Behavior of Radioactive Cesium in Municipal Solid Waste of Thermal Treatment Plants after Fukushima Nuclear Power Plant Accident – 16626

Municipal solid waste (MSW) contaminated by radioactive cesium (r-Cs) has been treated by various thermal treatment facilities in Japan. However, the behavior of r-Cs in their plants is scarcely known. Therefore, we investigated 4 types of the thermal treatment plants to understand the r-Cs behavior during different thermal treatments of the MSW. The distribution of r-Cs between residue discharged from bottom of furnace (bottom ash, incombustibles or slag) and fly ash was affected by furnace type. The leaching rate of r-Cs from the fly ash and the bottom residues were measured. The leaching rate from the former was high and ranged from 40 to 100%. In contrast, the most of r-Cs in the latter did not dissolve into water. Furthermore, we evaluated whether baghouse (an air-pollution control equipment) successfully removed r-Cs compound in flue gas. R-Cs was efficiently removed regardless of thermal treatment type. INTRODUCTION A huge amount of radioactive substances was released due to the Fukushima Daiichi Nuclear Power Plant accident on March 11, 2011. The environment in eastern part of Japan was widely contaminated by the fallout of r-Cs, resulting in generation of radioactively contaminated MSW. In Japan, a densely populated country, waste thermal treatments such as incineration and ash-melting process have played a significant role in MSW treatment, because they can reduce mass and volume of the waste by 10% to 20%. After the accident, it was found r-Cs in MSW was highly concentrated into ash from the incinerator. Safety handling of the ash requires understanding the behavior of r-Cs in the MSW thermal treatment plants. Therefore, we investigated the behavior of r-Cs in various types of the plants. METHOD In this study, actual thermal treatment plants dealing with contaminated MSW were investigated. The specification for each facility is summarized in TABLE I. Stoker type incinerator discharges bottom ash and fly ash as incineration ash, whereas the others such as fluidized bed type incinerator and melting furnace generate fly ash only. The former and latter discharge incombustibles and slag from individual furnace bottom, respectively. Therefore thermal treatment solid residues can be divided to two resides; 1) residue from bottom of furnace such as bottom ash, incombustibles and slag, 2) fly ash collected by baghouse. Although there are various kinds of radioactive nuclei, we focused on only r-Cs in this research because the amount of r-Cs emission was so much larger. Concentration of r-Cs was expressed as the sum of Cs-134 and Cs-137 activities. We measured the r-Cs concentration of the solid residues as well as flue gas. Using mass balance based on actual plant operation data, the r-Cs WM2016 Conference, March 6 – 10, 2016, Phoenix, Arizona, USA 2 concentration data were converted into the r-Cs distribution among the solid residues and flue gas. In addition, leachability of r-Cs from the solid residues was investigated as follows; first, an amount of the sample was added to pure water in a plastic bottle, where the weight ratio of water to the sample was 10. Second, the solution was stirred or shaken for 6 hours, and then solution was separated by filtration. The leaching rate was determined by measuring the r-Cs concentration of the filtrate. TABLE I. Overview of each thermal treatment plant. RESULT Distribution of r-Cs between fly ash and bottom residue is shown in Figure 1. 60 to 99% of R-Cs in MSW moved to fly ash in all type of plants. In the case of the stoker type incinerators, the distribution of r-Cs to bottom ash was relatively high compared with that to bottom residues in the other types. In terms of the r-Cs concentration ratio of fly ash to bottom residue, the ratio for melting furnaces (facility E and F) exceeded 20, while that for all the incinerators (facility A to D) up to 10. There is no significant difference between the two types of incinerators. These results indicate as follows; 1) Stoker type incinerator tends to leave r-Cs in bottom ash to some extent (30 to 40%). 2) In the melting furnaces most of the r-Cs is distributed to fly ash because they basically operate at a much higher temperature than the incinerators. Especially shaft-type melting furnace which sometimes operate over 1600 °C. Facility NO.of date Furnace type Bottom residue Temp. (°C) A 1 Stoker type incinerator Bottom ash