Long-term Geochemical Reaction Modeling for the Reactive Gas-groundwater-bentonite System in the SNF Deep Geological Repository

One of the critical barriers in the DGR (deep geological repository) is the buffer, which reduces stress on the storage canister and delays the release of radioactive nuclides.Bentonite is well known for a potential buffer material due to its low hydraulic conductivity, high swelling capacity, and strong adsorption properties for radionuclides.In this study, the geochemical reaction modeling was conducted to evaluate the long-term effects of reactive gases (CO 2 and H 2 S) on bentonite (a candidate buffer material for the domestic SNF (spent nuclear fuel) repository).As a simulation code for the modeling, the PHREEQC version 3.7.3 was used with the LLNL (Lawrence Livermore National Laboratory) thermodynamic database for a simulation period of 100,000 years.Reaction conditions were designed for the reactive gas-groundwater-bentonite system, based on groundwater quality data from the KURT (KAERI underground research tunnel) site and mineralogical data of bentonite.Equilibrium reaction modeling results in the gas-groundwater system supported that the groundwater inflow from the surrounding bedrock led to an acidic and aerobic environment in the bentonite pore spaces due to the dissolution of residual atmospheric gases (O 2 and CO 2 ).Based on the equilibrated water quality data, the kinetic reaction modeling for bentonite interactions with aerobic microbial respiration was performed.From results of the modeling, oxygen in groundwater was completely depleted after approximately 5,190 years, transitioning the system to the anaerobic condition.Montmorillonite and calcite were identified as the primary dissolving minerals in Bentonil-WRK, exhibiting mass losses of 0.024% and 0.37%, respectively.Secondary precipitated minerals by the geochemical reaction included kaolinite, dolomite, chalcedony, and pyrite.Over the 100,000 year simulation period, the reactive gases resulted in a 0.0029% decrease in mineral volume of Bentonite and a 0.0043% increase in pore volume.It suggested that the long-term stability of the buffer material in the SNF repository may be affected by geochemical reactions induced by reactive gas generation.