Radiochemical analysis of the drain water sampled at the exhaust stack shared by Units 1 and 2 of the Fukushima Daiichi Nuclear Power Station

Radioactive gas of Unit 1 of the Fukushima Daiichi Nuclear Power Station was released from the exhaust stack shared by Units 1 and 2 through the venting line on March 12th, 2011. In the present study, radiochemical analysis of drain water sampled at the drain pit of the exhaust stack was conducted to study radionuclides released during venting of the Unit 1. Not only volatile (129)I, (134)Cs and (137)Cs but also (60)Co, (90)Sr, (125)Sb and Unit 1-originated stable Mo isotopes were detected. Although Unit 1-originated stable Mo isotopes were clearly detected, their amounts were quite low compared to Cs, suggesting that the formation of Cs(2)MoO(4) was suppressed under the accident condition. Approximately 90% of iodine existed as I(−) and 10% as IO(3)(−) in November 2020. Furthermore, larger amount of (129)I than (137)Cs was observed, suggesting major chemical form of (131)I was molecular iodine rather than CsI at the accident time. The (134)Cs/(137)Cs radioactivity ratio decay-corrected to March 11th, 2011 was 0.86, supported the results that Unit 1 originated radiocesium in environment has smaller (134)Cs/(137)Cs radioactivity ratio than Unit 2 and 3 originated radiocesium.