Investigation and Research on Depth Distribution in Soil of Radionuclides Released by the TEPCO Fukushima Dai-ichi Nuclear Power Plant Accident

The accident of the TEPCO Fukushima Dai-ichi Nuclear Power Plant occurred by the 2011 off the Pacific coast of Tohoku Earthquake on 11 Mar. 2011. It is estimated that totally 1.2-1.5x10 16 Bq for 137 Cs and 1.5-1.6x10 17 Bq for 131 I were released until the beginning of Apr. and those radionuclides (RN) were deposited on soil surface and forest etc. widely around Fukushima Pref. This work was carried out as one of the investigations for making the distribution maps of radiation dose rate and soil contaminated by RNs which the MEXT promotes. The Geoslicer investigation on the depth distribution of RNs in soil was performed after 3 months from the accident. The investigation was conducted at 11 locations in Nihonmatsu City, Kawamata Town and Namie Town, and soil samples of depth 50 cm to 1 m were taken. Both of 134 Cs and 137 Cs were detected in all investigated locations, and 129m Te and 110m Ag were detected only in locations where radiation dose rates are high. At many locations investigated, radiocaesium more than 99 % distributed within a depth of 10 cm in soil in the surface layer. On the other hand, RNs tended to distribute to deeper part in soil at locations that are supposed to have been used as farmland than in soil in the surface layer, and radiocaesium more than 99 % in soil at locations that are supposed to have been used as farmland also distributed within a depth of around 14 cm. The apparent diffusion coefficients (D a ) of RNs derived from penetration profiles near the surface layer showed a tendency to be higher in soil at locations that are supposed to have been used as farmland (D a =0.1-1.5x10 -10 m 2 /s) than in soil in the surface layer (D a =0.65-4.4x10 -11 m 2 /s), and most D a -values were nearly 10 -11 m 2 /s. The distribution coefficients (K d ) by a batch method were in the range of K d =2,000-61,000 ml/g for Cs and K d =0.5-140 ml/g for I. Although the K d -values are different between cation (Cs + ) and anion (I - ), the D a -values ( 134 Cs, 137 Cs, 129m Te and 110m Ag) were similar levels. This is considered to be due to that the D a -values were controlled by dispersion by flow of rain water.