Japan radiation levels uncertain: Should evacuation zone be bigger?
Difficulties in measuring radiation are increasing confusion in Japan. The government must weigh incomplete and sometimes contradictory data before making decisions.
Conflicting data on radiation levels is making it difficult to judge the dangers posed by the damaged Fukushima Daiichi nuclear plant – and heightening anxiety among average Japanese.Skip to next paragraph
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On Thursday, store shelves across Tokyo were bare of bottled water one day after authorities warned that the level of radioactive iodine in tap water was twice the allowable level for infants. But follow-up tests conducted Thursday showed the iodine level had fallen back below acceptable limits.
Some nearby cities were showing elevated levels of radioactive iodine 131 in their municipal water supplies, however. And new estimates from Japan’s Nuclear Safety Technology Center seemed to indicate that atmospheric radiation levels might be too high for infants at some spots outside the 12-mile evacuation zone surrounding the Fukushima Daiichi nuclear complex.
“There is continuing scattered information about contamination exposures that is not entirely consistent,” said Edward Lyman, a senior staff scientist at the Union for Concerned Scientists, a nuclear watchdog group in Washington. “Confusion seems to be growing.”
Given the scale of the crisis, such confusion is to be expected, said Dr. Lyman. Similar problems would occur anywhere multiple reactors suffered total power blackouts and apparent fuel rod damage.
Why is it so hard to measure radiation? It is not like measuring temperature, or barometric pressure, or some other easily-discernable weather variable. Emissions from Fukushima have been a mix of different kinds of radioactive materials, which disseminate into the atmosphere differently, and travel in the air in different ways.
Though the analogy is not exact, it is somewhat like trying to map the spread of different aromas released from the same general area, taking into account wind, rain, and other environmental factors.
Plus, some of the measurements released so far measure things over different time frames. Others are projections or the results of computer monitoring.
For instance, the city of Fukushima itself had a radiation level of .00685 milliSieverts (mSv) per hour at 7 p.m. local time on Tuesday, according to Japanese authorities.
Given that an average person receives 2.4 mSv per year from sunlight and other natural sources, that particular Fukushima reading does not sound so bad.
But that is per hour, remember. At times during the crisis, it has spiked considerably higher.
Estimates from the Japanese Nuclear Safety Technology Center, in contrast, were an attempt to figure out possible cumulative doses that might have affected people in the path of a radiation plume.
According to these estimates, an infant just outside the 12-mile evacuation zone around the Fukushima plant could theoretically have absorbed more than 100 mSv of radiation since the crisis began.
To accumulate a worrisome dose, an infant would have had to be outdoors constantly since last the earthquake on March 11, noted Japanese Cabinet Secretary Yukio Edano. Buildings shield humans from some of radiation’s effects.
Edano said that it remains unnecessary to expand the 12-mile exclusion zone.
“As a precautionary measure, I would like to recommend that if people [near the exclusion zone] are on the leeward of the nuclear power plant, they close their windows and stay indoors inside sealed buildings as [much] as possible,” he said.
The US has recommended that US citizens within 50 miles of the Fukushima complex evacuate to a safer place. Lyman believes the Japanese government should follow suit, at least for children and pregnant women.
“It looks like there are going to be areas considerably further than the 12-mile zone that are going to require significant decontamination or condemnation,” he said Thursday.