Analysis of two forms of radioactive particles emitted during the early stages of the Fukushima Dai-ichi Nuclear Power Station accident

佐藤 志彦   ; 末木 啓介*; 笹 公和*; 吉川 英樹; 中間 茂雄 ; 箕輪 はるか*; 阿部 善也*; 中井 泉*; 小野 貴大*; 足立 光司*; 五十嵐 康人*

Satou, Yukihiko; Sueki, Keisuke*; Sasa, Kimikazu*; Yoshikawa, Hideki; Nakama, Shigeo; Minowa, Haruka*; Abe, Yoshinari*; Nakai, Izumi*; Ono, Takahiro*; Adachi, Koji*; Igarashi, Yasuhito*

Two types of radioactive particles have been isolated from environmental samples collected at various distances from the Fukushima Dai-ichi Nuclear Power Station. "Type A" particles are 2-10 m in diameter and display characteristic Cs X-ray emissions when analysed by energy-dispersive X-ray spectrometry (EDS). "Type B" particles are considerably larger, up to 400 m in diameter, with Cs concentrations too low to be detectable by EDS. These larger particles were isolated from the region to the north of the nuclear reactor site, which was contaminated on March 12, 2011. The specific activity of Type B particles is much lower than Type A, and the mean CsCs ratios are 0.93 and 1.04, respectively. The Type B ratio indicates power station Unit 1 as the source, implying that these larger radioactive particles were discharged on March 12. This study found that different type of radioactive particle was released not only on March 15 but also on March 12.