"According to Kawata, soil in a 600 square kilometer area mostly to the northwest of the Fukushima plant is likely to have absorbed radioactive cesium of over 1.48 million becquerels per square meter, the yardstick for compulsory migration orders in the 1986 Chernobyl catastrophe.
Kawata also said soil in a 700 square km area is likely to have absorbed 555,000-1.48 million becquerels per square meter, which was a criteria for temporary migration during the Chernobyl disaster.
Kawata estimated the soil contamination using data on radiation levels in the air monitored by the Ministry of Education, Culture, Sports, Science and Technology.
The size of the contaminated areas in the Fukushima crisis is one-tenth to one-fifth of those polluted in the Chernobyl disaster, Kawata said.
While the expected radiation exposure from 1.48 million becquerels of cesium is around five millisieverts a year, below the government’s benchmark of 20 millisieverts for evacuation orders, decontamination will still be necessary before evacuees can return as radioactive cesium binds strongly to soil, making it hard to reduce radiation levels, Kawata said."
Since this seems to be as legit as the Fukushima-Chernobyl comparisons get, let's pull up a map of the Chernobyl control-zone to get a visual idea of Dr. Kawata's numbers and their implications:
|1.48 million Becquerels/m² = 40 Curies/km²|
600 km² of area in the Fukushima Prefecture have radioactivity equivalent to the confiscated/closed zones on this map (red). An additional 700 km² of area fit the "permanent control zone" criteria (dark pink), as it stands in Chernobyl today.
Edit: Here's a recently released French IRSN map based on MEXT data. This map breaks down the isotope concentration in the Fukushima and surrounding area:
How long will it take for Cs-137 to work it's way out of the environment? Considering the Chernobyl situation further, we can see from the following chart that a long, slow slog in dose rate reduction is ahead for Japan as well. Measurements are in air, with Cs-137 as the main contributor.
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