Understanding the long-term geochemical evolution of engineered barrier system is crucial for conducting safety assessment in high-level radioactive waste disposal repository. One critical scenario to consider is the intrusion of seawater into the engineered barrier system, which may occur due to global sea level rise. Seawater is characterized by its high ionic strength and abundant dissolved cations, including Na, K, and Mg. When seawater infiltrates an engineered barrier, such dissolved cations displace interlayer cations within the montmorillonite and affect to precipitation/ dissolution of accessory minerals in bentonite buffer. These geochemical reactions change the porewater chemistry of bentonite buffer and influence the reactive transport of radionuclides when it leaked from the canister. In this study, the adaptive process-based total system performance assessment framework (APro), developed by the Korea Atomic Energy Research Institute, was utilized to simulate the geochemical evolution of engineered barrier system resulting from seawater intrusion. Here, the APro simulated the geochemical evolution in bentonite porewater and mineral composition by considering various geochemical reactions such as mineral precipitation/dissolution, temperature, redox processes, cation exchange, and surface complexation mechanisms. The simulation results showed that the seawater intrusion led to the dissolution of gypsum and partial precipitation of calcite, dolomite, and siderite within the engineered barrier system. Additionally, the composition of interlayer cation in montmorillonite was changed, with an increase in Na, K, and Mg and a decrease in Ca, because the concentrations of Na, K, and Mg in seawater were 2-10 times higher than those in the initial bentonite porewater. Further studies will evaluate the geochemical sorption and transport of leaked uranium-238 and iodine-129 by applying TDB-based sorption model.

The mobility of uranium (U) in various disposal environments of a deep geological repository is controlled by various geochemical conditions and parameters. In particular, oxidation state of uranium is considered as a major factor to control the mobility of uranium in most of geological environments. In this study, therefore, we investigated the geochemical behaviors of uranium in grounwater samples from natural analogue study sites located in the Ogcheon Metamorphic Belt (OMB). Groundwater samples were taken using a packer system from Boeun Hoenam-myun site and Geumsan Suyoung-ri site where several boreholes were dilled with various depths. The geochemical properties and parameters such as temperature, pH, Eh, EC, and DO were directly measured in the site using an in-line measurement method. The concentrations of major cations and anions in the groundwater samples were measured by using ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry) and IC (Ion Chromatography), respectively. The concentrations of trace elements including U and Th were measured by using ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) The concentrations of U in the groundwater samples are very low for the Hoenammyun site (0.03~0.69 ppb) and Suyoung-ro site (0.39~1.74 ppb) even though the two sites are uranium deposits and redox conditions are weakly oxidizing. The speciation, saturation index (SI), pH-Eh (Poubaix) diagram were calculated using the Geochemist’s Workbench (GWB 9.0) program and the recent OECD/NEA thermochemical database for U. Calculation results for U speciation in the groundwater samples show that major dissolved uranium species in the groundwater samples are mainly as calcium uranyl carbonate complexes such as Ca2UO2(CO3)3(aq) and CaUO2(CO3)3 2- for almost all groundwater samples. The calculated results for SI and Poubaix diagram also show that the dominant uranium solid phase is a uranyl silicate mineral, uranophane (Ca(H2O)(UVIO2)2 (SiO2)2(OH)6), not uraninite (UIVO2). Since the determination of Eh values for natural groundwater samples is very difficult and uncertain work, we analyzed and discussed the effect of Eh on the geochemical behaviors of U in the groundwater. However, these calculation results are not consistent with the observation for U minerals in rock samples using electron microscopic techniques. Thus, we need further studies to explain the discrepancy between calculation and observation results.

The natural barrier system surrounding the geological repository for high-level radioactive waste plays a crucial role in preventing or delaying the leakage of radionuclides. Therefore, the natural barrier should ensure low permeability to prevent groundwater flow into the engineered barrier system throughout the repository’s lifetime. Crystalline rock, considered as the host rock for the geological repository in Korea, exhibits low intact rock permeability, but the crystalline rock often contains the multiple discontinuities due to its high brittleness that can allow the unexpected fluid flow. Therefore, the long-term hydraulic behavior of the discontinuity should be characterized while considering additional thermal, mechanical, and chemical effects. In comparison to thermal, hydraulic, and mechanical processes, the chemical processes on the discontinuities progress relatively slowly, resulting in limited researches to include these chemical processes. This research introduces mechanisms the involving coupled thermal-hydraulic-mechanicalchemical processes focusing on the rough fracture surfaces and asperities. The chemically-induced changes in mechanical and hydraulic properties are described based on pressure solution and precipitation concepts. A comprehensive review of laboratory tests, field tests, and numerical simulations is conducted related to the chemically-induced coupled processes in fractured rock. Laboratory tests, in particular, concentrate on microscopic changes in fracture asperities induced by pressure solution to analyze chemically-induced aperture changes. The TOUGHREACT, an integral finite difference method program for thermal-hydraulic-chemical simulations, is generally employed to model the chemical response of pressure solution and precipitation on fracture surfaces. The TOUGHREACT includes a module to describe effective porosity and permeability changes based on the modified cubic law, so the real-time change of the fracture permeability can be reflected during the flow simulation. Considering the coupled thermal-hydraulic-mechanicalchemical processes of discontinuity, it becomes evident that the chemical processes under repository conditions (long-term, high temperature, and high pressure) can disturb the hydraulic performance of the natural barrier, so further research is required to characterize the chemically-induced coupled processes for assessing the long-term performance of the natural barrier system.

Solubility and species distributions of radionuclides in domestic groundwater conditions are required for the safety assessment of deep underground disposal system of spent nuclear fuel (SNF). Minor actinides including Am contribute significant extents to the long-term radiotoxicity of SNF. In this study, the solubility of Am was evaluated in synthetic groundwater (Syn-DB3), which were simulated for the groundwater of the DB3 site in the KAERI Underground Research Tunnel (KURT). Geochemical modeling was performed based on the ThermoChimie_11a (2022) thermochemical database from Andra to estimate the solubility and species distributions of Am in the Syn-DB3 condition. Dissolved Am concentrations in the Syn-DB3 were experimentally measured under oversaturation conditions. Am(III) stock solution in perchlorate media was sequentially diluted in Syn-DB3 to prepare 8 μM Am(III) in Syn-DB3. The pH of the solutions was adjusted to be in the range of 6.4–10.5. A portion of the samples was transferred to quartz cells for UV-Vis absorption and time-resolved laser fluorescence spectroscopy studies and the rest were stored in centrifuge tubes. The absorption spectra of the samples were monitored over 70 days and the results suggest that Am colloidal particles were formed initially in all the samples and precipitated rapidly within two days. Over the experimental period of 236 days, small volume (10 μL) of the samples in the centrifuge tubes were periodically withdrawn after centrifugation (18000 rpm, 1 hr) for the liquid scintillation counting to measure the concentrations of Am dissolved in Syn-DB3. In the end of the experiments, pH of the samples was checked again and the final dissolved Am concentrations were determined after ultrafiltration (10 kDa) to exclude the contribution of colloidal particles. In the pH range of 8-9, which is relevant to the KURT-DB3 groundwater condition, the measured dissolved Am(III) concentrations were converged to around 10-8 M. These values are higher than the solubility of AmCO3OH:0.5H2O(s), but lower than that of AmCO3OH(am). There was no indication of transformation of the amorphous phase to the crystalline phase in our observation time window.

In the engineered barrier system of deep geological disposal repository, complex physicochemical phenomena occur throughout the entire disposal time, consequently impacting the safety function. The bentonite buffer, a significant component of the engineered barrier system, can be geochemically altered due to the changes in host rock groundwater, temperature, and redox condition. Such changes may have direct or indirect effects on radionuclide migration in case of canister failure. Therefore, a modeling tool that accounts for coupled thermal-hydraulic-mechanical-chemical (THMC) processes is necessary for the safety assessment. To this end, the Korea Atomic Energy Research Institute (KAERI) has developed the APro, a modeling interface for conducting safety assessment of deep geological disposal repository. The APro considers coupled THMC processes that influence radionuclide migration. Here, the solute transport considering thermal and hydraulic processes are calculated using the COMSOL multi-physics, while geochemical reactions are carried out in PHREEQC. The two software are coupled using a sequential non-iterative operator splitting approach, and transport of non-water H, non-water O, and charge were additionally considered to enhance the coupling model stability. Finally, the applicability of APro to simulate long-term geochemical evolution of bentonite was demonstrated through benchmark studies to evaluate the effects of mineral precipitation/dissolution, temperature, redox, and seawater intrusion.

The mobility of uranium (U) in the environment of a deep geological repository is controlled by various geochemical conditions and parameters. In particular, oxidation state of uranium is considered as a major factor to control the mobility of uranium in most of geological environments. In this study, therefore, we investigated the mobility of uranium in a deep geological repository by a natural analogue approach using a uranium deposit in the Ogcheon Metamorphic Belt (OMB). Uranium contents of rock samples from the study site ranged from 1.3 to 71 ppm (average 17.4 ppm). Uranium minerals found in the study site were mostly uraninite (UIVO2+x) and uranothorite ((UIV, Th)SiO4). The concentrations of U in the groundwater samples were very low (0.025~0.690 ppb) even though redox conditions are weakly oxidizing. Calculation results for U speciation in groundwater samples showed that major dissolved uranium species in the groundwater samples are mainly as calcium uranyl (UO2 2+) carbonate complexes such as Ca2UO2(CO3)3(aq) and CaUO2(CO3)3 2-. However, the activity ratios between 234U and 238U (AR(234U/238U)) showed U behavior in reducing conditions although the groundwater conditions were not reducing conditions and major dissolved U species were U(VI) species. Results from electron microscopic analyses for rock samples showed that major uranium minerals were U(IV) minerals such as uraninite and uranothorite. We could not identify other uranyl minerals and altered minerals from uraninite. This means that the geochemical condition of the study site has been maintained a reducing condition although the groundwater condition was a weakly oxidizing condition. Thus, the dissolution of uranium is strongly limited by the low solubility of uraninite. It is not obvious how the reducing condition of the study site has been maintained. Reducing agents such as pyrite, organic materials, and reducing bacteria might contribute to maintaining the reducing condition although further studies will be necessary. Results from this study imply that uranium mobility will be greatly limited by low dissolution of uraninite into groundwater if the reducing condition is well reserved. This limited mobility of uranium will be also contributed by low possibility of uraninite alteration into uranyl minerals which have a higher solubility than uraninite.

The solubility and species distribution of radionuclides in groundwater are essential data for the safety assessment of deep underground spent nuclear fuel (SNF) disposal systems. Americium is a major radionuclide responsible for the long-term radiotoxicity of SNF. In this study, the solubility of americium compounds was evaluated in synthetic groundwater (Syn- DB3), simulating groundwater from the DB3 site of the KAERI Underground Research Tunnel. Geochemical modeling was performed using the ThermoChimie_11a thermochemical database. Concentration of dissolved Am(III) in Syn-DB3 in the pH range of 6.4–10.5 was experimentally measured under over-saturation conditions by liquid scintillation counting over 70 d. The absorption spectra recorded for the same period suggest that Am(III) colloidal particles formed initially followed by rapid precipitation within 2 d. In the pH range of 7.5–10.5, the concentration of dissolved Am(III) converged to approximately 2×10−7 M over 70 d, which is comparable to that of the amorphous AmCO3OH(am) according to the modeling results. As the samples were aged for 70 d, a slow equilibrium process occurred between the solid and solution phases. There was no indication of transformation of the amorphous phase into the crystalline phase during the observation period.

Disposal methods of radioactive waste can be mainly divided into near-surface disposal and deep geological disposal. If the radioactive waste is exposed to groundwater for a long time in the disposal environment, no matter how the decommissioning waste generated from the nuclear power plant is disposed of, the radionuclides may be released from the disposal site. Decommissioning waste from nuclear power plant contains radionuclides that are harmful to ecosystem including humans. Radionuclides released from disposal site behave in a complex and sensitive manner affected by geochemical conditions such as soil, geological media and groundwater. Sorption is one of the important mechanisms to retard the migration of radionuclides in a subsurface environment. In this study, geochemical properties of groundwater were collected and analyzed in the disposal environment considering disposal method in order to evaluate the geochemical behavior of radionuclides. The formation of aqueous and precipitated species of cesium and cobalt in a disposal condition were calculated and estimated. The sorption properties were also evaluated and predicted by considering the changes in the geochemical conditions such as pH, redox potential and geological media for the safety assessment.

Montmorillonite plays a key role in engineered barrier systems in the high-level radioactive waste repository because of its large sorption capacity and high swelling pressure. However, the sorption capacity of montmorillonite can be largely varied dependent on the surrounding environments. This study conducted the batch simulation for U(VI) sorption on Na-montmorillonite by utilizing the cation exchange and surface complexation coupled (2SP-NE-SC/CE) model and evaluated the effects of physicochemical properties (i.e., pH, temperature, competing cations, U(VI) concentration, and carbonate species) on U(VI) sorption. The simulation demonstrated that the U(VI) sorption was affected by physicochemical properties: the pH and temperature relate to aqueous U(VI) speciation, the competing cations relate to the cation exchange process and selectivity, the U(VI) concentration relates to saturation at sorption sites. For example, the Kd (L kg−1) of Na-montmorillonite represented the largest values of 2.7×105 L kg−1 at neutral pH condition and had significantly decreased at acidic pH<3, showing non-linear and diverse U(VI) sorption at the ranged pH from 2 to 11. Additionally, the U(VI) sorption on montmorillonite significantly decreased in presence of carbonate species. The U(VI) sorption for long-term in actual porewater chemistry and temperature of high-level radioactive waste repository represented that the sorption capacity of Na-montmorillonite was affected by various external properties such as concentration of competing cation, temperature, pH, and carbonate species. These results indicate that geochemical sorption capacity of bentonite should be evaluated by considering both geological and aquifer environments in the high-level radioactive waste repository.

Several previous simulation studies using various geochemical models have been carried out in several major analogue sites. The cases are beneficial when these studies provided the possibility of testing the geochemical models to be used to describe the migration of radionuclides in a future radioactive waste repository system. It was possible to interpret the complex transport behaviour of radionuclides such as uranium and thorium in an environment. We organize major natural analogue study sites from the previous literatures that provided information on the general geochemistry of the sites, in terms of groundwater composition and mineralogy. Also, we calculated aqueous speciation and the solid phases most likely to control their solubilities. The results obtained from the previous studies and this study vary depending on the tools used and on the conceptual models followed. Also, the results differed from the actual measured concentrations of trace metals or radionuclide analogues. The results obtained from these tests identify the main mathematical limitations of available geochemical models. However, the modelling results using a geochemical code with the thermodynamic database simulated well the observed behaviour of radionuclides, especially to identify the dominant processes controlling actinide mobilization and fixation. It was a useful outcome in terms of building confidence on the current geochemical tools to predict the concentrations of radionuclide analogues once the major geochemical characteristics were known. This study allows improving specific aspects of geochemical modelling using major natural analogue sites.

The deep geological repository consisting of a multi-barrier system (engineered and natural barriers) is generally designed to isolate the high-level radioactive waste. The natural barrier is outermost portion to secure safety of the disposal. Crystalline rocks are considered for potential geological repository media to retard and inhibit the migration of radionuclides when the radionuclides leak from the canister and break through the engineered barrier. Sorption and diffusion processes play a major role in retardation of the radionuclides in deep underground environment. In order to evaluate the migration of radionuclides in the safety assessment or geochemical modelling, distribution coefficient and diffusivity of radionuclides are required as input data. In this study, we performed mineralogical and geochemical analysis for a crystalline rock (e.g., granite) to use the sorption and diffusion experiment. The fresh rock samples are obtained from a deep core samples (DB-2) drilled up to 1 km from the surface at KURT (KAERI Underground Research Tunnel) site. For the optical and microscopic examination, thin sections of the rock sample were provided. The rock samples were crushed into powder size to analyze major and trace elements of the whole-rock aliquots. The powdered specimens also used for mineral identification and measurement of specific surface area. The major constituent minerals of the granite are plagioclase, quartz, and K-feldspar and the minor minerals are phlogopite, biotite, and chlorite. According to the results of geochemical analysis, the granite specimens generally contain more than 70wt% of SiO2 and 8wt% of total alkali oxides (Na2O + K2O). The trace elements normalized to primitive mantle compositions show positive Cs, Rb, U, K, and Pb anomalies and negative Nb and Ti anomalies. The rock samples have an average density of 2.62 g·cm−3 and an average porosity of 0.222%. The crushed samples represent the specific surface area of 0.2087 m2·g−1 for the 75–150 μm fraction and 0.1616 m2·g−1 for the 150–300 μm fraction by BET method, respectively. The granite specimens will be used for the sorption and diffusion experiments to evaluate the radionuclides’ geochemical behaviors. The mineralogical and geochemical properties provided in this study can be useful in understanding the sorption and diffusion processes of significant radionuclides under the geological disposal environments.

백두산 성층화산체와 천지 칼데라 외륜산 정상부에 분포하는 홀로세에 분화한 규장질 화산암 시료와 플라이스 토세 개마용암대지와 장백산순상화산체의 고철질 화산암 시료의 암석화학적 특징 분석을 통해 지체구조적 위치를 알아 보았다. 백두산 화산지대에서 초기 고철질 분출물들은 개마용암대지와 장백산순상화산체를 형성하였으며, 대부분 알칼 리계열의 현무암에서 조면현무암 또는 서브-알칼리(쏠레이아이트) 현무암에서 현무암질안산암의 성분이며, 백두산 성층 화산체와 천지 칼데라 정상부 부근의 홀로세 분출물은 대부분 규장질의 조면암에서 유문암 성분이다. 고철질 화산암류 와 규장질 화산암류 사이의 SiO2 54-62 wt.%가 결핍된 쌍모식 조성을 나타낸다. 이는 마그마작용이 지각내 신장형 지 체구조적 위치에서 발생하였음을 지시하는 단서가 될 수 있다. 지구조판별도에서 고철질 화산암류들은 판내부 또는 판 내부 알칼리암과 쏠레이아이트암의 영역에 구분되어 도시되는데 이는 TAS성분도의 결과와 잘 일치한다. 규장질 화산암 류들은 규장질 화강암류에 적용하는 판별도에서 판내부화강암(WPG)의 지체구조적 위치에 도시된다. 지구조 판별도에서 판의 섭입과 관련한 도호 또는 대륙연변호의 영역에는 도시되지 않으며, 모두 판내부 영역에 도시된다. 미량원소 함량 을 원시맨틀값으로 표준화한 거미도에서 섭입대 화산암류에서 특징적으로 나타나는 Nb, Ti 의 부(−) 이상을 나타내지 않으며, OIB와 유사한 패턴을 나타낸다. 미량원소 함량 조성은 섭입대에서 유래된 마그마 작용에 연관된 의미있는 증 거를 나타내지 않는다. 이는 백두산화산지대의 마그마작용이 판내부 환경에서 있었음을 지시한다. 이들 화산암류의 판 내부 지체구조 위치는 이 지역에서 발생하는 천발지진의 진원 깊이와도 조화적이다. 백두산화산지대의 화산암석들은 신 생대 동안 맨틀 물질의 용승에 의한 판내부 화산활동의 결과로 해석된다.

고준위방사성폐기물 심지층 처분 대상 암종으로 고려되는 화강암에서 방사성핵종의 장기 거동특성을 이해하기 위한 연구의 일환으로 KURT (KAERI Underground Research Tunnel) 화강암에 존재하는 우라늄의 용출특성에 대한 연구를 수행하였다. 반응 시작 후부터 10일 동안의 반응기간 중 다른 반응용액에 비해 CO3 2- 농도가 높은 UD-CO3 및 UD-Bg 반응용액에서 우라늄의 용출량이 다소 급격하게 증가하였다. 또한 Na 또는 Ca가 다량 함유된 반응용액에서 반응 60일 이후 우라늄 용출량이 다소 급격히 증가하였다. 각 반응용액에 의한 반응 270일까지의 우라늄의 용출량은 UD-CO3 (44.61 μg·L-1), UD-Bg (41.01 μg·L-1), UD-Na (26.87 μg·L-1), UD-Ca (20.26 μg·L-1), UD-CaSi (17.03 μg·L-1), UD-Si (10.47 μg·L-1)으로 지속적으 로 증가 하였으나, 반응 270일 이후 우라늄 용출량은 점차 감소하는 경향을 나타낸다. 이는 화강암 시료 내에 존재하는 우라늄이 반응용액과 상호반응에 의해 최대 용출될 수 있는 한계에 도달하였기 때문으로 판단된다. 우라늄 용출은 혼합된 반응 용액 내의 CO3 2- 존재와 수질의 지화학적 유형에 따라 우라늄의 용출 농도 및 용출 최대치가 나타나는 시점이 다르게 확인되 었다. 이는 시료와 반응용액의 상호반응 과정에서 용존이온의 영향에 의해 화강암시료와 반응용액 사이에 반응속도의 차이가 발생하는 것으로 판단된다.