A Dual Porosity Approach to Model Swelling of Bentonite Hydrated with Brine

Abstract of the technical presentation presented at:
ComGeo IV symposium, Assisi, Italy
May 2–4, 2018

Prepared by:
T.S. Nguyen, Z. Li, G. Su and Q. Zheng
Canadian Nuclear Safety Commission

Abstract

The Canadian Nuclear Safety Commission (CNSC), Canada's nuclear regulator, conducts regulatory research in order to build independent knowledge on safety aspects related to the deep geological disposal of radioactive wastes. In addition to the geological barrier, a major safety component of the repository system consists of engineered seal materials that contain highly expansive clay minerals, such as MX-80 bentonite. In Canadian sedimentary rock formations that are currently being considered as one of the candidate host rock types for geologic repositories, highly concentrated brine has been found. This concentrated brine affects the hydraulic-mechanical properties of the bentonite seal by reducing the swelling potential of the bentonite clay mineral. Therefore, the CNSC is conducting experimental and theoretical research in order to further understand how the concentrated brine influences the performance of bentonite-based seals as a barrier to radionuclide migration.

In this study, a model based on the dual porosity approach was developed and calibrated with laboratory swelling experiments on MX-80 bentonite, where the samples were infiltrated with brine and the evolution of the swelling pressure was monitored for more than a year. We defined micropores as the pores between the clay stacks within an aggregate, and macropores as the pores between the aggregates. The model considers porewater flow and solute transport in both the micropores and macropores, and exchange of water mass and solutes between the two pore types. The model can successfully simulate the observed early increase in the swelling pressure, followed by a gradual decrease to a value that is much smaller than the value of swelling pressure for deionized water.

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