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

Theresa Hennig,Michael Kühn

doi:10.3390/min11101087

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 Such a homogeneous assumption is no longer applicable on the host rock scale as can be seen from the pore water profiles of the potential host rock Opalinus Clay at Mont Terri (Switzerland).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.

Highlights

The protection of future generations and their environment from effects of highlyradioactive waste requires the isolation of the e.g., spent fuel in deep geological repositories in a suitable host rock [1]
Reactive transport simulations were applied to quantify uranium migration considering diffusion and sorption processes as well as interaction with the inherent minerals depending on the geochemical gradient in the Opalinus Clay formation at the Mont Terri
For scenario 1R, concentrations measured from samples of the Opalinus Clay at the Mont Russelin were used, where the Liassic aquifer has no contact with meteoric water and the pore water is considered as unaffected

Summary

Introduction

The protection of future generations and their environment from effects of highlyradioactive waste requires the isolation of the e.g., spent fuel in deep geological repositories in a suitable host rock [1]. Clay formations are one of the preferred host rocks, because they have high retention capacities with respect to radionuclides due to the large reactive surface area of the inherent minerals and provide very low permeabilities, only allowing diffusive transport. 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]. Numerical process coupling is required in order to give quantitative robust statements as the “homogeneous” results of the small-scale experiments are not transferable. Due to the large spatial and temporal scales to be considered, numerical simulations are indispensable for safety assessments of potential repositories to quantify radionuclide migration

Methods

Results

Discussion

Conclusion