In the evaluation of the stability of radioactive waste disposal, it is imperative to take into account the concept of the redox front. Initially, this front is typically observed near the surface. However, if the hydraulic gradient increases due to the construction of a disposal facility, the redox front can potentially transport deeper into the geological environment through groundwater flow. This transport triggers changes in the geochemical characteristics, potentially diminishing the natural buffering capacity of the bedrock. Consequently, it is necessary to characterize both the unsaturated and saturated zones in the disposal site. In this context, a tracer test is a useful method to identify the characteristics of the site from the surface to the deep geological environment where the disposal facility can be located. Therefore, this study also aims to establish a methodology enabling a comprehensive understanding of the hydrogeochemical characteristics through the tracer test that can be applied to future sites for research URL (Underground Research Laboratory) or radioactive waste disposal in Korea. For the tracer test, a UNIT (UNsaturated zone Insitu Test facility) was built within the KAERI and five wells with a depth of 24 m were installed in 2022. Before conducting the test, to determine the geochemical background characteristics of the site, topsoil and soils at depths of 30 cm, 60 cm, and 90 cm were collected. Additionally, a groundwater sample was obtained from the newly installed well. Soil samples were analyzed for soil texture, moisture content, total and exchange cations, anions, and heavy metals. Similarly, the groundwater sample was analyzed for cations, anions, and trace elements. The outcomes of these comprehensive analyses will serve as the baseline values in the hydrogeochemical changes after the tracer test. This includes changes in soil composition, water quality, precipitation/dissolution processes, and mineral phases. Furthermore, these results will be provided as input parameters for surface-underground interface models in future studies.

Long-term evolution of the surface environments can affect the safety of deep geological disposal. Therefore, it is important to understand the water balance components constituting the water cycle among atmosphere, surface, and subsurface. In Finand, the surface and near-surface hydrological model (SHYD) was developed to calculate the water balance of Olkiluoto Island. Through the intensive site investigations, the data sets as input for the site scale model in present-day conditions have been collected such as transpiration and meteorological data. In this study, weighing lysimeter method was selected to quantify small-scale soil water balance of the vadose zone in the UNsaturated zone In-situ Test facility (UNIT) around KAERI Underground Research Tunnel. Hydrological components such as precipitation, evapotranspiration (ET) and leachate were derived from water balance analysis on the lysimeter measurements in UNIT. Among the hydrological components, actual ET accounts for more than 50% of the annual precipitaion, and thus plays an important role on predicting the hydrological evolution in the future. In this context, actual ET measured from the weighing lysimeter was compared with potential ET estimated from meteorological data using FAO-56 Penman-Monteith method.

Surface environmental factors such as climate change can affect the safety of the disposal system by changing groundwater recharge or flow. Therefore, it is important to identify surface environmental factors and hydrogeological factors to evaluate long-term changes in hydrogeological environment of a disposal system. In particular, evapotranspiration is an important to be considered because it loses 70% of rainfall and has a great effect on groundwater recharge. Evapotranspiration can be estimated using simple or complex models based on meteorological data. Meteorological data from January 2010 to December 2022 were collected from 44 Automatic Synoptic Observation Systems (ASOS) of the Korea Meteorological Administration (KMA), which observe factors necessary for calculating evapotranspiration. For the estimation of evapotranspiration through simple models, temperature-based models (Blaney-Criddle method, modified Blaney-Criddle method, Hargreaves-Samani method) and radiation-based models (Simple Abtew method, Makkink method, Prietley-Taylor method, Turc method, Solar radiation-Maximum temperature method) were used. The calculation of evapotranspiration through the complex model used the Penman-Monteith method, which is used as a standard model in the USA, Japan, and FAO. By comparing the evapotranspiration calculated by complex and simple model, methods with small errors were identified each region. In addition, long-term climate change scenarios were applied to confirm changes in long-term evapotranspiration in South Korea. The results of this study will be used to find alternative models in the case of missing data in the Penman-Monteith model, which requires a lot of meteorological data, and can be used as basic data for calculating groundwater recharge that can affect the disposal system in the future.

The deep geologic repository (DGR) concept is widely accepted as the most feasible option for the final disposal of spent nuclear fuels. In this concept, a series of engineered and natural barrier systems are combined to safely store spent nuclear fuel and to isolate it from the biosphere for a practically indefinite period of time. Due to the extremely long lifetime of the DGR, the performance of the DGR replies especially on the natural geologic barriers. Assessing the safety of the DGR is thus required to evaluate the impacts of a wide range of geological, hydrogeological, and physicochemical processes including rare geological events as well as present water cycles and deep groundwater flow systems. Due to the time scale and the complexity of the physicochemical processes and geologic media involved, the numerical models used for safety evaluation need to be comprehensive, robust, and efficient. This study describes the development of an accessible, transparent, and extensible integrated hydrologic models (IHM) which can be approved with confidence by the regulators as well as scientific community and thus suitable for current and future safety assessment of the DGR systems. The IHM under development can currently simulate overland flow, groundwater flow, near surface evapotranspiration in a modular manner. The IHM can also be considered as a framework as it can easily accommodate additional processes and requirements for the future as it is necessary. The IHM is capable of handling the atmospheric, land surface, and subsurface processes for simultaneously analyzing the regional groundwater driving force and deep subsurface flow, and repository scale safety features, providing an ultimate basis for seamless safety assessment in the DGR program. The applicability of the IHM to the DGR safety assessment is demonstrated using illustrative examples.

The change of surface environments (e.g., climate change, uplift/subsidence, and erosion) can undermine the long-term safety of a high-level radioactive waste repository. Therefore, understanding the water cycle between atmosphere, surface, and subsurface is essential to ensure the long-term safety of deep geological disposal and consequently to gain public acceptance for the repository. Among hydrologic components (e.g., precipitation, interception, runoff, infiltration, evapotranspiration (ET), and recharge) which constitute the water cycle, ET is more than half of the total precipitation and plays a crucial role in the water and energy transfer among the three systems. Although various methods for ET evaluation (e.g., Bowen Ratio, Eddy Covariance, Optical Scintillation, and Weighing Lysimeter methods) have been developed, many influential factors such as vegetation, climate, and moisture content make its accurate evaluation still tricky. In this work, we chose weighing lysimeter and Penman-Monteith methods for direct/indirect estimation of ET, and installed a smart field lysimeter and a micro-meteorological station around KAERI Underground Research Tunnel. Water balance in the unsaturated zone and five climatic variables (air temperature, humidity, precipitation, radiation, and wind speed/direction) were measured more than once per 10 minutes for six months from April to September, 2022. From the measurements, daily actual and potential ET values at the study site were calculated and compared. We also discussed the applicability and limitation of current methods and ET assessments at different spatial scales regarding verifying and validating the developing numerical models.