Loss of radioactivity in CsPs by heating

Six CsPs, named CsP-1 to 6, were heated in a temperature range of 600–1000 °C and their radioactivity was determined before and after heating. The change of radioactivity normalized according to the original radioactivity is shown in Fig. 1. CsP-1 to 3 were put into a hole in a platinum plate using a micro-sampling unit attached to a focused ion beam (FIB) system, and presented for the heating experiments. CsP-4 to 6 were dropped into a platinum pan and presented for the heating experiments. As for CsP-6, soil with a negligible amount of radiocesium was added to the pan to cover the CsP before heating. The radioactivity was measured after each heating operation, in which CsPs were heated up to a target temperature at a rate of 10 °C/min and cooled in air. The data points of CsP-2 to 4 are connected by lines in Fig. 1 because heating and cooling were repeated at target temperatures of 600, 700, 800 and 900 °C for CsP-2 and 3, and of 900 and 1000 °C for CsP-4. The radioactivity of CsPs except CsP-2 and 3 was measured using a germanium detector, whereas that of CsP-2 and 3 was determined by imaging plate (IP) autoradiography4. Since radioactivity is calculated from the signal intensity in readout images of IPs in IP autoradiography, the error was not evaluated for CsP-2 and 3. As a result of the heating experiments, the radioactivity of CsPs decreased at a temperature of more than 600 °C and was almost lost at around 1000 °C although the decrease rates varied. Since the radioactivity of CsPs except CsP-1 to 3 also decreased, the loss of radioactivity originated not from the artifacts of FIB processes for picking up CsPs but from the properties of CsPs themselves.

Figure 1 Radioactivity change of CsPs by heating. Radioactivity normalized by that before heating is shown as a function of heating temperature. In each heating operation, CsPs were heated up to the target temperature at 10 °C/min and cooled in air. Points connected by lines mean that heating and cooling were repeated at subsequently increasing target temperatures. Error bars are not shown in CsP-2 and 3 because their radioactivity was determined by IP autoradiography. The radioactivity of other CsPs was measured using a germanium detector. Full size image

Effects of heating on the morphology and composition of CsPs

In order to investigate the effects of heating on the morphology of CsPs, we observed CsPs before heating using scanning electron microscopy (SEM). Figure 2a,b shows secondary electron (SE) and back-scattered electron (BSE) images of CsP-2, on Kapton tape. Although this CsP was almost buried in the glue of Kapton tape, the BSE image indicated that it had a spherical shape, which is typical of CsPs. The CsP was put into a hole made in a platinum plate using FIB and heated as mentioned above. Figure 1c shows the SE image of CsP-2 after heating at 900 °C. The surrounding Kapton tape was burned off and CsP-2 was exposed to view. CsP-2 remained its spherical shape, and its size was also unchanged even after heating. This result indicated that heating at 900 °C did not alter the morphological features of CsPs.

Figure 2 Morphologies of CsP-2 before and after heating. SE (a) and BSE (b) images of CsP-2 before heating. (c) SE image of CsP-2 after heating at 900 °C. White arrows in (a) and (c) indicate CsP-2. Full size image

The effects of heating on the composition of CsPs were also investigated using SEM-EDS (Fig. 3). The spectrum of CsP-3 before heating showed Si and O as the main elements, and Cl, K, Fe, Zn, Sn and Cs as minor elements, which is a typical composition for CsPs7. Although Al was also detected in CsP-3, it was probably due to adhering soil particles (see the results of STEM-EDS in Fig. 4). Carbon originated from the Kapton tape. Because Cl, K and Cs disappeared in CsP-3 by heating at 900 °C, these elements may have been released from the CsP mainly as a form of KCl and CsCl. Note that Pt and Ga were detected because of a platinum plate for a container of the CsP and a gallium ion beam in FIB, respectively. Although the compositions of CsP-2 changed similarly to CsP-3, a portion of Cs remained in the CsP even after heating (Supplementary Fig. 1). This is consistent with the result that CsP-2 retained 16% of its original radioactivity after heating, as shown in Fig. 1.

Figure 3 Composition change of CsP-3 by heating. SEM-EDS spectra acquired from CsP-3 before and after heating at 900 °C. The inset shows the enlarged spectra in the range of 2–7 keV. Full size image

Figure 4 Structures and compositions of CsP-2 after heating at 900°C. (a) STEM-ADF image of CsP-2. (b) Enlarged view of the white square in (a). (c) Electron diffraction pattern acquired from the rim of CsP-2, in which many tiny bright spots exist in the STEM-ADF image. The inset shows the calculated Debye-Scherrer pattern of franklinite (ZnFe 2 O 4 ). (d) Element maps of the same frame as (b). (e) EDS spectra acquired from the center and rim of CsP-2. The peak intensity is normalized for quantitative comparison. Full size image

More detailed examinations of heated CsPs were conducted using TEM/STEM. Figure 4a is a STEM-annular dark-field (ADF) image of CsP-2 after heating at 900 °C. In the enlarged view, nanometric crystals identified as franklinite (ZnFe 2 O 4 ) by electron diffraction were observed at the rim of the CsP (Fig. 4b,c). The element maps of the CsP were also acquired using STEM-EDS (Fig. 4d), indicating the enrichment of Fe, Zn and Sn in the same region as franklinite. Since such a distribution has not been reported to date, these elements are believed to have migrated within the CsP and to have crystallized as franklinite. Furuki et al.11 reported the existence of franklinite in original CsPs; however, we suspect that it was caused by irradiation damage because silicate glass is very sensitive to an electron beam7. We actually confirmed that Debye-Scherrer rings corresponding to a spinel structure appeared when an intense electron beam was used to irradiate CsPs although a halo corresponding to amorphous materials was initially observed in the electron diffraction pattern. Al existed at the periphery of the CsP, meaning that it did not originate from the CsP but foreign substances such as soil particles, as mentioned above. Cl, K and Cs were not detected in the EDS spectra obtained from the center and rim of the CsP although K and Cs slightly remained in the center region because CsP-2 retained 16% of its original radioactivity after heating (Fig. 4e). These results also suggest that these elements migrated within CsPs and were released from the surface of CsPs.

The examination of CsP-3 after heating at 900 °C using TEM/STEM revealed that the phase separation of CsP-3 proceeded further than that of CsP-2 because larger crystals of several dozen nanometers were distributed over CsP-3 (Fig. 5a,b). These crystals were willemite (Zn 2 SiO 4 ; Fig. 5c,d) and franklinite (Fig. 5e,f), which were precipitated as a result of heating. The element maps for Si, Fe, Zn and Sn also have heterogeneous distribution because of migration and crystallization of these elements (Fig. 5g). The element map of Sn indicates that acicular cassiterite (SnO 2 ) was also precipitated. Cl, K and Cs were completely undetectable in the EDS spectrum acquired from the whole CsP, which is corroborated by the result that the radioactivity of CsP-3 was almost lost by heating (Fig. 5h).

Figure 5 Structures and compositions of CsP-3 after heating at 900°C. (a) STEM-ADF image of CsP-3. (b) Enlarged view of the white square in (a). (c) Electron diffraction pattern acquired from the area indicated by c in (b). (d) Calculated electron diffraction pattern of willemite (Zn 2 SiO 4 ) observed along <122>. (e) Electron diffraction pattern acquired from the area indicated by e in (b). (f) Calculated electron diffraction pattern of franklinite (ZnFe 2 O 4 ) observed along <111>. (g) Element maps of the same frame as (b). (h) EDS spectrum acquired from whole CsP-3. Full size image

Heating of CsPs together with soil

To mimic a CsP in soil in the environment, we heated CsP-6 in the presence of weathered granitic soil collected in Fukushima. After heating, we turned the pan upside down, and found that only soil was out of the pan and CsP-6 remained inside. The radioactivity of 137Cs in the CsP and soil before and after heating is shown in Table 1. Consequently, half of the original radioactivity moved from the CsP to the soil by heating. Moreover, the radioactivity of the soil was halved when the soil was divided into two parts, suggesting that the radioactivity was distributed throughout the soil. The halved soil A was scattered on an IP and kept in the dark for six days. In the readout image of the IP, many bright spots corresponding to radioactive particles appeared, although their intensities varied (Supplementary Fig. 2). Since we confirmed that no bright spots appeared when the original soil was scattered on an IP and kept in the dark for six days, the bright spots in Supplementary Fig. 2b were caused by radiocesium moved from the CsP to the soil. The radioactivity of 137Cs in the most intense particles indicated by the white arrow was estimated to be approximately 0.005 Bq at most by IP autoradiography. These results indicate that radiocesium in CsPs is sorbed by the surrounding soil particles when actual contaminated soil including CsPs is heated in air.

Table 1 Radioactivity of 137Cs (Bq) for CsP-6 heated with soil. Full size table

CsP-6 in the pan after heating was transferred onto Kapton tape for the following SEM observation. Consequently, the morphology of CsP-6 did not change by heating even when it was heated together with soil as shown in Supplementary Fig. 3. Cs remained inside the CsP even after heating because it retained half of the original radioactivity (Supplementary Fig. 4). Some Fe and Zn in CsP-6 was crystallized as franklinite according to the TEM analysis (Supplementary Fig. 5a,b). The element maps of CsP-6 after heating at 900 °C showed that only Cs near the surface of the CsP disappeared (Supplementary Fig. 5c). On the contrary, Sn was concentrated at the periphery of the CsP, indicating that this CsP slightly dissolved and cassiterite was precipitated around the CsP8. These results are consistent with the results of other CsPs that were heated without soil, meaning that CsPs themselves show the same responses to heating regardless of the coexistence of soil.