Radioactive cesium (¹³⁷Cs) released from the accidence of the Fukushima Dai-ichi nuclear power plant contaminated areas of the eastern part of Japan, particularly Fukushima prefecture and has affected the life of local residents in many ways. During the release events, 137Cs (b.p. 678˚C) was emitted to the atmosphere as CsI in aerosols. After a couple of years soil is likely the most significant reservoir for it. As ¹³⁷Cs emits 652 keV γ-ray, ¹³⁷Cs in top-layers of soils may contribute to external exposure of local residents and biospheres. It can be absorbed by plants from contaminated soils, which may lead to internal exposure. Thus, It is of great importance to understand behaviors of ¹³⁷Cs.

In this research, we’ve evaluated the geochemical and grain-size distribution of ¹³⁷Cs in contaminated soils sampled in Fukushima Prefecture after the accident. The obtained results have been also compared with those of stable cesium (¹³³Cs) to infer the variation of the geochemical distribution in the near future.

¹³⁷Cs are most extracted to the acid-digestible factions and the extraction residues. It is also found that ¹³⁷Cs is associated with relatively small silt and clay  grains, which are rich in micaceous clay minerals and kaolins according to X-ray diffraction. We’ve found positive correlation between ¹³⁷Cs content in each fraction and the amount of micaceous minerals, suggesting crucial roles of the clay minerals on fixation of ¹³⁷Cs in soils. The geochemical distribution of ¹³⁷Cs determined by sequential extraction is different from that of ¹³³Cs. This indicates that the distribution of ¹³⁷Cs is in full isotopic equilibrium with that of ¹³³Cs; in the near future fixation of ¹³⁷Cs to the clay minerals will further proceeds.