Nuclear Waste Storage

Soil mechanics laboratory

Tools for the assessment of multi-barrier systems

Disposal in deep clay geological formation along with the confinement of the canisters containing the waste with proper buffer material is the most promising way for disposing of these high level wastes. Involved materials (host rock/clay and buffer) must be deeply analyzed in order to provide reliable prediction for the behaviour of storage facilities. Several interacting physical phenomena take place simultaneously during the waste disposal life, making any prediction for the reservoir behaviour a challenging subject. The involved materials are basically expected to undergo significant changes in temperature, stress state and water content. In this sense, the better understanding of the thermo-hydro-mechanical (THM) behaviour of these materials has to be addresses with reasonable accuracy.

Research Group for Nuclear Waste Storage:

Research Projects:

International Research Projects

GAST Experiment
The Gas-Permeable Seal Test (GAST) was initiated in 2010 and conducted at the Grimsel Test Site (GTS, Switzerland) to demonstrate the effective functioning of gas-permeable seals at realistic scales and under realistic hydraulic conditions.

FEBEX: Full-scale Engineered Barriers Experiment
Modelling of the bentonite and the repository design. Partners are ENRESA and UPC (Spain), NAGRA (Switzerland), SKB (Sweden).

SHARC2: Shale Research Centre consortium
This consortium unites the state-of-the-art knowledge and research on shales of both oil industries and nuclear agencies. The Shale Research Centre’s objectives are the experimental and theoretical characterisation of gas shales properties and the analysis of problems related to reservoirs performance. Partners are CSIRO (Australia), NAGRA (Switzerland), BP (U-K), Total (France), SINOPEC (China).

GAS SHALES
Experimental investigation for the analysis of the hydro-mechanical behaviour of gas shales. Partner is CHEVRON (US).

FE-Experiment in Mont Terri
A full scale emplacement project of high level nuclear waste canisters in Mont Terri. The emplacement follows the Swiss concept involving compacted granular bentonite as a buffer material and Opalinus Clay as a host rock. the in-situ experiment is leaded by NAGRA (Switzerland).

Analysis of geomaterial behaviour in unsaturated conditions

In the concept of nuclear waste repository, both the natural and the engineered barriers are in unsaturated states. Understanding and characterising the behaviour of unsaturated geomaterials composing the multi-barrier system is therefore one of the main issues. Several advanced devices are available in our laboratory allowing to analyse experimentally swelling behaviour, mechanical response in unsaturated conditions, water retention behaviour and liquid and gas transfers.

Unsaturated Soils

Characterisation of the Thermo-Mechanical response of geomaterials

Heat from nuclear wastes induces temperature changes in the multi-barrier system and justifies our interest for the thermo-mechanical behaviour of geomaterials. The determination of model parameters in coupled thermo-mechanical models with consideration of partial saturation requires advanced lab testing. The behaviour of the geomaterial in such complex conditions has to be characterised in order to understand the in-situ governing physical processes and to feed numerical models with the necessary information.

Non-iso Thermal Testing

Thermo-hydro-mechanical constitutive modelling

A variety of phenomena occurs during wetting, drying, heating or mechanical loadings. The swelling behaviour, thermal collapse, wetting collapse, and the isothermal mechanical behaviour must all be included in a constitutive model. Based on concepts from the Cam-Clay model, our model, ACMEG, has been extended and tested to reproduce the key behaviours of unsaturated soils. It is currently expanded to other problems such as anisotropy or chemical couplings.

Constitutive Modeling

Numerical modelling – a key process in understanding the behaviour of the multi-barrier systems

The development of advanced elasto-plastic constitutive models for soils and their application to nuclear waste repository design in a finite element framework is a key competence in the Soil Mechanics group at EPFL.
The analysis of thermally and hydraulically induced plastic strains is performed in order to evaluate the long-term sealing capabilities of buffer materials.

Numerical Modeling

 Thermo-Hydro-Chemo-Mechanical behaviour of Shales 

Shales are not only considered as a suitable soil for the storage of high-level nuclear waste. They also provide perfect cap rocks for oil and gas reservoirs. This makes their behaviour an exciting subject of research for the civil and environmental engineer. Hi-tech and advanced boring techniques are essential for the exploitation of the traditional and more recently discovered natural resources.

The mechanical issues occurring during a reduction of earth pressure form the basis for a challenging study of its effects on the surrounding material. Pressures of dozens of MPa have been recorded. Such in-situ stresses are not easy to assess in the laboratory. Nonetheless, LMS possesses adequate equipment. One example is the high-pressure oedometer cell, which can apply pressures up to 100 MPa. This kind of device allows for a mechanical characterization of the shales using methods similar to those applied to conventional soils.

A second aspect inspiring the research on shales and shale gas and/or oil exploitation, is the occurrence of high temperatures. At high depths and under high pressures, temperatures up to 100°C can be reached. LMS is equipped with an innovative device that can reproduce such temperatures, as well as extreme mechanical conditions, with an option for partial saturation. LMS’s thermo-hydro-mechanical (THM) triaxial cell is especially designed for this kind of situations.

THM Triaxial Cell

Bore companies always search for efficient ways of exploitation. This affects also the engineers. One example of borehole improvement is the use of a drilling fluid, which is often water with dissolved salts. This fluid interacts with the soil with which it is in contact. In the case of partial saturation, the fluid can enter the pores of the material, for example. If saturated, a chemical gradient can result in osmotic pressures and flow of water, or diffusion of ions. All cases involve a change in the local chemistry of the pore fluid. LMS is currently researching the influence of salts on saturated soils and shales.

Previous studies in the field of shale characterization focussed mainly on (chemo-)poro-elasticity. LMS’s approach is developed in such a way that it provides the opportunity to consider irreversible strains as well.

The mentioned points alltogether form an advanced, unitary framework for the analysis of the thermo-hydro-chemo-mechanical behaviour of shales. LMS is convinced that its research adds valuable knowledge to the civil and environmental engineering community by profoundly researching the elasto-plasticity of this material.

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