Modelling of Solute Transport under Flow Conditions Varying in Time, Using the Channel Network Model
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0RGHOOLQJRI6ROXWH7UDQVSRUWXQGHU)ORZ&RQGLWLRQV9DU\LQJLQ7LPH8VLQJWKH&KDQQHO 1HWZRUN0RGHO Luis Moreno, James Crawford, and Ivars Neretnieks Dept Chemical Engineering and Technology Royal Institute of Technology SE-100 44 Stockholm, SWEDEN Email: [email protected], [email protected], [email protected] $%675$&7 Transport of radionuclides from a repository to the biosphere is calculated in the presence of varying boundary conditions. This may occur in Sweden, for example, during post-glacial rebound. To calculate the transport of radionuclides, the flow field has to be calculated at each time step. An alternative approach is to determine the flow field at certain time intervals; i.e., modelling the dynamic flow system as a sequence of punctuated steady states. This can be done in a rather simple way by using the Channel Network model (CNM), in which radionuclide transport is calculated by particle tracking. Intuitively one would expect such a simple procedure to be permissible and lead to small errors if the time intervals are so short that only small changes in flow rates and flow directions take place. In this paper, the technique is applied to a study case. This describes a repository located at a sub sea level location outside the coastline; where, owing to the land lift that is taking place in Scandinavia the coastline is advancing towards the repository. An important issue to be determined is the minimum number of time intervals that need to be used in order to obtain a reliable solution to radionuclide transport. It is found that the number of time intervals needed is strongly dependent on the variation in the boundary conditions. For the changing conditions used in the example tested, a number on the order of 5-9 time intervals allows a sufficiently good representation of the transport. ,1752'8&7,21 When radionuclide release from a repository is calculated for long times, the boundary conditions may vary with time. This may occur in Sweden, for example, due to the land lift that is taking place in Scandinavia subsequent to the last deglaciation. To calculate the transport of radionuclides, the flow field has to be calculated at each time step. Another approach to avoid this difficulty is to calculate the flow field at certain time intervals; i.e., modelling the dynamic flow system as a sequence of punctuated steady states. This can be done in a rather simple way by using the Channel Network model (CNM), in which radionuclide transport is calculated by particle tracking. Intuitively one would expect such a simple procedure to be permissible and lead to small errors if the time intervals are so short that only small changes in flow rates and flow directions take place. The Channel Network Model (CNM) implemented in the program CHAN3D was developed to calculate fluid flow and solute transport in fractured media [1]. The flow is assumed to occur through channels in the fractures in the rock. The model includes advection in the channels, sorption on the channel surfaces, diffusion into the rock matrix and
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