Radionuclide Migration Analysis Using a Discrete Fracture Network Model
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model by linking the center of each fracture to the center of the adjacent fractures through the middle point of each intersection with circular tube channels. Their model, however, does not take into account matrix diffusion and sorption, only advection and dispersion. Dverstorp et al. [6] applied a similar approach to the flow and tracer experiments conducted at the Stripa mine, Sweden. This paper describes an approach for assessing geosphere performance in a fractured rock. In this approach, three-dimensional heterogeneous channel network models are constructed from DFN models by linking the fracture intersections on each fracture plane. Radionuclide migration is then solved in the channel network using the Laplace transform Galerkin finite element method developed by Sudicky [7]. The method permits the solution to be evaluated at
any future point in time without any need for time steps and permits the use of grids having a relatively coarse spatial discretization while avoiding significant numerical dispersion [7]. Preliminary radionuclide-migration analysis in a 200m cubic block region of hypothetical
granitic rock was made in order to better understand the impact of radionuclide migration processes in fractured rock. The processes considered include dispersion due to the network system and variability of velocity as well as the effects of matrix diffusion and sorption on
retarding radionuclide migration. Total radionuclide release from a whole repository is evaluated by integrating the results of the fifty realizations of the stochastic block-scale model under the hypothesis of ergodicity. A simplification of the complex three-dimensional DFN model using a one-dimensional parallel-plate model, needed for extensive Monte Carlo simulations and sensitivity studies in order to evaluate the effects of parameter uncertainty as well as parameter variation on radionuclide migration, is also discussed. 729 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
METHDOLOGY AND BASIC ASSUMPTIONS Modeling Approach First, a three-dimensional fracture network model is generated by stochastic realization of polygonal fractures based on realistic distributions of geometric and hydrogeological parameters which include orientation, size, intensity, location, transmissivity and transport aperture. The DFN model is then converted into a channel network model by linking the centers of fracture intersections on each fracture plane with rectangular pipe channels as shown in Figure 1. In this procedure, the transmissivity of each fracture is preserved through channels formed in the fracture. The width of each channel is defined by an empirical relation: 2(1) W =f•,x W1j +W2 2
where W is channel width [m], f, is the width factor [-] which is assumed uniform for all channels, W, and W2 are projected lengths of intersections [m] as shown in Figure 2. Width factor is determined from flow analysis of the channel network model so as to preserve the bulk-permeability of the system. Radionuclide migration is solved in th
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