Size distribution of radioactive particles collected at Tokai, Japan 6 days after the nuclear accident

• We started size-fractioned samplings 6 days after the Fukushima NPP accident.
• Activity median aerodynamic diameter (AMAD) of 131I was 0.45 μm.
• Median diameter of 129mTe, 140Ba, 134Cs, 136Cs and 137Cs activity was 1.5–1.6 μm.
• The diameters of 134Cs and 137Cs collected in May 2011 were <0.5, 0.94, and 7.8 μm.
• Activity ratios of Cs were helpful in understanding the change of Cs composition.

Airborne radioactive particles released by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in 2011 were collected with a cascade low-pressure impactor at the Japan Atomic Energy Agency (JAEA) in Tokai, Japan, 114 km south of the FDNPP. Size-fractionated samples were collected twice, in the periods of March 17–April 1, 2011, and May 9–13, 2011. These size-fractionated samplings were carried out in the earliest days at a short distance from the FDNPP. Radioactivity of short-lived nuclides (several ten days of half-life) was determined as well as 134Cs and 137Cs. The elemental composition of size-fractionated samples was also measured. In the first collection, the activity median aerodynamic diameter (AMAD) of 129mTe, 140Ba, 134Cs, 136Cs and 137Cs was 1.5–1.6 μm, while the diameter of 131I was 0.45 μm. The diameters of 134Cs and 137Cs in the second collection were expressed as three peaks at <0.5 μm, 0.94 μm, and 7.8 μm. The 134Cs/137Cs ratio of the first collection was 1.02 in total, but the ratio in the fine fractions was 0.91. A distribution map of 134Cs/137Cs – 136Cs/137Cs ratios was helpful in understanding the change of radioactive Cs composition. The Cs composition of size fractions <0.43 μm and the composition in the 1.1–2.1 μm range (including the AMAD of 1.5–1.6 μm) were similar to the calculated compositions of fuels in the reactors No. 1 and No. 3 at the FDNPP using the ORIGEN-II code. The Cs composition collected in May, 2011 was similar to the calculation results of reactor No. 2 fuel composition. The change of Cs composition implies that the radioactive Cs was released from the three reactors at the FDNPP via different processes.