Investigation of Physical and Chemical Characteristics of Radioactive Aerosols Emitted From Reactor Unit 1 by Fukushima Daiichi Nuclear Power Plant Accident

Deposition and Dispersion of Radio‐Cesium Released due to the Fukushima Nuclear Accident: 2. Sensitivity to Aerosol Microphysical Properties of Cs‐Bearing Microparticles (CsMPs)

“…On the other hand, the dose rate from ground shine should be enhanced due to fast settling velocities; therefore, the assumption of the relative fractions of cloud shine and ground shine should be modified. There were also type B CsMPs with diameters of 1–5 μm floating in the air (Onozaki et al., 2019). Thus, it is currently still not clear whether the estimated amount will be increased or decreased by considering the microphysical properties of type B CsMPs.…”

“…Onozaki et al. (2019) found type B CsMPs smaller than 10 μm in diameter from air samples collected at a station located 25 km north of the F1NPP. The relative contributions of radioactivity from type B CsMPs larger than 10 μm and smaller than 10 μm would have been a key factor controlling the surface air concentration and deposition of radio‐Cs during the emission event.…”

“…Approximately half of this underestimation can be explained by the presence of type B CsMPs (0.7 PBq), if the emission estimated by Katata et al (2015) during the hydrogen explosion of Unit 1 consisted solely of type B CsMPs and the diameters of all the particles were much larger than 10 μm (as suggested by Satou et al, 2018 [70-400 μm]) so that they were readily deposited onto the ground surface within less than 100 km from the F1NPP. Onozaki et al (2019) found type B CsMPs smaller than 10 μm in diameter from air samples collected at a station located 25 km north of the F1NPP. The relative contributions of radioactivity from type B CsMPs larger than 10 μm and smaller than 10 μm would have been a key factor controlling the surface air concentration and deposition of radio-Cs during the emission event.…”

“…The presence of type B CsMPs (70–400 μm in soil (Satou et al., 2018) and 1–5 μm in air (Onozaki et al., 2019) arising from the hydrogen explosion of Unit 1 occurred on March 12, 15:36 LT; UTC+9; 0.7 PBq) could partly explain the simulated underestimation of the total deposition over land when assuming 100% WSPs. The observed deposition over land totaled 2.64 PBq, whereas the simulated deposition assuming submicron carriers was 1.19 PBq.…”

“…Works on the isolation of CsMPs from hourly air filters at one hundred locations (Oura et al., 2015; Tsuruta et al., 2014) are currently ongoing (partly reported by Onozaki et al., 2019). Information on the relative magnitudes of radioactivity from CsMPs and WSPs is essential.…”