Fig. 1

Sr-90 and ¹³⁷Cs monthly deposition observed at the Meteorological Research Institute (MRI) from 1957 to 2014. Monthly deposition is expressed in millibecquerel per square meter on a logarithmic scale. Sr-90 and ¹³⁷Cs analyses from deposition samples taken 6 and 8 months before the accident, respectively, are ongoing to avoid possible sample contamination at the MRI because of the accident. Thus, these data are missing not only in Fig. 1 but also in Fig. 2. 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 effects of atmospheric nuclear bomb tests have been recorded since 1957. Until the Partial Test Ban Treaty (PTBT) became effective in 1963, the USA, Soviet Union, and UK conducted atmospheric tests. France and China continued atmospheric testing until 1974 and 1980, respectively. Since 1981, all the nuclear bomb tests have shifted underground, so additional radioSr and Cs contamination should be negligible. However, the Chernobyl accident in 1986 also affected the time series. 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