Temporal variations of 90Sr and 137Cs in atmospheric depositions after the Fukushima Daiichi Nuclear Power Plant accident with long-term observations via Scientific Reports

• Takeshi Kinase, 
• Kouji Adachi, 
• Tsuyoshi Thomas Sekiyama, 
• Mizuo Kajino, 
• Yuji Zaizen & 
• Yasuhito Igarashi 

Scientific Reports volume 10, Article number: 21627 (2020) 

Abstract

We have measured artificial radionuclides, such as ⁹⁰Sr and ¹³⁷Cs, in atmospheric depositions since 1957 in Japan. We observed the variations in ⁹⁰Sr and ¹³⁷Cs, which were emitted from atmospheric nuclear tests and nuclear power plant accidents, due to their diffusion, deposition, and resuspension. In March 2011, the Fukushima Daiichi Nuclear Power Plant accident occurred in Japan, and significant increases in ⁹⁰Sr and ¹³⁷Cs were detected at our main site in Tsukuba, Ibaraki. Our continual observations revealed that the ¹³⁷Cs monthly deposition rate in 2018 declined to ~ 1/8100 of the peak level, but it remained more than ~ 400 times higher than that before the accident. Chemical analysis suggested that dust particles were the major carriers of ⁹⁰Sr and ¹³⁷Cs during the resuspension period at our main site. Presently, the effective half-life for ¹³⁷Cs deposition due to radioactive decay and other environmental factors is 4.7 years. The estimation suggests that approximately 42 years from 2011 are required to reduce the atmospheric ¹³⁷Cs deposition to a state similar to that before the accident. The current ⁹⁰Sr deposition, on the other hand, shows the preaccident seasonal variation, and it has returned to the same radioactive level as that before the accident.

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