The integrity of the disposal repository structure must be guaranteed for few hundreds to few hundred thousand years until toxicity of radioactive waste is surely degraded. Acoustic emission (AE) method is widely utilized to evaluate the integrity of the structure because it can detect crack wave signals of the structures. It is well known that the cracking AE energy is proportional to the volume of the structure (Fractal theory). However, it is hard to destroy whole structures for obtaining AE energy. Therefore, the scaled specimens are prepared to obtain the relationship between volume of the structure and AE energy. The specimens are prepared with same of Wolsong Low and Intermediate Level Radioactive Waste Disposal Center (WLDC) silo concrete recipe. Their diameters are from 50 mm to 150 mm in each 10 mm and their heights are twice of the diameter. One set of 50 mm to 150 mm specimens (11 specimens in one set) are made in single mixers to maintain uniformity. Surface of the specimens are flatten with cement milk to prevent from applying load with eccentricity. The uniaxial compression test is performed by controlling displacement as 0.1 mm/min. The fractal constant is obtained using least square function from volume-cumulative AE energy relationship.

Backfill is one of the main components of engineered barrier in a high-level waste repository. The material selection of the backfill determines the barrier performance of the backfill. Overseas, its related research has been carried out mainly in Sweden, Finland, Canada, and Japan. However, Korea has recently started backfill research, and it is urgent to select a potential material for establishing the concept of backfill material and conducting backfill research. This study reviews NEA report, potential materials for overseas backfill research, advantages and disadvantages of single and mixed backfill materials, cases of license applications in Finland and Sweden for the selection of potential materials for backfill in Korea’s high-level waste repository. The review results indicated that it is reasonable to carry out backfill research according to the following plan: Both single and mixed materials are considered as potential materials for backfill research; experiments and performance studies are conducted with these materials; and, based on the results, a potential material or candidate material for the backfill suitable for the HLW repository in Korea is determined. For this plan, the single material is tentatively selected, as in Sweden, as bentonite with a montmorillonite content of about 40-50%. Then, if the selection criteria for montmorillonite content are determined through experiments and performance studies, we determine the final potential backfill material. As for the mixed backfill material, the bentonite/crushed rock mixture seems to be more advantageous than the bentonite/sand mixture considering the disposing problem of crushed rock generated from tunnel excavation and economic feasibility through its recycling. It is thought that the bentonite used in the bentonite/crushed rock mixture should have a higher montmorillonite content than bentonite used as a single backfill material since the crushed rock acts as an inert material in the mixture. The results of this study can be used as basic data for selecting the backfill material to be applied to the high-level waste repository in Korea, and can be used as a guideline for selecting the potential material required for backfill experiments and performance studies to be carried out in the future.

The 2-round Delphi survey and Focus Group Interview (FGI) survey method, in this study, are sequentially applied for the level analysis of the high-level radioactive waste (HLW) management technologies, that are classified into transport/storage, site evaluation, and disposal categories. The 2- round Delphi survey was conducted on domestic 56 experts in the HLW field in Korea, and survey answers were managed with questionnaires distributed by e-mail. In the FGI survey, domestic 24 experts from management field were formed into three groups to conduct in-depth interviews. Past research achievements including journal papers, intellectual properties and the expert opinions presented at expert hearing on HLW technology were used as reference materials. As a result of the survey, in this study, the average domestic technology level compared to the leading countries was 83.1% in transport area, 79.6% in storage area, 62.2% in site evaluation area, and 57.4% in disposal area, respectively. When compared to the former level analysis results in 2017, technology level of transport-storage area increased by 8.6%, and the site evaluation-disposal technology area decreased by 7.27%. The highest factor that increase the level of technology in the transport-storage field was due to the increased R&D program resulting on journal papers, intellectual properties. In addition, the decrease factor in the level of technology in the site evaluation-disposal field was mainly due to relatively low R&D program when compared to the leading countries. Suggested method for the level survey can be used to find out the basic data of the lower tech technologies, to estimate the efficient research budgets and to prepare the R&D human resources. With this regards, R&D roadmap can be matured with suggested prediction method for the domestic technology level on HLW.

Compacted bentonite buffer materials are a key component of the engineered barrier system for high-level radioactive waste disposal. The bentonite buffer is saturated via groundwater flow through the excavation damaged zone in the adjacent rock mass. Bentonite saturation results in bentonite swelling, gelation and intrusion into the nearby rock discontinuities. Groundwater flow can cause bentonite erosion and transportation of bentonite colloids. This bentonite mass loss can negatively impact the long-term integrity of the engineered barrier system. Hence, it is necessary to understand the effects of erosion on the properties of the bentonite buffer. In this study, a series of artificial fracture erosion experiments are conducted to investigate the erosion characteristics of compacted Ca-bentonite buffer materials for different initial dry density conditions. Compacted bentonite blocks and bentonite pellets were manufactured using the cold isostatic pressing technique and granulation compactor respectively. The specimens were placed in a custommade transparent artificial fracture cell and the bentonite intrusion characteristics were monitored for two months under free swelling conditions with no groundwater flow. The radial expansion of the bentonite specimens within the artificial fracture was measured using a digital camera. In addition, the swelling pressure, displacement, and saturation were determined using a load cell-piston system, LVDT, and electrical resistivity electrodes respectively. A hydro-mechanical-chemical coupled dynamic bentonite diffusion model was applied to model the bentonite erosion characteristics using COMSOL Multiphysics.

When a rapid groundwater inflow is introduced from the adjacent rock mass in the early stage of disposal, hydraulic pressure build-up occurs, which may cause piping erosion at the buffer material itself and the interface of the gap-filling material. Such piping erosion in compacted bentonite buffer via interaction between the buffer and the adjacent rock mass may deteriorate the performance of the buffer material. Therefore, it is necessary to understand the conditions and scenarios in which the piping phenomenon around the buffer material occurs for the long-term health of the repository. In this study, laboratory-scale experimental tests of piping erosion in buffer and interfacial rock was introduced. ø 100 mm × 200 mm height compacted bentonite specimens were placed in a cylindrical acetal cell, and the distilled water was continuously injected at a flow rate of 0.068 L/min using a dual syringe pump. The inflow of water was generated from the bottom and side cell of buffer material. During water injection, injected water pressure and amount were measured with visual observation. The results showed that the external saturation of buffer firstly occurs followed by piping crack generation along the wetting front. The additional piping channels were generated and merged with others. As the injection stopped, the swelling and self-sealing behavior of buffer material were observed. Moreover, X-ray CT scanning of the cell was conducted after the piping simulation to analyze the piping channels and saturation depth. The results highlight the piping erosion phenomenon mainly occurs due to the presence of a gap outside the buffer material. Further experimental cases is need to comprehensively understand piping phenomena in buffer material for assessing the long-term stability of underground radioactive waste disposal systems.

The backfill refills the deep geological disposal system after the installation of buffer in the disposal hole. SKB and Posiva have established the safety function for the backfill such as hydraulic conductivity of 10-10 m/s and swelling pressure of 0.2 MPa. The study on the thermal properties is required for the evaluation of performance design and long-term stability of backfill, since the thermal condition affects the hydraulic and mechanical behavior of backfill. Thermal conductivity is a key characteristic of thermal properties due to heat dissipation from spent fuel. In this study, thermal conductivities of bentonite-sand mixed blocks were measured. The silica sands were used instead of the crushed rock with bentonil-WRK, one of the candidate bentonite of the Korean repository system. The effects of size distribution and mass ratio of sand were evaluated. Four different size of silica sand (i.e., 0.18-0.25, 0.7-1.12, 1.6-2.5, 2.5-5.0 mm) and five mixing ratio (i.e., 1:9, 2:8, 3:7, 4:6, 5:5 of bentonite and sand) were used for characterization of thermal conductivity. As a result, the thermal conductivities were measured ranging from 1.6 to 3.1 W/m∙K depending on the size and mass ratio of the sand. The smaller the size or higher the mixing ratio of sand or the higher the water contents, the higher the thermal conductivity on the surface of backfill block. The higher compressing pressure induce higher thermal conductivity. Meanwhile, the feasibility study of backfill block productivity was reviewed according to the variables of this study. The excessive sand ratio and water contents lead to poor quality that results in the failure of the block. In Korea, the research of backfill is only now in fundamental steps, thus the results of this study are expected to use for setup the experimental conditions of hydraulic and mechanical performance, and can be used for the design of safety function and evaluation of long-term stability for deep geological disposal system.

Kori-1, the nuclear power plants in South Korea, first started operation in April 1978 and was suspended permanently in 2017. The saturation rate time of spent nuclear fuel generated by major nuclear power plants operating in Korea are getting closer. If we fail to dispose spent nuclear fuel, which is equivalent to high-level radioactive waste, the nuclear power plants will have to be shutdown. High-level radioactive waste is permanently disposed through a deep geological disposal system because it contains long-term half-life nuclides and emits high energy. To select the deep geological disposal site and construct the disposal facilities, it is necessary to establish appropriate regulatory policies accordingly. The status of database construction in OECD-NEA, NRC, SITEX, and IAEA, which provides safety regulations for deep geological disposal system, stipulates each requirement for dismantling nuclear power plants. However, details such as specific figures are not specified, and guidelines for the disposal of high-level radioactive wastes are not clearly distinguished. In Korea, the CYPRUS program, an integrated database system, has been developed to support comprehensive performance evaluation for high-level waste disposal. However, due to several difficult situations, maintenance and upgrades have not been performed, so the research results exist only in the form of raw data and the new research results have not been reflected. Other than that, there is no preemptive basis for regulating the deep geological disposal system. With real-time database, we can develop a regulatory system for the domestic deep disposal system by systematically analyzing the regulatory condition and regulatory case data of international organizations and foreign leading countries. The database system processed and stored primary data collected from nuclear safety reports and other related data. In addition, we used relational database and designed table to maximize time and space efficiency. It is provided in the form of a web service so that multiple users can easily find the data they want at the same time. Based on these technologies, this study established a database system by analyzing the legal systems, regulatory standards, and cases of major foreign leading countries such as Sweden, Finland, the United States, and Japan. This database aims to organize data for each safety case component and further prepare a safety regulatory framework for each stage of development of disposal facilities suitable for the domestic environment.

In high-level radioactive waste disposal, a high temperature is generated from the canister containing the waste in the engineered barrier, while groundwater flows into the buffer system from the host rock. The temperature increase and groundwater inflow result in the water phase change and saturation variation. Saturation change is related to the thermal conductivity of buffer material; hence the phase change and saturation strongly interact with the temperature evolution. The complex coupled behavior affects the stability of the whole disposal system, and the security of the repository is critical to human-being life. However, it is difficult to predict the long-term coupled behavior in the disposal system due to the considerable field-test scale, and therefore a numerical simulation is a suitable method having repeatability and cost-effectiveness. DECOVALEX is an international cooperating project for developing numerical methods and models for thermo-hydro-mechanical-chemical (THMC) interaction. DECOVALEX has a four-year cycle with various topics. At the current phase, Task C aims to simulate the full-scale emplacement (FE) experiment performed at Mont Terri underground rock laboratory. Nine research groups are participating in the task, and among them, KAERI simulates the experiment using OGS-FLAC. The simulator combines OpenGeoSys for TH simulation and FLAC3D for M simulation. Through the benchmark simulation, we verified OGS-FLAC for the two-phase flow analysis in the disposal system and finally modeled the FE experiment with a three-dimensional grid. We performed a simple sensitivity analysis to investigate the effect of input parameters on the two-phase flow system and confirmed that the compressibility and permeability affected the flow behavior. We also compared the simulation results to the field data and obtained well-matched results from a series of simulation.

CYPRUS is a web-based waste disposal research comprehensive information management program developed by the Korea Atomic Energy Research Institute over three years from 2004. This program is stored as existing quality assurance documents and data, and the research results can be viewed at any time. In addition, it helps to perform all series of tasks related to the safety evaluation study of the repository in accordance with the quality assurance system. In the future, it is necessary to improve the user convenience by clarifying the relationship between FEP and scenarios and upgrading output functions such as visualization and automatic report generation. This purpose of this study is to research and develop the advanced program of CYPRUS. This study is based on building FEP, DIM and scenario databases. It is necessary to develop an algorithm to analyze and visualize the FEP, DIM and scenario relationship. This project is an integrated information processing platform for DB management and visualization considering user convenience. The first development goal is to build long-term evolutionary FEP, DIM, and scenarios as a database. The linkage by FEP item was designed in consideration of convenience by using a mixed delimiter of letters and numbers. This design provides information on detailed interactions and impacts between FEP items. Scenario data lists a series of events and characteristic change information for performance evaluation in chronological order. In addition, it includes information on FEP occurrence and mutual nutrition by period, and information on whether or not the repository performance is satisfied by item. The second development goal is to realize the relationship analysis and visualization function of FEP and scenario based on network analysis technique. Based on DIM, this function analyzes and visualizes interactions between FEPs in the same way as PID, RES, etc. In addition, this function analyzes FEP and DIM using network analysis technique and visualizes it as a diagram. The developed platform will be used to construct and visualize the FEP DB covering research results in various disposal research fields, to analyze and visualize the relationship between core FEP and scenarios, and finally to construct scenarios and calculation cases that are the evaluation target of the comprehensive performance evaluation model. In addition, it is expected to support the knowledge exchange of experts based on the FEP and scenario integrated information processing platform, and to utilize the platform itself as a part of the knowledge transfer system for knowledge preservation.