Indoor deposition of radiocaesium in an evacuation area in Odaka district of Minami-Soma after the Fukushima nuclear accident

The indoor deposition of radiocaesium was investigated for 27 wooden houses in eight areas of Kanaya, Mimigai, Ootawa, Ooi, Kamiyama, Kamiura, Ebizawa, and Yoshina in Odaka district of Minami-Soma, Fukushima Prefecture from November 2013 to January 2015. Odaka district is within a 20 km radius of the Fukushima Daiichi nuclear power plant (FDNPP), which used to be designated as a restricted area and has been designated as an evacuation area. Dry smear test was performed over an area of 100 cm² on the surface of materials made of wood, glass, metal, and plastic in the rooms and the surface of wooden structure in the roof-space. Approximately 1000 smear samples were collected in total; 89% of the smear samples obtained in the rooms exceeded the detection limit (0.004 Bq/cm²) and a maximum value was evaluated to be 1.54 Bq/cm²; 77% of the smear samples taken from the wooden structure in the roof-space exceeded the detection limit and a maximum value was evaluated to be 1.14 Bq/cm². Area differences in surface contamination were observed. Assuming that two horizontal phases of the room have uniform surface contamination with the maximum median radioactivity observed in Kamiura (0.1 Bq/cm²) for 27 houses investigated, the ambient dose equivalent rate for ¹³⁴Cs and ¹³⁷Cs in November 2013 was calculated as approximately 0.002 μSv/h.