Isotopic ratio and vertical distribution of radionuclides in soil affected by the accident of Fukushima Dai-ichi nuclear power plants

The results of γ analyses of soil samples obtained from 50 locations in Fukushima prefecture on April 20, 2011, revealed the presence of a spectrum of radionuclides resulted from the accident of the Fukushima Dai-ichi nuclear power plant (FDNPP). The sum γ radioactivity concentration ranged in more than 3 orders of magnitude, depending on the sampling locations. The contamination of soils in the northwest of the FDNPP was considerable. The 131I/137Cs activity ratios of the soil samples plotted as a function of the distance from the F1 NPPs exhibited three distinctive patterns. Such patterns would reflect not only the different deposition behaviors of these radionuclides, but also on the conditions of associated release events such as temperature and compositions and physicochemical forms of released radionuclides. The 136Cs/137Cs activity ratio, on the other hand, was considered to only reflect the difference in isotopic compositions of source materials. Two locations close to the NPP in the northwest direction were found to be depleted in short-lived 136Cs. This likely suggested the presence of distinct sources with different 136Cs/137Cs isotopic ratios, although their details were unknown at present. Vertical γ activity profiles of 131I and 137Cs were also investigated, using 20–30 cm soil cores in several locations. About 70% or more of the radionuclides were present in the uppermost 2-cm regions. It was found that the profiles of 131I/137Cs activity ratios showed maxima in the 2–4 cm regions, suggesting slightly larger migration of the former nuclide. ► We report the results of γ analyses of soil samples around Fukushima Dai-ichi NPPs. ► The contamination of soils in the northwest of the power plant was considerable. ► The 131I/137Cs activity ratios exhibited three distinctive patterns. ► Most of 131I and 137Cs were present in the uppermost 2-cm regions of soil.