개별요소법을 활용한 고준위 방사성 폐기물 심층처분장 주변 균열투수량계수 변화 모델링

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 에너지시스템공학부, 2021. 2. 민기복.원자력발전에서 발생하는 고준위 방사성 폐기물은 계속해서 방사선과 열을 발생하기 때문에, 충분한 시간 동안 생태계로부터 격리되어야 한다. 고준위 방사성 폐기물을 영구적으로 처분하는 방법으로 공학적방벽과 천연방벽을 통해 격리시키는 심지층처분방식이 제시되었다. 심지층처분 시스템의 성능을 평가하기 위하여 공학적방벽과 천연방벽의 장기적 거동을 조사하는 다양한 실험 및 수치해석이 진행되고 있다. 특히 천연방벽의 투수량계수는 핵종의 누출을 평가할 수 있는 수단으로 다양한 열, 수리. 역학적 영향 요인에 대한 분석이 필요하다. 본 논문에서는 고준위 방사성 폐기물 심층처분장의 건설 및 운영 중에 천연방벽, 특히 결정질 균열암반의 투수량계수 변화에 대하여 검토하였다. 결정질 균열암반의 실제 거동을 모사하기 위하여 스웨덴 애스푀 지하연구시설에서 취득한 암석 및 균열 물성을 활용하여 삼차원 개별요소법 기반 균열암반모델을 구축하였다. 균열투수량계수에 영향을 주는 요인인 굴착손상영역, 열, 빙하, 지진에 의한 응력 변화를 수치해석 모델에 반영하였다. 굴착손상영역의 영향은 터널 자유면에 의한 응력재분배가 야기한 균열의 수직변형 및 전단 미끄러짐을 통하여 투수량계수의 변화를 분석하였으며, 이렇게 구축된 터널 모델에 각각 열원, 빙하에 의한 경계조건변화, 지진에 의한 동적 하중을 추가하여 각 영향요인에 의한 투수량계수 변화를 분석하였다. 굴착손상영역으로 인한 영향은 크게는 1,000배가량의 투수량계수 증진으로 나타났고, 수직 열림/닫힘과 전단 미끄러짐에 의한 영향이 균열의 방향에 따라서 복합적으로 작용한 것으로 나타났다. 열, 빙하, 지진에 의한 영향 분석에서는 모두 하중의 재하와 제하가 반복되는 비슷한 응력변화 양상을 수반하였으며 영향의 분석이 끝난 후에는 모두 초기 응력조건으로 회귀하는 과정이 포함되어 있었다. 열, 빙하, 지진에 의한 영향 분석 모두 일부 균열에서 비가역적인 투수량계수의 증진이 확인되었다. 시간에 따른 투수량계수 및 응력 상태를 분석한 결과 하중의 재하 시에는 균열의 수직 닫힘이 지배적으로 나타났지만, 제하 시에는 닫혔던 균열이 가역적으로 열림과 동시에 수직 닫힘에 의해 가려졌던 비가역적 전단팽창의 영향이 드러났다. 이러한 가역적 수직변형과 비가역적 전단팽창의 영향은 균열의 기하학적 특성에 따라 선별적으로 나타났다. 투수량계수의 변화를 터널 주변 절리의 기하학적 특성에 대하여 보다 일반적으로 분석하기 위하여, 균일절리군을 터널 주변에 추가하여 각각 굴착손상영역, 열, 빙하, 지진에 의한 영향을 정리하였다. 투수량계수의 증진은 주로 최대주응력과 평행인 절리에서 크게 나타났으며, 이는 최대주응력의 영향이 작아 수직방향으로 작용하는 압축응력이 상대적으로 낮았기 때문이었다. 또한 터녈면으로부터 약간 기울어진 방향의 절리에서 투수량계수의 증진이 크게 발생했으며, 이는 역학적으로 자유로운 터널면 때문에 추가적인 응력이 절리면 상에서 전단응력으로 집중되어 비가역적인 전단팽창이 크게 발생한 것으로 확인되었다. 본 논문의 결과는 고준위 방사성 폐기물 심층처분장의 계획 및 건설 시 굴착손상영역, 열, 빙하, 지진 시나리오를 고려한 성능 평가의 기초 자료로 활용될 수 있으며, 투수량계수와 같은 천연방벽의 수리학적 인자가 시간에 따라서 변화할 수 있다는 것을 제시하였다.High-level radioactive waste produced from nuclear power plant continuously emits radioactivity and heat after it has been disposed of, so it should be isolated from the biosphere for a sufficient amount of time. To dispose of high-level radioactive waste, deep geological repository systems consisting of engineered barriers and natural barriers are suggested. As a performance assessment, numerous experiments and numerical simulations are performed to characterize the long-term behaviors of engineered and natural barriers. Especially, the transmissivity of natural barriers is considered an important parameter related to radionuclide leakage. Therefore, the relation between transmissivity and a variety of thermal, hydraulic, and mechanical factors are raised, and quantitative studies to determine the effects on transmissivity are suggested for performance assessment. This thesis aims to quantify the transmissivity of natural barriers, especially crystalline fractured rock, under a variety of factors possibly happening during the construction and operation of a geological repository. Three-dimensional discrete element models were constructed based on the rock and fracture characteristics extracted from the Äspö Hard Rock Laboratory in Sweden to describe the realistic behaviors of the crystalline fractured rock. The effects of excavation damage zone, thermal loading, glaciation, and earthquake were applied to numerical models as the factors disturbing the fracture transmissivity. The excavation damage zone was considered the effect of stress re-distribution inducing the fracture normal deformation and shear slip. The heat sources, glacial boundary conditions, and dynamic load were separately applied to the tunnel model after the stress re-distribution to determine the transmissivity changes. The transmissivity changes induced by the excavation damage zone appeared as three-order increases due to the combined effects of normal opening/closure and shear dilation, depending on the fracture orientation. Thermal, glacial, and earthquake scenarios included the loading and unloading cycles and recovered the initial stress conditions after each scenario ended. Irreversible transmissivity increases were found for several fractures under thermal, glacial, and earthquake effects. According to the transient analysis of transmissivity and stress path, the normal closure dominated the transmissivity during loading cycles, while the irreversible effects of shear dilation were revealed after the dissipation of normal loads. These reversible normal deformations and irreversible shear dilations appeared depending on the geometrical characteristics of fractures. To define the relation between the geometry of fractures and transmissivity changes, the uniformly distributed joints were applied on the tunnel models to analyze the effects of excavation damage zone, thermal loading, glaciation, and earthquake. The transmissivity increases were larger on joints parallel to the direction of maximum horizontal stress due to the absence of effects from the largest principal stress inducing the normal stress on fractures. At the vicinity of the tunnel, the joints that are slightly inclined from the tunnel surface accompanied a large amount of permanent transmissivity increase, because the additional stresses were converted to the shear stress on fractures due to the mechanically free surface. The results extracted in this research can be the basis data for performance assessments of geological repositories under excavation damage zone, thermal loading, glaciation, and earthquake scenarios which can happen during the lifetime of repositories. This thesis implies that the hydraulic parameters of natural barriers should be considered as dynamic variables that change as a repository operates.Chapter 1. Introduction 1 1.1 Motivation 1 1.2 Objectives 4 Chapter 2. Literature review 7 2.1 Excavation Damage Zone 7 2.2 Thermal loading 15 2.3 Glaciation 21 2.4 Earthquake 26 Chapter 3. Theory and methodology 31 3.1 Discrete Element Method 31 3.2 Formulations 35 3.2.1 Deformable block motions 35 3.2.2 Formulations used in thermal features 37 3.2.3 Joint constitutive models 38 3.2.4 Fracture Transmissivity 40 Chapter 4. Geological and geomechanical data of Äspö HRL 43 4.1 Model descriptions 43 4.1.1 Äspö Hard Rock Laboratory 43 4.1.2 Three-dimensional tunnel model 46 4.1.3 Fracture geometry 48 4.2 Properties 50 4.2.1 Mechanical and thermal properties of the host rock 50 4.2.2 Mechanical characteristics of fractures 50 Chapter 5. Transmissivity evolution on the Äspö HRL model 56 5.1 Stress re-distribution by excavation on the Äspö HRL model 56 5.2 Thermal loading on the Äspö HRL model 62 5.2.1 Descriptions of heat source 62 5.2.2 Results of thermal simulations 64 5.3 Glaciation on the Äspö HRL model 71 5.3.1 Descriptions of the glaciation scenario 71 5.3.2 Results of glacial simulations 72 5.4 Earthquake on the Äspö HRL model 79 5.4.1 Descriptions of earthquake models 79 5.4.2 Results of the earthquake simulation 82 Chapter 6. Transmissivity evolution on the uniformly jointed model 86 6.1 Stress re-distribution by excavation on the uniformly jointed model 88 6.1.1 Model with a 0° joint dip direction 88 6.1.2 Model with a 90° joint dip direction 90 6.2 Thermal loading on the uniformly jointed model 93 6.2.1 Model with a 0° joint dip direction 93 6.2.2 Model with a 90° joint dip direction 96 6.3 Glaciation on the uniformly jointed model 99 6.3.1 Model with a 0° joint dip direction 99 6.3.2 Model with a 90° joint dip direction 103 6.4 Earthquake on the uniformly jointed model 108 6.4.1 Model with a 0° joint dip direction 108 6.4.2 Model with a 90° joint dip direction 112 Chapter 7. Discussions and conclusions 116 7.1 Discussions 116 7.1.1 Hydraulic coupling 116 7.1.2 Fracture descriptions 118 7.1.3 Repository design 119 7.2 Conclusions 122 Reference 126 초 록 141Docto

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Long-term solutions for the disposal of toxic wastes usually involve isolation of the wastes in a deep subsurface geologic environment. In the case of spent nuclear fuel, if radionuclide leakage occurs from the engineered barrier, the geological medium represents the ultimate barrier that is relied upon to ensure safety. Consequently, an evaluation of radionuclide travel times from a repository to the biosphere is critically important in a performance assessment analysis. In this study, we develop a travel time framework based on the concept of groundwater lifetime expectancy as a safety indicator. Lifetime expectancy characterizes the time that radionuclides will spend in the subsurface after their release from the repository and prior to discharging into the biosphere. The probability density function of lifetime expectancy is computed throughout the host rock by solving the backward-in-time solute transport adjoint equation subject to a properly posed set of boundary conditions. It can then be used to define optimal repository locations. The risk associated with selected sites can be evaluated by simulating an appropriate contaminant release history. The utility of the method is illustrated by means of analytical and numerical examples, which focus on the effect of fracture networks on the uncertainty of evaluated lifetime expectancy.Comment: 11 pages, 8 figures; Water Resources Research, Vol. 44, 200

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Shale disposal of U.S. high-level radioactive waste.

Approved for public release; further dissemination unlimited. Issued by Sandia National Laboratories, operated for the United States Department of Energy by Sandia Corporation. NOTICE: Neither the United States Government, nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors, or their employees, make any warranty, express or implied, or assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represent that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government, any agency thereof, or any of their contractors or subcontractors. The views and opinions expressed herein do not necessarily state or reflect those of the United States Government, any agency thereof, or any of their contractors. Printed in the United States of America. This report has been reproduced directly from the best available copy

MULTI-SPECIES MULTI-PHYSICS MODELING AND VALIDATION OF HYDRODYNAMIC ELECTROCHEMICAL SYSTEM FOR USED NUCLEAR FUEL

Department of Nuclear EngineeringAccurate predictions of processes in hydrodynamic electrochemical systems require an understanding of both the surface electrochemical reactions and the bulk mass transport. Complete coupling of electrochemistry and fluid mechanics is computationally very rich for multidimensional modeling since it involves multiple components across multi-phases at the same time. Therefore, this study develops a computational model that combines a 3D model for calculating single-species mass transport and a 2D model for calculating multi-species electrochemical reactions. The computational model is validated against lab-scale experimental data using a rotating cylinder solid metal cathode and liquid metal anode in the Argonne National Laboratory. The 3D model assumes that U, the representative component in the system, dominates the hydrodynamic behavior, and thus calculates mass transport caused by the rotating solid cylinder electrode. The 2D model still reflects the diffusion of U, Pu, and Nd within a diffusion boundary layer and the bulk concentration changes of these components. The 3D model provides a diffusion layer thickness reflecting convective mass transfer effects to the 2D model. The results of the proposed model show good agreement with the reference experiment, and the model can be considered an important tool for investigating the multidimensional distributions of hydrodynamic and electrochemical variables.clos