Modeling the Interaction Between Compacted Bentonite Clay and Hyperalkaline Solution

Spent nuclear fuel is planned to be disposed of in deep underground repositories in many countries, including Finland. An essential part of the Finnish final repository concept is a compacted bentonite buffer. Unfortunately, experimental work is only of limited use when predicting the long-term stability of bentonite in repository conditions, and therefore complementary modeling tools and method development are needed to gain confidence in the safety of the disposal. This work involves the modeling of the chemical interaction between inflowing hyperalkaline solution—envisaged to constitute a risk to the long-term chemical stability of the buffer—and bentonite clay in a column filled with crushed rock and compacted bentonite. The main interest was in the cation-exchange equilibria and mineral alterations in bentonite. TOUGHREACT was found to be a useful modeling tool in stability studies of the bentonite. Especially good results were gained for the cation-exchanger composition.