RD&D-Programme 2004 - SKB

Together with SKB Andra, Enresa, JNC, Nagra and Posiva, SKB has conducted a model
development project for modelling of gas migration in bentonite (Gambit). The third phase of
the project has been concluded. Unfortunately, the available experimental data does not provide
a clear picture of how gas migrates in bentonite. Model development has therefore assumed
three different possible mechanisms for gas flow:
1. The bentonite behaves like a conventional porous medium, where gas flow is governed by
capillary pressure and relative permeability.
2. The gas flows in microfissures in the bentonite.
3. The gas flows in macroscopic fractures in the bentonite. This differs from mechanism 2 in
that the fractures are of the same size as the specimen. In mechanism 2 the fissures are much
smaller than the specimen.
The models that have been devised and tested can reproduce experimental data, but there are
obvious deficiencies in certain respects. The conclusion is that future model development will
require a better understanding of the gas pathways in bentonite and the couplings between stress
and strain, gas and water fluid pressures, and gas-filled porosity.
Gasnet was a network within the EU’s Fifth Framework Programme. The purpose was to review
how gas-related issues were dealt with in safety and performance assessments. Among the
issues discussed were:
• Corrosion rates over long time spans.
• Microbial gas formation.
• Release of carbon-14 in gas phase.
• Gas migration in clay.
• Near-field evolution.
• Importance of the excavation disturbed zone (EDZ).
• Human intrusion.
• Importance of gas for water transport.
The treatment of these issues in safety assessments is still associated with considerable
uncertainties.
Programme
Our knowledge of gas transport in bentonite is based solely on experiments on a relatively
small scale. A clear conclusion from all projects in the area is that gas transport experiments on
a larger, preferably full, scale are necessary. SKB has therefore decided to carry out a full-scale
experiment in the Äspö HRL (Lasgit). The purpose of the project is to:
• Carry out and interpret a full-scale gas injection test based on the KBS-3 concept.
• Evaluate the questions surrounding scaling-up and their importance for gas transport and
buffer function.
• Provide more information about the gas transport process.
• Generate high-quality data for testing and validation of models.
• Demonstrate that gas evolution inside a canister does not have any appreciable negative
consequences for the barriers in the repository.
Lasgit consists of three phases:
• An installation phase with design and manufacture of the components for the test, plus
deposition of a full-scale canister and surrounding bentonite.
• A water saturation phase where the intention is to saturate the bentonite as fast as possible.
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