Abstract
In instances of damage to engineered barriers containing nuclear waste material, surrounding bedrock is a natural barrier that retards radionuclide movement by way of adsorption and delay due to groundwater flow through highly tortuous fractured rock pathways. At the Gyeongju nuclear waste disposal site, groundwater mainly flows through granitic and sedimentary rock fractures. Therefore, to understand the nuclide migration path, it is necessary to understand discrete fracture networks based on heterogeneous fracture orientations, densities, and size characteristics. In this study, detailed heterogeneous fracture distribution, including the density and orientation of the fractures, was considered for a region that has undergone long periods of change from various geological activities at and around the Gyeongju site. A site-scale discrete fracture network (DFN) model was constructed taking into account: (i) regional fracture heterogeneity constrained by a multiple linear regression analysis of fracture intensity on faults and electrical resistivity; and (ii) the connectivity of conductive fractures having fracture hydraulic parameters, using transient flow simulation. Geometric and hydraulic heterogeneity of the DFN was upscaled into equivalent porous media for flow and transport simulation for a large-scale model.
Highlights
In 2015, the Korea Radioactive Waste Agency at the Gyeongju nuclear waste facility started the first-stage of low- and intermediate-level radioactive waste disposal in a cavern at a depth of 130 m below the ground surface, and it is currently undergoing the process to gain permission from the regulatory agency in order to build a second-stage near-surface disposal facility
In order to reflect the heterogeneity of the study area, multiple linear regression analyses of P10 were performed using two independent variables: the inverse distance from the faults weighted by nearest fault size, and the minimum electrical resistivity value
A 3-D discrete fracture network (DFN) model was constructed taking into account regional fracture heterogeneity and the connectivity of conductive fractures for the Gyeongju radioactive waste disposal site in Korea
Summary
In 2015, the Korea Radioactive Waste Agency at the Gyeongju nuclear waste facility started the first-stage of low- and intermediate-level radioactive waste disposal in a cavern at a depth of 130 m below the ground surface, and it is currently undergoing the process to gain permission from the regulatory agency in order to build a second-stage near-surface disposal facility. The Gyeongju radioactive waste disposal facility is equipped with a multi-barrier system consisting of a primary engineered barrier and secondary natural barrier in order to safely isolate nuclear waste from human exposure. The natural barrier functions to reduce the risk of radionuclide leakage from the disposal facility by adsorbing the radionuclides as well as impeding their movement, as the radionuclides predominately migrate along groundwater flow paths if the engineering barrier is damaged. Radionuclide migration paths are intimately linked to discrete fracture networks that are characterized by heterogeneous fracture orientations, densities and sizes. Over the past several decades, the discrete fracture network (DFN) model has been utilized to characterize natural discrete fracture networks in relation to nuclear facilities [1,2,3], underground petroleum-storage systems [4,5], and enhanced geothermal systems [6,7]. The DFN model can simulate 2- or 3-dimensional networks
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