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Modelling and Testing of the Hydration of Backfill and Sealing Materials

Objective

If compacted clay-based materials are to be used for the backfill and sealing of radioactive waste repositories the hydro-mechanical behaviour of unsaturated materials have to be understood. When these materials are used as a backfilling around HLW packages their behaviour during saturation is further complicated by the thermal transient due to the heat emission of the waste.
Elasto-plastic models of the Cam-Clay type are not able to describe the irreversible volumetric strain (swelling or collapse) or the evolution of the mechanical property limits due to changes in the water content. In the case of partly saturated clay materials the suction phenomena, which have a strong influence on the hydraulic, mechanical and thermal properties of the material, are to be considered.
The goal of the study is to analyze and model the behaviour of a clay based engineered barrier during its hydration phase under real repository conditions. The hydro-mechanical and thermo-hydraulical models will be coupled to describe stress/strain behaviour, moisture migration and heat transfer.
When considering compacted clay based materials for sealing, the study of the hydromechanical behaviour of unsaturated materials becomes an essential point to be understood. Also being considered is the use of clay based materials as backfilling around high level radioactive waste (HLW) packages. In this case their behaviour during saturation is further complicated by the thermal transient caused by the heat emission of the waste.

The goal of this study is to analyze and model the behaviour of a clay based engineered barrier during this hydration phase under real repository conditions. The hydromechanical and thermohydraulical models will be coupled by data transfer techniques and will be able to describe stress and strain behaviour, moisture migration and heat transfer.

An X-ray transparent oedometer has been designed and a prototype has been realized. The theoretical formulation which combines the thermohydraulical behaviour with soil suction and the hydromechanical behaviour with soil suction has also been developed.
Work programme:

1. Hydration experiments
Uniaxial experiments determining the influence of the suction potential on the hydration rate, swelling or swelling pressure, will be performed. The progression of the hydration front will be followed by x-ray tomography.

2. Hydro-mechanical experiments
These experiments will determine the mechanical characteristics as a function of the water content and the suction potential of swelling clay at room temperature. Tests with imposed suction potential will be performed.

3. Modelling
The first step will be the adaptation of the existing models to dense, swelling clays. In the heat and moisture transfer model, strain and deformation effects will be included as independent variables, while in the moisture migration and stress/strain behaviour model, expansive soil effects will be included.
In the second step a coupling of the models will be done by automatic data transfer from one model to the other.

Coordinator

BELGIAN NUCLEAR RESEARCH CENTRE
Address
200,Herrmann Debrouxlaan 40-42
1160 Bruxelles
Belgium

Participants (3)

Commissariat à l'Energie Atomique (CEA)
France
Address
Centre D'etudes De Fontenay-aux-roses
92265 Fontenay-aux-roses
UNIVERSITAT POLITECNICA DE CATALUNYA
Spain
Address
S/n,jordi Girona 1-3, Modul D2, Campus Nord
08034 Barcelona
University of Wales, Cardiff
United Kingdom
Address
Newport Road
CF2 1XH Cardiff