In the geological disposal system whose host rock is crystalline rock, fractures play a significant role in the safety assessment as they are the main pathway of the radionuclide migration. From the perspective of long-term safety assessment, the properties of fractures can be changed by tectonic movement such as earthquake, uplift, etc. In general, methods for simulating fractures include Discrete Fracture Network (DFN), which directly simulates the fracture surface, and Equivalent Continuous Porous Media (ECPM), which is equivalent to the ratio of the fractures in a certain rock volume. DFN is generally appropriate for deterministic fractures with large scale and high flow velocity, but ECPM may be more appropriate for small scale and sporadically distributed stochastic fractures because the flow velocity is slow and thus the rock matrix diffusion needs to be considered. In fact, several commercial software, such as FracMan, are already in use to convert DFN to ECPM. However, in order to consider the change in properties of fractures due to tectonic movement in the long-term safety assessment, a model that converts DFN to ECPM needs to be modularized and embedded into the safety assessment model. In this study, therefore, an in-house MATLAB code was developed to convert DFN to ECPM, which can be used as a submodule. The algorithm of converting from DFN to ECPM basically followed the Oda’s method. As the first step of the algorithm, in order to obtain the volume ratio of the fracture in a certain mesh element, the cross-sectional area of the fracture and the mesh element was calculated. Then, porosities of each mesh element were calculated as the volume fraction of fractures passing through the mesh element. Based on the Oda’s method, the permeability tensors of each mesh element were calculated by using an empirical fracture tensor which is weighted by the cross-sectional area and transmissivity of each fracture. Finally, the newly developed module was verified by a benchmark test, in which the ECPM results converted from a certain DFN data by using the numerical module developed in this study were compared with those by using FracMan. The newly developed module will be installed in the process-based total system performance assessment framework (APro) being developed by KAERI.

• Jung-Woo Kim(Korea Atomic Energy Research Institute (KAERI)) Corresponding author
• Yong-Min Kim(Korea Atomic Energy Research Institute (KAERI))
• Jaewon Lee(Korea Atomic Energy Research Institute (KAERI))
• Sung-Hoon Ji(Korea Atomic Energy Research Institute (KAERI))