Fixation and mobilization of 60Co on sediments in coastal environments

Over the past 15 years an enormous amount of radioactive wastes from Japanese nuclear power generating stations on coastal areas has been piled up at storage facilities in sites facing the sea. A study was made of the underground migration of radioactive cobalt from the radioactive wastes that may be disposed of at burial sites in coastal regions in the future. The following subjects were emphasized: a) the mechanisms of fixation and mobilization of radioactive cobalt on sediments in coastal environments and b) possible chemical forms of 60Co remaining in the water phase after interaction and desorption. Under anaerobic conditions, 30% of 60Co added to a sand sediment and freshwater system was potentially mobile. In contrast, more than 98% of the 60Co under aerobic conditions was permanently fixed on the sediment. Meanwhile, most of the mobile 60Co, produced under anaerobic conditions in seawater, consisted of non-ionic cobalt associated with low molecular weight materials. In conclusion, 60Co released in aquatic environments was categorized as mobile, exchangeable or irreversible, depending on the magnitude of its mobility. In particular, the redox conditions play a significant role in the distribution of 60Co between the three aforementioned categories. (author)