Modelling of flow in fractured environment is very important problem for applied hydrogeology (for example for simulations of the neighbourhood of the radioactive waste repositories). We present the transport model composed of two parts. First, the fracture fluid flow model is based on the description by the Darcy's law and the mass balance equation with Dirichlet boundary conditions. The second part of the transport model Includes the convection term using the velocity computed in the fluid flow model, diffusion-dispersion term With dual porosity (representing micro fractures), and chemical term describing sorption, radioactive decay, and chemical interactions.
The weakest link in the nuclear energy production is the safe storage of highly radioactive spent fuel. One of the proposed repositories of dangerous nuclear waste are underground granitoid massifs. However, these massifs are always disrupted by a system of geological faults, fractures, therefore the majority of the fracture flow occurs.
In general, there are three main possibilities of modelling the fracture flow. In large-scale without need to now detail flow and transport behaviour in any site subarea, it is possible to use equivalent porous medium models. More complex are the double porosity models, with two distinct interacting subsystems: fractures and porous blocks. As a third and most accurate possibility, we can approximate the original 3-D fractures y planar elliptic or polygonal disks whose frequency, Size, assigned aperture, and orientation are statistically derived from field measurements (Fig. 2, 3), and then consider the 2-D Darcy flow through such a network. However, due to high computer requirements, it is possible to solve just local problems by using these stochastic discrete fracture network models. We refer for instance to Bear & Bachmati (1991) or Wanfang, Z., Wheater & Johnston (1997) for more details.
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In this text, a stochastic discrete fracture network model is presented. We generate the fracture network based on statistical data obtained from geological measurements. The original 3-D fractures are approximated by planar circle disks (Fig. 1), and each disk is subsequently discretized onto a triangular mesh respecting the intersections with its neighbours. In order to simplify the geometrical situation in fracture planes, the computed intersections can be slightly moved and stretched. One then obtains a better mesh, however for the price of vanishing of real 3-D correspondence; the connectivity information is preserved. Finally, an aperture distribution function assigns an imaginary aperture to each triangle element.
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Our contemporary simulation system makes it possible to solve real-world problems of contaminant transport in fractured-rock environment. But there is relatively strong limitation of the spatial dimensions of the computational domain. The problem is solvable only on the domains of volume in order of hundreds of cubic meters, which means only a close neighbourhood of the source of the contaminant. This limitation is caused by used model of fluid flow in the fractured-rock environment, which calculates the flow field on all fractures in the domain, even on the very small ones.
