우라늄 광상 내 물-암석 상호작용에 대한 자연유사 연구: 해외 연구 사례 및 국내 옥천대 우라늄 광상의 수리지구화학적 특성

Abstract

In order to evaluate the long-term safety of deep underground disposal, which is proposed as a disposal method for high-level radioactive wastes, it is necessary to understand the behavior of uranium, a representative radionuclide. Uranium is involved in various geochemical reactions, such as oxidation-reduction, complexation, precipitation-dissolution, and adsorption-desorption; thus, it is important to understand the behavioral characteristics related to these geochemical reactions. For this purpose, experiments or reactive transport model can be used. However, behavioral analyses through them could be limited in time and space or there could be uncertainties in model input parameters such as hydraulic conductivity and sorption coefficient. Therefore, it is necessary to monitor and analyze the long-term behaviors of radioactive elements in natural conditions during the past geological time. These studies are called as 'natural analogue' studies. In this review, we tried to understand the behaviors of uranium during the water-rock interactions based on the studies examining the geochemical characteristics of the seven overseas natural analogue research sites and the Okcheon belt with high uranium contents in Korea. Geochemical characteristics of groundwater in uranium-containing aquifers in the overseas natural analogue research sites indicated reduced environments represented by low Eh values, and showed that more reduced U(IV) species were predominantly present. Accordingly, it was found that the transport of uranium in the groundwater is limited due to precipitation of uranium minerals such as uraninite or coffinite. Groundwater from the uranium-containing coaly slate layer in the Okcheon belt in Korea also indicated a reduced environment, and showed that the transport of uranium is limited due to the precipitation of U(IV) into uraninite. This review, which identified the geochemical characteristics of groundwater in aquifers containing uranium under natural conditions, can derive implications for the behavior of uranium in the deep geological repository for high-level radioactive wastes.