Evaluating the Behavior of Uranium Through Column Experiments Using Artificial Groundwater and Coaly Slate from a Natural Analogue Study Site in Korea

The deep geological disposal method is a prominent approach for the management of high-level radioactive waste, and understanding the behavior of uranium under various geochemical conditions is essential for this purpose. To predict the behavior of uranium in the field, it is necessary to evaluate not only the transport of uranium but also the reactions between host rock and groundwater at the field site. In this regard, we evaluated the behavior of uranium in the underground environment by analyzing the temporal and spatial changes in uranium geochemistry in response to water-rock reactions. This was achieved through column experiments using rocks containing uranium ore bodies and groundwater from a natural analogue study site in Korea. Two columns (NA-PJ1 and NA-PJ2) were prepared by collecting coaly slate materials containing uranium minerals sourced from the Okcheon Metamorphic Belt in Korea. NA-PJ1 was filled with coaly slate (0.025 ~ 2 mm grain size) collected from the study area, while NA-PJ2 included a mixture of coaly slate and limestone (10 wt%) to provide pH buffering. The input solutions to the columns were artificial groundwater manufactured with a chemistry similar to that of the groundwater at the study site. The artificial groundwater was purged with Ar gas before the experiment to minimize the ingress of dissolved oxygen (DO) from the atmosphere. To observe spatial and temporal changes in geochemistry resulting from the interactions between the artificial groundwater and the reactive materials inside the columns, water samplings were performed at 0, 5, 10, 15, 20, 25, 30, 35, and 40 cm from the column influent. The results consistently showed low and stable DO concentrations in both columns. The pH in NA-PJ1 initially exhibited the highest value at 0 cm but gradually decreased to below 4.5. This was attributed to the insufficient carbonate buffering capacity to neutralize hydrogen ions generated by the oxidation of iron sulfide in coaly slate. In NA-PJ2, in contrast, the pH remained around 8. Uranium concentration in NA-PJ1 increased gradually with distance. It was determined that uranium was released through the dissolution of uranium minerals (i.e., uraninite and ekanite). Subsequently, the released uranium formed uranium aqueous complexes with dissolved F or SO₄ induced by iron sulfide oxidation. Furthermore, it was shown that uranium in NA-PJ1 formed UO₂CO₃(aq) complexes closer to the column influent, while it formed more UO₂SO₄(aq) complexes with increasing distance. This study contributes to understanding the transport and reaction characteristics of uranium in groundwater, ultimately aiding in the management of high-level radioactive waste in a deep geological disposal site.