Abstract
On March 12, 2011, the Fukushima Daiichi Nuclear Power Plant of the Tokyo Electric Power Company caused a sequent hydrogen explosion. After the explosion, the atmospheric radiation doses at measuring points in Mito City, Ibaraki Prefecture, have rapidly increased, which have caused the concerns about the bad effects on residents' life by the atmospheric radiation doses. For our analysis, we prepared a model for forecasting the future fluctuations of the atmospheric radiation doses in Mito City. This model was built based on the exponentially decreased radiation doses, depending on their own decay rates; these radiation doses were resulted from the nuclear plant accident and rectified by natural decontamination, by wind and rain, of the radiation doses environmentally accumulated. We made a forecast of the atmospheric radiation doses between April, 2012 and March, 2014, by applying this model. This forecast shows that the radiation doses of Mito City would be reduced until March, 2013, two years after the accident, to the level a little higher (0.065 μSv/hr) than the natural radiation doses before the accident (0.056 μSv/hr), and that the radiation doses of Mito City in March, 2014, three years after the accident, would be reduced to the level of natural radiation doses. The forecast values of the yearly accumulated atmospheric radiation doses measured by hourly basis shows that the values would be reduced to the natural radiation level (1 mSv/yr), both one year and two years after the accident. As for the suddenly increased radiation doses frequently observed, we clarified by multiple regression analysis that these phenomena were caused by the combination of both “heavy precipitation” and “high wind velocity”.
