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Prediction of the Species and Solubility of Uranium in Beishan Groundwater

In order to understand the sorption, diffusion and migration behaviors of U in Beishan water-rock system, the speciation distribution of uranium in Beishan granite groundwater, along with its solubility influenced by calcite were calculated, using PHREEQC software with the most recent thermodynamic data published by OECD / NEA.The results suggest that under acidic conditions, uranium mainly occurs as UO2F +,U O 2 SO 4,U O 2+ 2, UO 2 F 2 and UO 2(S O 4) 22, while under nearly neutral to alkaline conditions, the dissolved uranyl predominantly exists as anionic UO2(C O 3) 43,U O 2(C O 3) 22,U O 2(O H)3 and UO 2(O H) 24, which are expected to enhance the mobility of uranium.Since the potential high level radioactive waste repository in China will be constructed at the depths of 500-1 000 meters underground, the groundwater chemistry at such depths would be weakly alkaline, and the uranyl will be existed as anionic complexes that are mobile.When the pH of the groundwater is fixed at 7.56, the calculations demonstrate that uraninite is the main phase at Eh24mV.With the Eh elevation, uranyl and its various anionic complexes formed with CO 23 and OHbecome the predominant species.When the groundwater is exposed to air, uranyl and its various complexes are expected to be the predominant species due to the dissolution of O 2.In case the groundwater-host rock system contains calcite, uranium solubility can be enhanced significantly at pH8.0, while the influence is negligible at higher pH.