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MODELING TWO-PHASE-FLOW INTERACTIONS ACROSS A BENTONITE CLAY AND FRACTURED ROCK INTERFACE
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
2014 (English)In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 187, no 2, 147-157 p.Article in journal (Refereed) Published
Abstract [en]

Deep geological repositories are generally considered as suitable environments for final disposal of spent nuclear fuel. In the Swedish and Finnish repository design concept, canisters are to be placed in deep underground tunnels in sparsely fractured crystalline bedrock, in deposition holes in which each canister is embedded with an expansive bentonite-clay-mixture buffer. A set of semigeneric two-dimensional radially symmetric TOUGH2 simulations are conducted to investigate the multiphase dynamics and interactions between water and air in a bentonite-rock environment. The main objective is to identify how sensitive saturation times of bentonite are to the geometry of the rock fractures and to commonly adopted simplifications in the unsaturated flow description such as Richards assumptions. Results show that the location of the intersection between the fracture system and the deposition hole is a key factor affecting saturation times. A potential long-lasting desaturation of the rock matrix close to the bentonite-rock interface is also identified extending up to 10 cm inside the rock. Two-phase-flow models predict systematically longer saturation times compared to a simplified Richards approximation, which is frequently used to represent unsaturated flows. The discrepancy diverges considerably as full saturation is approached.

Place, publisher, year, edition, pages
2014. Vol. 187, no 2, 147-157 p.
Keyword [en]
multiphase flow, fractured rock, bentonite clay
National Category
Earth and Related Environmental Sciences
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:su:diva-107122ISI: 000340311600004OAI: oai:DiVA.org:su-107122DiVA: diva2:743148
Note

AuthorCount:3;

Available from: 2014-09-03 Created: 2014-09-03 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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