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Workreport 2018-31

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Name:

3D and 1D Dual-Porosity Reactive Transport Simulations - Model Improvements, Sensitivity Analyses, and Results from the Integrated Sulfide Project Inter-Model Comparison Exercise

Writer:

Pękala, M., Alt-Epping, P., Wersin, P.

Language:

English

Page count:

172

Summary:

The Integrated Sulfide Project (ISP) is a joint Posiva-SKB project aiming to assess the processes affecting sulfide cycling in the near field of the repository for Spent Fuel. This report describes the work carried out by the University of Bern within WP3 of the ISP. The goal of WP3 is to develop conceptual models and reactive transport tools to evaluate potential sulfide concentrations in different parts of the repository, and to estimate canister corrosion due to sulfide.

The Base Case conceptual model considers that Sulfate Reducing Bacteria (SRB) thrive within a zone of the host rock at its contact with the bentonite buffer and backfill (“the rock-clay interface” or RCI). The bentonite constitutes a source of sulfate and dissolved organic matter (DOM), which can diffuse towards the RCI. In the RCI, SRB can utilise DOM and sulfate to generate sulfide. Upon reaching the canister surface, sulfide reacts instantly with metallic copper to produce chalcocite (Cu2S). This reaction generates hydrogen, which is utilised by the SRB as a second electron donor. Parametric and conceptual uncertainties are evaluated by sensitivity cases testing the effect of the diffusivity of the buffer, backfill and the RCI, the concentration of DOM, the leachability and dissolution rate of solid organic matter (SOM,) and the effect of siderite (FeCO3) presence in the buffer. Variant scenarios are studied to evaluate the effect of SRB activity within the backfill, limited amount of reactive iron (siderite) in the backfill, localized loss of buffer, anaerobic corrosion of steel elements, and the effect of a fracture intersecting the deposition hole.

The models are implemented in the code PFLOTRAN, using fully unstructured finite volume grids. This allows 3D repository geometry and relevant details of the disposal system to be realistically represented. Effects related with spatial resolution of the grid are evaluated through a series of scoping calculations. Electrostatic effects associated with solute transport within the compacted clay are quantified using a simplified 1D geometry, implementing electrochemical solute transport (Nernst-Planck equation) within a two-porosity continuum.

The results highlight the importance of mackinawite solubility for controlling the concentrations of dissolved sulfide. The impact of reactive iron is illustrated by a case where siderite is present in the buffer, which results in significantly lower sulfide fluxes and canister corrosion depth than when the mineral is absent in the buffer. Furthermore, a case considering siderite absence in the backfill leads to markedly higher sulfide fluxes and corrosion depths compared to the case where siderite is present in the backfill. Another key model aspect is the availability of organics as the electron donor to SRB and the maximum rate of SRB activity. Comparison of the single- and two-porosity models performed using the simplified 1D model geometry suggest that, under the specific assumptions made (e.g. electrically neutral DOM species), electrostatic effects related with solute transport through compacted bentonite are relatively minor.

The model predicts that the effect of hydrogen generated by anaerobic steel corrosion is small and limited to the initial several thousand years. Likewise, calculations considering localized buffer loss indicate that the effect would be negligible on the scales of tens of thousands of years. The model suggests that the presence of a single discrete fracture intersecting the deposition hole would likely have a small to negligible effect on the calculated sulfide fluxes to the canister. The biggest impact is predicted for a theoretical case, where SRB activity directly in the backfill coincides with siderite absence in the backfill.

The work carried out shows that reactive transport modelling is a useful tool for constraining the impact of sulfide on canister corrosion and enables meaningful estimates of sulfide fluxes in the near field of the repository. Main challenges include the characterization of in-situ microbial activity rates, the kinetics and solubility/bio-availability of solid and dissolved organic matter, the reactivity of Fe-bearing minerals not considered in the model (such as Al-silicates in the rock and ferric oxy-hydroxides in the bentonite), pH buffering processes, and the transport properties of the RCI. Despite the difficulties, the models provide a quantitative insight into a complex and intrinsically coupled system, and offer a convenient means of hypothesis testing. As such, they are useful as an auxiliary tool for decision making regarding the design of the engineered barrier system of the future repository.

Keywords:

Microbial sulfate reduction, copper canister corrosion due to sulfide, reactive transport modelling, 3D model geometry using unstructured grids, two-porosity bentonite model

File(s):

WR 2018-31_web (pdf) (70.8 MB)


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