Yuanzineng kexue jishu (May 2024)
Geochemical Simulation on U Species and Their Influence Factors in Deep Groundwater from Xinchang of Beishan, Gansu
Abstract
The rapid development of nuclear power reduced the consumption of fossil fuels, effectively reduced the environmental pollution of liquid waste, solid waste, and harmful gases, playing a crucial role in reducing carbon dioxide emissions. However, it also generated a large amount of high-level radioactive wastes (HLWs). HLWs contain a quantity of elements, high toxicity, high-level radioactivity, long half-life period, and large heat release, posing a huge threat to the ecological environment and human health. Its safe disposal has become a key and difficult issue in international radioactive waste management. Deep geological disposal is widely regarded as the safest, most reliable, and technically feasible method for disposing HLWs. Deep geological disposal refers to the establishment of a geological disposal repository of HLWs in rocks at a depth of 500-1 000 meters underground, a site buring the processed HLWs, and isolating HLWs from the biosphere through a multiple barrier system to achieve long-term and effective safe disposal. At present, the construction of an underground research laboratory (URL) has been initiated for the geological disposal of HLWs in Xinchang of Beishan, Gansu. The distribution of U species in deep groundwater and its influence factors can provide basic data for the performance assessment of a deep geological disposal system at the micro level, but they remain unclear, urgently needed relevant geochemical simulation. Based on the database (llnl.dat) in PHEEQC, with an addition of thermodynamic data of U(Ⅵ) from NEA-TDB and ThermoChimie databases, the U species and their distribution in the deep groundwater of BS28 borehole of the URL were simulated using PHEEQC.v3 and PhreePlot.v11, to clarify the U species in the deep groundwater from Xinchang of Beishan and elucidate the influence of pH, pE, and concentration ratio of Ca2+ and HCO3-(c(Ca2+)/c(HCO3-)) on the distribution of U species. The simulation results indicate that U occurs as U(Ⅵ) in this deep groundwater environment, with the main species of Ca2UO2(CO3)3(aq) and CaUO2(CO3)32-, accounting for 84.14% and 15.16%, respectively. The valence states of U can gradually change from U(Ⅵ) to U(Ⅳ) when pE<1. The simulation also shows that UO2(s) and soluble uranyl-carbonate complexes can coexist meanwhile at pE=－1-1. CaUO2(CO3)32- is more easily formed at low c(Ca2+)/c(HCO3-), while Ca2UO2(CO3)3(aq) is more easily formed at high c(Ca2+)/c(HCO3-). Importantly, the Ca2+-UO22+-CO32- ternary system can greatly affect the solubility and migration ability of U in the deep groundwater of Beishan. This study can help to find out the migration and adsorption mechanism of U in the Beishan granite fissures.
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