Th(IV) is a stable actinide that can act as a chemical analogue of U(IV) and Pu(IV), which are important radionuclides in safety assessments of deep geological repositories (DGR). Therefore, to understand the geochemical behaviour of U(IV) and Pu(IV), batch sorption of Th(IV) onto crystalline rocks were performed in oxidising conditions. The distribution coefficients (Kd) of Th(IV) were of particular interest. Gyeongju fresh groundwater (GF) and Gyeongju brackish groundwater (GB) were obtained at the Gyeongju Low and Intermediate Level Radioactive Waste (LILW) Disposal Facility. Crystalline granite (gr) and biotite gneiss (bg) were collected in Gyeongju and Gwacheon respectively and were grounded to a particle size smaller than 150 μm. Sorption samples were continuously shaken for 7 days under 200 rpm at 25°C. The liquid-to-solid ratio (V/m) was 200 L·kg-1. Th(IV) concentrations of the sorption samples were determined by UV-Vis-NIR absorption colorimetry from the formation of Th(IV)-arsenazo III complexes. Although the method allowed the initial Th(IV) concentrations to be determined, the final Th(IV) concentrations fell below the limit of detection (LOD), 6.27×10-9 mol·L-1. Taking the LOD as the final concentrations, conservative Kd were calculated to be 4,410 L·kg-1 for GF-gr and GF-bg, and 7,830 L·kg-1 for GB-gr and GB-bg. The result indicates a strong sorption affinity of Th(IV) onto granite and biotite gneiss within Gyeongju groundwater, suggesting a similar behaviour for U(IV) and Pu(IV). Furthermore, comparison of the conservative Kd obtained from the experiment were compared with existing Kd values of Th(IV). Such analysis and comparison of Th(IV) Kd in various types of groundwater could help locate the optimal site for a DGR in South Korea.

The safe disposal of high-level radioactive waste is a critical concern in many countries, especially in the context of the increasing use of nuclear power to overcome climate change. To provide a comprehensive understanding of the behavior of the radionuclides in the crystalline natural barrier, sorption of the artificially synthesized high-level radioactive waste (HLW) leachate was conducted. Granite (-1,000 m from ground level) and biotite gneiss (-100 m from ground level) rock cores were collected from Gyeongju and Gwacheon, respectively. The rock cores were milled with a jaw crusher and steel disk mill and then sieved. The crushed rocks with a diameter of 0.6 – 1.0 mm were selected, washed three times with deionized water, and then dried. To synthesize the simulated HLW leachate, representative elements (U(VI), Se(IV), Mo(VI), and Ni(II)) were added to natural groundwater collected from Gyeongju. The kinetic sorption experiment was performed in a polypropylene bottle with a solid-to-liquid ratio of 100 g/L in the orbital shaking incubator (200 rotations per min, 25.0°C). After the sorption, the supernatants were filtered by a 0.2-μm polytetrafluoroethylene syringe filter and subsequently analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). Through the kinetic change of aqueous concentration, the contact time has been determined to be 7 days. Ni(II) showed the highest distribution coefficients (Kd = 0.81 L/m2 for granite and 8 – 16 L/m2 for biotite gneiss), followed by U(VI) (Kd = 0.03 – 0.04 L/m2 for granite and 0.04 – 0.05 L/m2 for biotite gneiss). Highly mobile nuclides such as Se(IV) (Kd = 0.02 L/m2 for granite and 0.03 L/m2 for biotite gneiss) and Mo(VI) (Kd = 0.01 – 0.02 L/m2 for granite and 0.01 L/m2 for biotite gneiss) showed the lowest distribution coefficient. Our study provides insights into the migration-retention behaviors of the HLW leachate with granite and biotite gneiss in geological systems and verifies the sorption parameters, e.g., distribution coefficients, experimentally produced by other groups to ensure the safe disposal of HLW.

흑운모 편마암과 화강암에 대한 풍화정도에 따른 광물조성과 화학성분 변화를 X-선회절분석, 전암분석을 통하여 연구하였다. 흑운모 편마암의 주 구성광물은 흑운모, 석영, 사장석이며 화강암의 주 구성광물은 석영, 사장석, 백운모과 약간의 정장석을 포함하고 있다. 풍화가 진행될수록 흑운모 편마암은 버미큘라이트와 할로이사이트가 증가하고 화강암은 일라이트와 캐올리나이트가 증가한다. 풍화가 진행될수록 대체로 Na2O, CaO, K2O가 감소하고 Al2O3의 함량이 증가하지만, Fe2O3의 값은 흑운모 편마암과 화강암에서 큰 차이를 나타낸다.

화강암질 편마암의 풍화단면에서 풍화초기에 흑운모-질석-캐올리나이트로의 변질과정을 보여 주었으나, 지표층으로 가면서 질석 중간 상이 거의 인지되지 않고 흑운모-캐올리나이트 변질과정을 나타낸다. 흑운모의 풍화작용에 의해 생성된 1:1 규산염 층상광물은 캐올리나이트와 할로이사이트이며, 다른 메커니즘에 의해 형성되어 각기 다른 풍화조직을 보여준다. 캐올리나이트화작용은 입자의 가장자리로부터 내부로 진행되었다. 변질과정에서 10a의 흑운모 다층과 7a의 캐올리나이트 다층이 교호하고 있고, 두 상의 c-축이 일치한다. 변질된 캐올리나이트의 가상은 흑운모 입자의 외곽선에 기준하여 부피의 변화 없이 일정하고, 많은 공극이 벽개면을 따라 발달해 있어 판상의 캐올리나이트는 흑운모와의 1:1의 엽층 대 엽층 교대작용에 의해 형성된 것으로 보인다. 할로이사이트화작용은 흑운모의 엽편들은 보다 얇게 분리되어 벽개면에 수직방향으로 휘어져 렌즈상의 공극을 형성하고, 할로이사이트는 그 가장자리에서 외곽 방향으로 부채모양으로 발달하여 할로이사이트의 가상은 흑운모 입자의 현저한 부피 증가를 초래했다. 관상의 할로이사이트는 사장석의 용해작용에 의한 외부용액으로부터 Si와 Al을 공급받아 흑운모의 표면에 침전되어 성장되었다 이렇게 형성된 할로이사이트는 비정상적으로 높은 Fe(~11%)를 함유하고 있다.