INFLUENCE OF WEATHERING OF BOTTOM ASH ON THE LEACHING BEHAVIOR OF CESIUM

Abstract

After nuclear accident in 2011, incineration residue contaminated by radioactive cesium had been produced. Government decided in 2011 that incineration residue with less than 8000 Bq/kg of radioactivity could be disposed of into conventional MSW landfill. Since radioactivity of bottom ash was reported to be low compared with fly ash, it had been disposed of MSW landfill based on the government regulation. However, long-term leaching behavior of cesium contained in bottom ash when being disposed of into landfill was not known. Thus, in this study, phase transformation and leaching of cesium from incineration bottom ash were investigated by conducting accelerating weathering experiment. Four kinds of weathering condition were set; namely, blank (under nitrogen gas condition), wetting and drying, freezing and thawing, and exposure to carbon dioxide gas. Leaching of cesium under blank condition didn't change. On the other hand, drastic decrease of leaching of cesium was identified under carbon dioxide exposure. From XRD analysis, formation of calcite was identified. It was deemed that newly formed calcite inhibited the leaching of Cs by covering reactive surface of bottom ash. In both wetting-drying and freezing-thawing condition, leaching of Cs increased initially. But it decreased after several weeks then it became lower than blank finally. This decrease of leaching also seems to be caused by formation of calcite. Besides, in order to confirm the assumption in which the restrain of cesium leaching occurred by calcite formation, SEM-EDS analysis was performed. By the analysis, calcite formation on the surface was clearly identified.