Reactive Transport Modelling of the Long-Term Interaction between Carbon Steel and MX-80 Bentonite at 25 °C

M Carme Chaparro,Volker Metz,Nicolas Finck,Horst Geckeis

doi:10.3390/min11111272

M Carme Chaparro, Volker Metz + Show 2 more

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https://doi.org/10.3390/min11111272

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Journal: Minerals	Publication Date: Nov 16, 2021
Citations: 12	License type: CC BY 4.0

Affiliation: Karlsruhe Institute of Technology

Abstract

The geological disposal in deep bedrock repositories is the preferred option for the management of high-level radioactive waste (HLW). In some of these concepts, carbon steel is considered as a potential canister material and bentonites are planned as backfill material to protect metallic waste containers. Therefore, a 1D radial reactive transport model has been developed in order to better understand the processes occurring during the long-term iron-bentonite interaction. The numerical model accounts for diffusion, aqueous complexation reactions, mineral dissolution/precipitation and cation exchange at a constant temperature of 25 °C under anoxic conditions. Our results suggest that Fe is sorbed at the montmorillonite surface via cation exchange in the short-term, and it is consumed by formation of the secondary phases in the long-term. The numerical model predicts precipitation of nontronite, magnetite and greenalite as corrosion products. Calcite precipitates due to cation exchange in the short-term and due to montmorillonite dissolution in the long-term. Results further reveal a significant increase in pH in the long-term, while dissolution/precipitation reactions result in limited variations of the porosity. A sensitivity analysis has also been performed to test the effect of selected parameters, such as corrosion rate, diffusion coefficient and composition of the bentonite porewater, on the corrosion processes. Overall, outcomes suggest that the predicted main corrosion products in the long-term are Fe-silicate minerals, such phases thus should deserve further attention as a chemical barrier in the diffusion of radionuclides to the repository far field.

Highlights

IntroductionThe waste is foreseen to be emplaced in metallic canisters (e.g., made of carbon steel), which would be surrounded by bentonite as buffer material (engineered barrier system or EBS)
A 1D radial reactive transport model has been developed in order to better understand the processes occurring in the long-term iron–bentonite interaction
Results suggest that Fe partly diffuses and is partly adsorbed to montmorillonite by cation exchange, in the shortterm

Summary

Introduction

The waste is foreseen to be emplaced in metallic canisters (e.g., made of carbon steel), which would be surrounded by bentonite as buffer material (engineered barrier system or EBS). During the geochemical evolution of such a repository system, groundwater is expected to move through the barriers and contact the metallic canisters, which will start corroding. During container corrosion, long-term geochemical processes at the corroding container/bentonite interface will proceed and have an impact on the properties of the geotechnical barrier. Such information may be of relevance for the Safety Case of the repository. To gain insights into container corrosion and bentonite alteration, several experiments and numerical models studying the iron-bentonite interactions have been reported in literature (see, e.g., in [1,2,3,4,5])

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Conclusion