Corrosive Species Transport through Bentonite Clay: Model Development and Sensitivity Study

M. A. Asad1, S. Couillard1, I. L. Molnar2, M. Behazin3, P. G. Keech3, M. M. Krol1
1York University, Toronto, Ontario, Canada
2University of Edinburgh, James Hutton Road, Edinburgh, United Kingdom
3Nuclear Waste Management Organization, Toronto, Ontario, Canada
Published in 2020

Canada’s plan for the long-term management of spent nuclear fuel in a deep geologic repository (DGR) designed by the Canadian Nuclear Waste Management Organization (NWMO) consists of a steel used fuel cannister (UFC) with a copper coating surrounded by a highly compacted bentonite (HCB) block, placed in placement rooms. Under some conditions, sulphide, produced through microbial processes at or beyond rock-bentonite interface could diffuse through the HCB and corrode the UFC’s copper coating which is called microbiologically influenced corrosion (MIC). The HCB is designed to limit the flux of corrosive species from the surrounding subsurface environment. A thorough understanding of sulphide transport through the HCB is required to estimate MIC rates and ensure the long-term safety of the DGR. Additionally, sensitivity study of design parameters such as rock permeability, bentonite density, bentonite saturation, sulphide diffusion coefficient, sulphide concentration and subsurface temperature can aid in the refinement of the corrosion allowance.

In this study, a 2-D, variably saturated, non-isothermal model has been developed in COMSOL Multiphysics®, to examine sulphide transport through the HCB under anticipated conditions of Canada’s DGR. The developed model uses three COMSOL® interfaces: Richards’ Equation (RE), Heat Transfer in Porous Media (HT) and Transport of Diluted Species in Porous Media (TDS). The RE interface describes the water infiltration into unsaturated bentonite from saturated rock, the HT interface describes conductive heat transfer in the domain and the TDS interface describes diffusion dominated sulphide transport through the bentonite due to a concentration gradient. These physics interfaces are coupled by saturation and temperature dependent parameters. The UFC surrounded by the HCB is modelled at a depth 500 m below the ground surface and the model domain extends 9,500 m beyond this level. The model considers a finer mesh around the UFC to effectively capture the sulphide diffusion and progressively larger mesh in the host rock away from the placement room to reduce computational cost. The model is symmetric about the vertical axis (half of the placement room with surrounding rock is simulated) which reduces computation cost and adiabatic symmetry boundary conditions are applied on the outside vertical boundary to represent an infinite repository with multiple heating sources. The model was run for a reference period of 1 million years and was used to conduct a sensitivity analysis of how placement room saturation behaviour and rates, sulphide flux, and MIC rates vary with different hydrogeologic, and fuel conditions. The results show that saturation behaviour and rate changes with rock type and heating behaviour, and it saturates relatively quickly with implications for other corrosion types (e.g., uniform corrosion, stress corrosion cracking). Sulphide flux and MIC rates are greatly affected by the period when UFC temperature is high and placement room approaches to full saturation.