Speciation of neptunium migration in under groundwater

The migration of radionuclides with groundwater into the environment of a radioactive waste repository is an important aspect in evaluating the safety assessment of such repositories. The potential transport and fate of these radioactive species are dependent upon their speciation and the geochemical behavior of the predominant ones. The speciation of different valence states of neptunium [Np(IV, V and VI)] and their migration behavior are investigated. The data revealed that the uptake is highly dependent upon pH and the distribution coefficient (K-d) is depressed by the presence of organic ligands. The relative migration velocities (V-n) of the valence states relative to that of groundwater, in the concerned area, clarified that Np(V) is the most mobile species and their transport rates are highly accelerated by the presence of humic acid. Also, a radionuclide dispersion model was applied to elucidate the migration behavior of these radioactive species in the selected site and to quantitatively predict their concentrations at different distances in both x and z directions over long time scales.