Structure, Composition, and Physicochemical Properties of Radiocesium-Bearing Microparticles Emitted by the Fukushima Daiichi Nuclear Power Plant Accident

Abstract

During the accident at TEPCO’s Fukushima Daiichi Nuclear Power Plant, radiocesium-bearing microparticles (CsMPs) were released from damaged reactors into the environment. These micron-sized spherical particles with high specific radioactivity have not been reported in previous nuclear accidents. Herein, the current understanding of the structure, composition, and physicochemical properties of CsMPs is summarized. Electron microscopy revealed that the CsMP matrix is composed of silicate glass containing Na, Cl, K, Fe, Zn, Rb, Sn, and Cs as major constituents. These elements are often inhomogeneously distributed, depending on the particle radius, and Cs was concentrated around the outer side of the particles. In addition, nanocrystals including Cr-rich oxides and chalcogenides were frequently found inside CsMPs. The average valence state of Fe in the CsMP glass matrix was almost Fe²⁺, indicating formation under a reducing atmosphere through condensation from the gas phase. Radiocesium diffused away from the CsMPs when heated to >600 °C. Accordingly, CsMPs may lose their high specific radioactivity when related radiation-contaminated waste is incinerated at sufficiently high temperatures. Although CsMP solubility is low, they cannot be regarded as “insoluble” materials owing to their small size. CsMP dissolution rates depend on the pH and dissolved species in the solution, and their dissolution behavior is comparable to that of silica-rich glass. Based on these dissolution properties, a method for estimating CsMP abundance and spatial distribution in the environment was proposed. The findings detailed herein contribute to the comprehensive elucidation of CsMP environmental dynamics.

8.1 Introduction

A significant amount of radionuclides was released into the environment during the accident at TEPCO’s Fukushima Daiichi Nuclear Power Plant in March 2011, resulting in radioactive pollution in the surrounding area (Kinoshita et al. 2011). Among the released radionuclides, ¹³⁷Cs is a major source of the high air dose rate in the evacuation zone, because it was released in large quantities and has not yet decayed owing to its long half-life of approximately 30 years. The radiocesium released in a gaseous state during the accident fell to the ground with raindrops and was sorbed mainly to minerals in soil. For example, the Abukuma Mountains located west of the nuclear power plant are covered with weathered granitic soil called “masa” and most of the radiocesium deposited in this region was sorbed to partially vermiculitized biotite, which is abundant in masa (Mukai et al. 2014, 2016). Radiocesium is now strongly fixed to these mineral particles and is practically nonleachable by ion exchange (Mukai et al. 2018).

In addition, part of the released radiocesium was contained in microparticles that were formed in the damaged reactor and subsequently released into the environment. These particles, referred to as radiocesium-bearing microparticles (CsMPs) or “cesium balls,” were first discovered in 2013 in aerosol filters collected in Tsukuba, 170 km southwest of the nuclear power plant (Adachi et al. 2013). CsMPs are almost spherical and less than a few microns in diameter, and the radioactivity ratio of ¹³⁴Cs/¹³⁷Cs suggests formation in reactor Units 2 or 3 (Satou et al. 2018). Furthermore, CsMPs have been suggested to originate from Unit 2 according to their constituent elements (IRID 2018). Although the CsMP formation process has not yet been fully elucidated, they may have been formed by condensation from the gas phase, because most possess a spherical shape. Extremely high specific radioactivity (radioactivity per volume) distinguishes CsMPs from the abovementioned radiocesium-sorbing minerals (hereafter abbreviated as CsSMs). CsMPs were transported and deposited over a wide area, extending into the Kanto region, because of their small size (Abe et al. 2021). CsMPs are unique in that they have never been reported in past nuclear power plant accidents. Therefore, understanding the structure, composition, and physicochemical properties of CsMPs, such as their thermal and dissolution characteristics, is an urgent issue. In this chapter, recent CsMP characterization efforts using electron microscopy are detailed. A lingering issue is the need to establish a method for quantitatively estimating CsMP abundance in environmental samples to accurately understand their contribution to radioactive pollution and influence on the environment and human health. Here, we propose an approach to resolve this issue based on the different dissolution properties of CsSMs and CsMPs in acidic solution, as they cannot be discriminated only by γ-ray measurements. Meanwhile, larger radiocesium-bearing particles of more than several tens of microns were found within 20 km of the nuclear power plant. These particles, referred to as “type-B” particles and probably released from Unit 1, have distinct characteristics from CsMPs (type-A) and have been characterized in other studies (e.g., Satou et al. 2018; Igarashi et al. 2019).

8.2 CsMP Structure and Composition

The CsMPs investigated in the studies described below were collected from sources including contaminated soil, plant tissues, bird feathers, and aerosols, but most abundantly from nonwoven fabric cloth laid on an agricultural field in the central part of Fukushima Prefecture. The procedure for collecting CsMPs has been described previously (Yamaguchi et al. 2016). Because the CsMPs are less than a few microns in size, their structure has been investigated mainly by scanning electron microscopy (SEM) and conventional and scanning transmission electron microscopy (TEM/STEM) with energy-dispersive X-ray spectroscopy (EDS). In particular, improvement in the detection efficiency of X-rays in EDS has enabled the determination of element distribution in very small CsMPs (Lechner et al. 2001). Electron-transparent thin films for the TEM/STEM analyses of CsMPs were prepared using a focused ion beam (FIB) system with microsampling equipment.

8.2.1 Silicate Glass Matrix of CsMPs

Following the discovery of CsMPs, synchrotron X-ray microbeam diffraction suggested that the CsMPs were amorphous (Abe et al. 2014). Subsequently, TEM/STEM observations and elemental analyses by EDS showed that CsMPs are mainly composed of silicate glass containing Cl, K, Fe, Zn, Rb, Sn, and Cs as the major constituents (Fig. 8.1) (Yamaguchi et al. 2016). More recently, X-ray absorption near-edge structure (XANES) analyses using scanning transmission X-ray microscopy (STXM) and STEM-EDS analysis revealed that Na is also a major constituent of CsMPs, and its atomic percentage is larger than that of the other alkali elements (K, Rb, and Cs) (Okumura et al. 2020b). The trace presence of Mn, Mo, Te, Ba, and U has been reported using synchrotron-beam X-ray fluorescence and high-energy-resolution EDS with microcalorimetry (Abe et al. 2014; Kogure et al. 2016). However, it is not clear whether these trace elements are contained within the silicate glass matrix or in the nanoparticles present inside the CsMPs, as discussed below. Meanwhile, B was not detected in the CsMPs even using electron energy-loss spectroscopy, which is quite sensitive to the detection of light elements, implying that B₄C control rods might have created a eutectic alloy with stainless steel cladding (Okumura et al. 2019a). It should be noted that Al and Ca were not present inside the CsMPs, although some studies have detected them using EDS analysis with SEM (Satou et al. 2016, 2018; Higaki et al. 2017; Miura et al. 2018). These elements may have been derived from fine mineral particles attached to the surface of CsMPs.

Fig. 8.1

Structure and composition of a typical CsMP. (a) Secondary-electron SEM image of a CsMP. (b) Bright-field TEM image and (c) electron diffraction pattern of a thin-sectioned CsMP prepared using FIB. (d) EDS spectrum acquired from a CsMP. Cu was emitted from the supporting grid. (Adapted from Yamaguchi et al. 2016)

Elemental mapping of CsMPs by STEM-EDS revealed that not all elements were uniformly distributed within the particles (Fig. 8.2) (Kogure et al. 2016; Okumura et al. 2019a). Cesium showed the most prominent inhomogeneous distribution, with a concentration higher near the surface and lower in the central of the particle. The nonuniform distribution of Cs can be explained by the inward diffusion of gaseous Cs into the suspended silicate melt balls during condensation. Following the inhomogeneous distribution of Cs, Cl showed a similar pattern. In contrast, other alkali elements (such as K and Rb) tend to concentrate in the inner part. In addition, a few CsMPs exhibited high concentrations of Fe and Zn near the surface. These nonuniform distributions of the constituent elements are frequently observed in CsMPs, but their concentration gradients vary greatly between particles (Fig. 8.3) (Kogure et al. 2016). Therefore, it can be assumed that the environment in which the CsMPs were formed inside the damaged reactor is not uniform. In some CsMPs, a high concentration of Sn was observed around particle surfaces, likely due to the precipitation of insoluble SnO₂ during CsMP dissolution after their release into the environment (Rai et al. 2011).

Fig. 8.2

(upper) Bright-field TEM images of three CsMPs and (lower) their elemental maps acquired using STEM-EDS. (Adapted from Kogure et al. 2016)

Fig. 8.3

Radial distribution of constituent elements in the three CsMPs shown in Fig. 8.2. (Reprinted from Kogure et al. 2016)

The atmosphere in which the CsMPs were formed in the damaged reactor could also be estimated from the valence state of the constituent elements. XANES analysis was performed using STXM to investigate the valence state of Fe (Fig. 8.4) (Okumura et al. 2020b). Inside the CsMPs, Fe²⁺ was dominant, while Fe³⁺ increased toward the upper region. The thin specimens for STXM were prepared using FIB and the sample thinned toward the upper region, implying that the increased Fe³⁺ is an artifact. In addition, the Fe in silicate glass was gradually oxidized by X-ray irradiation during the STXM experiments. Considering this change induced by the measurement, the Fe state in CsMPs was probably almost Fe²⁺ at the time of formation, suggesting that the reactor was in a significantly reducing atmosphere. The valence state map of Fe in Fig. 8.4 also shows that Fe was highly oxidized near the particle surface likely due to oxidation in the environment.

Fig. 8.4

(a) Fe valence state map acquired from a CsMP using STXM. (b) XANES spectra acquired from the L, M, and U areas in (a). The spectrum from the rim region is also shown. (Reprinted from Okumura et al. 2020b)

8.2.2 Nanoparticles in CsMPs

Using TEM/STEM observations, various types of nanoparticles have been found inside CsMPs, including oxides with a spinel structure containing Cr and Fe (Fig. 8.5) (Okumura et al. 2019a). Generally, metallic ions with a small ionic radius and high valence, such as Cr³⁺, hardly dissolve in silicate glass, forming or segregating oxide nanoparticles in the CsMPs. In addition to oxides, sulfides such as Cu_1.8S, Ag₂S, and MoS₂, and tellurides including Ag₂Te, have been found (Fig. 8.5) (Yamaguchi et al. 2016; Okumura et al. 2019a). It should be noted that the specimens observed were thin films processed by FIB; therefore, only a small portion of a single CsMP particle was examined using TEM/STEM. Nonetheless, these nanoparticles have been frequently detected, indicating that they are quite common in CsMPs.

Fig. 8.5

(a) STEM image of a CsMP. (b) Electron diffraction pattern acquired from the spherical inclusion indicated by arrow “b” in (a), corresponding to hessite (Ag₂Te) observed along <113>. (c) Electron diffraction pattern acquired from the inclusion indicated by arrow “c” in (a), corresponding to a spinel structure observed along <114>. (d) Elemental maps of the CsMP. (Adapted from Okumura et al. 2019a)

8.2.3 CsMPs with Irregular Forms and Different Compositions

As described above, most CsMPs are almost spherical and their glass matrices exhibit similar compositions with several notable exceptions (Yamaguchi et al. 2017). Two representative examples of these anomalies are presented in Fig. 8.6. The left one, with an intensely foamed appearance, was found in 2013 in atmospheric dust. The specific radioactivity and ¹³⁴Cs/¹³⁷Cs ratio were comparable to those of the normal CsMPs, and TEM/STEM analysis indicated that crystalline materials were not present inside the particles, and the oxide glass was mainly composed of silicate and chromium-oxide. The former contained Al and Cs with an irregular distribution, while Fe, Zn, and Sn were present in both glass types but were more concentrated in the chromium-oxide glass. The CsMP on the right side of Fig. 8.6 was recovered from plant tissue collected in Fukushima Prefecture in 2015. These images show an aggregate of finer particulates 100–200 nm in size composed of silicate glass containing Sn and Cs but without Fe and Zn. Iron was present as individual fine particles of hematite and magnetite in these aggregates.

Fig. 8.6

Two examples of CsMPs with nonspherical forms. (Left) CsMP with an intensely foamed appearance. (Right) CsMP with an aggregate of fine particles. The upper images are the secondary-electron SEM images and the bottom shows bright-field TEM images of these particles. Note that the right CsMP is half-buried in glue. (Adapted from Yamaguchi et al. 2017)

The origin of these irregular and uncommon CsMPs must be completely different from that of the spherical counterparts. For instance, the foamed particle likely experienced rapid depressurization due to hydrogen explosions that occurred in Units 1 and 3. Considering its ¹³⁴Cs/¹³⁷Cs ratio, it is suspected that this particle originated from Unit 3.

8.3 Physicochemical Properties of CsMPs

As mentioned above, radioactive particulate materials like CsMPs have not been reported until the Fukushima nuclear accident and their physicochemical properties remain largely unknown. Therefore, several groups have investigated the properties of CsMPs to predict their dynamics and fates in the environment.

8.3.1 Thermal Properties of CsMPs

Radiation-contaminated waste is often incinerated to reduce its volume and is expected to contain CsMPs. However, the effects of incineration on CsMPs is unknown so isolated CsMPs were held on a platinum plate and heated in air at various temperatures to better understand their thermal properties (Okumura et al. 2018). The radioactivity of the CsMPs gradually decreased starting at 600 °C and was almost lost when the temperature reached 1000 °C. The size and spherical morphology of the CsMPs remained almost unchanged after heating, but the alkali elements (K, Rb, and Cs) and Cl were lost (Fig. 8.7). This indicates that these elements, including radiocesium, diffused away from the CsMPs and were volatilized upon heating, resulting in decreased radioactivity. Iron, zinc, and tin, which were originally dissolved in the glass matrix, formed various oxides including ZnFe₂O₄, Zn₂SiO₄, and SnO₂ inside the CsMPs. These elements diffused inside the CsMPs via heating and crystallized during cooling. In contrast, when the CsMPs were heated together with the soil, radiocesium released from the CsMPs was sorbed to the surrounding soil. Thus, these studies indicate that CsMPs lose their high specific radioactivity when radiation-contaminated waste is incinerated at sufficiently high temperatures.

Fig. 8.7

(a) SEM image of a CsMP before heating. (b) SEM image of the CsMP after heating at 900 °C. (c) EDS spectra acquired from the CsMP before and after heating. (Adapted from Okumura et al. 2018)

8.3.2 Dissolution Properties of CsMPs

CsMPs have been recognized as insoluble particles, because their shapes and radioactivity did not change when immersed in water (Adachi et al. 2013). However, CsMPs are mainly composed of silicate glass, which can slowly dissolve in aqueous solution. Therefore, CsMP solubility should be further investigated to predict the environmental fates of these particles.

Dissolution experiments were performed using CsMPs in pure water and artificial seawater at several temperatures and the dissolution progress was monitored by the decrease in the ¹³⁷Cs radioactivity originating from the CsMPs, allowing the dissolution rate and activation energy to be estimated (Okumura et al. 2019b). The dissolution rate of the CsMPs in seawater was approximately one order of magnitude higher than that in pure water (Fig. 8.8). Assuming that a 1 μm radius CsMP is immersed in seawater at 13 °C (the approximate annual mean temperature in Fukushima City), it will completely dissolve within 10 years. Furthermore, the CsMPs were observed using SEM, TEM, and STEM after the dissolution experiments. The shapes of the CsMPs dissolved in pure water were considerably altered, suggesting that the decreased radioactivity was caused not by elution of ¹³⁷Cs from the silicate glass through ion-exchange, but by dissolution of the glass itself. Tin and iron originally in the glass matrix formed oxide nanoparticles on the CsMP surfaces owing to their low solubility (Fig. 8.9). This is similar to observations of CsMPs collected in Fukushima Prefecture (Yamaguchi et al. 2017), indicating that CsMP dissolution actually occurs in the environment. For CsMPs dissolved in seawater, a crust composed of secondary Mg- and Fe-rich minerals was formed on the surface and glass matrix dissolution proceeded inside the crust (Fig. 8.9).

Fig. 8.8

Arrhenius plot of the logarithm of k (rate of decrease in the CsMP radii) as a function of the reciprocal temperature 1/T for pure water and seawater. (Reprinted from Okumura et al. 2019b)

Fig. 8.9

STEM images and corresponding element maps of CsMPs after the dissolution experiments in pure water (a) and artificial seawater (b). (Adapted from Okumura et al. 2019b)

Subsequently, further dissolution experiments in solutions of various compositions and pH values were conducted at 60 °C. The dissolution behavior of CsMPs was largely comparable to that of silica-rich glass under these variable conditions (Fig. 8.10). In neutral and basic solutions, dissolution was accelerated by alkali ions such as Na⁺, which played a catalytic role. In contrast, dissolution in acid was slow, even in the presence of alkali ions. In addition, the dissolution rate in Ringer’s solution at 37 °C was determined to be 1.00 ± 0.37 μm/year, implying that CsMPs with radii of approximately 1 μm will dissolve completely within a few years at most if they are inhaled.

Fig. 8.10

Dependence of dissolution rates (k) on pH at 60 °C, where k (m/s) is the decrease rate of the CsMP radii in the presence (open symbols) and absence (closed symbols) of Na ions

8.4 Discrimination of CsMPs in Contaminated Samples

CsMPs have spread widely in eastern Japan, with several atmospheric plumes emitted from damaged reactors on March 14–15, 2011 (Tsuruta et al. 2015; Nakajima et al. 2017). Hence, their deposition areas and abundance were controlled by environmental factors such as the wind transporting the plumes, rainfall, and landscapes. Furthermore, CsMPs deposited in fields, forests, and cities have been exposed to environmental factors that may have caused movement during approximately a decade after the accident. However, abundance and distribution of CsMPs in the environment remains largely unknown. Autoradiography using an imaging plate (IP) is a common method to identify CsMPs as “radioactive particles,” but a large number of CsSMs are also present in environmental samples. Recently, a method to distinguish CsMPs from CsSMs using autoradiography was developed based on the assumption that radioactivity per individual particle is higher in CsMPs than in CsSMs (Ikehara et al. 2018). However, CsSMs with higher radioactivity than that of CsMPs have been reported (Okumura et al. 2019c), indicating that this distinction method could be inaccurate because of the overlapping radioactivity distributions between CsMPs and CsSMs.

As an alternative approach, a discrimination method based on the difference in response to acidic solutions between CsMPs and CsSMs was developed (Okumura et al. 2020a). The dissolution experiments of the CsMPs mentioned above indicated that the dissolution rate of CsMPs in acidic solutions is very slow, with only mild reductions of radioactivity. In contrast, CsSMs treated with acidic solutions under the same conditions completely lost their radioactivity (Okumura et al. 2020a). Radiocesium sorbed or fixed on the surface of the CsSMs was almost completely released, because acidic solutions dissolve the surface atomic layers. Furthermore, contaminated samples (nonwoven fabric cloth and wheat leaves) were immersed in 1 mM HCl at 90 °C for 24 h, and then in 100 mM HCl at 90 °C for 24 h. The samples were exposed to IP after each treatment (Fig. 8.11) and the IP read-out image before treatment showed several hot spots and uniform luminescence intensity throughout the cloth. However, uniform luminescence was almost completely extinguished by immersion in 1 mM HCl, likely due to the release of radiocesium from the CsSMs. Meanwhile, almost all hot spots remained even after immersion in 100 mM HCl, which could be attributed to CsMPs, because these hot spots would have been extinguished if they originated from CsSMs. In addition to the discrimination of CsMPs using IP autoradiography, their contribution to the total radioactivity of contaminated materials can be estimated quantitatively using γ-ray measurements. When the contaminated samples were immersed in acidic solutions and radioactivity periodically measured, the decrease rate slowed after the initial rapid drop (Fig. 8.12). The initial drop was ascribed to CsSM dissolution and the contribution of CsMPs to the total radioactivity was easily estimated by measuring the residual radioactivity after the acid treatment.

Fig. 8.11

IP read-out images (a–c) and optical micrograph (d) of a fragment of nonwoven fabric cloth. (a) Before acid treatment. (b) After immersion in 1 mM hydrochloric acid. (c, d) After immersion in 100 mM hydrochloric acid. The ¹³⁷Cs radioactivity corresponding to the entire area of the IP read-out images is noted below each image and the residual radioactivity after immersion is noted in brackets. (e) ¹³⁷Cs radioactivity determined from the hot spots in the IP read-out image shown in b. (Reprinted from Okumura et al. 2020a)

Fig. 8.12

Residual ratio of ¹³⁷Cs radioactivity of nonwoven fabric cloth fragments (circles and triangles) and single CsMP (squares) as a function of immersion time. They were immersed in 1 or 100 mM hydrochloric acid at 60 or 90 °C. (Reprinted from Okumura et al. 2020a)

8.5 Concluding Remarks

Knowledge of the composition and structure of CsMPs has increased considerably since their discovery in 2013 and it is generally agreed that CsMPs were formed in Unit 2. Fuel debris is recovered and investigated to understand the current conditions of the damaged reactors and guide their decommissioning processes. Debris investigations may provide further information regarding the CsMP formation processes and accident progression. The physicochemical properties of CsMPs, including their thermal and dissolution behaviors, have also been characterized to some extent, which will contribute significantly to the prediction of their dynamics in the environment and influence on human health. However, information regarding the abundance and distribution of CsMPs in Fukushima Prefecture and Kanto area is necessary to fully understand radioactive pollution by ¹³⁷Cs. For this, an approach to estimate the contribution of CsMPs to the radioactivity of environmental specimens was developed. In future research, this method should be improved to be more efficient and practical for the recovery of Fukushima and its surrounding regions.