Measurement of air dose rates over a wide area around the Fukushima Dai-ichi Nuclear Power Plant through a series of car-borne surveys

Ando, Masaki  ; Nakahara, Yukio ; Tsuda, Shuichi  ; Yoshida, Tadayoshi ; Matsuda, Norihiro  ; Takahashi, Fumiaki  ; Mikami, Satoshi   ; Kinouchi, Nobuyuki ; Sato, Tetsuro*; Tanigaki, Minoru*; Takamiya, Koichi*; Sato, Nobuhiro*; Okumura, Ryo*; Uchihori, Yukio*; Saito, Kimiaki  

A series of car-borne surveys using the KURAMA and KURAMA-II systems was conducted in a wide area in eastern Japan from June 2011 to December 2012 to evaluate the distribution of air dose rates around the Fukushima Dai-ichi Nuclear Power Plant, and to determine the time-dependent trend of decrease in air dose rates. An automated data processing system was established, which enables analyses of large amounts of data obtained with the use of about 100 units of the measurement system in a short time. The initial data for studying the migration status of radioactive cesium was obtained in the first survey, followed by the other car-borne surveys having wider measurement ranges. Comparing the measured air dose rates obtained in each survey, it was found that the decreasing trend of air dose rates measured through car-borne surveys was larger than that expected from the physical decay of radioactive cesium and that measured using NaI (Tl) survey meters around the road.