Model study on geochemical reactions in the reactive gas-groundwater- bentonite system under geological SNF repository conditions

Abstract

Bentonite is widely recognized as a key buffer material in spent nuclear fuel (SNF) repositories due to its high swelling capacity, low hydraulic conductivity, and high radionuclide adsorption properties. However, reactive gases such as CO₂ and H₂S, generated during the hydrogeological evolution process of the SNF repository, can alter chemical and physical properties of the buffer material (Bentonite), potentially impacting the long-term stability of the repository. In this study, the long-term geochemical reaction modeling was conducted to evaluate the effects of reactive gases (CO₂ and H₂S) on the properties of the bentonite (Bentonil-WRK), one of candidate buffer materials for the domestic SNF repository. Geochemical reaction modeling was performed to simulate the reactive gas-groundwater-bentonite reaction system using PHREEQC version 3.7.3, geochemical reaction code with the LLNL (Lawrence Livermore National Laboratory) database as the thermodynamic data source. Mineralogical properties of the Bentonil-WRK and water quality data of the KURT (KAERI Underground Research Tunnel) groundwater sample were used in this model study. Reaction of gases in the aqueous system, microbial respiration at redox condition, and mineral dissolution/precipitation in bentonite were mainly considered as geochemical reactions in this modeling process. Equilibrium modeling results showed that infiltrating groundwater from the host rock transitioned to an acidic and aerobic environment in the buffer zone due to the dissolution of residual atmospheric gases in pore spaces of the bentonite block. Subsequently, microbial respiration consumed O₂ and SO₄²⁻ ions in pore water at redox conditions, generating CO₂ and H₂S gases. The aerobic environment in the buffer zone was transitioned to the anaerobic condition after approximately 5,190 years of aerobic microbial reaction. As primary minerals in the Bentonil-WRK, montmorillonite and calcite showed mass losses of 0.024% and 0.37%, respectively, during the dissolution process, which led to the precipitation of secondary minerals such as kaolinite, dolomite, chalcedony, and pyrite. Over the 100,000-year simulation period, mineral volume in the bentonite decreased by 0.0029%, and pore volume of the Bentonil-WRK increased by 0.0043%. These results indicate that dissolution reactions were dominant throughout the 100,000-year geochemical reaction period and suggest that the reactive gases potentially generated in the SNF repository environment can alter the properties of the buffer material at the SNF repository. Key words: Bentonil-WRK, Bentonite, Buffer, Geochemical reaction, Geochemical reaction modeling, Reactive gases, SNF repository.