The buffer is installed around the disposal canister, subjected to heating due to decay heat while simultaneously experiencing expansion influenced by groundwater inflow from the surrounding rock. The engineering barrier system for deep geological disposal require the evaluation of longterm evolution based on the verification of individual component performance and the interactions among components within the disposal environment. Thus, it is crucial to identify the thermalhydro- mechanical-chemical (THMC) processes of the buffer and assess its long- and short-term stability based on these interactions. Therefore, we conducted experimental evaluations of saturationswelling, dry heating, gas transport, and mineralogical alterations that the buffer may undergo in the heated-hydration environment. We simulated a 310 mm-thick buffer material in a cylindrical form, simulating the domestic disposal system concept of KRS+ (the improved KAERI reference disposal system for spent nuclear fuel), and subjected it to the disposal environment using heating cartridges and a hydration system. To monitor the thermal-hydro-mechanical behavior within the buffer material, load cells were installed in the hydration section, and both of thermal couples and relative humidity sensors were placed at regular intervals from the heat source. After 140 days of heating and hydration, we dismantled the experimental cell and conducted post-mortem analyses of the samples. In this post-mortem analysis, we performed functions of distance from the water contents, heat source, wet density, dry density, saturation, and X-Ray diffraction analysis (XRD). The results showed that after 140 days in the heated-hydration environment, the samples exhibited a significant decrease water contents and saturation near the heat source, along with very low wet and dry densities. XRD Quantitative Analysis did not indicate mineralogical changes. The findings from this study are expected to be useful for input parameters and THMC interaction assessments for the long-term stability evaluation of buffer in deep geological disposal.

Backfill is one of the key elements of deep geological disposal. The backfill material is used to fill disposal tunnels and is mainly composed of swellable clay, preventing the migration of nuclide and structurally supporting the tunnel. The selection and application of backfill material are critical for the stable and efficient disposal of spent fuel. Therefore, it is essential to secure various candidate materials for backfill and to comprehensively understand the properties and behavior of these materials. Recently, the Korea Atomic Energy Research Institute has selected a candidate material called Bentonil-WRK and is evaluating its applicability. To utilize this material as backfill, the safety function of a mixed backfill concept, consisting of sand and Bentonil-WRK, was assessed. The swelling pressure was measured as a function of dry density for a bentonite/silica sand mix ratio of 3/7. The results showed that the swelling pressure ranged from 0.15 to 0.273 MPa, depending on the dry density, with higher dry densities resulting in higher swelling pressures. The measured swelling pressure met the target performance criteria suggested by SKB and Posiva (i. e., 0.1 MPa), but did not meet the design requirement for swelling pressure (i. e., 1 MPa). This indicate the need for further research after increasing the mass fraction of bentonite (e. g., mix ratio 4/6 or more). The results of this study are expected to be used in the selection of candidate backfill materials and the establishment of design guidelines for engineered barrier backfill.

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 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.

Expansive clays (for examples, bentonites) are favored as buffer and backfill materials because of their low hydraulic conductivity, high swelling potential, and good mechanical properties, and are installed in highly compacted blocks in repositories. Compacted expansive clays have a dual-structure system: macrostructural system which is a complex of clay aggregates with the inter-aggregate pores (macropores) which can be filled by either liquids or gases; microstructural system with the intraaggregate pores between or within clay particles (micropores) which is usually considered to be saturated by liquid. Understanding the dual-strucure system of expansive clays is essential for characterizing and modeling multiphysics (stress-strain, swelling pressure, etc.) in buffers and backfills. Existing multiphysics studies of expansive clays, as in non-expansive soils, were mostly conducted with a single structure approach based on the behavior of macropores, and there have been limitations in the comprehensive interpretation and modeling of experimental results. However, with the recent development of measurement techniques, a lot of available information on the pore structure of compacted expansive clays has been reported, and with the results, a dual-structure approach considering both microstructural and macrostructural systems has been increasingly applied to improve the modeling of multiphysics of expansive clays. This study reviewed the dual-structure system of compacted expansive clays, analyzed previous studies on its evolution according to hydromechanical loading (loading-unloading and wetting-drying paths), and based on these, intended to provide technical knowledge and information needed for multiphysics research of expansive clays-based buffer and backfill for the KRS repository.

The backfill close the deep geological disposal system by filling the disposal tunnel and the connecting tunnel after the installation of buffer in the disposal hole. SKB and Posiva have established and designed the safety function of the backfill for the common goal of the deep geological disposal system. The safety function of backfill material has been set hydraulic conductivity of less than 10−10 m·s−1, a swelling pressure of 0.2 MPa, a compressive modulus of 10 MPa or a buffer density of 1,950 kg·m−3 or more, and freezing resistance. For the selection of the optimum backfill material, SKB and Posiva developed the concept of the backfill and evaluated the candidate that satisfies the requirements in four steps. In the first step, the performance and function that the backfill material should have were conceptualized. For the second step, laboratory tests and in-depth analysis of the candidate material properties were conducted. At this step, the focus has been on testing with the concept of the block method, using key candidate materials. In step 3, laboratory and large-scale experiments were performed to test engineering feasibility. In addition, design specifications for backfill materials were set based on site conditions, installation methods, and short- and long-term functions of materials. In Korea, it is only now in the step of selecting the concepts of the safety function. Therefore, it is necessary to benchmark the development process based on the previous studies of SKB and Posiva. In this study, candidate materials, experimental methods, and results were analyzed. As a result, the research steps and conditions for the selection of the optimum backfill material were reviewed. Using this study, the research steps of domestic backfill was suggested to develop within a short time for the Korean deep geological disposal system.