Uranium migration lengths in Opalinus Clay depend on geochemical gradients, radionuclide source term concentration and pore water composition

In the context of safety assessments for nuclear waste repositories, it is essential to quantify potential radionuclide migration. This can only be done by the application of numerical simulations due to the required spatial (>100 m) and temporal scales (1 Ma), whereby resulting migration lengths highly depend on the underlying model concept and data [1-4].

Migration of uranium, the main component of spent fuel, is used here in regard to the potential host rock Opalinus Clay as an example for models close to a real case application. For this, one-dimensional diffusion simulations were conducted with PHREEQC applying mechanistic surface complexation models to account for sorption processes as a function of the geochemical conditions [1].

Extensive numerical studies for the hydrogeological system at the underground rock laboratory Mont Terri (Switzerland) have shown that migration lengths can vary from 5 m using an experimentally determined distribution coefficient K_d (m³/kg) up to 80 m applying more advanced approaches [1-4]. However, these results represent maximum scenarios. At Mont Terri, geochemical gradients established towards the embedding aquifers due to the Jura folding and associated erosion history [2, 3]. For a potential disposal site, more constant conditions without a gradient are favoured. Furthermore, the impact of the engineered barriers and with that a reduction of the source term was not taken into account in previously. Therefore, simulations are conducted for a site with less steep geochemical gradients compared to Mont Terri as well as for decreased source term concentrations.

First, measured pore water profiles from Schlattingen (Switzerland) were modelled, where geochemical gradients are less pronounced. Second, they serve as initial conditions for subsequent uranium migration driven by decreasing source term concentrations. The comparison of resulting migration lengths with the mentioned maximum scenarios shows that uranium migration is decreased by several metres. Consequently, the selection of initial and boundary conditions is essential for a reliable quantification of radionuclide migration.

References:

[1] Hennig, T. et al. (2020): Simulation of diffusive uranium transport and sorption processes in the Opalinus Clay. Applied Geochemistry 123, 104777. DOI: 10.1016/j.apgeochem.2020.104777

[2] Hennig, T. and Kühn, M. (2021): Potential uranium migration within the geochemical gradient of the Opalinus Clay system at the Mont Terri. Minerals 11 (10), 1087. DOI: 10.3390/min11101087

[3] Hennig, T. (2022): Uranium migration in the Opalinus Clay quantified on the host rock scale with reactive transport simulations, PhD Thesis, Potsdam: Universität Potsdam, 161 p. DOI: 10.25932/publishup-55270

[4] Hennig, T. and Kühn, M. (2022): Reactive transport simulations of uranium migration in the Opalinus Clay depend on ion speciation governed by underlying thermodynamic data. Advances in Geosciences 58, 11–18, DOI: 10.5194/adgeo-58-11-2022