Numerous stories are circulating today about a potential exclusion zone around the Fukushima plant. It will mainly include villages less than 19 kilometers from the plant, according to the New York Times.
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? Click here).
That number appears to have come from a recent survey conducted by the science ministry (MEXT). In a move to confuse, MEXT has chosen to give its doses in microsieverts per hour (μSv/hour), but it’s simple enough to convert: divide by 1,000, to convert to millisieverts, then multiply by 24 (hours in a day) and 365 (days in a year). And voila! You do indeed find areas around Okuma in the 500 mSv/year-range.
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? A lot will probably depend on the nature of the radioactive fall-out. The government has already announced a plan to remediate large areas of the countryside, and it may be that they can remove the topsoil in radioactive hot-spots outside the 19km zone. But if the contamination is more widespread, it may also involve changing the government safety limits, something that it has done already to the dismay of its citizens.