www.nature.com/scientificreports

OPEN

received: 03 June 2015
accepted: 06 January 2016
Published: 03 February 2016

Internal structure of cesiumbearing radioactive microparticles
released from Fukushima nuclear
power plant
Noriko Yamaguchi1,*, Masanori Mitome2, Akiyama-Hasegawa Kotone2, Maki Asano1,3,
Kouji Adachi4 & Toshihiro Kogure5,*
Microparticles containing substantial amounts of radiocesium collected from the ground in Fukushima
were investigated mainly by transmission electron microscopy (TEM) and X-ray microanalysis withscanning TEM (STEM). Particles of around 2 μm in diameter are basically silicate glass containing Fe
and Zn as transition metals, Cs, Rb and K as alkali ions, and Sn as substantial elements. These elements
are homogeneously distributed in the glass except Cs which has a concentration gradient, increasingfrom center to surface. Nano-sized crystallites such as copper- zinc- and molybdenum sulfide, andsilver telluride were found inside the microparticles, which probably resulted from the segregationof the silicate and sulfide (telluride) during molten-stage. An alkali-depleted layer of ca. 0.2 μm thickexists at the outer side of the particle collected from cedar leaves 8 months after the nuclear accident,
suggesting gradual leaching of radiocesium from the microparticles in the natural environment.
Although almost five years have passed since the accident of Fukushima Daiichi Nuclear Power Plant (FDNPP),
radioactive contamination in the surrounding area is still a serious problem in Japan. The largest radionuclide
deposition event occurred on March 15–16 and the second largest on 21–23, 2011. Wet deposition was a major
source of radiocesium contamination of terrestrial environment1, while contribution of dry deposition was larger
near the FDNPP2. In order to understand and predict the fate of radioactive materials contaminating the terrestrial environment, it is important to clarify the physicochemical properties of the deposited materials. Fromprevious cases of radionuclide release, it is known that the chemical species of released radiocesium is monovalent cation (Cs+) which is soluble3. Deposition of radiocesium as insoluble particles has also been pointed out.
Autoradiography analyses using imaging plate (IP) showed spots of particulate materials on plant tissues collectedfrom Fukushima4–6. On the aerosol filter collected from March 14–15, 2011 in Tsukuba, 170 km south-southwestof FDNPP, Adachi et al.7 discovered spherical particulate radiocesium of 2.0–2.6 μ m in diameter, with particles
insoluble in water having a glass-like structure8. These microparticles contain several fission products of U-235
other than radiocesium, and Fe and Zn which are also used in nuclear reactors8. Hence, they were considered tobe released directly from nuclear reactors.
Kaneyasu et al.9 suggested that vaporized radiocesium was transported with sulfate aerosol in the air, dissolved
to cloud droplets and fell as rain. On the aerosol filter collected on March 20–21, 2011, rainy days in Tsukuba,
the majority of radiocesium was in water-soluble form7. Such water-soluble radiocesium that reached the groundsurface as a solute was fixed to soils, especially to clay minerals10. In the terrestrial environment, the majority ofradiocesium is present in solid form regardless of the initial form of deposition. However, compared to clay minerals originally contaminated by soluble radiocesium in soil, the solid radiocesium, which was initially depositedas radioactive microparticles, had stronger radioactivity. Although the contribution or percentage of such radioactive microparticles in the contamination level of Fukushima has not been evaluated, its influence on human1

National Institute for Agro-Environmental Sciences, Kannondai, Tsukuba 305-0864, Japan. 2National Institutefor Materials Science, Namiki, Tsukuba 305-0044, Japan. 3Graduate School of Life and Environmental Sciences,
University of Tsukuba, Tennodai, Tsukuba, 305-8572, Japan. 4Meteorological Research Institute, Nagamine, Tsukuba,
305-0052, Japan. 5Graduate School of Sciences, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan. *Theseauthors contributed equally to this work. Correspondence and requests for materials should be addressed to N.Y.
(email: nyamag@affrc.go.jp) or T.K. (email: kogure@eps.s.u-tokyo.ac.jp)
Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

1www.nature.com/scientificreports/

Figure 1. (a) Bright-field (BF) image of NWC-1. The opaque material outside the microparticle is tungsten(W) and copper (Cu) deposited in the FIB process. (b) Electron diffraction (ED) pattern from the particle.
(c,d) EDS spectrum acquired from almost the whole area of the particle for the energy range of (c) 0–11 keV
and (d) 10–30 keV. (e) BF image of CB-8. The opaque material outside the sphere is W and the thin transparent
material is Kapton tape. (f) ED pattern from the particle.

health may be serious in terms of its intense radioactivity. Moreover, the structural detail of the microparticles
may give insights into the state of the broken reactor and fuel debris.
In the present study, we investigated radioactive microparticles, similar to those reported by Adachi et al.7, butcollected from the ground, by observing their internal structure with transmission electron microscopic (TEM)
techniques.

Results

Structure and composition of Cesium-bearing radioactive microparticles.

Cesium-bearing radioactive microparticles that had been deposited on non-woven fabric cloth (NWC-1) and on a needle of Japanesecedar (Cryptomeria japonica) (CB-8) were investigated. They were in the field for five and eight months, respectively, until sampling. Scanning electron microscope (SEM) images of NWC-1 of the whole microparticle before
preparing thin sections for TEM analyses; and elemental composition of the whole particle determined by synchrotron radiation microbeam X-ray fluorescence (SXRF) are shown in Supplementary Figs S1 and S2 online,
respectively. The activities of 137Cs for the NWC-1 and CB-8 were 5.04 ±  0.472 and 3.14 ±  0.178 Bq, respectively.
Bright-field (BF) images and selected-area electron diffraction (SAED) patterns recorded in TEM from thewhole area of NWC-1 and CB-8 are shown in Fig. 1, along with the energy-dispersive X-ray spectrum (EDS)
acquired in TEM from NWC-1. Preparation of thin specimens from NWC-1 microparticle by focused-ion-beam(FIB) process was successful only for the upper half of the sphere. The BF contrast of NWC-1 (Fig. 1a) was almost
uniform except for two dark nanoparticles inside the sphere. These nanoparticles will be described in a later subsection. The SAED pattern from the whole particle consists of only a halo (Fig. 1b) and the EDS from the whole

Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

2www.nature.com/scientificreports/
particle (Fig. 1c,d) mainly consist of peaks of Si and O (Cu is from the supporting mesh), indicating that the particle was basically silicate glass. Additionally, Cl, K, Cs, Fe, Zn, Rb, and Sn were definitely identified. The presence ofNa was not confirmed since its peak overlapped with the L-peak of Zn. These elements were previously reported7,8
except for K. EDS Semi-quantitative analysis for almost the entire area of the TEM specimen, without considering
the absorption effect estimated the glass composition (wt.%) to be SiO2; 69.3, K2O; 1.9, Fe2O3; 8.6, ZnO; 11.0,
Rb2O; 1.3, SnO2; 1.4, Cs2O; 3.4 with a small amount of Cl (1.4 wt.%). The valence states of cations were assumed to
be in line with results in an X-ray absorption spectroscopic study8. Rubidium, Cs and Sn are the fission productsof U-235. Tin is also used for fuel cladding11. Iron is used as steel for reactor pressure vessels, and Si and O aremain components of concrete at the bottom of the containment vessel, where the melt-down fuel or core debris
is thought to exist. Zinc had been added to the primary cooling water12. Potassium and Cl may originate fromconcrete and/or seawater, which was used to cool down the reactor.
The BF TEM image of CB-8 showed a two-layer structure having an inner core with darker contrast, an outercrust with a lighter one, and a small dark nanoparticle near the center (Fig. 1e). The thickness of the crust wasaround 0.2 μ m. SAED pattern indicated that both are amorphous (Fig. 1f). Locally, a bubble-like structure wasobserved at a certain radius in the crust, or the outside part was peeled off at the radius (Fig. 1e). The chemicalcomposition of the entire area of the specimen was SiO2; 73.3, K2O; 1.4, Fe2O3; 7.2, ZnO; 11.4, Rb2O; 1.2, SnO2;
1.5, Cs2O; 3.3 and Cl; 0.7, which is similar to that of NWC-1.

Distributions of elements in microparticles. Elemental maps from NWC-1 using STEM-EDS are
shown in Fig. 2a. Brighter color indicates higher concentration of the elements. All elements were almost uniformly distributed in the particle with Cs the only exception. Cesium concentration was lower near the center
than the outside of the particle. The ratio of the amounts between the center and outside is around two. It is
possible that vapor-phase cesium in the reactor was absorbed to the molten silicate microparticles, from theirsurface. The low diffusion velocity of cesium in the glass may have left the concentration gradient. On the contrary, elemental mapping for CB-8 (Fig. 2b) indicates that K, Rb and Cs were distinctively depleted in the crust
but concentrated in the vicinity of the surface. Although the reason for the high concentration at the surface is
not certain, it might be an artifact by diffusion of alkali cations caused by electron-beam radiation in the analysis.
Chlorine was concentrated in the crust.
Crystalline nanoparticles in the glass. Two nanoparticles with dark contrast were observed in the TEM
image of NWC-1 (Fig. 1a). Although EDS spectrum only from the particles cannot be obtained since they arestill buried in the glass in spite of the thinning by FIB, S was distinctively identified from the particles (Fig. 3a).
Elemental mapping by STEM-EDS indicated enrichment of Cu, Zn and Mo at the nanoparticles along with S,
indicating that they are sulfide. The possibility of sulfate was excluded because oxygen is deficient in the nanoparticles in the oxygen map (Fig. 3a). Moreover, the locations of Cu, Zn and Mo do not overlap within thenanoparticles, suggesting that these metal elements form different sulfide phases in the nanoparticle (Fig. 3a).
SAED from the left particle showed a single-crystal diffraction pattern (Fig. 3b), which can be explained by the
crystallographic parameters for a high-temperature polymorph of digenite (Cu2−xS). The origin of Cu and S may
be minor elements in the concrete. On the other hand, Ag and Te were distinctively detected by EDS in TEM
from the nanoparticle in CB-8 (Supplementary Fig. S3 online), indicating the particle to be silver telluride. Bothelements can be fission products of U. Diffraction spots were observed in the SAED pattern from the particle,
indicating that it is crystalline but the phase could not be identified. These sulfide and telluride were probably
segregated and crystallized in the microparticles at the molten state.

Discussion

Our most significant finding is that the matrix of the Cs-bearing microparticles is silicate glass, based on the
TEM-EDS analysis with FIB sample preparation. Previous studies suggested that Fe, Mo, Sn and Zn in the
Cs-bearing microparticles had a similar X-ray absorption near-edge structure to those composed of glass8, however the presence of Si in the microparticles has not been verified7,8. It is probable that the high-temperature
melt-down fuel from the reactor came into contact with and melted the concrete, and then splashed microparticles of silicate melt, which were solidified by cooling to form silicate glass in the atmosphere. However, there areseveral questions with respect to the selection of the constituting elements in the glass from various ones in thereactor. For instance, Ca which is one of the major elements in concrete, was almost absent in the microparticles
of NWC-1. Since TEM observed only a small portion of the microparticles, by making them thin using FIB, there
may have been other elements in the microparticles, for instance, as a form of chalcogenide nanoparticle.
The next important finding is the alkali-depleted crust in CB-8 microparticle. This is probably the result of
elution of alkali ions by contact with acidic solution in the field, commonly observed in silicate glass13. On theother hand, such alkali-depleted crust was not observed in NWC-1. This may be attributed to the different environments of the two microparticles after release from the nuclear plant. NWC-1 was on non-woven cloth andCB-8 on a cedar leaf before it was collected. It is well-known that silicate glass elutes alkali components fromtheir surface by ion-exchange with proton or hydronium ions to form an alkali-leaching layer on the surface if
pH of reacting solution is low, whereas the silicate framework of the glass itself is dissolved with high-pH solution13,14. Coniferous forest canopy induces acidic condition due to ammonia uptake, nitrification and leachingof plant-derived acid15. It was likely that CB-8 deposited on the cedar leaf had been in acidic conditions, which
derives alkali depletion in the crust, for eight months. The finding of the alkali-depleting crust on the surface of
the Cs-bearing radioactive microparticle indicates that radiocesium in the particles can be released by “weathering” of the glass in natural environments, and considering its small size, duration for the total release of theradioactive cesium from the particles is probably not long, from several years to a few decades, though it willstrongly depends on the environment.
Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

3www.nature.com/scientificreports/

Figure 2. (a) Element maps of NWC-1 acquired by STEM-EDS with a STEM Dark-field (DF) image at thesame area, and (top) TEM Bright-field image in which the rectangle indicates the area analyzed by STEM-EDS.
The thin area around the top of the particle which was not observed in Fig. 1a was formed by a further thinning
process by FIB. (b) Element maps and images of CB-8 with the same framing as in (a).

In order to investigate the dissolution rate and detailed Cs-leaching properties of the Cs-bearing radioactive
microparticles, a leaching experiment should be conducted as a function of temperature and pH. However, collecting and isolating the Cs-bearing microparticles is time-consuming and it is difficult to obtain a large enoughnumber of Cs-bearing microparticles to investigate dissolution properties. Alternatively, synthesized silicate glasswith the same composition as the microparticles presented in this study may help to obtain information on thefate of Cs-bearing radioactive microparticles. However, in our preliminary experiment, we were unable to makeuniform glass with the same composition at present. This is probably due to the liquid immiscibility at the composition. A solution to this problem is now being considered.
There are mainly two types of solid-phase radiocesium in the terrestrial environment affected by FDNPP accident; that fixed to clay minerals in the soil via wet deposition and that contained in the microparticles of silicate
glass flown directly from the nuclear reactors. The radioactivity of the former is rather in low-density but distributed widely, therefore it is a major source of external radiation from ground. Part of the radiocesium adsorbed on
Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

4www.nature.com/scientificreports/

Figure 3. (a) Element maps and STEM bright-field (BF) image of the two nanoparticles inside NWC-1.
The circular contrasts outside the nanoparticles in the BF image are radiation damage in glass formed by theelectron probe for chemical analyses. (b) (Left) ED pattern from the left nanoparticle. (Right) Drawing of ED
pattern for the high-temperature polymorph of digenite (Cu2−xS) along < 121> .

the soil clay is desorbed and transferred to crops thereby causing internal radiation. On the other hand, contribution of the microparticles to the air radiation is most likely not significant, but their radiation density is very high,which is particularly problematic for organisms including humans if the microparticles are inhaled or ingested.
The plant availability of radiocesium in the microparticles should depend on its solubility. Consequently, further
research on this material should be carried out as soon as possible.

Methods

Samples. Radioactive microparticles attached on non-woven fabric cloth and needles of Japanese cedar, collected from Fukushima, were identified by autoradiography followed by point-by-point analyses with scanningelectron microscope (SEM; Hitachi high-Technologies SU3500) equipped with an energy dispersed X-ray spectrometer (EDS; Horiba X-max 50 mm). The identified particles were further determined by SXRF with excitation
X-ray energy of 37.5 keV to ensure the detection of Cs-K line from the particle. The activities of 137Cs was determined by a Ge detector (GCW2523S Canberra, USA). Detailed methods to identify the Cs-bearing microparticles are described in the supplementary information.
Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

5www.nature.com/scientificreports/
TEM Analyses.

Cross-sectional thin TEM specimens were prepared from radioactive microparticles using
a focused ion beam (FIB) instrument with micro-sampling system (Hitachi FB-2100) as described in the supplementary information. Then specimens were initially examined using a TEM (JEOL JEM-2010UHR) operated at
200 kV with an EDS analyzer system (JEOL JED-2200). Elemental mapping in the microparticles and quantitative
analyses were performed using a JEOL JEM-3100FEF operated at 300 kV in the STEM mode, with an EDS analyzer system (Thermo Fisher Scientific NORAN System SIX). Finally, elemental maps for nanoparticulates inside
the microparticles were acquired using a JEOL JEM-2800 operated at 200 kV with double wide-area (0.95 sr.)silicon drift detectors (SDD) for EDS analyses.

References

1. Yoshida, N. & Takahashi, Y. Land-surface contamination by radionuclides from the Fukushima Daiichi Nuclear Power Plant
accident. Elements 8, 201–206 (2012).
2. Katata, G., Ota, M., Terada, H., Chino, M. & Nagai, H. Atmospheric discharge and dispersion of radionuclides during the Fukushima
Dai-ichi Nuclear Power Plant accident. Part I: Source term estimation and local-scale atmospheric dispersion in early phase of theaccident. J. Environ. Radioactiv. 109, 103–113 (2012).
3. Santschi, P. H. et al. Chernobyl radionuclides in the environment- tracers for the tight coupling of atmospheric, terrestrial, andaquatic geochemical processes. Environ. Sci. Technol. 22, 510–516 (1988).
4. Nakanishi, T. M., Kobayashi, N. I. & Tanoi, K. Radioactive cesium deposition on rice, wheat, peach tree and soil after nuclear
accident in Fukushima. J. Radioanal. Nuclear Chem. 296, 985–989 (2013).
5. Nakanishi, H. et al. Radioactive cesium distribution in bamboo Phyllostachys reticulata (Rupr) K. Koch shoots after the TEPCO
Fukushima Daiichi Nuclear Power Plant disaster. Soil Sci. Plant Nutr. 60, 801–808 (2014).
6. Itoh, S., Eguchi, T., Kato, N. & Takahashi, S. Radioactive particles in soil, plant, and dust samples after the Fukushima nuclear
accident. Soil Sci. Plant Nutr. 60, 540–550 (2014).
7. Adachi, K., Kajino, M., Zaizen, Y. & Igarashi, Y. Emission of spherical cesium-bearing particles from an early stage of the Fukushima
nuclear accident. Sci. Rep. 3, No. 2554 (2013).
8. Abe, Y. et al. Detection of uranium and chemical state analysis of individual radioactive microparticles emitted from the Fukushima
nuclear accident using multiple synchrotron radiation X-ray analyses. Anal. Chem. 86, 8521–8525 (2014).
9. Kaneyasu, N., Ohashi, H., Suzuki, F., Okuda, T. & Ikemori, F. Sulfate aerosol as a potential transport medium of radiocesium fromthe Fukushima Nuclear accident. Environ. Sci. Technol. 46, 5720–5726 (2012).
10. Mukai, H. et al. Speciation of radioactive soil particles in the Fukushima contaminated area by IP autoradiography andmicroanalyses. Environ. Sci. Technol. 48, 13053–13059 (2014).
11. Zinkle, S. J. & Was, G. S. Materials challenges in nuclear energy. Acta Mater. 61, 735–758 (2013).
12. Hori, S. & Suzuki, A. TEPCO’s challenges for occupational exposure reduction - installation of additional CF in Fukushima Daiichi
NPP., < www.nsra.or.jp/isoe/english/…/pdf/atc2009-3-1ga.pdf> (2009). (Accessed 18th May 2015).
13. Bunker, B. C. Molecular mechanisms for corrosion of silica and silicate-glasses. J. Non-Cryst. Solids 179, 300–308 (1994).
14. Gin, S. et al. Origin and consequences of silicate glass passivation by surface layers. Nat. Commun. 6, No. 6360 (2015).
15. Cronan, C. S. & Reiners, W. A. Canopy processing of acidic precipitation by coniferous and hardwood forests in New England.
Oecologia 59, 216–223 (1983).

Acknowledgements

Part of this work was supported by a Grant-in-Aid for science research (15H04222 and 24110003) by JSPS, Japan.
This work was also supported by 2014 JAEA contract research for Fukushima environment recovery, “Studyon Cs adsorption and desorption process on clay minerals” and by MAFF contract research “Development of
Decontamination technologies for radioactive substances in agricultural land”. The synchrotron radiation
experiment was performed under approval of the SPring-8 Program Advisory Committee (Proposal Number:
2014B1282). The authors wish to thank Dr. M. Kobayashi for providing the Japanese cedar samples, Ms. E. Fujii
for preparing TEM specimens, Dr. T. Hara for providing the opportunity to use the STEM instrument, Prof. Clive
Langham for English edition, and Dr. Y. Igarashi for comments on the manuscript.

Author Contributions

N.Y. and T.K. designed the research and wrote the paper; N.Y. performed the IP, SEM and SXRF analyses; M.A.
performed SEM and SXRF analyses; M.M. and K.A.-H. performed TEM analyses; K.A. performed SEM analyses;
T.K. performed SEM and TEM analyses and supervised the study; all authors contributed to the manuscript. N.Y
and T.K. contributed equally to this work.

Additional Information

Supplementary information accompanies this paper at http://www.nature.com/srep
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Yamaguchi, N. et al. Internal structure of cesium-bearing radioactive microparticles
released from Fukushima nuclear power plant. Sci. Rep. 6, 20548; doi: 10.1038/srep20548 (2016).
This work is licensed under a Creative Commons Attribution 4.0 International License. The images
or other third party material in this article are included in the article’s Creative Commons license,
unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license,
users will need to obtain permission from the license holder to reproduce the material. To view a copy of thislicense, visit http://creativecommons.org/licenses/by/4.0/

Scientific Reports | 6:20548 | DOI: 10.1038/srep20548

6