Chemical states of Cs in sewage sludge ash and radioactivity concentration reduction method

Kozai, Naofumi   ; Suzuki, Shinichi; Aoyagi, Noboru   ; Sakamoto, Fuminori  ; Onuki, Toshihiko

Huge quantities of radionuclides were emitted by the accident at the Fukushima Daiichi Nuclear Power Plant and descended in wide areas of east Japan. A great amount of sewage sludge ashes (SSAs) containing fallout radionuclides (mainly Cs) at high concentration was generated and has been stored in sewage treatment plants until being disposed of. To contribute for the safety landfill disposal, we investigated the chemical states of Cs in SSAs by dissolution and static leach experiments. A fraction of Cs was contained in alkali metal salts, the major fraction of Cs was in iron oxides, and the rest was in silicates (quartz and feldspar). About 80-90% of Cs was contained in alkali metal salts and iron oxides and most of these solid phases were dissolved by heating in HCl aqueous solutions. These HCl-dissolvable phases were completely dissolved by pre-pulverizing SSAs and the subsequent heating in HCl aqueous solution at 95C. The residues were mostly silicates and contained about 10-20% of Cs in the original SSAs. Several percent of Cs in the original SSA was leachable to synthetic seawater, while the Cs in the residue was non-leachable.