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

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. - Highlights: • 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