Long-term diffusion of U(VI) in bentonite: Dependence on density

Area of Science:

• Geochemistry
• Nuclear Engineering
• Materials Science

Background:

• Nuclear waste repositories require robust safety assessments.
• MX-80 bentonite is a potential buffer material for repositories.
• Understanding uranium (U(VI)) transport in bentonite is crucial for safety.

Purpose of the Study:

• Investigate U(VI) diffusion in MX-80 bentonite over six years.
• Determine the impact of clay dry density on U(VI) diffusion.
• Compare long-term in-diffusion with short-term experimental results.

Main Methods:

• Performed in- and out-diffusion experiments with U(VI) in MX-80 bentonite at three dry densities (1.3, 1.6, 1.9 g/cm³).
• Utilized synthetic MX-80 model pore water as the background electrolyte.
• Analyzed U(VI) diffusion depths and calculated effective (D_e) and apparent (D_a) diffusion coefficients.

Main Results:

• U(VI) diffusion depth decreased with increasing dry density (3mm at 1.3 g/cm³ to 1mm at 1.9 g/cm³).
• No through-diffusion of U(VI) tracer was observed; natural uranium leaching occurred.
• Long-term in-diffusion D_a values were two orders of magnitude lower than short-term results, suggesting reduced effective porosity.

Conclusions:

• Increasing dry density significantly reduces U(VI) diffusion in MX-80 bentonite.
• Long-term U(VI) in-diffusion is slower than predicted by short-term experiments, possibly due to speciation changes or altered porosity.
• Long-term studies are essential for accurately assessing nuclear waste repository safety.