
@wtm Hi wtm!
Here's a Scientific American article that will explain it much better than I can...
How do fast breeder reactors differ from regular nuclear power plants?
www.scientificamerican.com
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"1.According to the official number from the Tokyo Metropolitan government measured at 18 meters off the ground at the Tokyo Metropolitan Institute of Public Health [in Hyakunin-cho, Shinjuku], the average radiation level for the month of May was 0.068 to 0.062 microsievert/hour. The ground level (1 meter from the ground) showed the similar number, according to the government. However, when we measured the radiation at 1 meter from the ground, the radiation level similar to the official level was only observed in very narrow, limited areas in Ota-ku, Suginami-ku, and Machida City. The areas with relatively high level of radiation were Ome City, Akiruno City, Nerima-ku, at 0.09 microsievert/hr, the Tokyo Bay area of Edogawa-ku and Koto-ku at 0.1 microsievert/hr, and the highest level of radiation was observed in Adachi-ku and Katsushika-ku, at 0.2 to 0.3 microsievert/hr." ex-skf.blogspot.com

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"There is a possibility that the radiation level in areas east of the line that connects Koto-ku and Nerima-ku is at the level that will result in 1 millisievert/year." ex-skf.blogspot.com