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Reconstruction of the water content at an interface between compacted bentonite blocks and fractured crystalline bedrock
Stockholm University, Faculty of Science, Department of Physical Geography.
Stockholm University, Faculty of Science, Department of Physical Geography.
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Number of Authors: 5
2017 (English)In: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, Vol. 142, 145-152 p.Article in journal (Refereed) Published
Abstract [en]

High-density sodium bentonite combines a low permeability with a swelling behavior, which constitute two important qualities for engineered barriers in geological disposal of spent nuclear fuel. For example, the KBS-3V method developed in Sweden and Finland is planned to include compacted bentonite as the buffer material to embed canisters containing the spent nuclear fuel packages in deposition holes in deep crystalline bedrock. The partially saturated bentonite buffer will then swell as it takes up groundwater from the surrounding rock. It is important to quantify the water content evolution of the installed buffer to correctly predict the development of the swelling pressure and the prevailing conditions (thermal, mechanical, chemical and biological). This study aimed at quantifying the water content profile at the surface of a cylindrical bentonite parcel retrieved after in situ wetting in fractured crystalline bedrock. We demonstrate the possibility of using regression-kriging to quantitatively include spatial information from high-resolution photographs of the retrieved bentonite parcel, where more water saturated areas appear as relatively dark shades, along with bentonite samples, where detailed measurements of water content were performed. The resulting reconstruction is both exact regarding local sample measurements and successful to reproduce features such as intersecting rock fracture traces, visible in the photographs. This level of detail is a key step to gain a deeper understanding of the hydraulic behavior of compacted bentonite barriers in sparsely fractured rock. An improved scanning procedure could further increase the accuracy by reducing errors introduced by the geometrical transformations needed to unfold and stitch the different photographs into a single gray scale map of the bentonite surface. The application of this technique could provide more insights to ongoing and planned experiments with unsaturated bentonite buffers.

Place, publisher, year, edition, pages
2017. Vol. 142, 145-152 p.
Keyword [en]
Engineered barrier system, Bentonite, Fractured rock, Regression-kriging, Unsaturated flow
National Category
Earth and Related Environmental Sciences
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:su:diva-145212DOI: 10.1016/j.clay.2016.10.002ISI: 000405048000016OAI: oai:DiVA.org:su-145212DiVA: diva2:1128814
Available from: 2017-07-28 Created: 2017-07-28 Last updated: 2017-07-28Bibliographically approved
In thesis
1. Numerical modeling of groundwater and air flow between compacted bentonite and fractured crystalline rock
Open this publication in new window or tab >>Numerical modeling of groundwater and air flow between compacted bentonite and fractured crystalline rock
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The geological repository for final storage of spent nuclear fuel, envisioned by the Swedish Nuclear Fuel and Management Company (SKB), relies on several barriers: copper canisters deposited in holes in the floor of underground tunnels in deep bedrock, embedded in a buffer of compacted bentonite. The initially unsaturated buffer would take up water from the surrounding rock mass and swell to seal any potential gap. This initial two-phase (gas and liquid) regime with two components (air and water) may impact the final density, swelling pressure and biogeochemical conditions in the buffer. A main objective of this work is to identify factors and mechanisms that govern deposition hole inflow and bentonite wetting under the prevailing two-phase flow conditions in sparsely fractured bedrock. For this purpose, we use the numerical code TOUGH2 to perform two-phase flow simulations, conditioned by a companion field experiment (the Bentonite Rock Interaction Experiment or BRIE) performed in a 417 m deep tunnel of the Äspö Hard Rock Laboratory in southeastern Sweden. The models predict a significant de-saturation of the rock wall, which was confirmed by field data. To predict the early buffer wetting rates and patterns, the position of local flowing fractures and estimates of local rock matrix permeability appear more important than the total open hole groundwater inflow. A global sensitivity analysis showed that the buffer wetting time and the persistence of unsaturated conditions over extended periods of time in the rock depend primarily on the local fracture positions, rock matrix permeability, ventilation conditions in the tunnel and pressure far in the rock. Dismantling photographs from BRIE were used to reconstruct a fine-scale snapshot of saturation at the bentonite/rock interface, showing tremendous spatial variability. The high level of heterogeneity in the rock generates complex two-phase flow phenomena (air trapping, dissolution), which need to be accounted for in buffer design and rock suitability criteria. In particular, results suggest that uncertainties regarding two-phase flow behavior are relatively high close to residual air saturation, which may also have important implications for other applications involving two-phase flows, such as geological storage of carbon dioxide.

Place, publisher, year, edition, pages
Stockholm: Department of Physical Geography, Stockholm Univeristy, 2016. 35 p.
Series
Dissertations from the Department of Physical Geography, ISSN 1653-7211 ; 52
Keyword
Two-phase flow, porous media, fractured media, compacted bentonite, crystalline rock, numerical modeling, geological disposal
National Category
Geosciences, Multidisciplinary
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-124428 (URN)978-91-7649-294-9 (ISBN)
Public defence
2016-02-25, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 4: Manuscript.

Available from: 2016-02-02 Created: 2015-12-21 Last updated: 2017-07-28Bibliographically approved

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