COMPARISON BETWEEN EXPERIMENTALLY MEASURED AND THERMODYNAMICALLY CALCULATED SOLUBILITIES OF UO2 AND THO2 IN KURT GROUND WATER

Solubility of a radionuclide is important for defining the release source term of a radioactive waste in the safety and performance assessments of a radioactive waste repository. When the pH and redox potential of the KURT groundwater were changed by an electrical method, the concentrations of uranium and thorium released from UO2(cr) and ThO2(cr) at alkali pH (8.1 similar to 11.4) and reducing potential (Eh < -0.2 V) conditions were less than 10(-7) mole/L. Unexpectedly, the concentration of tetravalent thorium is slightly higher than that of uranium at pH = 8.1 and Eh = -0.2 V conditions, and this difference may be due to the fort-nation of hydroxide-carbonate complex ions. When UO2(s) and UO2(am, hyd.), and ThO2(s) and Th(OH)(4)(am) were assumed as solubility limiting solid phases, the concentrations of uranium and thorium in the KURT groundwater calculated by the PHREEQC code were comparable to the experimental results. The dominating aqueous species of uranium and thorium were presumed as UO2(CO3)(3)(4-) and Th(OH)(3)CO3- at pH = 8.1 similar to 9.8, and UO2(OH)(3)(-) and Th(OH)(4)(aq) at pH = 11.4.