Solubility study of NpO2OH(am) under oxidizing groundwater conditions in the KAERI underground research tunnel

Neptunium is a key radionuclide of concern in the safety assessment of spent nuclear fuel repository systems. Under groundwater conditions, neptunium exists primarily in the tetravalent and pentavalent oxidation states, which differ significantly in solubility. In this study, the solubility of the pentavalent solid NpO₂OH(am) was measured under oxidative groundwater conditions simulating those of the Korea Atomic Energy Research Institute (KAERI) Underground Research Tunnel (KURT). Amorphous NpO₂OH(am) was prepared from ²³⁷Np(V) solution and equilibrated in synthetic KURT groundwater (pH ≈ 9, ionic strength ≈ 2.2 mmol/kg). The dissolved Np concentration at the end of experiment was measured to be 1.67 ± 0.07 mM at pH 8.27, E_h 0.445 V, and room temperature (22 ± 5 °C). Vis–NIR spectrophotometric analysis identified NpO₂⁺ and NpO₂(CO₃)⁻ as the dominant aqueous species. X-ray power diffraction suggested that the residual greenish solid at the end of the experiment was amorphous NpO₂OH(am). The experimental results were in good agreement with the thermodynamic modeling, confirming that integrated experimental and modeling approaches reliably describe the solubility behavior of Np(V) under the given groundwater condition.