Investigation on distribution of radioactive substances in Fukushima within former evacuation areas, 6; Temporal changes in reduction rates of air dose rate in and around residential houses

Yoshida, Hiroko*; Hayashi, Masateru*; Makita, Taisuke*; Azuma, Tetsushi*; Sasano, Makoto*; Tsuda, Shuichi  

Investigation on distribution of radioactive substances in Fukushima within former evacuation areas has been ongoing since 2012. Reduction rates of air dose rate indoor and outdoor were compared for 54 residential houses in Iidate-mura and Minami-Soma. Just after the decontamination, the averaged value of the outdoor reduction rates was higher than those indoor, while the averaged outdoor reduction rate in 2019 and 2020 was almost identical to the indoor one within uncertainties. This means that indoor air dose rate has decreased less than outdoors. One of the reasons would be the existence of additional radionuclides coming from outside of the decontamination area around the houses, according to spectra analyses of air dose measurement and deposition of radionuclides on the roof.