Vol.9 No.3 2017

Research paper : Radioactive cesium decontamination technology for ash (T. Kawamoto et al.)−143−Synthesiology - English edition Vol.9 No.3 (2017) poor selectivity for cesium uptake and the high concentration of other ions in the ash washing water.2.2.2 Mass production of the PB nanoparticles and adsorbent developmentMass production of the PB nanoparticles was conducted through joint research with Kanto Chemical Co., Inc. Our fundamental strategy was the following: AIST determined the appropriate composition and crystal structure for the radioactive adsorbent. The manufacturing method was also investigated among the methods available for mass-production. Finally, this technology was transferred to the company, and Kanto Chemical improved the manufacturing method for mass production.We were successful at developing the PB nanoparticles exhibiting high cesium adsorption capacity, but this material was normally obtained in the form of powder or slurry. For radioactive cesium removal from ash washing water, solid–liquid separation with powder or slurry must be done after adding the powder to washing water.[46] However, safety concerns were often raised for the powder addition method for the concentration of radioactive materials. Especially, the risk of system trouble exists because an engineer must handle the adsorbent powder with high concentrations of radioactive cesium. To avoid such a risk, with cooperation from companies, we started the development of another system with the adsorbent sealed in a container such as a column. If the adsorbent is enclosed in a column, even if clogging occurs, it would only be necessary to exchange the column. There would be no need to handle the adsorbent directly.First, we originally developed the adsorbent supported by cotton cloth, and confirmed its sufficient adsorbent properties.[47] Next, through collaboration with Kanto Chemical and Japan Vilene Co., Ltd., we developed adsorbents of two types: a granular one[34][36] and an adsorbent-immobilized nonwoven one[38][48] (Fig. 6). The granules contain 80 % PB nanoparticles with high adsorption capacity. The nonwoven adsorbent has a large surface area that supports high-speed adsorption. These adsorbents are used for another purpose: concentration of radioactive material in environmental water for pretreatment of the trace analysis and pond water decontamination. For this development, computational approaches such as a simulation of adsorption performance were also used.[49]2.2.3 System development of volume reduction of incinerated ashWe sought to create an ash washing system using the adsorbents. Our goal was to reduce the volume of contaminants generated in environmental decontamination work by the government. The government planned that the contaminated materials would be collected and incinerated in a newly built temporary incinerator.[50] Therefore, all facilities including the washing device and the storage will be newly constructed. Considering this situation, we designed the most efficient method for volume reduction of waste. We integrated the components from incineration, ash washing, recovery of radioactive cesium by adsorbents, and a storage of used adsorbents.For the pilot plant for demonstration at a realistic scale, we designed each component as follows: as an incinerator, a biomass boiler was used. To remove radioactive cesium completely from the exhaust gas and to study the contaminant behavior, a three-step treatment device was installed for the exhaust gas, a cyclone, a bag filter, and a HEPA filter. We also confirmed that the addition of calcium chloride during incineration improves the extraction rate of radioactive cesium by washing at a later stage. Particularly, the “main ash” recovered from the bottom of the incinerator presents difficulties for extraction of radioactive cesium by washing. Those difficulties are different from those related to the “fly ash” recovered from the chimney. Nevertheless, we were able to achieve a high extraction rate of 60 % by adding calcium chloride.[6]Regarding the washing process, we tried to reduce the water demand. For decontamination activities, great concern has arisen in relation to contaminated water processing because the handling of contaminated water storage at the Fukushima Daiichi Nuclear Power Plant is a serious matter, and because drinking water and agricultural water might be directly affected if environmental water is contaminated. Therefore, it is important to reduce the amount of water used to the greatest degree possible. For cases in which water discharge is not permitted even if it is properly treated, water treatment without discharge is necessary. By reduction of the water amount, it is possible to avoid discharge, e.g. by evaporation. A schematic diagram of the plant for ash washing and recovery by adsorbents is portrayed in Fig. 7. Our design has three important points: (1) use of a vibrating granulator to mix the ash with as little water as possible, (2) avoiding clogging by increasing the Fig. 6 Radioactive cesium adsorbents that contain PB nanoparticlesLeft: Granular adsorbent made by Kanto Chemical. Right: Nonwoven fabric supported adsorbent made by Japan Vilene.5 mm5 mmWith nonwoven-berWith Granules

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