This is a really good article at the link on the amount of radiation fallout from the Fukushima nuclear accident.
The good news: as everyone said from the start, it's not another Chernobyl:
Initial estimates suggest that Fukushima's reactors have emitted one-tenth of all the radioactive material released during the Chernobyl disaster in 1986, and prevailing winds have swept most of the radioactivity over the Pacific Ocean.The bad news, however, is that there does seem to be a band of particularly heavy contamination in a path to the north-west of the reactors, extending well outside the 20 km evacuation zone:
The survey showed that the highest radioactivity doses on the ground (greater than 0.125 millisieverts per hour; mSv h−1) were restricted to a narrow band within 40 km of the plant, stretching to the northwest (see 'Fukushima's fallout'). No values anywhere exceeded 0.3 mSv h−1, a dose likely to cause adverse health effects in anyone continually exposed for a few months. Still, doses at some sites over the course of a year would top 1,000 mSv, enough to cause symptoms of radiation sickness, including nausea, hair loss and reduced white-blood-cell counts.
Much of the 20-km evacuation zone around the plant had far lower dose levels, below 0.012 mSv h−1. Nevertheless, that corresponds to a potential annual dose of more than 100 mSv, more than five times the annual limit permitted for UK nuclear-industry workers. The patchy distribution of fallout reflects the role of wind patterns and rainfall in washing out radioisotopes to the ground. Overall, Smith says he was "relieved" by the data, as they suggest that contamination around Fukushima will be much lower than that seen around Chernobyl.
Apart from the still very unclear issue of whether workers on site have received (or will receive) very dangerous doses of radiation, if anyone thinks that the need to permanently evacuate patches of land up to 40 km away is not a serious issue, they want their head read.But some areas of high contamination seem to lie outside the exclusion zone. Soil samples taken on 20 March from a location 40 km northwest of the plant showed caesium-137 levels of 163,000 becquerels per kilogram (Bq kg−1) and iodine-131 levels of 1,170,000 Bq kg−1, according to Japan's science ministry. Acceptable contamination levels for areas used to grow crops are much lower, typically in the range of a few hundred Bq kg−1. "If there are significant areas of caesium-137 soil concentration of the order of 100,000 Bq kg−1, evacuation of these areas could be effectively permanent," says Smith.
Update: an article at Physorg discusses how serious seawater contamination may be. Short answer: well, it shouldn't spread much, but expect some exclusion zone for quite some time:
the contamination from iodine 131 is short-lived because the element has a half life -- the pace at which it loses half of its radioactivity -- of only eight days."This means that after a few months, it will be harmless, basically," said Simon Boxall, a lecturer at Britain's National Oceanography Centre at the University of Southampton, southern England, who praised early measures to stop fishing around the plant after the March 11 disaster.
"What worries me more is if caesium and plutonium get into the system," he said, referring to two radioactive heavy metals whose half-lives are around 30 years and potentially thousands of years respectively.
"That's more concerning, because that can build up in the sediments" of the sea bed at Fukushima, said Boxall.
At high levels, this could lead to the imposition of an exclusion zone of catches of fish and seafood, a measure that could last "years and years," he said.
"It's hard to know (how long) until they start taking measurements and determine how extensive the pollution is.
"You would basically not fish in an exclusion zone, period. And beyond the exclusion zone there would be an additional zone where you would come from time to time and see if there's any radioactivity."
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