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Development of coupled models and their validation against experiments in nuclear waste isolation: Modelling of THM (Thermo-Hydro-Mechanical) Behaviour for Fractured Rocks


To study the importance of THM (thermo-hydro-mechanical) processes in the transport of radionuclides from the repository up to the biosphere.
To improve and to compare the different calculations codes, and to compare and compile experimental results.
The DECOVALEX project is composed in its first phase (October 1991 to September 1992) of two benchmark tests and one test case. The first benchmark test is a far-field study of a very fractured rock mass, the second one is a near-field study of a reduced number of fractures, and the test-case studies the real behaviour of a single fracture.
In its second phase, (October 1992 to September 1993), the project is composed of one bench-mark-test and two test-cases. The bench mark-test is a near-field study of a very fractured rock mass, the two test-cases respectively concern the behaviour of a single fracture, and the thermo-hydro-mechanical experiment of Fanay-Augères (uranium mine in France).
The aim of this experiment was to study the effects of the heating on fractured and unsatured granite rock mass, and to improve computer codes.
A rock mass has been modelled by a discrete approach, ie, by representing all the blocks and discontinuities. The informatical tools of the Ecole Nationale Superieure des Mines de Paris have been adapted to the specific needs of water circulation in a set of fractures. Variation of the temperature and of the geometry of the joints have been taken into account for the calculation of the hydraulic parameters.

Two basic assumptions have been made: all the water flows take place only in the fractures and flows in the porosity of the blocks have been neglected; all the heat flows take place only in the blocks heat transfers through the water flows have been neglected.

A calculation without temperatures has been carried out, in order to validate the behaviour of joints in the case of a big mesh.
Work programme:

Bench-Mark Test No.1 is a 2-Dimensional modelling of the reactions, by THM processes, of a rock mass, (3000m x 1000m) to a radioactive wastes disposal about 500m deep.
The work programme for phase 1 is as follows:
- Selection of a discrete approach to model the rock mass.
- Selection of three 2-D codes based on the finite elements method: one for the mechanical calculation (VIPLEF), one for the thermal calculation (CHEF), and one for hydraulic (HYDREF).
- Addition to the HYDREF code of a joint-element taking into account both a thermo-hydraulic approach by a quadratic estimation of the density, the viscosity and the temperature of the fluid, and a hydro-mechanical approach by a quadratic estimation of the joint aperture.
- Coupling THM with CHEF, VIPLEF and HYDREF.
The work programme for phase 2 is as follows:
- development of tri-dimensional hydro-mechanical joint taking into account the thermal variations of the rock environment. The discontinuities are modeled using special joint finite elements,
- determination of principal characteristics of fractures for joint modelling,
- geometric definition of discontinuities allowing mesh of fractures by finite elements method: geometry, aperture, toughness, mechanical parameters of fractures,
- hydraulic description of fractures: channelling and connecting,
- thermo-hydrological coupling taking into account heat transfer by convection,
- search of tension behaviour laws in compression and in shearing taking into account the displacement of the rock matrix,
- search of the connection between stress-strain and mechanical aperture - hydraulic aperture for mechanical behaviour modelling.

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Commissariat à l'Energie Atomique (CEA)
EU contribution
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Centre d'Études de Fontenay-aux-Roses
92265 Fontenay-aux-Roses

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