Concentration level of radiocesium in agricultural soils (mainly, paddy soils) in neighboring district of the huge nuclear facilities as the atomic nuclear reactor of Japan before the accident of the Fukushima Daiichi nuclear power plant. 4. Fall out of radiocesium and radioiodine originated from an open-air nuclear test and nuclear accident on the land surfaces, and the movement of their nuclides in the soil

This report focused on the radiocesium and radioiodine originated from nuclear tests and nuclear reactor accidents (mainly Chernobyl accident, including Fukushima Nuclear accident), and examined their release to the atmosphere, fallout onto the ground surface, and transition from the soil surface to lower layer. The authors clarified the difference between the radiocesium that descends in an insoluble particulate form and has low migration property to the soil lower layer and the radioiodine that descends in a gaseous or water-soluble particle form and has high migration property. The ¹³⁷Cs concentrations were all in the range of several Bq/kg to 100 Bq/kg dry soil, and when above this figure, it was suggested that cumulative leakage/contamination from nuclear reactors, etc. would have occurred. This survey showed that the results can be used as the background data of radiocesium in the survey of farmland soils after the Fukushima nuclear accident. As the regional difference of ¹³⁷Cs concentrations, Hokuriku and Akita Prefecture showed more than twice the values compared with those in other areas. It was presumed that this difference is due to the number of precipitation days, rainfall volume, latitude, and east/east positional correlation. As mentioned in Reports I to III, the radioactivity concentration of ¹³⁷Cs is lower than the concentration of 40 K of natural radionuclide, and is considered to be a level that is not a problem as an external radiation source. From now on, the authors would like to publish as separate materials the long-term transition and penetration (in several decades order) of ¹³⁷Cs into agricultural lands (mainly paddy fields) and forest land lower layers. (A.O.)