Investigation of Redox Potential of Beishan Site and Its Impact on Mobility of Redox-Sensitive Radionuclides

The redox potential(Eh) is one of the key parameters that can affect the long-term safety of the HLW repository. However, direct determination of the Eh value of the groundwater faces many uncertainties. Beishan granite contains a certain amount of iron containing minerals, which may control the concentration of aqueous iron in the groundwater. In this study, based on the aqueous iron of 0.033 mg/L analyzed in the groundwater of BS03 borehole at 400 m deep and the Fe²⁺/Fe_tot ratio of 96.86% found in the corresponding rock core, a possible method for deducing the redox potential of Beishan groundwater is proposed, and a value of 83.2 mV is calculated for the groundwater of BS03 borehole. This value is comparable to 101.8 mV that is estimated using the precipitation saturation index of goethite, but in situ Eh measurement should be carried out for verification and comparison. In light of the obtained Eh, the speciation and solubility of redox-sensitive radionuclides, i.e., U, ⁹⁹Tc, ⁷⁹Se, and Np are calculated using PHREEQC with the Nuclear Energy Agency thermodynamic database by adding stability constants of MUO₂(CO₃)^2-₃ and M₂UO₂(CO₃)⁰₃ (M=Ca, Mg, Sr). The results indicate that the solubility is expected to be relatively high (≈10^-5-10^-4 mol/L) for U and Tc, whereas relatively low for Se (≈10^-8 mol/L) and fairly low for Np (≈10^-18 mol/L). Furthermore, the dissolved U, Tc, and Se mainly exist as anions that have strong mobility in geological media. On the other hand, because of the abundance of ferrous iron in Beishan granite, in order to have a comprehensive evaluation on the safety performance of the disposal site, further experimental works concerning the retardation of these redox-sensitive radionuclides via heterogeneous reduction are needed.