2:30 PM - 2:45 PM
[HCG20-04] Long-Term Geochemical reaction model study for the reactive gas-groundwater-bentonite system under SNF geological repository site conditions
Keywords:Bentonite, Geochemical reaction, Geochemical reaction modeling, Radioactive nuclide, Reactive gases, SNF repository
A deep geological repository (DGR) is widely recognized as the most feasible solution for the long-term safe disposal of spent nuclear fuel (SNF). Among the engineering barriers in the repository, the buffer plays a crucial role in mitigating mechanical stress on the canister and delaying the migration of radionuclides. Bentonite is regarded as an optimal buffer material due to its extremely low hydraulic conductivity, high swelling ability, and strong radionuclide adsorption capacity. However, during the hydrogeological evolution of an SNF repository, reactive gases such as CO2 and H2S can be generated and they may alter the chemical and physical characteristics of bentonite, potentially affecting the long-term stability of the repository.
In this study, geochemical reaction modeling was carried out to assess the long-term impact of reactive gases (CO2 and H2S) on the bentonite (Bentonil-WRK: from Clariant Korea), a candidate buffer material for the domestic SNF repository. The PHREEQC version 3.7.3 simulation code was employed with the LLNL (Lawrence Livermore National Laboratory) thermodynamic database, simulating the reactive gas-groundwater-bentonite geochemical reactions over a period of 100,000 years. Reaction conditions for the modeling were determined based on groundwater quality data from the KURT (KAERI Underground Research Tunnel) site and mineralogical data of the bentonite. Geochemical processes incorporated into the model included the dissolution of atmospheric gases, microbial respiration under both aerobic and anaerobic conditions, as well as mineral dissolution and precipitation within the bentonite matrix.
Results of the equilibrium reaction modeling indicated that the groundwater inflow from the surrounding bedrock initially created an acidic and oxygen-rich environment within the bentonite pore spaces, primarily due to the dissolution of atmospheric gases (O2 and CO2). As reactions progressed, microbial respiration consumed dissolved O2 and SO42- in groundwater, resulting in the production of CO2 and H2S gases. Consequently, the system transitioned from an aerobic to an anaerobic state after approximately 5,190 years. The kinetic modeling results identified montmorillonite and calcite as the primary minerals undergoing dissolution in bentonite, with respective mass reductions of 0.024% and 0.37%. These dissolution reactions facilitated the precipitation of secondary minerals, including kaolinite, dolomite, chalcedony, and pyrite. Over the 100,000-year simulation period, the presence of reactive gases led to a 0.0029% decrease in the overall mineral volume of the bentonite and a 0.0043% increase in its pore volume. These findings indicate that dissolution reactions primarily dictated the geochemical processes throughout the simulation period, driven by the influence of reactive gas generation.
Results in this study underscore the necessity of considering gas-induced geochemical alterations in buffer when evaluating the long-term stability and performance of buffer materials in SNF repositories.
Acknowledgements
This study was conducted with the Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education (No. 2021R1A6C101A415).
This research was supported by the Institute for Korea Spent Nuclear Fuel (IKSNF) and National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science and ICT, MSIT) (2021M2E1A1085202).
In this study, geochemical reaction modeling was carried out to assess the long-term impact of reactive gases (CO2 and H2S) on the bentonite (Bentonil-WRK: from Clariant Korea), a candidate buffer material for the domestic SNF repository. The PHREEQC version 3.7.3 simulation code was employed with the LLNL (Lawrence Livermore National Laboratory) thermodynamic database, simulating the reactive gas-groundwater-bentonite geochemical reactions over a period of 100,000 years. Reaction conditions for the modeling were determined based on groundwater quality data from the KURT (KAERI Underground Research Tunnel) site and mineralogical data of the bentonite. Geochemical processes incorporated into the model included the dissolution of atmospheric gases, microbial respiration under both aerobic and anaerobic conditions, as well as mineral dissolution and precipitation within the bentonite matrix.
Results of the equilibrium reaction modeling indicated that the groundwater inflow from the surrounding bedrock initially created an acidic and oxygen-rich environment within the bentonite pore spaces, primarily due to the dissolution of atmospheric gases (O2 and CO2). As reactions progressed, microbial respiration consumed dissolved O2 and SO42- in groundwater, resulting in the production of CO2 and H2S gases. Consequently, the system transitioned from an aerobic to an anaerobic state after approximately 5,190 years. The kinetic modeling results identified montmorillonite and calcite as the primary minerals undergoing dissolution in bentonite, with respective mass reductions of 0.024% and 0.37%. These dissolution reactions facilitated the precipitation of secondary minerals, including kaolinite, dolomite, chalcedony, and pyrite. Over the 100,000-year simulation period, the presence of reactive gases led to a 0.0029% decrease in the overall mineral volume of the bentonite and a 0.0043% increase in its pore volume. These findings indicate that dissolution reactions primarily dictated the geochemical processes throughout the simulation period, driven by the influence of reactive gas generation.
Results in this study underscore the necessity of considering gas-induced geochemical alterations in buffer when evaluating the long-term stability and performance of buffer materials in SNF repositories.
Acknowledgements
This study was conducted with the Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education (No. 2021R1A6C101A415).
This research was supported by the Institute for Korea Spent Nuclear Fuel (IKSNF) and National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science and ICT, MSIT) (2021M2E1A1085202).