Coupled modeling of cement/claystone interactions and radionuclide migration

Area of Science:

• Geochemistry
• Materials Science
• Environmental Science

Background:

• Underground repositories are designed for intermediate-level radioactive waste disposal.
• Cementitious materials are used in repository construction, interacting with host-rock formations.
• Understanding these interactions is vital for long-term performance assessment and safety.

Purpose of the Study:

• To investigate the long-term geochemical interactions between cement and clayey host-rock.
• To model mineral transformations and their impact on radionuclide transport.
• To support the performance assessment of underground radioactive waste repositories.

Main Methods:

• Utilized the reactive transport code HYTEC for simulations.
• Considered time scales up to 100,000 years and relevant spatial dimensions.
• Incorporated three hypotheses on mineralogical composition and neo-formed phases.

Main Results:

• Observed long-term pH buffering and significant mineral transformations in both cement and host-rock.
• Identified localized destruction of primary minerals and precipitation of secondary phases (illite, zeolite) with retention properties.
• Demonstrated disturbance of pore water chemistry over several meters due to ion fluxes (OH-, K+, Ca2+).
• Highlighted four interdependent mechanisms controlling the system: alkaline plume diffusion, mineralogical buffering, ion exchange, and pore space clogging.

Conclusions:

• Cement-clay interactions lead to substantial mineralogical evolution and chemical changes in the repository system.
• Secondary mineral phases contribute to radionuclide retention.
• The study provides essential data on distribution coefficients (Kd) and solubility limits for key radionuclides (Cs, Ra, Tc, U).
• These findings are critical for validating and improving performance assessment models for radioactive waste disposal.