont et al, 2003;Grenthe et al, 2020).…”

Impact of Uniaxial Mechanical Perturbation on Structural Properties and Smectite Porosity Features: Ion Exchanger Efficiency and Adsorption Performance Fate

“…Montmorillonite fraction, which is a dioctahedral species of the smectite mineral group, can constitute up to 95% of bentonite. ese functional materials provide restriction of groundwater access to radioactive waste, promote conditions for mass transfer between waste and groundwater by diffusion, overwhelm the migration of radionuclides in colloidal form into groundwater, ensure effective sorption after possible depressurization of radioactive waste containers, and so on [40][41][42][43][44][45]. In addition to their presence in the formations housing the storage facilities and their use as backfill material or as sealing material to isolate parts of disposal facility, montmorillonite is also present as a buffer material in the empty space between the package and the host rock [46][47][48].…”