Does Japan’s new Fukushima exclusion zone add up? Does singling out villages inside the 19km zone really make sense?

It appears that the Japanese government is close to announcing what many of us have suspected since the nuclear crisis at Fukushima Daiichi began: some people won’t be going home.

But try to dig a little deeper into this, and as happens over-and-over during the Fukushima crisis, you come up against confusing numbers and not much in the way of explanation from the Japanese government. The New York Times story mentions the village of Okuma 3 km southwest of the plant. The story states a resident there would receive an annual dose of 508.1 millisieverts of radiation per year, far more than the 20 mSv/yr limit set by the government (need a refresher on sieverts?

In fact, anywhere on the map where dose levels are above 2.5 μSv/hour is already above the government’s annual limit (click image to enlarge). It does indeed seem like, for the most part, rates on that edge of the 20 kilometer exclusion zone set up immediately after the crisis began are near the government limits.

One notable exception is post 14 where an rate of 27.4 μSv/hr (240 mSv/yr) can be seen right at the edge of the zone. This corresponds to a large plume of radiation that settled to the northwest of the plant. Taking a look at this recent survey data outside the evacuation zone, and you can see that the contamination stretches to at least Futaba county, 25-30 km to the west-northwest. There, rates are as high as 306 mSv/year, well above other parts of the 19 km exclusion zone around the plant.

We know that, contrary to the wishes of physicists, fallout doesn’t settle in an even circle. So does singling out villages inside the 19km zone really make sense?

Source:, via Nature News Blog
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