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Content archived on 2024-04-16

Development of coupled models and their validation against experiments in nuclear waste isolation: Modelling of THM Behaviour for Granitic Rocks

Objective

HTM (Hydro-Thermo-Mechanical) processes in jointed rock may be of importance in nuclear waste performance assessment.
So DECOVALEX is dealing with the study of coupled hydro-thermo-mechanical modelling processes in fracturated rock and associated computer codes.
A computer program has been designed to simulate the processes in a large rock mass with a repository located at depth of about 500 metres. The model is two dimensional, measures 3000 m x 1000 m and contains two sets of intersecting fractures. Three fracture spacings are defined or 25 m, 50 m, 100 m. A non-uniform hydraulic head acts at the ground surface and zero flux is imposed on the bottom and lateral boundaries. The heat flux from the repository is assumed to decay exponentially with time.

A second computer program has been written, consisting of a model of an assemblage of nine blocks, separated by two sets of discontinuities (planar fractures). This model measures 0.75 m x 0.50 m and is confined along all boundaries. The rock mass is subjected to in situ stress and thermal loading as well as hydraulic gradient. No flow and adiabatic conditions are imposed at top and bottom of the model. The heat flux acting along a section of one of the lateral boundaries includes expansion of the rock mass and cause shearing in the model.

To obtain the experimental data needed to quantify the effects of joint deformation and joint conductivity, a testing facility has been designed and built by the Norwegian Geotechnical Institute. With this apparatus, joints can be closed and sheared under controlled conditions while fluids can be flushed through the joint. Deformations, flow rates and stresses are recorded simultaneously. The boundary stresses applied by flat jacks result in pure normal stress when the same pressure is applied in the flat jack. An increasing shear stress occurs when differential pressure is applied. The proposed coupled stress flow models have input data derived from Stripa studies. The options of linear and nonlinear joint deformability are given and different loading conditions are specified.
Work programme:

Phase 1
(1) Bench-mark Test 1, called "Far field model"

This BMT is designed to simulate the processes in a large rock mass with a repository located at depth of about 500 meters. The model is two-dimensional, measures 3000 m x 1000 m and contains two sets of intersecting fractures. Three fracture spacings are defined 25 m, 50 m, 100 m. A non-uniform hydraulic head acts at the ground surface and zero flux is imposed on the bottom and lateral boundaries. The heat flux from the repository is assumed to decay exponentially with time.

(2) Bench-mark Test 2, called "Multiple fracture model"

This BMT consist of an assemblage of nine blocks, separated by two sets of discontinuities (planar fractures). This model measures 0.75 m x 0.50 m and is confined along all boundaries. The rock mass is subjected to in-situ stress and thermal loading as well as hydraulic gradient. No-flow and adiabatic conditions are imposed at top and bottom of the model. The heat flux acting along a section of one of the lateral boundaries will include expansion of the rock mass and cause shearing in the model.

(3) Test Case 1, called "Coupled Stress-Flow model"

To obtain the experimental data needed to quantify the effects of joint deformation and joint conductivity, a testing facility has been designed and built. With this apparatus, joints can be closed and sheared under controlled conditions while fluids can be flushed through the joint. Deformations, flow rates and stresses are recorded simultaneously. The boundary stresses applied by flat jacks result in pure normal stress when the same pressure is applied in the flat jack. An increasing shear stress occurs when differential pressure is applied. The proposed coupled stress-flow model have input data derived from Stripa studies. The options of linear and non-linear joint deformability are given and different loading conditions are specified.

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Coordinator

NATIONAL RADIOACTIVE WASTE MANAGEMENT AGENCY OF FRANCE
EU contribution
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Address
Rue Jean Monnet 1-7
92298 CHATENAY MALABRY
France

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