Modelling of transport in fractures with complex matrix properties
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Modelling of transport in fractures with complex matrix properties Luis Moreno, James Crawford, and Ivars Neretnieks Department of Chemical Engineering Royal Institute of Technology 100 44 Stockholm, Sweden ABSTRACT In the ongoing Swedish site investigations it has been found that the rock matrix adjacent to many open fractures has been altered. The alteration can extend from mm to several cm. The altered rock can have different sorption and diffusion properties compared to the undisturbed rock and this may influence the retardation of the nuclides. The paper presents how the Channel Network model has been adapted to handle diffusion into a matrix composed of several layers with different properties in addition to the infinite undisturbed matrix. For one channel, the solution for the Residence Time Distribution, RTD, may be found in the Laplace-plane. For the transport in the Channel Network, a particle tracking technique is used to determine the paths followed by solute particles. The RTD for this path is obtained using convolution, which in the Laplace-plane means multiplication of the transfer functions for each channel. The inversion to the time-plane is made by numerical inversion of the Laplace transforms for each path. The method has been tested with data from the TRUE (Tracer Retention Understanding Experiments) project, Task 6F, fluid flow and solute transport in two features in 100 m scale where a complex matrix was modelled. The model was used to predict the transport of the tracers (I-129), Cs-137, and Am-241) over some 20 m. The paper also addresses how the RTD is influenced by the different retardation mechanisms under Site Characterisation (SC) as well as Performance Assessment (PA) conditions. INTRODUCTION The Channel Network Model (CNM) was developed to calculate fluid flow and solute transport in fractured media [1]. The flow is modelled to occur through channels in the fractures within the rock. The model includes advection in the channels, sorption on the channel surfaces, diffusion into the rock matrix and sorption within the matrix. These are important mechanisms to be considered when predicting the retardation of radionuclides. In the ongoing Swedish site investigations it has been found that the rock matrix adjacent to many open fractures has been altered and the matrix may have a very complex structure. The alteration can extend from mm to several cm. The altered rock can have different sorption and diffusion properties compared to the undisturbed rock, which may influence the retardation of the nuclides. Several types of materials can usually be distinguished in the fracture and zone close to the fracture (e.g., fracture coating, fault gouge, cataclasite, and altered zone). Modelling of diffusion into a matrix formed by several layers or materials [2, 3] and transport in fracture network have previously been studied [4]. Since at present the Channel Network model considers only diffusion into a semi-infinite homogeneous matrix, the model is adapted to handle diffusion into a matrix composed
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