Thermodynamic controls on plutonium(Pu) speciation in deep groundwater: Implications for HLW disposal safety at Beishan, China

Abstract

The safe disposal of high-level radioactive wastes (HLWs) is a critical concern due to the environmental risks posed by radionuclides like plutonium (Pu), which is highly radiotoxic and has a long half-life. Deep geological disposal, such as at the Beishan site in Gansu Province, China, is considered a viable solution for isolating HLWs from the biosphere. However, the potential for tectonic disturbances and fracture networks raises concerns about the migration of radionuclides, making it essential to understand the speciation and mobility of Pu in deep groundwater. Previous studies have largely focused on surface environments, with limited attention to Pu behavior under deep geological conditions. This study employs thermodynamic simulations using PHREEQC to examine Pu speciation in deep groundwater of Beishan, evaluating the effects of pH, redox potential (pe), temperature, and ionic concentrations. Results demonstrate that Pu predominantly exists as Pu(OH)₄ at neutral to alkaline pH, with significant changes in speciation under varying conditions. At acidic pH (2–2.5), Pu exists as a free ion in the +III oxidation state, while at higher pH (>7), Pu(OH)₄ becomes dominant. Redox potential influences Pu speciation, with Pu(III) and Pu(IV) species forming at low and high pe values, respectively. Temperature influences the concentration of Pu(OH)₄ but has a minimal effect on other species. Additionally, sulfate (SO₄²⁻) concentration significantly alters Pu speciation, while changes in bicarbonate (HCO₃⁻), chloride (Cl⁻), and fluoride (F⁻) concentrations have minimal effects. The findings highlight the importance of considering local hydrogeochemical conditions in the design of HLW disposal facilities to optimize safety and minimize potential radionuclide migration.