Abstract
After the Fukushima nuclear accident, atmospheric 134Cs and 137Cs measurements were taken in Fukushima city for eight years, from March 2011 to March 2019. The surface air concentrations and deposition of radio-Cs were high in winter and low in summer; these trends are the opposite of those observed in a contaminated forest area. The half-lives of 137Cs in the concentrations and deposition before 2015 (275 d and 1.11 y) were significantly shorter than those after 2015 (756 d and 4.69 y). The dissolved fractions of precipitation were larger than the particulate fractions before 2015, but the particulate fractions were larger after 2016. The half-lives of 137Cs in the concentrations and deposition were shorter before 2015, probably because the dissolved radio-Cs was discharged from the local terrestrial ecosystems more rapidly than the particulate radio-Cs. X-ray fluorescence analysis suggested that biotite may have played a key role in the environmental behavior of particulate forms of radio-Cs after 2014. However, the causal relationship between the seasonal variations in particle size distributions and the possible sources of particles is not yet fully understood. The current study also proposes a method of evaluating the consistency of a numerical model for radio-Cs resuspension and suggests that improvements to the model are necessary.
Highlights
We conducted eight-year measurements of atmospheric 134Cs and 137Cs in Fukushima city after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident that occurred in March 2011 to understand the time variations in and emission sources of 134Cs and 137Cs and to propose effective ways to reduce atmospheric radioactivity
The concentration quickly decreased to a level of 10-4 Bq/m3, and the minimum concentration of 4.05 × 10-6 Bq/m3 was obtained on December 5, 2018
The coefficient of determination, R2, is 0.993. This demonstrates that the surface concentration decreased exponentially and halved in approximately 4 years; the decrease rate was higher than the rate of radioactive decay of 137Cs
Summary
We conducted eight-year measurements of atmospheric 134Cs and 137Cs in Fukushima city after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident that occurred in March 2011 to understand the time variations in and emission sources of 134Cs and 137Cs and to propose effective ways to reduce atmospheric radioactivity. 30 % of the radio-Cs released in March 2011 was deposited onto the ground surface in Japan (the 10 aircraft-measured deposition on the ground was 2.7 PBq for 137Cs; NRA, 2012, and the most updated estimate of 137Cs emissions by the Japan Atomic Energy Agency is 10 PBq; Terada et al, 2020). Once radio-Cs is deposited onto the ground surface, it circulates within local terrestrial ecosystems, so the discharge from the local environment to downstream or downwind regions may not be substantial (0.02 - 0.3 % y-1 to river; Iwagami et al, 2017, approximately 1 % y-1 to atmosphere). Long-term monitoring of 15 atmospheric radio-Cs at even one station may allow us to understand the mechanisms of its circulation in the local terrestrial ecosystems, to estimate the external and inhalation exposure risks to the local residents, to propose efficient ways to reduce health risks to the residents, and to assess the effectiveness of decontamination efforts
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