The objective of the present study is to investigate the use of the coupled model from a methodological point of view. Modelling of fractured medium in the framework of the safety analysis of a deep geological disposal for radioactive waste generally relies on two families of models:
- the Equivalent Porous Medium model relies on the assumption that properties of the fractures medium (hydraulic conductivities, porosity, ...) can be averaged in a meaningful manner on rock volumes which can be regarded as small on the regional scale of interest;
- the Discrete Fracture Model seeks to represent each fracture of the medium, or at least a statistically equivalent set of fractures, and to describe more accurately the flow in the fractures.
In the framework of contracts with the CEA/IPSN, the CIG/ENSMP, has developed and applied two numerical tools:
- the FRACAS model is a Discrete Fracture Network (DFN) model describing flow, mass transport, heat transfer and hydromechanical interrelations in a network of disk-shaped fractures (Beacher model);
- the TRISEC code is an Equivalent Porous Medium (EPM) model which solves in three space dimensions the equations for flow and heat transfer on a variety of finite elements.
A simulation tool for flow in a fractured rock mass was developed to be used in the safety analysis related to deep underground nuclear waste disposal. The model is based on a fracture network type approach in the vicinity of the repository and an equivalent porous medium type approach on the regional scale. Both models are merged in a single simulator for steady state flow.
This model is applied to a synthetic data set inferred from French study sites (Auriat for the regional hydraulic conductivity, and Fanay-Augeres for the structure of the fracture). Regional flow from a recharge zone to 2 discharge zones is modelled. The discrete fracture network zone, which is supposed to contain the repository, is situated in the descending part of the flow, at a depth of 1000 m.
Several aspects of the model are studied: regional as well as local flow are compared to those obtained from a fully homogeneous porous medium, for various sizes of the discrete fracture network zone (coupling radius). The distance at which the influence of the discrete fracture network vanishes is investigated. This approach gives an estimation of the size of a representative elementary volume for the considered fracture structure.
A numerical tool based on the connexion of the FRACAS and TRISEC models has been developed by CIG/ENSMP for CEA/IPSN. Our approach is to apply this coupled model at the scale of a real site with data as close as possible to "real life". The aim of these evaluations is to see whether a distance exists from which the DFN perturbation is no more visible, and how this distance depends on the "coupling radius".