Fate of actinides in the presence of humic substances under conditions relevant to nuclear waste disposal

The modelling of radionuclide transport in the geosphere is a key issue for the safety assessment of nuclear waste repositories. This paper focuses on the role of humic substances as potential vectors of radiocontaminants through natural aquifers. By using interaction constants of tri-, penta- and hexavalent actinides (Am, Cm, Np, U) with humic acids (determined experimentally in the laboratory), their speciation under conditions relevant to geological formations (granitic, sedimentary) for nuclear waste repositories is presented as well as the limitations of these calculations. The role of organic complexation is discussed as well as its consequence on their mobility through the geosphere. It arises that under these environmental conditions, only trivalent actinides are influenced by organic complexation, whereas penta- and hexavalent actinides are present as carbonate or hydroxide complexes depending on the physico-chemical conditions. The processes which can modify these conclusions are underlined, in particular the possible formation of mixed complexes.