INCINERATION OF OFF-SITE CONTAMINATED WASTE AFTER THE FUKUSHIMA DAIICHI NUCLEAR POWER PLANT ACCIDENT

Transcription

1 INCINERATION OF OFF-SITE CONTAMINATED WASTE AFTER THE FUKUSHIMA DAIICHI NUCLEAR POWER PLANT ACCIDENT H. FUJIWARA*, K. YUI*, K. ITO*, K. NODA*, H. KURAMOCHI* AND M. OSAKO* * Center for Material Cycles and Waste Management Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki, , Japan, kuramochi.hidetoshi@nies.go.jp SUMMARY: After the accident at the Fukushima Daiichi Nuclear Power Plant in Japan in 2011, three types of off-site radioactive waste (municipal solid waste (MSW), decontamination waste (DW) from decontamination activity, and disaster waste) have been generated. These wastes must be incinerated to reduce their mass and volume. MSW has been incinerated by either stoker-type or fluidized-bed-type incinerators, and also melted by fluidized-bed gasification and melting or direct melting (shaft-type) systems. In our previous studies, we investigated the characteristics of solid residues such as fly ash () and bottom ash (BA), incombustible materials, and slag discharged from such thermal treatment facilities, focusing on the radiocesium (r-cs) concentration level and r-cs leaching rate as characteristics, and the r-cs distribution to the solid residues. Both the r-cs concentration in (dust in flue gas) and the r-cs leaching rate from were much higher than in the case of materials discharged from the furnace bottom. Although the r-cs distribution to was higher than to the other materials, the magnitude of distribution depended on the type of thermal treatment, in the following order: shaft-type melting > gasification and melting fluidized-bed incineration > stoker incineration. In the case of DW, most has been incinerated in temporary stokertype incineration facilities, with some melted in ta temporary direct melting system. Since DW and MSW differed significantly in their constituent components, there were large differences in the r-cs behavior during thermal treatment, and in the respective r-c leaching rates from treatment residues. The r-cs concentration in DW-BA was close to that in DW-. As a result, the r-cs distribution to DW-BA was enhanced by up to 70-80%. We revealed that the leaching rate from DW- was lower than from MSW- as an unique nature of DW thermal treatments. 1. INTRODUCTION After the accident at the Fukushima Daiichi Nuclear Power Plant in Japan in 2011, the environment of north-eastern Japan was widely contaminated by radioactive cesium (r-cs) fallout from the plant; including, unfortunately, even sewage sludge and municipal solid waste (MSW). As a result, a few months after the accident, significant levels of r-cs were discovered in this region, in incineration ash discharged from sewage sludge and MSW incineration plants [1]. In particular, high levels of r-cs in Proceedings CRETE 2018, Sixth International Conference on Industrial & Hazardous Waste Management Chania Crete Greece; 4 7 September 2018 ISSN:

2 fly ash () [2] and a high leaching rate of r-cs from [3] were reported. In Japan, thermal treatments such as incineration and melting processing have played an important role in effective treatment of MSW. Currently, 80% of MSW is incinerated [4], and local municipalities are responsible for such treatment. Thus, the contamination was a serious concern for local municipalities and citizens. To effectively treat and dispose of such contaminated waste, therefore, the Ministry of the Environment (MOE) created technical standards for contaminated waste treatments, and corresponding technical guidelines, under an act governing special measures for the handling of contaminated waste [5]. However, these guidelines did not provide information for better understanding of r-cs behavior during MSW incineration, or of the causes of highly contaminated and high r-cs leaching rate from. Hence, we began to investigate the behavior of r-cs during the incineration of contaminated MSW, and the characteristics of the relevant incineration residues. In the present study, we reviewed the r-cs concentration in solid residues, the r-cs distribution to the residues, and the respective r-cs leaching properties, for various types of treatments, and discussed the differences among these treatment types. In addition to the contaminated MSW, combustible disaster waste, such as demolition wood generated by the massive tsunami had to be rapidly incinerated to reduce the volume in some prefectures around Fukushima prefecture. Although this waste contained a high concentration of ash, the radioactivity level was relatively low and the distribution to was roughly similar to that observed in MSW incineration [6]. Thus, the present study did not focus on such disaster waste. Since 2015 (four years after the accident), the decontamination waste (DW) discharged from decontamination activities in the Special Decontamination Area has been incinerated at temporary facilities [7]. The national government is responsible for the implementation of this treatment. The DW has differed from the contaminated MSW in terms of radioactivity level and components, with the former being much higher in radioactivity, and composed of contaminated soil and vegetation, as shown in Figs. 1a and 1d. Therefore, we here investigated DW incineration and melting processing at a number of temporary facilities, with respect to the same items as in our previous investigation of treatments for contaminated MSW. In addition, we compared the investigation results for the MSW and DW treatments, and will discuss the effect of waste type on the r-cs behavior and r-cs leaching properties of solid residues. 2. METHOD 2.1. R-Cs concentration in incineration residues, and r-cs distribution to each residue Earlier investigations of thermal treatment plants dealing with contaminated MSW, including our own studies [8 14], focused on the r-cs concentration in and furnace-bottom residues such as BA, incombustibles, and slag; as well as the r-cs distribution to and the various furnace-bottom residues. In the present study, 12 stoker-type incinerators (SIs), 2 fluidized-bed-type incinerators (FBIs), 1 fluidized-bed gasification and melting system (FGMS), and 1direct melting system (DMS) were investigated as the target MSW incinerators and melting systems (Figs. 1a-1d). Although the predominant contaminants were two kinds of cesium radioisotopes (Cs-134 and Cs-137), we focused only on the total activity of r-cs. Thus, r-cs activity in this paper is expressed as the sum of Cs-134 and Cs-137 activity. In our previous studies, the r-cs concentration in solid samples was measured using Ge semiconductor detectors. Here, employing a mass balance equation based on actual plant operation data, the r-cs distribution ratios between and the various bottom residues were determined from the r-cs concentration data, as follows: CRETE Sixth International Conference on Industrial & Hazardous Waste Management 2

3 R-Cs distribution ratio in (or bottom residue (BR)) (%) = R-Cs concentration of (or BR)(Bq/kg) weight of (or BR)(kg) 100 (1) R-Cs concentration of (Bq/kg) weight of (kg) + R-Cs concentration of BR (Bq/kg) weight of BR (kg) 2.2. R-Cs leaching rate from incineration residues To evaluate the safety of incineration residues, we measured the r-cs leaching rate from each form of residue, according to a domestic standard leaching test method (JIS K0058-1). First, a solid sample amount was measured and added to pure water in a plastic bottle, where the weight ratio of the water to the sample was fixed at 10. The solution was then stirred or shaken at 200 rpm for six hours, and separated by filtration. The leaching rate was determined by measuring the r-cs concentration in the filtrate: R-Cs leaching rate (%) = (R-Cs concentration in filtrate (Bq/kg) weight of filtrate (kg)) (R-Cs concentration in solid residue (Bq/kg) weight of solid residue (kg)) 100. (2) In our previous studies [15 17], similar characterization and r-cs distribution analysis was performed in the case of three DW-SIs and one DW-DMS. Here, we discuss the effect of feedstock type on their respective characteristics, and the r-cs distribution to, BA, and slag. MSW or DW fly ash (from MSW) MSW fly ash bottom ash (from MSW) Incombustible, metal, sand Figure 1a. Stoker incinerator (SI) Figure 1b. Fluidized-bed incinerator (FBI) lime and cokes MSW melting furnace fly ash MSW or DW fly ash (from DW) Incombustible, metal, sand combustion chamber Figure 1c. Fluidized-bed gasification melting system (FGMS) slag + O 2 slag (from DW), and metals Figure 1d. Direct melting system (DMS) CRETE Sixth International Conference on Industrial & Hazardous Waste Management 3

4 3. RESULTS AND DISCUSSION 3.1. R-Cs concentration in solid residues The r-cs concentrations in and bottom residues in the aforementioned reports are summarized in Figs. 2a and 2b. Basically, r-cs radioactivity was much higher in the than in the bottom residues. In the case of stoker incineration, the is typically composed of solidified volatile chemicals, additives for flue gas cleaning such as slaked lime for removal of acid gas species (hydrochloric acid and sulfur oxides), and fine particles blown up from the primary combustion zone into the flue gas. The weight ratio of the blown dust to the BA is expected to be small [18]. Hence, we suggest that the main source of r-cs distribution to is the solidification of gaseous r-cs species on the surface of dust particles during the flue gas cooling process (< 200 C) after the combustion of contaminated MSW. This seems reasonable because alkaline metals such as Cs are considered to be relatively volatile elements [19]. However, it should be noted that Cs usually forms electrolytes, such as CsCl, due to its very low ionization energy. Furthermore, there were obvious differences in the r-cs concentration ratios between and the bottom residues, among the various types of MSW thermal treatment, as shown in Figs. 2a and 2b. The ratios for FGMS and DMS exceeded 18 and 130, respectively, while the ratio for all of the incinerators was around 5-6. This significant enrichment of due to these melting processes is attributed to promotion of r-cs vaporization by the higher treatment temperatures. From the point of view of safety, we must pay careful attention to this higher r-cs concentration in, even if the radioactivity in MSW is relatively low. Slag, meanwhile, is safer in this respect than BA. In the case of DW treatments, the r-cs concentration levels in and BA during DW incineration were relatively similar in each case. DW incineration residues may be relatively easy to manage for storage because of their similar radioactivity levels. However, DW incineration produces BA with a much higher concentration of r-cs than MSW incineration, as shown in Fig. 2a. Thus, we should consider radiation protection not only from but also BA in temporary DW incineration facilities. Figure 2a. Relationship of r-cs concentration in bottom ash (BA) and fly ash () discharged from a stoker incinerator (SI). DW: decontamination waste; MSW: municipal solid waste; solid line: average value for the r-cs concentration ratio of to BA; dashed line: correlation results for y= ax. Figure 2b. Relationship of r-cs concentration in bottom residue [incombustibles for FBI (fluidized-bed incinerator) and slag for FGMS (fluidized-bed gasification and melting system) and DMS (direct melting system)] and fly ash (). DW: decontamination waste; MSW: municipal solid waste; solid line: average value for the r-cs concentration ratio of to BA; dashed line: correlation results for y= ax. CRETE Sixth International Conference on Industrial & Hazardous Waste Management 4

5 3.2. R-Cs distribution behavior during thermal treatments The results for r-cs distribution to solid residues in 11 MSW treatment plants (Fig. 2) revealed that 60-94% of the r-cs in the MSW was transferred to during MSW thermal treatments. The order of r-cs distribution to was DMS > FGMS FBI > SI. Although the two melting processes and FBIs showed greater r-cs distribution to, the reason for the former differs from that of the latter. In the FBI, most of the ash content is discharged as instead of BA; whereas, in the melting processes, more vaporized r-cs is transferred to dust, as described above. In addition, the distribution to in the FGMS was about 10% less than in the DMS. The distribution in the melting process is affected by the furnace type, because the vapor pressure of Cs electrolytes rises as the temperature increases [20]. We consider that the relatively lower melting temperature (1,300-1,400 C) in the FGMS leads to the lower distribution to. The DMS, operating at its highest temperature from 1,350 to 1,650 C, can vaporize more than 96% of r-cs in waste, and simultaneously produce cleaner slag, some of which can be used without regulation as construction materials. Fig. 3 shows the r-cs distribution between and BA during DW incineration in 3 facilities, and also that between and slag in a DMS for DW. Roughly 80% of the r-cs in DW was distributed to DW-BA, in spite of there being little difference in the r-cs concentration between DW- and DW- BA (see Fig. 2a). This distribution behavior is completely opposite to that of MSW incineration, and is the reason why DW has less Cl available for r-cs vaporization as gaseous CsCl, as described below. The high distribution to BA indicates that there is a large amount of BA with high r-cs content stored in temporary facilities. In future, the BA must be treated for further volume reduction, due to limited Interim Storage Facility (ISF) storage space; and the DMS may offer a means to address this, because the direct melting process in the presence of CaCl2 (r-cs distribution to slag < 0.2% ) has the potential to produce the cleanest slag [17]. MSW treatments DW treatments Figure 3. R-Cs distribution to fly ash () and bottom residue including bottom ash (BA), incombustibles, and slag, at various thermal treatment facilities. MSW: municipal solid waste; DW: decontamination waste; SI: stoker incinerator; FBI: fluidized-bed incinerator; FGMS: fluidized-bet gasification and melting system; DMS: direct melting system R-Cs leaching properties of solid residues The r-cs leaching rates from solid residues are shown in Fig. 4. The leaching rate from MSW- ranged from 60-80%. Therefore, MSW- must be carefully disposed in landfill sites, and carefully stored in the ISF. On the other hand, most of the r-cs in MSW-BA did not dissolve into water. Our CRETE Sixth International Conference on Industrial & Hazardous Waste Management 5

6 previous theoretical studies [20, 21] and earlier work [e.g., 22] indicate that the main chemical form of r-cs is the chloride (r-cscl) with a high aqueous solubility in MSW-, while r-cs in BA exists in aluminosilicates (including glass phase). Although the leaching rate from DW-BA was similar to that from MSW-BA, the rate from DW- was much less than from MSW-. Even DW- from a DMS had a low r-cs leaching rate, though the DW-DMS vaporized most of the r-cs into flue gas. Compared with a MSW-DMS, the r-cs leachability of was significantly different. Therefore, the r-cs leaching rate from is dependent on the waste type, likely because the generation of alkali chlorides, including r-cscl, is suppressed by the lower Cl content in DW. Figure 5 shows an example of the element composition of and BA from DW and MSW incineration. The Cl content in MSW- was much higher than in DW-. The element composition of DW- was roughly the same as that of both DW-BA and MSW-BA, and this similarity suggests that the r-cs may be fine particles of DW-BA blown up in flue gas. With respect to safety management of r-cs leaching into the environment, DW incineration ash is safer than MSW incineration ash. However, there are massive amounts of DW-BA and DW-. These residues have thus far been transported to the ISF, and will be melted, beginning in 2020, to reduce their volume BA BA Incomb. slag slag slag R-Cs leaching rate (%) MSW-SI DW-SI MSW-FBI MSW-FGMS MSW-DMS DW-DMS Figure 4. Leaching rate of r-cs from various treatment residues. MSW: municipal solid waste; DW: decontamination waste; SI: stoker incinerator; FBI: fluidized-bed incinerator; FGMS: fluidizedbed gasification and melting system; DMS: direct melting system; : fly ash; BA: bottom ash; Incomb: incombustibles. ). 100 Element composition (%) BA BA Na Mg Al Si P S Cl K Ca Fe DW-SI MSW-SI Figure 5. Element composition of fly ash () and bottom ash (BA) discharged from MSW-SI (municipal solid waste stoker incineration) and DW-SI (decontamination waste stoker incineration). ). CRETE Sixth International Conference on Industrial & Hazardous Waste Management 6

7 4. CONCLUSION The incineration, including melting processing, of contaminated MSW and DW was investigated in terms of the r-cs concentration in solid residues, the r-cs distribution to the residues, and the r-cs leaching rate from them. The r-cs concentration in (represented by dust in the flue gas), the r-cs distribution to, and the r-cs leaching rate from were all much higher than in the case of furnace-bottom residues. However, the magnitude of the r-cs distribution to was affected by the type of thermal treatment, in the following order: DMS (direct melting system) > FGMS (fluidizedbed gasification and melting system) FBI (fluidized-bed incineration) > SI (stoker incineration). In particular, the treatment temperature and ash-discharge flow are crucial factors in the distribution. In the case of DW-SIs, the r-cs distribution behavior and the characteristics of the solid residues both differed significantly from MSW treatments, due to the difference in constituent elements; the r-cs concentration in DW-BA (bottom ash) was similar to that in DW-, and thus the r-cs distribution to DW-BA was enhanced by up to 70-80%. The leaching rate from DW- was less than from MSW- ; and in a DW-DMS, a much lower r-cs leaching rate from was observed, in spite of having the highest distribution ratio of r-cs to. These results suggest that the formation of gaseous r-cscl was suppressed by the lack of Cl content in DW. The current knowledge of DW (vs. MSW) thermal treatment is very important for effective ISF storage of solid residues from such treatment, with respect to safety management for radiation protection and the design of further volume reduction methods for these residues. Finally, in incineration facilities treating contaminated waste, refractory materials within the facilities are also contaminated, and the dose rates inside the furnaces and gas cooling towers increase as r-cs accumulates in such materials [23], with this accumulation being influenced by the waste type [24]. Based on the accumulation level and the differences among waste types, we should consider useful methods for decontaminating such contaminated refractory materials efficiently and safely, when maintaining or demolishing the relevant facilities. ACKNOWLEDGEMENT This work was supported by the Japan Environmental Storage and Safety Corporation (JESCO) as part of the decontamination activities of the Ministry of the Environment (Japan). REFERENCES [1] Takigami, H., Endo, K., Osako, M. (2013) The Great East Japan Erthquake Disaster and treatment of disaster waste (treatment of radioactively contaminated waste). Gakujutsu-to-doukou, (in Japanese) [2] df (in Japanese) [3] (in Japanese) [4] MOE (Ministry of the Environment) (2015) Ippan Haikibutsu no Haisyutsu oyobi Syori Jyoukyounado (Heisei 25 Nendo) nitsuite. (in Japanese) < recycle/ waste_ tech/ ippan/ h25/ data/ env_ press. pdf> [5] MOE (2011) Guidelines for Waste, < (in Japanese) [6] (in Japanese) [7] (in Japanese) [8] Abe, S., S. Kanbayashi, A. Sato, Y. Kamata and K. Nishimura (2012) Behavior of radioactive cesium in rotation surface melting furnace. Proceeding of 1st Annual Meeting of Society for Remediation of Radioactive Contamination in Environment, 48. (in Japanese) CRETE Sixth International Conference on Industrial & Hazardous Waste Management 7

8 [9] Yamamoto T., H, Takigami, T. Tanosaki, Y. Takeuchi, G. Suzuki, M. Osako, A. Kida and S. Sakai (2012) Behavior of radioactive cesium during incineration process. Proceedings of 23th Annual Conference of Japan Society of Material Cycles and Waste Management, (in Japanese) [10] Kawamoto, K., H. Kuramochi, T. Tanosak and Y. Takeuch (2012) Behavior of radioactive elements in an incineration facility with an ash-melting furnace. Proceedings of 23th Annual Conference of Japan Society of Material Cycles and Waste Management, (in Japanese) [11] Kawamoto, K., S. Mizuhara, M. Fukushima, T. Tanosaki and Y. Takeuchi (2013) Fate and characteristics of radioactive elements at two types of MSW incineration plants. Proceedings of 23th Symposium on Environmental Engineering, (in Japanese) [12] Osako, M., H. Kuramochi and H. Sakanakura (2012) Incineration of MSW containing radiocesium. Journal of Japan Waste Management Association, 65 (305): (in Japanese) [13] Kuramochi, H. (2014) Thermal treatment of waste contaminated with radioactive chemicals due to the accident at Fukushima Nuclear Power Stations: A review of recent research findings and introduction of some key literatures. Journal of the Society for Remediation of Radioactive Contamination in the Environment, 2 (2): (in Japanese) [14] Fujiwara, H., Kuramochi, H., Maeseto, T., Nomura, K., Takeuchim, Y., Kawamoto, K., Yamasaki, S., Kokubun, K., Osako, M. Influence of the type of furnace on behavior of radioactive cesium in municipal solid waste thermal treatment. Waste Management, submitted. [15] Fujiwara, H., Kuramochi, H., Maeseto, T., Nomura, K., Osako, M. (2017) Behavior of radioactive cesium during incineration of radioactively contaminated wastes from decontamination activities in Fukushima, J. Environ. Radioact., , [16] Fujiwara, H., Kuramochi, H., Ito, Maeseto. T., Osako, M., (2017) Investigation on radiocesium partitioning behavior and its leaching property of incineration residues during temporary incineration plants: difference among types of waste. Proceedings of the 6th Annual Meeting of the Society for Remediation of Radioactive Contamination in the Environment, 68. (in Japanese) [17] Noda, H., Kuramochi, H., Ito, K., Suzuki, H., Yoshimoto, Y., Yoshimoto N., Nagata, T., Koshida H., Takaoka M., Osako M. BEHAVIOR OF RADIOACTIVE CESIUM(R-Cs) IN A DIRCT MELTING SYSTEM CILITY AND A METHOD FOR PROMOTING EVAPORATION OF R-Cs. Proceedings of the 6th International Conference on Industrial and Hazardous Waste Management (2018). [18] Ginsberg, T., Liebig, D., Modigell, M., Sundermann, B., (2012) Multizonal thermochemical modelling of heavy metal transfer in incineration plants. Process Saf. Environ. 90 (1), [19] MacKay, K.M., R.A. MacKay and W. Henderson (2002) Introduction to Modern Inorganic Chemistry 6th ed., Nelson Thomas Ltd. Cheltenham, UK [20] Kuramochi, H., Fujiwara, H., Yui, K., (2016) Behavior of radioactive cesium during thermal treatment of radioactively contaminated wastes in the aftermath of the Fukushima Daiichi nuclear power plant accident. Global Environ. Res. 20, [21] Yui, K., Kuramochi, H., Osako, M., Understanding the behavior of radioactive cesium during the incineration of contaminated municipal solid waste and sewage sludge by thermodynamic equilibrium calculation. ACS Omega, submitted. [22] Shiota, K., Takaoka, M., Fujimori, T., Oshita, K., Terada, Y. (2015) Cesium speciation in dust from municipal solid waste and sewage sludge incineration by synchrotron radiation micro-xray analysis. Anal. Chem. 87, [23] Mizuhara, S., K. Kawamoto, T. Maeseto, H. Kuramochi and M. Osako (2015) Accumulative behavior of radioactive cesium during the incineration of municipal solid waste. Journal of the Society for Remediation of Radioactive Contamination in the Environment, 3(3): (in Japanese) [24] Mizuhara, S., T. Maeseto, Takeuchi, Y., Ito, K., H. Kuramochi and M. Osako (2017) Investigation on radiocesium accumulation inside a temporary incineration facility. Proceedings of the 6th Annual Meeting of the Society for Remediation of Radioactive Contamination in the Environment, 2. (in Japanese) CRETE Sixth International Conference on Industrial & Hazardous Waste Management 8