Results of the environmental radiation monitoring following the accident at the Fukushima Daiichi Nuclear Power Plant. Interim report. Ambient radiation dose rate, radioactivity concentration in the air and radioactivity concentration in the fallout

As a response to the accident at the Fukushima Daiichi Nuclear Power Plant, the environmental radiation monitoring was performed at the Nuclear Fuel Cycle Engineering Laboratories, Japan Atomic Energy Agency (JAEA). This report provisionally presents the monitoring results of ambient radiation dose rates, radioactivity concentrations in air and in fallout and meteorological observation until May 31, 2011. The ambient radiation dose rate has increased since 1:00 in March 12 2011, with mainly three peaks of several thousand nGy/h at 7:00 in March 15, at 5:00 in March 16, and at 4:00 in March 21. The variation on the radioactivity concentrations in the air and in the fallout almost showed the same tendency as that of the dose rate. The radioactivity ratio of I-131 to Cs-137 in the air varied largely every day. The fallout of Cs-137 for 1 month from March 15 to April 15 was about 120 times higher than that in May 1986 after the Chernobyl Accident, and about 30 times higher than that at Koenji in Tokyo in June 1963 during the atmospheric nuclear weapon tests. The dose of internal exposure due to inhalation was estimated from the air concentration observed. (author)