Levels of radioactivity from Japan's damaged Fukushima Daiichi nuclear complex are above government limits for infants in some areas outside the plant's 12-mile evacuation zone, according to the latest estimate to fuel an international debate over how close civilians should be allowed to the plant.
The new estimate, by a state-funded monitoring body, came as fears over Tokyo's tap water eased. Tests Thursday showed radioactive material in a major plant supplying water to the capital has fallen beneath the level the government says could pose long-term health risks to infants. Elevated levels at the plant Tuesday and Wednesday sparked official warnings and bottled-water sales.
Japan's Nuclear Safety Technology Center, a government monitoring group, released the estimate late Wednesday of the cumulative exposure to radiation in zones surrounding the Fukushima Daiichi complex. The estimate covered the 12 days since Japan's March 11 earthquake and tsunami spurred fires, explosions and spikes in radioactivity levels at the complex.
The estimate—produced by modeling radiation readings collected at various points around the plant—suggested that most areas with radiation that exceeded government thresholds fell within the 12-mile evacuation zone.
But the model showed that areas where cumulative exposure over 12 days reached 100 millisieverts—the government's maximum for infants—extended beyond the evacuation zone. A map based on the data showed areas that received a cumulative 100 millisieverts extended as far as about 25 miles northeast and south from the plant.
Government officials said the center's estimate didn't require a larger evacuation, under even the most conservative standards. They said a person would have to have been in the area, and outdoors for the entire time since the March 11 earthquake, to receive that full dose.
Still, the test results demonstrate the uncertainty surrounding the measuring of radioactive emissions from the power plant.
Elevated levels of radioactive elements have been found as far away as Tokyo in its tap water, prompting worries over the nation's food and water supplies.
In Japan's capital on Thursday, new tests showed a sharply lower level of radioactive iodine-131 at the northern Tokyo water-purifying plant where tests earlier this week revealed levels roughly twice those recommended for infants.
The Thursday test showed the plant's water contained 79 becquerels per kilogram—a measure of radioactive events—below the 100 becquerels per kilogram level considered acceptable for infants. Tokyo officials Thursday lifted the city's warning about consumption by infants.
It isn't clear why radiation levels in the tap water rose and fell within this short span. Iodine-131 has a half-life of about a week, meaning that levels in water could fall quickly if new doses aren't introduced. Tokyo metropolitan government officials have posited, but have not proved, that recent rains could have flushed airborne iodine-131 into rivers that supply the vast majority of Tokyo's water, and also increased radiation levels in Tokyo.
On Thursday, Tokyo's government said radioactive cesium exceeding the government's limit was detected in komatsuna, or Japanese mustard spinach, in a sample taken in Tokyo. It is the first time a radioactive substance exceeding the government limit has been found in farm products in Tokyo since the beginning of the crisis.
Also Thursday, Chiba prefecture, east of Tokyo, said it detected levels of radioactive iodine from two water samples that exceeded the government's threshold for consumption by infants. The samples came from two of the city's water purification plants collected Wednesday.
And on Friday, officials in Singapore said they found radioactive contamination in four samples of vegetables imported from Japan—though at levels that don't pose a health hazard.
Tokyo's air Thursday showed radiation lingered at nearly four times the normal level, though the hourly exposure is still about one-thousandth of the dose from a typical chest X-ray
Regulators in Japan, the U.S. and elsewhere have grappled with differing data and estimates of the plant's dangers. "Even within a small area, radiation levels can be very uneven," said Hisaaki Kudo, nuclear chemistry professor at Niigata University, citing variables ranging from wind to location to different types of equipment and radioactive material.
In another example, the U.S. Embassy Thursday advised Americans in Tokyo to use only bottled water for feeding children under 3. In its radiation alert Wednesday, the Tokyo government said its warning applied to children a year old or younger.
Wednesday's estimate by Japan's Nuclear Safety Technology Center differs from daily data that Japan's Ministry of Science and local governments have been releasing since March 11. Those daily measures have shown radioactive material at slightly elevated levels of generally under 10 microsieverts per hour.
The Nuclear Safety Technology Center's estimate Wednesday judges the full amount of exposure over the period since the radioactive material was released, which health experts say could be a better gauge of health threats. Its computer simulation by software called System for Prediction of Environmental Emergency Dose Information, or Speedi. Speedi estimates how far radioactive materials have spread based on factors including landscape, weather and the direction and speed of winds. According to people at the center, the system has been running almost nonstop since the accident and has provided the government with a number of forecasts on how radioactive materials may spread in the area.
Officials stressed that the simulation is based on an unlikely scenario of a person staying outside the entire time to be exposed to radioactive materials. Once people go indoors, radiation levels typically drop to a quarter to one-tenth of outdoor levels, it said.
The simulation was conducted with limited available data, they added. One variable includes radiation levels at the Fukushima Daiichi plant itself. Those numbers vary greatly based on where the plant's operator, Tokyo Electric Power Co., takes its results at the plant.
The government has been under pressure after the U.S. released data this week that suggests radiation in the surrounding countryside is higher than what Japanese officials have found. Japanese officials have declined to comment on the U.S. results, saying they need to see details, while continuing to defend the current evacuation zone.
Chief Cabinet Secretary Yukio Edano said at a press conference Thursday that the government at some point may have to consider alternatives for those Fukushima residents staying indoors, due to the difficulty of ensuring stable supply of food and other essentials. But he stood by the government's current evacuation-zone limits. The government needs to be careful "not to send a misleading message that the area is becoming more dangerous," said Mr. Edano.
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