Researchers in Japan found new materials they described as tiny spherical glass particle that was highly radioactive. Researchers found that the radioactivity was highest in the center of the particle, indicating the cesium was incorporated into the glass particle during the molten phase of the meltdown. The glass particle also contains materials that indicate it includes either concrete from the containment vessel or seawater that was injected.
Microparticles containing substantial amounts of radiocesium collected from the ground in Fukushima were investigated mainly by transmission electron microscopy (TEM) and X-ray microanalysis with scanning 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. Nano-sized crystallites such as copper- zinc- and molybdenum sulfide, and silver telluride were found inside the microparticles, which probably resulted from the segregation of the silicate and sulfide (telluride) during molten-stage. 0.2 μm thick exists 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.
Photo of the glass sphere from Nihonmatsu, from the Yamaguchi et al study.
Cross section of the NWC-1 glass sphere from Nihonmatsu, photo credit Yamaguchi et al.
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. Wet deposition was a major source of radiocesium contamination of terrestrial environment1, while contribution of dry deposition was larger near the FDNPP2. Deposition of radiocesium as insoluble particles has also been pointed out. On the aerosol filter collected from March 14–15, 2011 in Tsukuba, 170 km south-southwest of FDNPP, Adachi et al.7 discovered spherical particulate radiocesium of 2.0–2.6 μm 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 to be 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 ground surface as a solute was fixed to soils, especially to clay minerals10. In the terrestrial environment, the majority of radiocesium 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 deposited as 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 human 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, but collected from the ground, by observing their internal structure with transmission electron microscopic (TEM) techniques.
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. 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.
Photo credit Yamaguchi et al.
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. 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.
Photo credit Yamaguchi et al.
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. This may be attributed to the different environments of the two microparticles after release from the nuclear plant. It is well-known that silicate glass elutes alkali components from their 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. 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 the radioactive cesium from the particles is probably not long, from several years to a few decades, though it will strongly depends on the environment.
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 enough number of Cs-bearing microparticles to investigate dissolution properties. Alternatively, synthesized silicate glass with the same composition as the microparticles presented in this study may help to obtain information on the fate of Cs-bearing radioactive microparticles. 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. 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.
Photo credit Yamaguchi et al.
Photo of black sand substances found in Namie, from research paper by Marco Kaltofan. Photo credit Marco Kaltofan.
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).
Marco Kaltofen, MS, PE : Radiological analysis of Namie street dust
Japan’s Black Dust, with Marco Kaltofen