Speciation of neptunium migration in under groundwater

The migration of radionuclides with groundwater into the environment of a r adioactive 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 geochemica l behavior of the predominant ones. The speciation of different valence sta tes of neptunium [Np(IV, V and VI)] and their migration behavior are invest igated. The data revealed that the uptake is highly dependent upon pH and t he distribution coefficient (K-d) is depressed by the presence of organic l igands. The relative migration velocities (V-n) of the valence states relat ive 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 appl ied to elucidate the migration behavior of these radioactive species in the selected site and to quantitatively predict their concentrations at differ ent distances in both x and z directions over long time scales.