Modelling the geochemical behaviour of aqueous solutions in the Opalinus Clay as part of the Mont Terri BN Experiment using PHREEQC

In several countries, claystone is a possible host rock for a radioactive waste repository. The reducing capacity of claystone has a strong impact on the speciation, solubility, retention and transport properties of redox-sensitive radionuclides. Since redox-sensitive radionuclides are more mobile in a more oxidised state, the reducing capacity of claystone is an important feature regarding the long-term safety of nuclear waste disposal.

To study the influence of oxidising components such as nitrate (which is for example a component of bituminised waste) on the reducing capacity of claystone and radionuclide mobility, ongoing in situ tests are carried out in the Opalinus Clay at the Mont Terri Rock Laboratory (Switzerland) as part of the Bitumen-Nitrate-Clay interaction (BN) experiment. These in situ tests are performed in a vertical borehole containing three packed-off intervals (water chambers), which allow to monitor and change the hydrochemical conditions in the different intervals. Moreover, in recent years, the fate of selenate (SeO₄^2-) in the Opalinus Clay has been studied within the BN experiment. This includes the possible microbial reduction of Se(VI) to Se(IV) (in presence of nitrate), as this could contribute to the retention of selenium within the geosphere. Selenate was selected since ⁷⁹Se is one of the main dose-contributing radionuclides in long-term safety analyses for radioactive waste disposal in claystone.

In each of the past in situ tests, artificial pore water with a pH of approximately 7.8 and containing nitrate and/or selenate and sometimes electron donors (acetate and H₂), is injected in the borehole. Thereby all ongoing processes are studied at a near-neutral pH. In future experiments, the influence of an increased pH on the fate of nitrate (and later on selenate) and on the reducing capacity of the claystone will be investigated, since cement, for example used as an engineered barrier in a repository, may cause alkaline conditions. Therefore, it is envisaged to increase the pH of the water within the borehole slowly in a stepwise manner - first up to pH 9-10 - by injecting solutions containing NaOH, KOH or cementitious pore water.

In this study, geochemical calculations are performed using the program PHREEQC to evaluate the increased pH in the intervals on the surrounding claystone and the borehole water. The result of this modelling will show the aqueous speciation and mineral dissolutions and precipitations. An evaluation of mineral precipitations under the given conditions is necessary since clogging of the water lines in the experiment should be avoided. Therefore, an understanding of the geochemical interactions of the injected aqueous alkaline solutions with the surrounding claystone and pore water is necessary. Within the context of nuclear waste disposal in claystone, the database ThermoChimie provides suitable thermodynamic data and will be used for the simulations. To assess the precipitation of secondary phases for cementitious materials, the thermodynamic database CEMDATA might be applied for comparison. Finally, the range of pH that can be reached in the interval solutions will be assessed and the amount of precipitated minerals will be calculated.