Scholarly article on topic 'Physical properties, structure, and shape of radioactive Cs from the Fukushima Daiichi Nuclear Power Plant accident derived from soil, bamboo and shiitake mushroom measurements'

Physical properties, structure, and shape of radioactive Cs from the Fukushima Daiichi Nuclear Power Plant accident derived from soil, bamboo and shiitake mushroom measurements Academic research paper on "Biological sciences"

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Abstract of research paper on Biological sciences, author of scientific article — Nobuo Niimura, Kenji Kikuchi, Ninh Duc Tuyen, Masakazu Komatsuzaki, Yoshinobu Motohashi

Abstract We conducted an elution experiment with contaminated soils using various aqueous reagent solutions and autoradiography measurements of contaminated bamboo shoots and shiitake mushrooms to determine the physical and chemical characteristics of radioactive Cs from the Fukushima Daiichi Nuclear Power Plant accident. Based on our study results and data in the literature, we conclude that the active Cs emitted by the accident fell to the ground as granular non-ionic materials. Therefore, they were not adsorbed or trapped by minerals in the soil, but instead physically adhere to the rough surfaces of the soil mineral particles. Granular Cs* can be transferred among media, such as soils and plants. The physical properties and dynamic behavior of the granular Cs* is expected to be helpful in considering methods for decontamination of soil, litter, and other media.

Academic research paper on topic "Physical properties, structure, and shape of radioactive Cs from the Fukushima Daiichi Nuclear Power Plant accident derived from soil, bamboo and shiitake mushroom measurements"

Journal of Environmental Radioactivity xxx (2014) 1—6

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Journal of Environmental Radioactivity

journal homepage: www.elsevier.com/locate/jenvrad

Physical properties, structure, and shape of radioactive Cs from the Fukushima Daiichi Nuclear Power Plant accident derived from soil, bamboo and shiitake mushroom measurements^

Nobuo Niimura2,*, Kenji Kikuchia, Ninh Duc Tuyen3,1, Masakazu Komatsuzakib, Yoshinobu Motohashic

a Frontier Research Center for Applied Atomic Sciences, Ibaraki University, 162-1 Shirakata, Tokai, Naka, Ibaraki 316-1106, Japan b Center for Field Science Research and Education, College of Agriculture, Ibaraki University, 3-21-1 Chuou, Ami, Inashiki, Ibaraki 300-0393, Japan c Countermeasure Department on Radiation, Takahagi City Hall, Kasuga 3-10, Takahagi, Ibaraki 318-8511, Japan

ARTICLE INFO

ABSTRACT

Article history:

Received 21 September 2013 Received in revised form 21 December 2013 Accepted 21 December 2013 Available online xxx

Keywords: Soil

Reagent solution Autoradiography Radioactive cesium Granular Fukushima Daiichi Nuclear Power Plant

We conducted an elution experiment with contaminated soils using various aqueous reagent solutions and autoradiography measurements of contaminated bamboo shoots and shiitake mushrooms to determine the physical and chemical characteristics of radioactive Cs from the Fukushima Daiichi Nuclear Power Plant accident. Based on our study results and data in the literature, we conclude that the active Cs emitted by the accident fell to the ground as granular non-ionic materials. Therefore, they were not adsorbed or trapped by minerals in the soil, but instead physically adhere to the rough surfaces of the soil mineral particles. Granular Cs* can be transferred among media, such as soils and plants. The physical properties and dynamic behavior of the granular Cs* is expected to be helpful in considering methods for decontamination of soil, litter, and other media.

© 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

1. Introduction

Discharge of radioactive Cs (134Cs and 137Cs, termed Cs*) from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident triggered by the earthquake and tsunami on 11 March 2011 contaminated a wide area of northeastern Japan (Kinoshita et al., 2011; Yasunari et al., 2011; Yoshida and Takahashi, 2012; Ohkura et al., 2012). Because the Cs* has been found to be non-water-soluble, it has been very difficult to decontaminate contaminated areas (Ohnuki and Kozai, 2013; Kozai et al., 2012). To elucidate the dynamics of transfer of Cs* between soil and flora as well as to

q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Corresponding author. Tel.: +81 29 352 3240; fax: +81 29 287 7872.

E-mail addresses: niimura@mx.ibaraki.ac.jp (N. Niimura), kikuchik@mx.ibaraki. ac.jp (K. Kikuchi), ductuyencnt@yahoo.com (N.D. Tuyen), komachan@mx.ibaraki. ac.jp (M. Komatsuzaki), ymotoha@net1.jway.ne.jp (Y. Motohashi).

1 Present address: Centre for Nuclear Techniques, 217 Nguyen Trai, 1 Dict, Hochiminh City, Vietnam.

ensure that uncontaminated food is protected against future contamination by Cs*, it is critical to establish the physical properties, structure, and shape of the Cs*. Modern techniques of structural analysis such as X-ray crystallography and fluorescent analysis could be used for these purposes, but require minimum picograms or nanograms of Cs* (about 1010 atoms of pure Cs*). Spatial concentrations, distributions, and depth profiles of Cs* have been measured to estimate doses (Kato et al., 2012; Tanaka et al., 2012). Those studies determined that the highest concentration of Cs* in the soil was ~ 105 Bq/kg (Tanaka et al., 2012), equivalent to ~2.3 x 10~10 mole/kg. Large-scale equipment is required to collect pure Cs* in the amount required for these experiments from contaminated soil. Even if the experimentally required amount of Cs* could be collected, the radiation level would be on the order of GBq—TBq, which would be challenging to handle in a typical laboratory. Therefore, they tend to use materials that include Cs, but which do not originate from the FDNPP accident. However, we must carefully consider whether these materials are appropriately representative.

Kaneyasu et al. collected Cs* in aerosols 47 days after the FDNPP accident at Tsukuba and measured the activity size distributions of

0265-931X/$ — see front matter © 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/joenvrad.2013.12.020

N. Niimura et al. / Journal of Environmental Radioactivity xxx (2014) 1—6

134Cs and 137Cs in the aerosols (Kaneyasu et al., 2012). They found that the activity median aerodynamic diameters in the first sample (28 April—12 May) were 0.54 and 0.53 mm, respectively, and those in the second sample (12—26 May) were both 0.63 mm. The activity size distributions of these radiocesium samples were similar to the mass size distribution of non-sea-salt sulfate but not to that of soils (Kaneyasu et al., 2012). The results indicated that the Cs* emitted from the FDNPP accident may have been granular, which is consistent with autoradiography images of contaminated plant leaves using imaging plates (IP) (Sakamoto et al., 2012; Nakajima et al., 2012). Black spots were observed on the contaminated leaves; the origin of the black spots may have been granular radioactive materials such as Cs*. In particular, images showing black spots on the leaves of tall cedar trees indicated that the Cs* fell on the leaves directly from the sky. Therefore, the Cs* likely also fell to the ground as granular radioactive materials. Very recently, Adachi et al. have reported that the shape of the Cs* collected using aerosol sampler was found to be spherical by a scanning electron microscope (Adachi et al., 2013).

In contrast, it has been stated that the Cs* that fell on the ground was adsorbed and trapped in soil minerals, explaining why the Cs* was not soluble in water. There have been many previous reports that radioactive and/or non-radioactive Cs is adsorbed into soil minerals (Tamura and Jacobs, 1989; Sawhney, 1970; Comans et al., 1991; Westrich et al., 1995; Ejeckama and Sherriff, 2005; Saiers and Hornberger, 1999; Ohnuki, 1994; Singh and Tandon, 1977; Hasany and Chaudhary, 2005). However, these papers were published before the FDNPP accident, and according to the mechanism described for Cs adsorption into the soil minerals, the Cs should exist in an ionic or atomic state.

After the FDNPP accident, there were also several reports that the Cs* was adsorbed into the soil minerals (Kogure et al., 2012; Kozai et al., 2012; Ohnuki and Kozai, 2013). For example, Kogure et al. carried out x-ray diffraction and high-resolution transmission electron microscopy experiments of the Cs ions trapped in new vermiculite clay. They showed that "ionic Cs" was fixed in the centers of hexagonal rings in the upper and lower silicate tetra-hedral sheets (Kogure et al., 2012). However, they used commercially available CsNO3 for the Cs+ ions in the samples (Kogure et al., 2012). It is not yet clear whether the Cs* emitted from the FDNPP was actually in an ionic state and thus, the experiment of Kogure et al. may not have simulated actual conditions. Ohnuki et al. studied the adsorption behavior of radioactive Cs by non-mica minerals, including kaolinite and 5 other minerals. For the Cs, they used commercially available radioactive and non-radioactive CsCl. This type of radioactive Cs and normal Cs are present in water in an ionic state and would be expected to be adsorbed into non-mica minerals (Ohnuki and Kozai, 2013). This experiment was also not intended to simulate the behavior of the Cs* emitted from the FDNPP accident. Kozai et al. examined the Cs* fallout on soils collected in Fukushima, Japan and characterized them with desorption experiments using appropriate reagent solutions (Kozai et al., 2012). They used contaminated soils containing Cs* emitted from the FDNPP accident, but analyzed all the obtained data assuming that the Cs* was adsorbed into the matrix of the soil minerals.

Because there are reports that the Cs* was in the form of an aerosol (Kaneyasu et al., 2012) and was observed as black spots in IP autography images (Sakamoto et al., 2012; Nakajima et al., 2012), additional experiments are required to determine whether the Cs* has been adsorbed into soil minerals. Based on the IP autoradiography of the leaves of tall trees (Sakamoto et al., 2012) and the activity size distributions of the aerosols (Kaneyasu et al., 2012), we may assume that granular Cs* fell onto the trees as well as on the ground. The physical and chemical characteristics of

the Cs* need to be examined using soils contaminated with Cs* from the FDNPP accident and moreover, we must consider whether the Cs* is present inside or outside the soil minerals, because it has not yet been clarified whether granular Cs* can be adsorbed into soil minerals. Therefore, in this study, we characterized the desorption behavior of Cs* in soil from the FDNPP accident using various reagent solutions. In addition, we considered autoradiography results for soils, plants, and other materials to determine the dynamic behavior of Cs*.

2. Experimental

2.1. Water washing of soils contaminated by the FDNPP accident

Contaminated soils from the playground of a primary school in Fukushima prefecture were collected on 5 May 2012. The place of the primary school is indicated in the map as shown in Fig. 1. Twenty grams of soil were immersed in 30 ml water while stirring at 200 rpm for 3 h at 20 °C and filtered through a 0.5 mm mesh filter. The radioactivities of the soil residues and filtrates were measured by a Ge-semiconductor detector, (CANBERRA GC4020: Energy resolution at 1.33 MeV is smaller than 2.0 keV.) termed Ge-measurement below.

2.2. Washing of soils contaminated by the FDNPP accident with Cs+ ion excess aqueous solution

After water washing, 20 g of the soils were immersed in 30 ml 0.1 M CsNO3 aqueous solution while stirring at 200 rpm for 3 h at 20 °C and filtered through a 0.5 mm mesh filter. The radioactivities of the soil residues and filtrates were measured using Ge-measurement.

2.3. Washing of soils contaminated by the FDNPP accident with various reagent solutions

After water washing, 10 g of the soils were immersed in 30 ml of various aqueous solutions, including HCl (1 M), H2SO4 (1 M), CH3COOH (1 M), (NH4)2SO4 (1 M), NH4Cl (1 M) while stirring at 200 rpm for 3 h at 20 ° C and filtered through a 0.5 mm mesh filter. The radioactivities of the soil residues and filtrates were measured using Ge-measurement.

2.4. Autoradiography of contaminated soils, bamboo shoots, and shiitake mushrooms with imaging plates

The distributions of Cs* in soils, bamboo shoots, and shiitake mushrooms contaminated by the FDNPP accident were measured using imaging plates (IPs; BAS-SR 2040, 200 mm wide x 400 mm long). The IPs were inserted into 12 mm thick aluminum foil. The samples were placed on 12 mm thick aluminum foil and sandwiched by IPs. The samples and IPs were stored in a shielding house constructed of lead bricks, a 0.5 mm thick Cd sheet, and 200 mm thick water layer for shielding from environmental radiation (gamma rays and neutron beams). The environmental radiation background was reduced to 0.004 mSv/h from 0.103 mSv/h. IP readings were conducted with a Fuji Film BAS-2500, termed IP measurement.

Shiitake mushrooms were harvested at a cultivation field in Yokokawa, Takahagi about 84 km south of the FDNPP on 14 November 2012. Bamboo shoots were harvested from a bamboo forest in Nihonmatsu, about 30 km west of the FDNPP on 19 May 2012. The places of Yokokawa and Nihonmatsu are indicated in the map (Fig. 1). Although Yokokawa locates at 80 km south from FDNPP, the deposition densities of total of cesium -134 & 137 at

N. Niimura et al. / Journal of Environmental Radioactivity xxx (2014) 1—6

Fig. 1. Map of the area where the samples used in the experiment were collected (Airborne monitoring results, 2012).

Yokokawa are reported as 30—60 kBq/m2 (Airborne monitoring results, 2012).

3. Results

as shown in the map

3.1. Water washing of soils contaminated by the FDNPP accident

After water washing, 134Cs (604.66 keV) and 137Cs (661.64 keV) radioactivities in the filtrate were not detected (<1.2230 x 10~2 Bq/ g) and detected at 1.6996 x 10~2 ± 3.6946 x 10~3 Bq/g, respectively, and those of the soil residues were 1.7097 ± 8.9696 x 10~2 Bq/g and 3.3697 x 10 ± 1.2204 x 10_1 Bq/g, respectively. The percent transfer of 134Cs and 137Cs to water was 0 and 0.05% respectively; thus, 134Cs and 137Cs in soils contaminated by the FDNPP accident can be regarded as insoluble in water.

3.2. Washing of soils contaminated by the FDNPP accident with Cs+ ion excess aqueous solution

After washing the soils with 0.1 M CsNO3 aqueous solution, 134Cs (604.66 keV) and 137Cs (661.64 keV) radioactivities in the filtrate liquid were 1.4618 x 10_1 ± 9.2687 x 10~3 Bq/g and 3.34501 x 10_1 ± 1.3525 x 10~2 Bq/g, respectively. The percent transfer of 134Cs and 137Cs to solution was 0.8% and 0.6%, respectively; thus, 134Cs and 137Cs in soils contaminated by the FDNPP accident can be regarded as nearly insoluble in 0.1 M CsNO3 aqueous solution. Therefore, the possibility that 134Cs and 137Cs released by the FDNPP accident were not trapped in soils as indicated by the previous X-ray diffraction and high-resolution transmission electron microscopy experiments is very low.

3.3. Washing of soils contaminated by the FDNPP accident with various reagent solutions

After washing the soils with various reagent solutions, the Cs* radioactivities of the filtrates and the filtration soil residues and the percent transfer of Cs* to the solutions are shown in Table 1. The Cs-

137/Cs-134 ratio of the Cs* from the FDNPP accident is reported that most of the ratio of atmospheric concentration of Cs-134 to that of Cs-137 was found between 0.9 and 1.1 from 15 March to 7 April 2011. The Cs-137/Cs-134 ratios of the Cs* were obtained in July 2013 between 1.93 and 2.09 from Table 1, and these values were in good agreement with those of Ohkura et al.'s report by considering the half-life of Cs-137/Cs-134. The Cs* was soluble in strong acid solutions such as HCl (1 M) and H2SO4 (1 M), but not in weak acid solutions such as CH3COOH (1 M). The Cs* was slightly soluble in certain salt solutions such as (NH4)2SO4 (1 M) and NH4Cl (1 M).

3.4. Autoradiography of contaminated soils, bamboo shoots, and shiitake mushrooms using imaging plates

Fig. 2 shows an IP image of the soil, exhibiting many black spots. The soil has been treated in various ways as indicated in figure captions. The IP image of each was found to be almost similar and this indicates that the existing form of Cs* in the soil depends on neither the size of soil nor washing of soil. Figs. 3 and 4 show photographs of portions of a bamboo shoot and their corresponding IP images, respectively.

The bamboo shoot body was cut vertically and horizontally. There were no black spots on the body in the IP image. However, there were black spots in the skin and litter images. The Cs* on the surface of the bamboo skin was likely transferred from the litter

Table 1

Cs* radioactivities of the filtrates and filtration residues, and percent transfer of Cs* to solution after washing with various reagent solutions.

Solvent Filtrate (Bq/g) Residue (Bq/g) Transition rate (%)

134Cs 137Cs 134Cs 137Cs (604.66 keV) (661.64 keV) (604.66 keV) (661.64 keV)

HCI 0.63 ± 0.03 1.38 ± 0.04 20.6 ± 0.2 44.3 ± 0.4 7

H2SO4 0.64 ± 0.03 1.24 ± 0.04 6.40 ± 0.1 13.7 ± 0.2 11

(NH4) 2SO4 0.22 ± 0.01 0.46 ± 0.02 23.6 ± 0.3 49.3 ± 0.4 2

NH4CI 0.16 ± 0.01 0.32 ± 0.02 14.8 ± 0.2 31.1 ± 0.2 1

CH3COOH ND (<0.03) ND (<0.02) 38.0 ± 0.4 81.5 ± 0.7 0

4 N. Niimura et al. / Journal of Environmental Radioactivity xxx (2014) 1—6

(a) (b) (c) (d)

(e) (f) (g) (n)

Fig. 2. IP images of several kinds of contaminated soils. (a) Raw soil, (b) after sieving of soil with a diameter of 0.5 mm or less, (c) after sieving of soil with a diameter of 0.5—4 mm, (d) after sieving of soil with a diameter of 4 mm or more, (e) washing soil, (f) after sieving of washing soil with a diameter of 0.5 mm or less, (g) after sieving of washing soil with a diameter of 0.5—4 mm, and (h) after sieving of washing soil with a diameter of 4 mm or more.

when the bamboo shoots sprouted through the litter and touched to the Cs* on the liter.

Figs. 5 and 6 show photographs of shiitake mushrooms and their corresponding IP images, respectively. One black spot on the lamella of the shiitake mushrooms (Fig. 6(a1)), several black spots on the outside of the cross section of the log (Fig. 6(b2)) and many black spots on the end face of the mushroom-cultivating log (Fig. 6(b1)) were observed in the IP images, respectively. However, there were no black spots inside the mushroom-cultivating log. Therefore, The Cs* on the lamella of the shiitake mushrooms did not come from the inside of the log, but it was likely transferred from the surface of the mushroom-cultivating log when the shiitake mushrooms emerged and touched to the Cs* on the surface of the log.

4. Discussion

4.1. Nature of the i34Cs and ,37Cs released from the FDNPP accident in the soil

It has previously been reported that Cs can be trapped and adsorbed into soil minerals. Kogure et al. determined the crystal structure of minerals capturing Cs by transmission electron

microscopy and x-ray diffraction (Kogure et al., 2012) and Ohnuki et al. showed that radioactive Cs could be trapped in several kinds of minerals (Ohnuki and Kozai, 2013). However, the Cs used as samples in these experiments were ions, such as Cs+ or 134Cs+ and 137Cs+, which did not originate from the FDNPP accident (Kogure et al., 2012; Ohnuki and Kozai, 2013). We agree that ionic Cs+ such as 134Cs+ and 137Cs+ can be captured by minerals. Cs trapped as ionic Cs+ in minerals would need to be dissolved in aqueous solution containing excess amounts of non-radioactive Cs+ to achieve chemical equilibrium between the minerals and the aqueous solution. For example, Ohnuki et al. showed that ionic 134Cs+ and 137Cs+ dissolved in CsCl solution were captured in minerals. However, as stated in Section 3.2, the 134Cs and 137Cs in the soils contaminated by the FDNPP accident were nearly insoluble in 0.1 M CsNO3 aqueous solution. This indicates that the 134Cs and 137Cs in the contaminated soils would not be available to be adsorbed into soils, in contrast to previous results. It appears that the 134Cs and 137Cs were not in ionic states, but were in the form of very small grains when they were released from the FDNPP. It is essential that 134Cs and 137Cs be in an ionic state to be trapped into soil minerals, as supported by the structure determined by Kogure et al., in which they showed that "ionic Cs" was fixed at the centers

N. Niimura et al. / Journal of Environmental Radioactivity xxx (2014) 1—6

Fig. 5. Photographs of (a1) lamella, (a2) stipe of the shiitake mushroom, and (a3) pileus, respectively, and (b1) an end face of the log, and (b2) a cross section of the log on which the mushrooms are cultivated, respectively.

of hexagonal rings in the upper and lower silicate tetrahedral sheets (Kogure et al., 2012). The reason of the different results between ours and theirs is as follows: We have used the Cs* emitted from FDNPP as samples in our experiment of the IP autoradiog-raphy and solubility assessment. There were no evidences that the Cs* emitted from FDNPP was ionic. On the other hand, Ohnuki et al. and Kogure et al. have used commercially available radioactive CsCl and commercially available CsNO3, respectively, both of which were soluble in water and the Cs used in their experiments were in ionic states.

4.2. Structure and shape of the radioactive Cs from the FDNPP accident

Based on the activity size distributions of 134Cs and 137Cs in aerosols (Kaneyasu et al., 2012), the results of a scanning electron microscope of the Cs* aerosols (Adachi et al., 2013), and the IP images taken of contaminated plant leaves (Sakamoto et al., 2012) and soils, bamboo shoots, and shiitake mushrooms (Figs. 2, 4 and 6), the shape of the radioactive Cs from the FDNPP accident is granular and around 1 mm in diameter, based on the activity size distributions of the aerosols (Kaneyasu et al., 2012; Adachi et al., 2013). The IP image spots seen in soils, on the plant leaves, bamboo shoots, and shiitake mushrooms are interpreted as granular substances containing Cs* (hereinafter referred to as "granular Cs*"). It appears that the granular Cs* can easily adhere physically to tiny hollows on the surfaces of soil mineral particles, plant leaves, and other materials. The size of the granular Cs* might be expected

to be equivalent to the size of the black spots on the IP images; however, this is not necessarily the case. The IP image is affected by the positional resolution of the imaging plate detector (50 mm, much larger than the size of the granular Cs*) as well as the optics of the autoradiography system, which are rather complex.

4.3. Chemical and physical properties of the granular Cs*

The actual microscopic and atomic scale structure of the granular Cs* has not yet been clarified. Its chemical properties were examined through solubility measurements with various reagent solutions (see Section 3). The granular Cs* is soluble in strong acid solutions such as HCl (1 M) and H2SO4 (1 M), but not in weak acid solutions such as CH3COOH (1 M). The granular Cs* is also slightly soluble in certain salt solutions such as (NH4)2SO4 (1 M) and NH4Cl (1 M). This might suggest that the granular Cs* consist of Cs as well as NHj and SO4~, which were reported as Cs* aerosol components by Kaneyasu et al. (2012).

Solubility measurements in other reagent solutions are planned for future research to assist in determining the microscopic and atomic scale structure of the granular Cs*.

Some physical properties of the granular Cs* can be inferred from the IP measurements of the bamboo shoots and shiitake mushrooms. The black spots observed in the IP images are

Fig. 6. IP images of (a1) lamella, (a2) stipe of the shiitake mushroom, and (a3) pileus, respectively, and (b1) an end face of the log, and (b2) a cross section of the log on which the mushrooms are cultivated, respectively, as shown in Fig. 5.

N. Niimura et al. / Journal of Environmental Radioactivity xxx (2014) 1—6

interpreted as granular Cs*. It appears that the granular Cs* is easily transferred from litter to bamboo shoots and from the surface of the logs used for cultivation of the shiitake mushrooms as interpreted in section 3.4. Therefore, the Cs* grains are very small (around 1 mm in diameter), light, and physically adhere to soils and so on, and can be easily transported to other places or media. As a matter of fact, we had several experiences that granular Cs* on shiitake mushrooms and bamboo shoots have been wiped off with a cloth.

The granular Cs* was insoluble in water as indicated in Section 3.1. If the Cs* were trapped and captured in soil minerals, it would not be as easily transported.

It has been confirmed that radioactive Cs is present within various plants to look like in the ionic states. We have not yet known how the water-insoluble granular Cs* becomes soluble and ionic, but this would be the next important subject, because once it becomes soluble and ionic, this ionic Cs* would become available to and absorbed into plants. Such soluble and ionic Cs* might also be trapped and adsorbed into the soil minerals over time.

The physical properties and dynamic behavior of the granular Cs* is expected to be helpful in considering methods for decontamination of soil, litter, and other media.

5. Conclusions

Based on our observed IP images of Cs* taken of contaminated soils, bamboo shoots and shiitake mushrooms, and the activity size distribution of Cs* in aerosols, Cs* emitted by the FDNPP accident fell onto the ground and plant surfaces in a granular form (around 1 mm in diameter). Since the granular Cs* is not soluble in water, and it does not become ionic Cs, it cannot be trapped into soil minerals, but physically adheres to the rough surfaces of soil mineral particles. This has been confirmed by the experiment that the 134Cs and 137Cs in the soils contaminated by the FDNPP accident were nearly insoluble in 0.1 M CsNO3 aqueous solution. This means that the 134Cs and 137Cs in the contaminated soils would not be available to be adsorbed into soils. Since it is soluble in strong acid solutions and slightly soluble in certain salt solutions such as (NH4)2SO4 (1 M) and NH4Cl (1 M), these might suggest that the granular Cs* consist of Cs as well as NHf and SO4-. Since Cs* physically adhere to soils and so on, it can be transferred among media, such as soils and plants. The physical properties and dynamic behavior of the granular Cs* is expected to be helpful in considering methods for decontamination of soil, litter, and other media. Moreover the reason why the granular Cs* emitted by the FDNPP accident are non-water soluble should be elucidated from the microscopic and atomic scale structural aspects, and the process how the water-insoluble granular Cs* become soluble and ionic in the soils and then are absorbed into plants should also be clarified in the future important subjects.

Acknowledgments

We thank Dr. S. Ishiyama of the Japan Atomic Energy Agency and Makino Co. Ltd. for providing contaminated soils.

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