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

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 Sr and Cs depositional fluxes in March 2011 reached approximately 5 Bq/m/month and 23 kBq/m/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 Cs. The Cs atmospheric concentration reached a maximum of 38 Bq/m during March 20–21, 2011. After that, the concentrations quickly decreased until fall 2011 when the decrease slowed. The pre-FDNPP accident Cs concentration levels were, at most, approximately 1 μBq/m. The average level 3 years after the accident was approximately 12 μBq/m 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.