Kinetics of smectite dissolution at high pH conditions for long-term safety assessment of radioactive waste disposal; Effect of Gibbs free energy and secondary minerals

Sato, Tsutomu*; Oda, Chie  

Bentonite buffer and cement fluid interaction has been a key research issue in long-term safety assessment of radioactive waste disposal. The dissolution rate of smectite, the main constituent mineral of bentonite, has therefore been investigated under hyperalkaline conditions. A dissolution rate equation of smectite has been derived considering the effect of the Gibbs free energy of reaction and is applicable to pH = 7-13 and t = 25-80C. Reactive-transport model analyses to elucidate the consequences of coupled changes in the porewater chemistry, mineralogy and, ultimately, the mass transport properties of the bentonite buffer were conducted using the smectite dissolution rate. Except in the close proximity of the cement interface, it was found that regardless of the choice of secondary minerals, the effective diffusion coefficient and hydraulic conductivity remained largely unchanged after 100,000 years.