A Study on the Development of a Thermal-Hydraulic Model and Performance Evaluation for High-Temperature Deep Geological Repositories = 고온 심지층처분장 조건을 고려한 열-수리 모델 개발 및 최적의 고온 처분 조건 평가를 위한 연구

다국어 초록 (Multilingual Abstract)

To ensure the safe management of Spent Nuclear Fuel (SNF), Deep Geological Repository (DGR) has received widespread attention. Among the various DGR concepts, the most promising is the KBS-3 concept developed by Finland and Sweden. The core of the KBS-3 concept is its multi-barrier system, which includes a cask iron insert, copper canister, bentonite buffer, and host rock. Each component plays a crucial role in maintaining the overall integrity of the DGR system. Additionally, a critical design criterion is the 100 ℃ thermal limit, which is necessary to preserve the long-term stability of repository. Consequently, the KBS-3 concept requires a large disposal area to disposal of SNF within this thermal constraint.
However, countries with high population density and limited available land, such as Korea, face significant challenges in securing large disposal sites. As a result, high-efficiency DGR concepts have garnered increasing attention, with the high-temperature DGR concept emerging as one of the most promising alternatives. This concept has been studied extensively from both simulation and experiment perspectives. For example, in the HotBENT project, various numerical codes were benchmarked against experimental results, and material properties were measured and incorporated into these codes to improve predictive accuracy.
In this study, to estimate the multi-physics behavior, including thermal-hydraulic-chemical-electrochemical behavior, under high-temperature DGR conditions, the following tasks were conducted: (i) development and validation of a numerical code, (ii) measurement of material property under high-temperature DGR conditions, and (iii) estimation of multi-physics behavior by incorporating the measured high-temperature material properties.
To estimate the multi-physics behavior of high-temperature DGR, a code HADESNU (High-level rAdiowaste Disposal Evaluation Simulator by Seoul National University) has been developed by using MOOSE framework, developed by Idaho National Laboratory. The HADESNU utilizes finite element method with Newton-Raphson method. The numerical code was validated by comparing to two benchmarks, including analytical solution and DECOVALEX-THMC, and two experiments, including lab (CTF1) and field-scale (HotBENT) experiments.
Additionally, to incorporate material properties into the HADESNU, thermal-hydraulic properties of GTC4 bentonite were measured. These include thermal conductivity, specific heat, hydraulic conductivity, and soil-water retention curve, which were evaluated under varying water contents and temperatures up to 150 ℃. The measured properties were correlated with both degree of saturation and temperature.
Based on the developed HADESNU code incorporating the measured high-temperature thermal-hydraulic properties, two high-temperature disposal strategies were evaluated: (i) compact disposal, which reduces the distance between disposal holes and tunnels, and (ii) fuel assembly, which involves combining fuel assemblies into a fuel assembly. According to the results, the compact disposal strategy with a thermal limit of 200 ℃ improves disposal efficiency by approximately 32, which is 3.5 times greater than that of the KRS (KAERI Reference disposal System) concept proposed by KAERI. Additionally, the other evaluated disposal strategy also shows higher disposal efficiency compared to the KRS concept.
In the case of copper corrosion under high-temperature conditions, the corrosion behavior was analyzed from two perspectives: (i) thermodynamic and (ii) kinetic. Thermodynamically, the corrosion mechanisms under high-temperature conditions are similar to those observed in conventional DGR environments, based on Pourbaix diagram. Copper oxide (Cu2O) and copper sulfide (Cu2S) remain the dominant corrosion products in oxic and anoxic conditions, respectively. Kinetically, the corrosion rate under high-temperature conditions is slightly higher than that under conventional DGR conditions during the first 1,000 years, primarily due to the more pronounced temperature evolution. However, after 1,000 years, the difference in corrosion depth becomes negligible, as the temperatures in both environments stabilize at approximately 30 ℃.

목차 (Table of Contents)

• Table of Contents
• Abstract i
• List of Tables vi
• List of Figures vii
• Chapter 1. Introduction 1
• 1.1. Background and Significance 1
• 1.2. Research Motivation 4
• Chapter 2. Literature Review 9
• 2.1. Analytical Models 9
• 2.2. Numerical Codes 12
• 2.3. High-temperature Material Properties 15
• 2.4. High-Temperature Disposal Strategies 21
• 2.5. Copper Corrosion under High-Temperature Condition 23
• 2.6. Research Gap 26
• Chapter 3. Research Objectives & Design 29
• Chapter 4. HADESNU: High-level rAdiowaste Disposal Evaluation Simulator by Seoul National University 32
• 4.1. Background on the Subsurface Environment 32
• 4.2. Multi-physics Coupling in High-temperature DGR 38
• 4.3. Governing Equations: Thermal-Hydraulics 41
• 4.3. Code Structure 46
• 4.4. Code Verification and Validation 50
• 4.4.1. Analytical solution 52
• 4.4.2. DECOVALEX-THMC 56
• 4.4.3. CTF1 experiments 65
• 4.4.4. HotBENT 75
• Chapter 5. Thermal-Hydraulic Property 95
• 5.1. Thermal property 100
• 5.2. Hydraulic property 119
• Chapter 6. Thermal-Hydraulic Behavior in High Temperature DGR 129
• 6.1. First Scenario: Compact Disposal 134
• 6.2. Second Scenario: Assembly Combination 139
• 6.3. Comparison b/w High-temperature Disposal Strategies 147
• 6.4. Sensitivity study 149
• 6.4.1. Temperature effects 149
• 6.4.2. Mesh study 151
• Chapter 7. Corrosion Behavior in High Temperature DGR 153
• 7.1. Thermodynamic Corrosion Behavior of Copper Canisters in High-Temperature DGR 153
• 7.2. Kinetic Corrosion Behavior of Copper Canisters in High-Temperature DGR 156
• Chapter 8. Conclusion 160
• Appendix I. Governing Equation of Copper Corrosion Behavior 162
• Appendix II. Analytical Solution 165
• Appendix III. Colloidal Property of GTC4 Bentonite 167
• Appendix IV. Unsuccessful Measurement Data of Thermal Property 171
• Appendix V. Cross-Validation b/w TPS and THW for Thermal Property 173