Evolution of depth distribution of radionuclides in soil contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident

Sato, Haruo; Niizato, Tadafumi  ; Tanaka, Shingo*; Abe, Hironobu ; Aoki, Kazuhiro 

This work was conducted as one of the researches relating to distribution maps of radiation dose rate, etc. which the government has promoted as one of the counter-measures to the Fukushima Dai-ichi Nuclear Power Plant (1F-NPP) accident in March 2011, and the 1st and 2nd investigations on the depth distribution of radionuclides (RNs) were conducted in June 2011 after about 3 months and in February and March 2012 after about 1 year from the 1F-NPP accident, respectively. The authors investigated at 11 locations in Nihonmatsu-city, Kawamata-town and Namie-town in the 1st investigation and obtained the distribution coefficients (Kd) of Cs-137 and I-131 onto soil and the apparent diffusion coefficients of RNs. The 2nd investigation was also conducted at the same locations basically. Sorption-desorption experiments of Cs-137 and I-131, CEC and AEC measurements and mineralogical analyses by XRD were conducted for 3 types of soils (sandy, clayey, organic) and those elutriated components (clay, silt, sand). Radiocaesium (Cs-134 and Cs-137) and Ag-110m were detected at all locations investigated and only at locations where radiation dose rate is high, respectively. Radiocaesium more than 95% and 99% of the inventory distributed within 5cm and 10cm deep in soil in the surface layer (mainly sandy soil is the support layer), respectively, and little evolution in the penetration profile was found. Whilst, radiocaesium more than 95% and 99% of the inventory distributed within 16cm and 20 cm deep in organic soil and in soil at locations where are supposed to have been used as farmland, respectively, and penetration profile tended to extend to deeper parts in soil that organic and clayey soils are the support layer, particularly in organic soil. The Kd-values of Cs-137 onto organic soil and its elutriated components were also lower than that onto other soils. This is consistent with trend of penetration profile.