Water Migration and Swelling in Engineered Barrier Materials for Radioactive Waste Disposal

Area of Science:

• Geological Engineering
• Materials Science
• Nuclear Engineering

Background:

• Deep underground repositories are essential for isolating radioactive waste.
• Bentonite clay is a key component in multibarrier repository systems.
• Understanding bentonite's hydraulic behavior is critical for long-term repository performance modeling.

Purpose of the Study:

• To analyze the hydraulic behavior of bentonite samples.
• To investigate the effects of aggregate size and hydrothermal treatments (NaCl, KCl, deionized water) on bentonite's water sorption and swelling.
• To quantify water transport dynamics within bentonite.

Main Methods:

• Neutron and X-ray imaging for quantifying water sorption and swelling.
• Analysis of water uptake and wetting front progression over time.
• Small-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS) for aggregate size distribution.
• Inelastic neutron scattering (INS) for understanding sorbed water's physicochemical environment.

Main Results:

• Water uptake in fresh samples followed a square-root-of-time dependence.
• Samples previously in contact with water showed more variable water uptake rates.
• Hydrothermal treatment with KCl demonstrated the most significant increase in water transport.
• K+ interaction with smectite layers is a potential mechanism for enhanced water transport.

Conclusions:

• Bentonite's hydraulic properties are influenced by aggregate size and hydrothermal history.
• Potassium chloride treatment notably enhances water transport in bentonite.
• These findings are vital for accurate modeling of radioactive waste repository performance and safety.