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 Japanese cedar (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 the whole 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 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 of Na 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 SiO 2 ; 69.3, K 2 O; 1.9, Fe 2 O 3 ; 8.6, ZnO; 11.0, Rb 2 O; 1.3, SnO 2 ; 1.4, Cs 2 O; 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 products of U-235. Tin is also used for fuel cladding11. Iron is used as steel for reactor pressure vessels and Si and O are main 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 from concrete and/or seawater, which was used to cool down the reactor.

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. Full size image

The BF TEM image of CB-8 showed a two-layer structure having an inner core with darker contrast, an outer crust with a lighter one and a small dark nanoparticle near the center (Fig. 1e). The thickness of the crust was around 0.2 μm. SAED pattern indicated that both are amorphous (Fig. 1f). Locally, a bubble-like structure was observed at a certain radius in the crust, or the outside part was peeled off at the radius (Fig. 1e). The chemical composition of the entire area of the specimen was SiO 2 ; 73.3, K 2 O; 1.4, Fe 2 O 3 ; 7.2, ZnO; 11.4, Rb 2 O; 1.2, SnO 2 ; 1.5, Cs 2 O; 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 their surface. 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.

Figure 2 (a) Element maps of NWC-1 acquired by STEM-EDS with a STEM Dark-field (DF) image at the same 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). Full size image

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 are still 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 the nanoparticles, 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 (Cu 2−x S). 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. Both elements 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.