Investigation on Solubility and Speciation of Valence-Variable Radionuclides in Synthetic Tamusu Claystone Groundwater

Abstract

Tamusu claystone is one of the preselected host rocks for geological disposal of high-level radioactive waste in China. In this work, two claystone samples collected from borehole Tzk1 at the depth of 542 m(Tzk1-542) and borehole Tzk2 at the depth of 516 m (Tzk2-516) of Tamusu site in Inner Mongolia were used to synthesize groundwater by soaking-balance method. The pH values of the synthetic groundwater were 9.45 and 8.24, respectively, and their components were analysed. Based on the Fe2+ and total Fe mass ratios of 78.36% and 91.92% in Tzk1-542 and Tzk2-516, the Eh values of -169.4 mV and 11.5 mV were proposed, respectively. In light of the obtained Eh, the solubility and speciation of U, 99Tc, 79Se and Np were calculated using PHREEQC with the latest OECD/NEA thermodynamic data and the thermodynamic data of uranyl carbonate complexes with Mg and Ca. When the pH range is 5.0-11.0 and the partial pressure of oxygen remains unchanged, the results show that the solubility of U, Se, Tc and Np in Tzk1-542 groundwater are 10-11-10-8, 10-16-10-9, 10-14-10-12 and 10-18-10-17 mol/L, respectively, while in Tzk2-516 groundwater they are 10-14-10-5, 10-14-10-5, 10-11-10-4 and 10-18-10-17 mol/L, respectively. The dissolved U, Se, Tc and Np in both groundwaters are mainly in the form of neutral molecules or anions. The claystone groundwater is generally under alkaline condition. When the pH is fixed(at 9.45 and 8.24 for Tzk1-542 and Tzk2-516, respectively) and the Eh varies from -169.4 mV to 169.4 mV, the solubility ranges of U, Se, Tc and Np in groundwater Tzk1-542 are 10-10-2.28×10-4, 10-12-10-3, 10-14-10-2 and 10-18-10-12 mol/L, respectively. The dissolved U, Se, Tc and Np mainly exist as negatively charged or neutral species such as UO2(CO3)4-3, CaUO2(CO3)2-3, SeO2-3, HSeO-3, TcO-4, NpO2OH, etc. For groundwater Tzk2-516, the solubility ranges of U, Se, Tc and Np are 10-11-10-5, 10-11-10-3, 10-16-10-2 and 10-19-10-13 mol/L, respectively. The dissolved U, Se, Tc and Np mainly exist as Ca2UO2(CO3)3, CaUO2(CO3)2-3, SeO2-3, HSeO-3, TcO-4, NpO2OH, etc. This work provides the simulation results of the homogeneous solution system. Further experimental works in combination with the real groundwaterwater condition for the related radionuclides are needed.

Keywords

• tamusu
• synthetic groundwater
• valence-variable radionuclides
• solubility
• speciation