Eight-year variations in atmospheric radiocesium in Fukushima city

Akira Watanabe,Mizuo Kajino,Yoshitaka Nagahashi,Kazuhiko Ninomiya,Atsushi Shinohara

doi:10.5194/acp-22-675-2022

Abstract

Abstract. After the Fukushima nuclear accident, atmospheric 134Cs and 137Cs measurements were taken in Fukushima city for 8 years, from March 2011 to March 2019. The airborne surface concentrations and deposition of radiocesium (radio-Cs) were high in winter and low in summer; these trends are the opposite of those observed in a contaminated forest area. The effective half-lives of 137Cs in the concentrations and deposition before 2015 (0.754 and 1.30 years, respectively) were significantly shorter than those after 2015 (2.07 and 4.69 years, respectively), which was likely because the dissolved radio-Cs was discharged from the local terrestrial ecosystems more rapidly than the particulate radio-Cs. In fact, the dissolved fractions of precipitation were larger than the particulate fractions before 2015, but the particulate fractions were larger after 2016. 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 an evaluation method of the consistency of a numerical model for radio-Cs resuspension and suggests that improvements to the model are necessary.

Highlights

We conducted measurements of atmospheric 134Cs and 137Cs in Fukushima city for 8 years following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident that occurred in March 2011 in order 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 m−3, and the minimum concentration of 4.05×10−6 Bq m−3 was obtained on 5 December 2018
The current study clearly shows that the concentrations of 137Cs are high from winter to spring, with peaks in March, and low from summer to autumn in the urban/rural area of Fukushima city (60 km northwest of the FDNPP)

Summary

Introduction

We conducted measurements of atmospheric 134Cs and 137Cs in Fukushima city for 8 years following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident that occurred in March 2011 in order to understand the time variations in and emission sources of 134Cs and 137Cs and to propose effective ways to reduce atmospheric radioactivity. Among the various radionuclides released to the environment, radiocesium (radio-Cs) is important due to its abundance in terrestrial ecosystems (the impacts of other nuclides were negligibly small 100 d after the accident; Yoshimura et al, 2020), long half-life (2.06 years for 134Cs and 30.17 years for 137Cs), and bioaccumulation (accumulation in muscle tissues, with a biological half-life of 30–150 d; WHO, 2001). A. Watanabe et al.: Eight-year variations in atmospheric radiocesium in Fukushima city

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Discussion

Conclusion