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    Atmospheric radioactivity over Tsukuba, Japan: A summary of three years of observations after the FDNPP accident

    Igarashi Y, Kajino M, Zaizen Y, Adachi K, Mikami M

    Temporal change, 90Sr, 137Cs, Atmospheric deposition, Atmospheric concentration, FDNPP accident

    Monthly atmospheric 90Sr and 137Cs depositions at the MRI.

    Monthly deposition is expressed in mBq/m2 on a logarithmic scale. Sr-90 and 137Cs analyses from deposition samples taken several months before the accident are ongoing to avoid possible sample contamination at the MRI because of the accident. The measurement uncertainty (1σ) is shown only for the data obtained after the FDNPP accident, and is reasonably small compared to the analytical data. For comparison, uncertainty for the monthly data in 2010 is also given. The simple summation of the deposition from 1957 to the time before the FDNPP accident (mid-2010) and decay-corrected summations for 90Sr and 137Cs can be compared to the FDNPP-derived deposition.

    A severe accident occurred in March 2011 at the Fukushima Dai-ichi nuclear power plant (FDNPP) operated by the Tokyo Electric Power Company (TEPCO), causing serious environmental pollution over a wide range covering eastern Japan and the northwestern Pacific. This accident created a large mark in the atmospheric radionuclide chronological record at the Meteorological Research Institute (MRI). This paper reports the impacts from the FDNPP accident over approximately 3 years in Tsukuba, Ibaraki (approximately 170 km southwest from the accident site), as a typical example of the atmospheric pollution from the accident. The monthly atmospheric 90Sr and 137Cs depositional fluxes in March 2011 reached approximately 5 Bq/m2 /month and 23 kBq/m2 /month, respectively. They are 3–4 and 6–7 orders of magnitude higher, respectively, than before the accident. Sr-90 pollution was relatively insignificant compared to that of 137Cs. The 137Cs atmospheric concentration reached a maximum of 38 Bq/m3 during March 20–21, 2011. After that, the concentrations quickly decreased until fall 2011 when the decrease slowed. The pre-FDNPP accident 137Cs concentration levels were, at most, approximately 1 μBq/m3 . The average level 3 years after the accident was approximately 12 μBq/m3 during 2014. The atmospheric data for the 3 years since the accident form a basis for considering temporal changes in the decreasing trends and re-suspension (secondary emission), supporting our understanding of radioCs’ atmospheric concentration and deposition. Information regarding our immediate monitoring, modeling, and data analysis approaches for pollution from the FDNPP accident is provided in the Appendices.