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Three radioactive microparticles were separated from particles on filter tape samples collected hourly at a suspended particulate matter (SPM) monitoring site located at ∼25 km north of the Fukushima Daiichi Nuclear Power Plant (FDNPP), after the hydrogen explosion of reactor 1 on 12th March 2011. The 134Cs/137Cs radioactivity ratios of the three radioactive aerosol particles showed that they were derived from the FDNPP reactor 1, rather than reactors 2 or 3. The physical characteristics of these particles with < 10 μm in diameter and non-uniform shape are clearly different from those of radioactive particles generated by the hydrogen explosion of the FDNPP reactor 1. A significant amount of Cl was detected by energy dispaersive X-ray spectrometery. Synchrosron radiation microbeam (SR-μ-) X-ray fluoresence (XRF) analysis showed that these particles contain a series of heavy elements related to the nuclear fules and their fission products with a non-homogeneous distribution within the particles. In addition, the SR-μ-XRF identified trace amounts of Br in these particles; the element has firstly been found in radioactive particles derived by the FDNPP accident. In contrast to the hydrogen explosion-generated radioactive particles containing Sr and Ba, both of which are easily volatile under a reduction atmosphere, these elements were not rich in the particles found in this study. By the SR-μ-X-ray absorption near edge structure analysis and SR-μ-X-ray powder diffraction, it was found that these particles consist of an amorphous (or low crystalline) matrix containing metal elements with chemical states in a comparatively high state of oxidation or chloride. Based on these physical and chemical characteristics and a trajectory analysis of air parcels that passed over the SPM monitoring site, we concluded that these radioactive particles were generated and emitted into the atomosphere at the time of seawater injection for cooling the reactor after the hydrogen explosion.

Additional file 1: Table S1. 134Cs and 137Cs radioactivities and 134Cs/137Cs activity ratio of eight particles (A1~G1). Figure S1. SR-μ-XANES spectra of four representative CsMPs (A1, B1, C1, and E1) isolated from the SPM filter and reference materials. (a) Fe-K edge, (b) Zn-K edge, (c) Mo-K edge, and (d) Sn-K edge. Figure S2. SR-μ-XRD patterns of four representative radioactive particles (A1, B1, C1, and E1) isolated from the SPM filter and reference material. (a) Particle A1, (b) particle B1, (c) particle C1, (d) particle E1, and (e) silicon powder (NIST SRM 640c). In contrast to sharp diffraction peaks detected in silicon powder, no obvious peaks were detected in four radioactive particles.