Evaluation of Americium Solubility in Synthesized Groundwater: Geochemical Modeling and Experimental Study at Over-Saturation Conditions

• Published in: Journal of nuclear fuel cycle and waste technology (Online) Vol. 20; no. 4; pp. 399 - 410
• Main Authors: Kim, Hee-Kyung, Cho, Hye-Ryun
• Format: Journal Article
• Language: English
• Published: 한국방사성폐기물학회 01.12.2022
• Subjects: 원자력공학; Americium; Groundwater; Radionuclide migration; Thermochemical data; Solubility; Colloidal particle
• ISSN: 1738-1894; 2288-5471
• DOI: 10.7733/jnfcwt.2022.041

The solubility and species distribution of radionuclides in groundwater are essential data for the safety assessment of deep underground spent nuclear fuel (SNF) disposal systems. Americium is a major radionuclide responsible for the long-term radiotoxicity of SNF. In this study, the solubility of americium compounds was evaluated in synthetic groundwater (Syn- DB3), simulating groundwater from the DB3 site of the KAERI Underground Research Tunnel. Geochemical modeling was performed using the ThermoChimie_11a thermochemical database. Concentration of dissolved Am(III) in Syn-DB3 in the pH range of 6.4–10.5 was experimentally measured under over-saturation conditions by liquid scintillation counting over 70 d. The absorption spectra recorded for the same period suggest that Am(III) colloidal particles formed initially followed by rapid precipitation within 2 d. In the pH range of 7.5–10.5, the concentration of dissolved Am(III) converged to approximately 2×10−7 M over 70 d, which is comparable to that of the amorphous AmCO3OH(am) according to the modeling results. As the samples were aged for 70 d, a slow equilibrium process occurred between the solid and solution phases. There was no indication of transformation of the amorphous phase into the crystalline phase during the observation period.