Plutonium (Pu) and non-natural uranium (U) originating from the Fukushima Daiichi Nuclear Power Plant (FDNPP) were identified in the atmosphere at 120 km distance from the FDNPP analyzing the ratio of number of atoms, following written as n(isotope)/n(isotope), of Pu and U. The n((240)Pu)/n((239)Pu), n((241)Pu)/n((239)Pu), n((234)U)/n((238)U), n((235)U)/n((238)U) and n((236)U)/n((238)U) in aerosol samples collected before and after the FDNPP incident were analyzed by accelerator mass spectrometry (AMS) and inductively coupled plasma mass spectrometry (ICPMS). The activity concentrations of (137)Cs and (134)Cs in the same samples were also analyzed by gamma spectrometry before the destructive analysis. Comparing the time series of analytical data on Pu and U obtained in this study with previously reported data on Pu, U, and radioactive Cs, we concluded that Pu and non-natural U from the FDNPP were transported in the atmosphere directly over a 120 km distance by aerosol and wind within a few days after the reactor hydrogen explosions. Effective dose of Pu were calculated using the data of Pu: (130 ± 21) nBq/m(3), obtained in this study. We found that the airborne Pu contributes only negligibly to the total dose at the time of the incident. However the analytical results show that the amount of Pu and non-natural U certainly increased in the environment after the incident.

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The observation network of atmospheric radon-222 ( 222 Rn) concentration established in East Asia region is introduced, and the characteristics of the observations at two continental sites Beijing and Seoul and three remote sites Cape Ochiishi, Hachijo Is. and Hateruma Is. are discussed in this paper. Higher levels of 222 Rn concentrations with typical diurnal variation with early morning maxima were observed on the continent, and lower levels with no diurnal variation at remote islands. Seasonal variations with summer minima and winter maxima were commonly obtained at all five observatories, and they suggested contribution of 222 Rn originated from the continent to atmospheric 222 Rn over the remote islands isolated in the ocean. A backward trajectory analysis showed clear relationship between variation in wind field and in 222 Rn concentration at Hachijo Is., and proved availability of the observation for analysis of atmospheric transport in East Asia.

A three-dimensional Eulerian numerical model for 222 Rn and its decay products coupled with a mesoscale meteorological model has been developed and applied to the reproduction of the daily and monthly variations of 222 Rn concentration, the monthly variation of 210 Pb deposition in Japan and the temporal variation of gamma dose rate after the cold front passage in the coastal area of the Japan Sea for the verification of model capability. The results are as follows: (1) The model reproduced the monthly variation of surface 222 Rn concentration in remote islands, but underestimated inland concentration due to a coarse vertical resolution near the surface of the model. (2) The model reproduced the seasonal variation of the observed and the long-term yearly averaged 210 Pb depositions as long as precipitations are predicted precisely. (3) The model reproduced the rise of gamma dose rate in precipitation accompanied by the cold front passage. In particular, 222 Rn decay products in melted snow and graupel contributed the rise of gamma dose rate. KEYWORDS: numerical model, concentrations of 222 Rn and its decay products, deposition of 210 Pb, gamma dose rate

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