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Journal Articles

A Review of Cs-bearing microparticles in the environment emitted by the Fukushima Dai-ichi Nuclear Power Plant accident

Igarashi, Yasuhito*; Kogure, Toshihiro*; Kuribara, Yuichi; Miura, Hikaru*; Okumura, Taiga*; Satou, Yukihiko; Takahashi, Yoshio*; Yamaguchi, Noriko*

Journal of Environmental Radioactivity, 205-206, p.101 - 118, 2019/09

 Times Cited Count:29 Percentile:76.47(Environmental Sciences)

Scientists face challenge in identifying the radioactive materials which are found as dotted images on various imaging plate (IP) autoradiographic photos of radioactively contaminated materials by the Fukushima Dai-ichi Nuclear Power Plant (F1NPP, or FDNPP) accident, such as air filter, fugitive dust, surface soil, agricultural materials, and water-shed samples. It has been revealed that they are minute particles with distinct morphology and elemental composition with high specific radioactivity, and different from those of the so-called Chernobyl hot particles. Basically, they are glassy particles once molten, composed of Si, O, Fe, Zn etc. with highly concentrated radiocaesium, which can be called as radiocaesium-bearing microparticles (CsMP). At present, CsMP can be classified into two types, Types-A and -B, which are characterized by different specific radioactivity, $$^{134}$$Cs/$$^{137}$$Cs ratio, size and morphology, and geographic distribution around F1NPP. Such studies on the CsMP from various aspects have provided valuable information about what happened in the nuclear reactors during the F1NPP accident and fates of the CsMP in the environment. This review first provides a retrospective view on the research history of the CsMP, which is helpful to understand the unique character of the CsMP. Subsequently, more details about the current understanding of the natures of these hot particles, such as origin, morphology, chemical compositions, thermal properties, water-solubility, and secondary migration of CsMP in river and ocean systems are described with future prospects.

Journal Articles

Inner structure and inclusions in radiocesium-bearing microparticles emitted in the Fukushima Daiichi Nuclear Power Plant accident

Okumura, Taiga*; Yamaguchi, Noriko*; Dohi, Terumi; Iijima, Kazuki; Kogure, Toshihiro*

Microscopy, 68(3), p.234 - 242, 2019/06

 Times Cited Count:5 Percentile:70.99(Microscopy)

Radiocesium-bearing microparticles (CsMPs), consisting substantially of silicate glass, were released to the environment during the Fukushima nuclear accident in March 2011. We investigated a total of nine CsMPs using transmission electron microscopy (TEM) and inferred the atmosphere in the reactors during the accident. From elemental mapping using energy-dispersive X-ray spectrometry, Fe and Zn showing radial inhomogeneities were found in the CsMPs, in addition to the Cs that had been previously reported. Four of the CsMPs included submicron crystals, which were identified as chromite, franklinite, acanthite, molybdenite, and hessite. The chromium-containing spinels, chromite and franklinite, indicated the presence of ferrous iron (Fe$$^{2+}$$), suggesting that the inside of the reactors was reductive to some extent. Electron energy-loss spectroscopy also confirmed that the CsMPs did not contain boron, and therefore the atmosphere in which they were formed might be boron-free.

Journal Articles

Dissolution behaviour of radiocaesium-bearing microparticles released from the Fukushima nuclear plant

Okumura, Taiga*; Yamaguchi, Noriko*; Dohi, Terumi; Iijima, Kazuki; Kogure, Toshihiro*

Scientific Reports (Internet), 9(1), p.3520_1 - 3520_9, 2019/03

 Times Cited Count:22 Percentile:90.33(Multidisciplinary Sciences)

Radiocesium-bearing microparticles (CsMPs) were released by the FDNPP accident. We conducted dissolution experiments of CsMPs by reaction with pure water absorbing CO $$_{2}$$ from the atmosphere and seawater. The activation energy for the dissolution of CsMPs was estimated to be 67 and 89 kJ/mol, and the dissolution rate at 13$$^{circ}$$C was 0.011 and 0.130 $$mu$$m/y for pure water and seawater, respectively. Probably the faster dissolution rate in seawater than in pure water is mainly owing to the difference in pH. The shapes of CsMPs dissolved in pure water were considerably altered. Tin oxide and iron oxide nanoparticulates were formed on their surfaces. Such features were similar to those observed in a CsMP collected recently in Fukushima Prefecture, indicating that dissolution of CsMPs is also occurring in the environment. In the case of CsMPs dissolved in seawater, a crust of secondary minerals rich in Mg and Fe was formed and the glass matrix became smaller inside the crust.

Journal Articles

Secondary radiocesium contamination of agricultural products by resuspended matter

Nihei, Naoto*; Yoshimura, Kazuya; Okumura, Taiga*; Tanoi, Keitaro*; Iijima, Kazuki; Kogure, Toshihiro*; Nakanishi, Tomoko*

Journal of Radioanalytical and Nuclear Chemistry, 318(1), p.341 - 346, 2018/10

 Times Cited Count:1 Percentile:17.98(Chemistry, Analytical)

Journal Articles

Loss of radioactivity in radiocesium-bearing microparticles emitted from the Fukushima Dai-ichi Nuclear Power Plant by heating

Okumura, Taiga*; Yamaguchi, Noriko*; Dohi, Terumi; Iijima, Kazuki; Kogure, Toshihiro*

Scientific Reports (Internet), 8, p.9707_1 - 9707_8, 2018/06

 Times Cited Count:8 Percentile:47.43(Multidisciplinary Sciences)

Radiocesium-bearing microparticles (CsP) substantially made of silicate glass are a novel form of radiocesium (RCs) emitted from the Fukushima Dai-ichi NPP. CsPs have a potential risk of internal radiation exposure caused by inhalation. Radiation-contaminated waste (Rcw) including CsPs is being burned in incinerators; therefore, this study has investigated the responses of CsPs to heating in air. The radioactivity of CsPs gradually decreased from 600 $$^{circ}$$C and was almost lost when the temperature reached 1000 $$^{circ}$$C. The size and spherical morphology of CsPs were almost unchanged after heating, but Cs including RCs, K and Cl were lost, probably diffused away from the CsPs. When the CsPs were heated together with weathered granitic soil that is common in Fukushima, the RCs released from CsPs was sorbed by the surrounding soil. From these results, it is expected that the radioactivity of CsPs will be lost when Rcw including CsPs is burned in incinerators.

Oral presentation

Decontamination test of radioactive contaminated soil by Cs-free mineralization with alkaline chlorides

Shimoyama, Iwao; Kogure, Toshihiro*; Okumura, Taiga*; Baba, Yuji; Okamoto, Yoshihiro

no journal, , 

In this work, we examined Cs-free mineralization (CFM) for decontamination of Fukushima soil with four alkaline chloride additives below 800$$^{circ}$$C in air and under low-pressure conditions. In the case of CaCl$$_{2}$$, radioactivity concentration largely decreased in both air and low-pressure conditions, and decontamination ratio reached to 97% at 790$$^{circ}$$C. In the case of MgCl$$_{2}$$, we found a tendency that decontamination ratio was higher under low-pressure condition than in air and it was more than 99% after three cycles of heat treatments at 695$$^{circ}$$C. KCl and NaCl reagents also showed similar tendency, and decontamination ratio reached to 93% and 97% for KCl and NaCl, respectively, after five cycles of heat treatments at 790$$^{circ}$$C. Structural analysis of products clarified that heat treatments with CaCl$$_{2}$$ and MgCl$$_{2}$$ decomposed biotite which is the major Cs sorbent in the soil in Fukushima. On the other hand, similar decomposition was not observed when KCl and NaCl were used as additives. Based on these results, CFM was confirmed to be effective for soil decontamination, and we found that Cs removal process depends on the valence of metal ions as chlorides.

Oral presentation

Synergetic effects of chloride reactants and low-pressure environment for soil decontamination using cesium-free mineralization

Shimoyama, Iwao; Kogure, Toshihiro*; Okumura, Taiga*; Baba, Yuji

no journal, , 

Cs removal processes from soil largely depend on reagents and pressure in cesium free mineralization. We have proposed a hypothesis that monovalent cations remove Cs from clay minerals through ion exchange and divalent cations remove Cs through phase transformation of clay minerals based on results using CaCl$$_{2}$$ and KCl reagents. In this work, we used MgCl$$_{2}$$ and NaCl reagents to verify this hypothesis and studied decontamination ratio and structural change of mineral phase in soil. We found that the examination with these reagents supports our hypothesis, but both phase transformation and ion exchange were enhanced under low-pressure conditions. Because sea water includes NaCl and MgCl$$_{2}$$ as major components, we attempted to use sea water salt as a reagent and demonstrated that soil can be decontaminated by about 99% after heat treatment at 790 $$^{circ}$$C under low pressure conditions. This result indicates the processing cost of heat treatment can be decreased using cheap sea water.

Oral presentation

Change of structure and radioactivity in radioactive particles generated by the Fukushima nuclear accident by heating

Okumura, Taiga*; Kogure, Toshihiro*; Yamaguchi, Noriko*; Dohi, Terumi; Iijima, Kazuki

no journal, , 

no abstracts in English

Oral presentation

Soil decontamination using cesium-free mineralization with alkaline chloride; Cs removal processes depending on valence of cations in reactants

Shimoyama, Iwao; Kogure, Toshihiro*; Okumura, Taiga*; Baba, Yuji

no journal, , 

We study decontamination ratio and structural change of soil after heat treatments with four kinds of alkaline chloride reagents in air and under low-pressure conditions to optimize soil decontamination method. Each reagent was added to contaminated soil with the weight ratio of 1/1, and we measured radioactivity and analyzed crystal phases of the samples after the heat treatments for 2 hours. In the case of CaCl$$_{2}$$, only small differences in decontamination ratio were observed in air and low-pressure conditions, and decontamination ratio reached to about 95% at 790$$^{circ}$$C. After the processing, biotite in soil decomposed and products depended on in air and under low-pressure conditions. Whereas, in the case of MgCl$$_{2}$$, decontamination ratios were 40 and 90% after the heat treatments at 695$$^{circ}$$C in air and under low-pressure conditions, respectively. We found that decomposition and phase transformation of biotite were more facilitated under low-pressure conditions than in air. Also, in the cases of NaCl and KCl, soil decontamination was more facilitated under low-pressure conditions; decontamination ratios were 15 and 94% after the heat treatments with NaCl at 790$$^{circ}$$C in air and under low-pressure conditions, respectively. However, biotite remained after the heat treatments both in air and under low-pressure conditions. Based on this result, we conclude that ion exchange was the dominant Cs removal process for NaCl and KCl.

Oral presentation

Dissolution of radiocesium-bearing microparticles released by the Fukushima Nuclear accident

Okumura, Taiga*; Yamaguchi, Noriko*; Dohi, Terumi; Iijima, Kazuki; Kogure, Toshihiro*

no journal, , 

To determine dissolution rates of radiocesium-bearing microparticles (CsMPs) released by the Fukushima Nuclear accident, we conducted dissolution experiments of CsMPs by reaction with pure water and seawater at several temperatures. According to the experiments, CsMPs may be completely dissolved and extinguished in several decades and several years in pure water and seawater, respectively.

Oral presentation

Soil decontamination by heat treatments with chlorides under low-pressure conditions

Shimoyama, Iwao; Kogure, Toshihiro*; Okumura, Taiga*; Baba, Yuji

no journal, , 

More than 99% of radioactive Cs can be removed from soil by heat treatment at more than 1000$$^{circ}$$C, however, its high cost inhibits application of this method. In this work, we attempted heat treatment with four kinds of chloride reagents under low-pressure condition to decrease processing temperature. Each reagent was added to contaminated soil and heat treatment was applied in air and under low-pressure conditions for two hours. In the case of CaCl$$_{2}$$, the difference in decontamination ratio was small for different pressure conditions and decontamination ratio reached to 95% at 790$$^{circ}$$C. On the other hand, MgCl$$_{2}$$, NaCl, and KCl had higher decontamination ratios under low-pressure conditions than in air. When MgCl$$_{2}$$ was added, decomposition and phase transformation of biotite in soil was facilitated under low-pressure conditions, and decontamination ratios at 695$$^{circ}$$C were 40 and 90% in air and under low-pressure conditions, respectively. In the cases of NaCl and KCl, biotite remained after the heat treatments and Cs was removed through ion exchange. When NaCl was added, decontamination ratios at 790$$^{circ}$$C were 15 and 94% in air and under low-pressure conditions, respectively. Based on these results, we propose heat treatment with sea water salt as a reactant.

Oral presentation

Dissolution behavior of radiocesium-bearing microparticles released from the Fukushima Nuclear Plant

Okumura, Taiga*; Kogure, Toshihiro*; Yamaguchi, Noriko*; Dohi, Terumi; Fujiwara, Kenso; Iijima, Kazuki

no journal, , 

Radiocesium-bearing microparticles (CsMPs) released from the Fukushima Nuclear Power Plant were revealed to be dissolved much more rapidly in seawater than pure water. In the case of hydrochloric acid, the radioactivity of CsMPs considerably decreased at the beginning, but ceased to change with time. Secondary minerals were precipitated in the CsMPs dissolved in seawater, whereas a passive layer was formed around the surface of CsMPs dissolved in hydrochloric acid.

Oral presentation

Distinction of radioactive particles derived from the Fukushima nuclear accident based on different heating characteristics

Okumura, Taiga*; Kogure, Toshihiro*; Yamaguchi, Noriko*; Dohi, Terumi; Fujiwara, Kenso; Iijima, Kazuki

no journal, , 

Most of the radiocesium released by the Fukushima nuclear accident was sorbed to clay minerals such as weathered biotite (WB), whereas radiocesium-bearing microparticles (CsMPs) were also emitted from the damaged reactors. In this study, we report CsMPs with smaller size and lower radioactivity and suggest the way to distinguish CsMPs from other radioactive particles. Radioactive particles were collected from non-woven fabric cloth and contaminated soil and identified by SEM-EDS, and their radioactivity was measured by IP autoradiography. As a result, five CsMPs of less than 0.05 Bq and two WB particles of more than 0.05 Bq were identified, indicating the presence of CsMPs with lower radioactivity than WB. Next, the contaminated soil was uniformly scattered on the substrate to which a heat-resistant adhesive was applied, and IP autoradiography was performed before and after heating at 800 $$^{circ}$$C. It was suggested that CsMPs could be distinguished by the heating treatment.

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