Settlement process of radioactive dust to the ground inferred from the atmospheric electric field measurement

摘要: Abstract. Radioactive materials from the accident at Fukushima Dai-ichi nuclear power plant (FNPP) in March 2011 spread over a large area, increasing atmospheric electric conductivity by their ionizing effect, and reducing vertical (downward) component of DC field near ground, or potential gradient (PG). PG data Kakioka, 150 km away FNPP, showed independent changes compared to radiation dose rate, comparison these revealed local dynamics radioactive dust. (1) The initial drop almost zero during 14–15 is most likely due dust suspended air ground cloudy weather. (2) An episode increase more than 50 V m−1 on 16 re-suspension surface subsequent removal Kakioka strong wind non-contaminated area. (3) Low but finite values 16–20 reflect reduced amount material after above transported majority (4) Very low substantial rain 20–22 settlement rain-induced fallout. (5) Temporal recovery daily variations end middle April with nighttime fair-weather baseline reflects into trees, transport other region fallout until late April. (6) Weakening variation gradual mid-April suggests complete partial migration subsurface.