Bentonite, primarily composed of montmorillonite, plays a crucial role as one of the engineering barrier materials in a deep geological repository (DGR). The advantageous properties of montmorillonite, such as its swelling capacity, low permeability, and low thermal conductivity, make it a key component as a buffer material for isolating high-level radioactive waste from the surrounding natural environment. It has been known that the stability of montmorillonite is influenced by a wide range of pressure-temperature-composition (P-T-X) conditions relevant to the DGR environment. When considering potential geological events, of notable concerns are its interactions with groundwater or seawater at elevated temperatures, leading to safety hazards within the system. In this study, therefore, we investigated the hydration and dehydration reactions of Ca-montmorillonite with saline fluids such as NaCl and KCl solutions at elevated pressures and temperatures by conducting in-situ X-ray diffraction experiments using both a capillary sample heating cell and a resistive-heated diamond anvil cell. As a result, we observed different hydration states of montmorillonite depending on the chemical composition of fluids, i.e., tri-hydrated layers in NaCl and bi-hydrated layers in KCl solutions, respectively. Furthermore, we identified that montmorillonite undergoes distinct stepwise dehydration with increasing temperature, and the dehydration temperature of montmorillonite significantly increases with increasing water pressure. Consequently, this study would provide insights into the stability of hydrated montmorillonite in a seawater-dominated fluid environment and its implications for the long-term safety of the disposal system.

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.

This study aimed to provide better understanding of the bedrock aquifer bacterial communities and their functions in deep geological repository (DGR) environment. Two study sites of uranium deposits in the Ogcheon Metamorphic Belt were selected: Boeun and Guemsan. From two study sites, six groundwater samples were obtained with different boreholes and depths: OB1 (Boeun, 25 m), OB3 (Boeun, 80 m), GS1 (Guemsan, 25 m), GS2 (Guemsan, 85-90 m), GS3-I (Guemsan, 32- 38 m), GS3-II (Guemsan, 70-74 m). The physicochemical properties of groundwater were analyzed by multi-parameter sensors, ion chromatography (IC), and inductively coupled plasma optical emission spectroscopy (ICP-OES). Illumina Miseq sequencing was performed to investigate bacterial community in six groundwater samples. In addition, the number of sulfate-reducing bacteria (SRB) was quantified by a quantitative PCR (qPCR). Bacterial community composition varied in response to boreholes and depths. A total of 14 different phyla and 36 classes were detected from six groundwater samples. Overall, Proteobacteria, Actinomycetota, and Bacteroidota were dominant in the phylum level. SRB and iron-reducing bacteria (IRB) were detected in all groundwater samples even though organic carbon sources were not abundant (0.7-3.3 mg-total organic carbon/L). This result shows a potential to immobilize uranium in DGR environment. In particular, SRB, Desulfosporosinus fructosivorans and Humidesulfovibrio mexicanus were mainly detected in GS1 and GS2 groundwater samples, which attributed to higher dissimilatory sulfite reductase functional gene copy number in GS1 and GS2 groundwater samples. Statistical analysis was performed to understand the correlation between environmental factors and core bacterial species. Dissolved oxygen (DO), Fe, and Mn concentrations were positively correlated with Curvibacter fontanus while Undibacterium rivi had a negative correlation with pH. These results indicate that bacterial community could be changed in response to environmental variation. Further study with a greater number of samples is necessary to obtain statistically reliable and meaningful results for a safe DGR system.

Raman characteristics of various minerals constituting natural rocks collected from uranium deposits in Okcheon metamorphic zone in Korea are presented. Micro-Raman spectra were measured using a confocal Raman microscope (Renishaw in Via Basis). The focal length of the spectrometer was 250 mm, and a 1800 lines/mm grating was installed. The outlet of the spectrometer was equipped with a CCD (1,024256 pixel) operating at -70°C. Three objective lenses were installed, and each magnification was 10, 50, and 100 times. The diameter of the laser beam passing through the objective lens and incident on the sample surface was approximately 2 m. The laser beam power at 532 nm was 1.6 mW on the sample surface. Raman signal scattered backward from the sample surface was transmitted to the spectrometer through the same objective lens. To accurately determine the Raman peak position of the sample, a Raman peak at 520.5 cm-1 measured on a silicon wafer was used as a reference position. Since quartz, calcite, and muscovite minerals are widely distributed throughout the rock, it is easy to observe with an optical microscope, so there is no difficulty in measuring the Raman spectrum. However, it is difficult to identify the uraninite scattered in micrometer sizes only with a Raman microscope. In this case, the location of uraninite was first confirmed using SEM-EDS, and then the sample was transferred to the Raman microscope to measure the Raman spectrum. In particular, a qualitative analysis of the oxidation and lattice conditions of natural uraninite was attempted by comparing the Raman properties of a micrometer-sized natural uraninite and a laboratory-synthesized UO2 pellet. Significantly different T2g/2LO Raman intensity ratio was observed in the two samples, which indicates that there are defects in the lattice structure of natural uraninite. In addition, no uranyl mineral phases were observed due to the deterioration of natural uraninite. This result suggests that the uranium deposit is maintained in a reduced state. Rutile is also scattered in micrometer-sizes, similar to uraninite. The Raman spectrum of rutile is similar in shape to that of uraninite, making them confused. The Raman spectral differences between these two minerals were compared in detail.

Uranium (U) is a hazardous material that can lead to both chemical and radiological toxicity, including kidney damage and health issues associated with radiation exposure. In South Korea. In Korea, where shallow weathered granitic aquifers are widespread, several previous studies have reported high levels of radioactivity in shallow groundwater. This ultimately led to the closure of 60 out of 4,140 groundwater production wells in South Korea. In this study, we examined aquifers currently dedicated to drinking water supply and investigated a dataset of 11,225 records encompassing 103 environmental parameters, based on the random forest classifier. This dataset comprises 80 physical parameters associated with the hydraulic system and 23 chemical parameters linked to water-rock interactions. Among the hydraulic parameters, the presence of a coarse loamy texture in the subsoil displayed a notable positive relationship with the concentration of uranium, implying that it plays a significant role in forming redox conditions for the leaching of uranium from host rocks. Fluorine (F), a major product of water-rock interaction in granitic aquifers, exhibited a positive correlation with the distribution of uranium concentrations. The positive relationship between F concentration and uranium levels suggests that the dissolved uranium originates from groundwater interacting with granites. In conclusion, our findings indicate that two key factors, namely the infiltration capacity of soil layers and the aqueous speciation in groundwater resulting from interactions with local solids, play important roles in determining uranium concentrations in granitic aquifers.

The bioreduction process from soluble U(VI) to insoluble U(IV) has been extensively studied in the field of radionuclides migration. Since acetic acid (AcOH) is widely used as an electron donor for bioreduction of U(VI), it is necessary to understand the effect of U(VI)-AcOH complexes that exist in different species depending on pH on this process. Changes in samples before and after bioreduction can be compared using time-resolved laser luminescence spectroscopy (TRLLS), which measures the characteristic luminescence spectra of different U(VI) species. Although luminescence properties of U(VI)-AcOH species were reported, experiments were conducted under conditions below pH 4.5. In this study, spectrophotometry and TRLLS for U(VI)-AcOH species (10−100 μM U(VI) and 20 mM AcOH) were performed in pH ranges extending to neutral and alkaline pH regions similar to groundwater conditions as well as acidic pH region. Two different complexes (UO2(AcO)+, UO2(AcO)2 with U(VI) and acetate ratios of 1:1, 1:2) were observed in the acidic pH region. The 1:1 complex, which appears as the pH increases, has no luminescence properties, but its presence can be confirmed because it serves to reduce the luminescence intensity of UO2 2+. In contrast, the 1:2 complex exhibits distinct luminescence properties that distinguish it from UO2 2+. The 1:3 complex (UO2(AcO)3 -) expected to appear with increasing pH was not observed. Two different complexes ((UO2)3(OH)5 +, (UO2)3(OH)7 - with U(VI) and OH ratios of 3:5, 3:7) were observed as the major species in the neutral pH region, but their luminescence lifetimes are remarkably short compared those in the absence of AcOH. Solid U(VI) particles were observed in the alkaline pH region, and they also had completely different luminescence properties from the aforementioned U(VI)-AcOH and U(VI)-hydrolysis species. Based on these results, the effect of pH in the presence of AcOH on the bioreduction process from U(VI) to U(IV) will be discussed.

Bacterial metabolisms influence the behavior of uranium (U) in deep geological repository (DGR) system because bacteria are ubiquitous in the natural environment. Nevertheless, most studies for the U(VI) bioreduction have focused on a few model bacterium, such as Shewanella putrefaciens, Desulfovibrio desulfuricans, and Geobacter sulfurreducens. In this study, the potential of aqueous U(VI) ((U(VI)aq) reduction by indigenous bacteria was examined under anaerobic conditions with addition of 20 mM sodium acetate for 24 weeks. Three different indigenous bacterial communities obtained from granitic groundwater at depths of 44–60 m (S1), 92–116 m (S2), and 234–244 m (S3) were applied for U(VI)aq reduction experiments. The S2 groundwater contained the highest U concentration of 885.4 μg/L among three groundwater samples, where U mainly existed in the form of Ca2UO2(CO3)3(aq). The S2 groundwater amended 20 mM of sodium acetate was used for the U(VI)aq bioreduction experiment. Variations in the U(VI)aq concentration and redox potential were monitored for 24 weeks to compare U(VI)aq removal efficiency in response to indigenous bacteria. The U(VI)aq removal efficiencies varied among three indigenous bacteria: 57.8% (S3), 43.1% (S2), and 37.7% (S1). The presence of the thermodynamically stable uranyl carbonate complex resulted in the incomplete U(VI)aq removal. Significant shifts in indigenous bacterial communities were observed through highthroughput 16S rRNA gene sequencing analysis. Two SRB species, Thermodesulfovibrio yellowstonii and Desulfatirhabdium butyrativorans, were dominant in the S3 sample after the anaerobic reaction, which enhanced the bioreduction of U(VI)aq. The precipitates produced by bacterial activity were determined to be U(IV)-silicate nanoparticles by a transmission electron microscope (TEM)-energy dispersive spectroscope (EDS) analysis. These results demonstrated that considerable U immobilization is possible by stimulating the activity of indigenous bacteria in the DGR environment.

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.

Uranium inventory in Boeun aquifer is facing the artificial reservoir that intended for supplying water to nearby cities (40-70 m apart) where, toxic radionuclides might mobile and enter the reservoir. In order to understand U mobility in the system, groundwater and fracture filling materials (FFMs) were analyzed for microbial signatures, C, O, Fe, S and U-series isotopes. The δ18O-H2O and 14C signatures suggested groundwater was originated from upland recharges dominantly and not affected by mixing with the surface water. However, the 234U/238U activity ratios (ARs) and 230Th/234U ARs in FFMs ranged from 0.93 to 1.67 and from 0.22 to 1.97, respectively, indicating that U was mobile along the fractures. In shallow FFMs, the U accumulations (~157 mg/kg) were found with Fe enrichments (~226798 mg/kg) and anomalies of δ56Fe and δ57Fe, implied U mobility in shallow depths was associated with Fe-rich environment. Also, in the shallow depths, Fe-oxidizers, Gallionella was prevailing in groundwater while Acidovorax was abundant near U ore depth. The Fe-rich environment in shallow depths was formed by pyrite dissolution, demonstrated using δ34S-SO4 and δ18O-SO4 distribution. Conclusively, the Fe-rich aquifer was capable of immobilizing the dissolved U through biotic and abiotic processes, without significant discharge into the nearby reservoir.

Several countries have been operating radioactive waste disposal (RWD) programs to construct their own repositories and have used natural analogues (NA) studies directly or indirectly to ensure the reliability of the long-term safety of deep geological disposal (DGD) systems. A DGD system in Korea has been under development, and for this purpose a generic NA study is necessary. The Korea Atomic Energy Research Institute has just launched the first national NA R&D program in Korea to identify the role of NA studies and to support the safety case in the RWD program. In this article, we review some cases of NA studies carried out in advanced countries considering crystalline rocks as candidate host rocks for high-level radioactive waste disposal. We examine the differences among these case studies and their roles in reflecting each country’s disposal repository design. The legal basis and roadmap for NA studies in each country are also described. However because the results of this analysis depend upon different environmental conditions, they can be only used as important data for establishing various research strategies to strengthen the NA study environment for domestic disposal system research in Korea.

Deep geological disposal is generally accepted to be the most practical approach to handling radioactive wastes. Bentonite has been considered as a buffer material in deep geological disposal repositories (DGR) for high-level radioactive wastes. Evaluating the effect of short-term bentonite alteration on EBS performance has limitations in safety assessment over thousands of years. Information on bentonite characteristics under various conditions obtained from natural systems can be used to evaluate long-term safety of bentonite buffer. The purpose of this study was to investigate mineralogical and physicochemical characteristics of bentonite in the Naah mine located in Yangnam-myeon, Gyeongju-si for a natural analogue of the bentonite barrier in DGR. A total of 15 samples were collected at regular intervals from the bentonite layer and andesitic lapilli tuff (i.e., parent rock) at the boundary with the bentonite layer. The bentonite layer is located at a depth of about 1 m below the ground surface. Each sample was separated into particles < < 75 μm and particles < 2 μm through grinding and sedimentation processes. The separated subsamples were characterized mineralogically and physiochemically using various analytic techniques. Bentonite samples have a similar SiO2/Al2O3 ratio to the parent rock and a lower (Na+K)/Si ratio than the parent rock, indicating depletion of alkali components during bentonitization. The parent rock and bentonite samples have similar mineral composition (i.e., quartz, feldspars, opal-cristobalite-tridymite and montmorillonite). Results of XRD analysis on the randomly distributed particles < 2 μm indicate that bentonite is mostly composed of Ca-montmorillonite, which is a typical dioctahedral smectite. Results of FTIR and VNIR analysis indicate that montmorillonite contained in bentonite is Al-dioctahedral montmorillonite, and Al is substituted with Mg in some octahedron units. The mineralogical and physicochemical characteristics are similar regardless of sampling location. These results suggest that bentonite potentially exposed to weathering, located near the ground surface, has hardly altered.

In south Korea, most of uranium deposits are distributed in the Ogcheon belt, which is one of two late Precambrian to Paleozoic fold belts (the Imjingang and Ogcheon belts). A study site of the Ogcheon metamorphic belt (OMB) in Hoenam-myun, Boeun-gun was selected for the natural analogue study by preliminary site investigation for several candidate study sites. Three boreholes were drilled in the site and some rock cores and groundwater samples were taken from the boreholes. Various analytical studies for the samples are now being performed. Thus, in this study, various basic characteristics of the study site such as occurrence, geological, mineralogical, and chemical properties were investigated for a future study. Base rocks containing uranium in the OMB are usually black slate and coaly slate. Coaly slate usually shows a higher content of uranium and larger grain size of uranium than black slate. Uranium minerals found in the OMB are uraninite, uranothorite, brannerite, ekanite, coffinite, francevillite, uranophane, autunite, and torbernite depending on the base rock types. Uranothorite is abundant in black slate whereas uraninite is mostly abundant in coaly slate. Chemical compositions of the solid and groundwater samples from the study site were also analyzed by using ICP-MS/OES (Inductively Coupled Plasma Mass Spectrometry) and XRF (X-ray Fluorescence). This will contribute to determine uranium minerals in the solid samples and uranium speciation in the groundwater. The results of this study will contribute to performing future natural analogue studies in domestic uranium deposits and provide basic information and knowledge for understanding long-term geochemical behaviors of radionuclides in a high-level radioactive repository.

Spent nuclear fuel (SNF) is the main source of high-level radioactive wastes (HLWs), which contains approximately 96% of uranium (U). For the safe disposal of the HLWs, the SNF is packed in canisters of cast iron and copper, and then is emplaced within 500 m of host rock surrounded by compacted bentonite clay buffer for at least 100,000 years. However, in case of the failure of the multi-barrier disposal system, U might be migrated through the rock fractures and groundwater, eventually, it could reach to the biosphere. Since the dissolved U interacts with indigenous bacteria under natural and engineered environments over the long storage periods of geologic disposal, it is important to understand the characteristics of U-microbe interactions under the geochemical conditions. In particular, a few of bacteria, i.e., sulfate-reducing bacteria (SRB), are able to reduce soluble U(VI) into insoluble U(IV) under anaerobic conditions by using their metabolisms, resulting in the immobilization of U. In this study, the aqueous U(VI) removal performance and change in bacterial community in response to the indigenous bacteria were investigated to understand the interactions of U-microbe under anaerobic conditions. Three different indigenous bacteria obtained from different depths of granitic groundwater (S1: 44–60 m, S2: 92–116 m, and S3: 234–244 m) were used for the reduction of U(VI)aq. After the anaerobic reaction of 24 weeks, the changes in bacterial community structure in response to the seeding indigenous bacteria were observed by high-throughput 16S rDNA gene sequencing analysis. The highest uranium removal efficiency of 57.8% was obtained in S3 sample, and followed by S2 (43.1%) and S1 (37.7%). Interestingly, SRB capable of reducing U(VI) greatly increased from 4.8% to 44.1% in S3 sample. Among the SRB identified, Thermodesulfovibrio yellowstonii played a key role on the removal of U(VI)aq. Transmission electron microscopy (TEM) analysis showed that the dspacing of precipitates observed in this study was identical with that of uraninite (UO2). This study presents the potential of U(VI)aq removal by indigenous bacteria under deep geological environment.

Time-resolved laser fluorescence spectroscopy (TRLFS) and excitation-emission matrix (EEM) spectroscopy were used to study the interaction of U(VI) and natural organic matters (NOMs) in groundwater. Various types of groundwaters (DB-1, DB-3 from KURT site and OB-1, OB-3 from a U deposit in Ogcheon metamorphic belt) were used as samples. Pulsed Nd-YAG laser at 266 nm (Continuum Minilite) was used as the light source of TRLFS. The laser pulse energy of 1.0 mJ was fixed for all measurements. The luminescence spectrum was recorded using a gated intensified chargecoupled device (Andor, DH-720/18U03 iStar 720D) attached to the spectrograph (Andor, SR-303i). EEM spectra were measured using a spectrofluorometer (Horiba Scientific, Aqualog) equipped with a 150 W ozone-free xenon arc lamp. Excitation spectra were recorded by scanning the excitation wavelength from 200 to 500 nm. Emission spectra were measured using a CCD in the wavelength range of 242–823 nm. In the case of the recently collected DB-1 samples, it was observed that the U and NOM quantities decreased compared to the samples collected before 2016. For some DB-1 samples, the amount of dissolved organic carbon indicating the presence of NOM was significantly reduced, and changes consistent with this phenomenon were observed in the EEM spectrum. The time-resolved luminescence characteristics (peak wavelengths and lifetime) of U(VI) in the DB-1 samples agree well with those of Ca2UO2(CO3)3(aq). This U(VI) species remains stable, even in samples taken five years ago. The estimated amounts of U and NOM from the spectroscopic data of DB-3, OB-1, and OB-3 samples are relatively low compared to DB-1 samples. When a known amount of U(VI) was mixed in each groundwater, the time-resolved luminescence spectrum exhibited a characteristic spectral shape different from the expected luminescence intensity. This phenomenon is presumed to be due to the interaction between U(VI) and NOM in groundwater. The results of this study suggest that the chemical speciation of NOM as well as U(VI) is required to understand U behavior in groundwater.

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 geological disposal of spent nuclear fuel is one of the important problems to be solved worldwide. For the safety of the geological disposal, disposal facility is recommended to be constructed in the deep reducing environment of host rocks. As host rocks, rock salt, argillaceous (clay) rock, and crystalline rock have been considered as stable geological formations in various countries. Although various studies have been conducted on crystalline rocks in Korea, there are still few studies on hydrogeochemical evolution in the deep and reducing environment related to the disposal of spent nuclear fuel. Therefore, this study was conducted to identify hydrogeochemical evolution process in granite aquifer which can affect the stability of disposal facility. Groundwater samples for isotope and chemical analysis were collected quarterly adjacent to KURT (KAERI Underground Research Tunnel). As the depth increased, the groundwater changed from Ca-HCO3 type to Na-HCO3 type under the influence of silicate mineral weathering, and the fluorine concentration increased due to the dissolution of fluorine-bearing minerals. However, hydrogeochemical evolution according to the depth was not observed in some wells because of a hydraulic connection through the fracture zone. In addition, the behavior of nitrate and redox-sensitive metals (Fe, Mn, U, Mo) in groundwater was clearly different in the redox condition. Considering these hydrogeochemical processes and hydrogeological factors, a conceptual model of granite aquifers in and around KURT was established. The results of this study will be used as basic data to understand the hydrogeochemical processes and to evaluate and predict the behavior of radionuclides in granite aquifer system.

Deep geological repository (DGR) has been considered as a globally accepted strategy to dispose high-level radioactive wastes. During long storage periods of 100,000 years, uranium (U) could be migrated through fractures in deep granite aquifers and interact with indigenous bacteria under anaerobic condition. Anaerobic bacteria can reduce U(VI) and further precipitate in the form of U(IV)-oxide minerals by transferring electrons through c-type cytochrome. In this point of view, a comprehensive understanding of uranium-microorganisms interaction is necessary to guarantee the safety of high-level radioactive waste disposal. Although diverse bacterial communities are present in DGR environment, a number of studies have been focused on some model bacteria, such as Desulfovibrio, Geobacter, and Shewanella spp.. In this study, indigenous bacterial community in deep granitic groundwater at 234–244 m was inoculated to sterile uranium-contaminated granitic groundwater amended with 20 mM of sodium acetate, and then incubated under anaerobic condition for 12 weeks. Bio-reduction of U(VI) to U(IV) by indigenous bacteria in uranium-contaminated groundwater was investigated during whole operation period. Initial U(VI) concentration of 885.4 μg·L−1 gradually decreased to 586.1 μg·L−1, resulting in approximately 33.8% of aqueous U(VI) removal efficiency. Oxidation-reduction potential (ORP) value was gradually decreased from 175.4 mV to –243.0 mV after the incubation of 12 weeks. The decrease in ORP value was attributed to the presence of aerobic bacteria and facultative anaerobic bacteria in indigenous bacterial community. The shift in bacterial community structure was observed by 16S rRNA gene high-throughput sequencing analysis. Proteobacteria (55.6%), Firmicutes (24.1%), Actinobacteria (5.5%), and Bacteroidetes (5.4%) were dominant in initial indigenous bacterial community, while Proteobacteria (94.8%) was found to be the only abundant phylum after the reaction. In addition, great increase in the relative abundance of sulfate-reducing bacteria (SRB) was observed: the relative abundance of SRB increased from 11.4% to 44.3% after the reaction. This result indicates that the SRB played a key role in the removal of aqueous U(VI). This finding shows the potential of aqueous U(VI) removal by indigenous bacteria in DGR environment.

Uranium isotopes (238U, 235U, and 234U) found in natural environments and their activity ratios (235U/238U and 234U/238U) have been used as an important tool in investigating various geological processes, especially in natural analogue studies. Occurrence and fractionation of uranium isotopes in nature between 238U, 235U, and 234U were investigated. Various measurement methods have been used for the determination of isotopic ratios and geochronology. Thus, we reviewed and summarized the measurement methods such as alpha spectrometry, gamma spectrometry, thermal ionization mass spectrometry (TIMS), secondary ion mass spectrometry (SIMS) with sensitive high resolution ion microprobe (SHRIMP), and multiple-collector inductively coupled plasma mass spectrometry (MCICPMS). Research status of natural analogue studies carried out using uranium isotopes and their isotopic ratios were also reviewed and summarized in terms of long-term behaviors of radionuclides in various foreign uranium deposits as analogues of high-level radioactive waste repositories. Research results for mineralogical, geochemical, and biogeochemical behaviors of uranium in various natural analogue sites were collected and analyzed to investigate the migration and retardation processes of uranium through geological media. These long-term behaviors of uranium and uranium isotopes include dissolution/precipitation of uranium minerals, interactions of uranium with various fracture minerals including sorption and incorporation, redox reactions by minerals and microbes, and hydrological groundwater flow thorough rock fracture systems including identification of flow paths and groundwater circulation. The results of this study will contribute to performing future natural analogue studies in domestic uranium deposits and provide basic information and knowledge for understanding long-term geochemical and geochronological behaviors of radionuclides in a high-level radioactive repository.