The disposal of spent nuclear fuel (SNF) in a deep geological repository (DGR) is a widely accepted strategy for the long-term sequestration of radiotoxic SNF. Ensuring the safety of a DGR requires the prediction of various reactions and migration behaviors of radionuclides (RNs) present in SNF within its geochemical surroundings. Understanding the dissolution behaviors of mineral phases harboring these RNs is crucial, as the levels of RNs in groundwater are basically linked to the solubility of these solid phases. Accurate measurements of solubility demand the use of welldefined solid materials characterized by chemical compositions and structures. Herein, we attempted the synthesis of sklodowskite, a magnesium-uranyl (U(VI))-silicate, employing a twostep hydrothermal synthetic approach documented previously. Subsequently, we subjected this synthesized sklodowskite to various analytical techniques, including powder X-ray diffraction (pXRD), scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDX), and vibrational spectroscopies (FTIR and Raman). Based on our findings, we confidently identify the obtained mineral phase as sklodowskite (Mg[UO2SiO3OH]2·5H2O). This identification is primarily based on the similarity between its pXRD pattern and the reference XRD pattern of sklodowskite. Furthermore, the measured infrared and Raman spectra show the vibrational modes of UO2 2+ and SiO4 4- ions, particularly within the 700~1,100 cm-1 region, which support that the synthetic mineral has a characteristic layered uranyl-silicate structure of crystalline sklodowskite. Finally, we utilized synthetic minerals to estimate its solubility up to about three months in a model groundwater, where the dissolved species composition is analogous to that of granitic groundwater from the KAERI Underground Research Tunnel. In this presentation, we will present in detail the results of spectroscopic characterizations and the methodology employed to assess the solubility of the U(VI)-silicate solid phase.

The solubilities of different multicomponent lanthanide oxide (Ln2O3) solid solutions including binary (Ln1 and Ln2 = La, Nd, Eu, or Tm), ternary (Ln1, Ln2, and Ln3 = La, Nd, Eu, or Tm), and higher systems (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were studied after aging for four weeks at 60°C. Our recent study revealed that the phase transformations in binary ((La, Nd) and (La, Eu)) and ternary (La, Nd, Eu) systems are responsible for the formation of (La, Nd)(OH)3, (La, Eu)(OH)3, and (La, Nd, Eu)(OH)3 solid solutions, respectively. The variations in the mole fractions of La3+, Nd3+, and Eu3+ in the sample solutions of these hydroxide solid solutions indicated that a thermodynamic equilibrium might account for the apparent La, Nd, and Eu solubilities. Conversely, the binary and ternary systems containing Tm2O3 as the heavy lanthanide oxide retained the oxide-based solid solutions, and their solubility behaviors were dominated by their congruent dissolutions. In the higher multicomponent system, the X-ray diffraction patterns of the solid phases, before and after contact with the aqueous phase indicated the formation of a stable oxide solid solution and their solubility behavior was explained by its congruent dissolution.