It would make the disaster the most severe since Chernobyl
The Kyodo News Service reports that the the Nuclear Safety Commission of Japan released a preliminary calculation April 11 saying that the Fukushima Daiichi nuclear plant had been releasing up to 10,000 terabecquerels of radioactive materials per hour at some point after a massive quake and tsunami hit northeastern Japan on March 11.
According to the wire service, the disclosure may require the government to consider raising the accident's severity level to 7, the worst on an international scale, from the current 5, government sources said. The level 7 on the International Nuclear Event Scale (INES) has only been applied to the 1986 Chernobyl catastrophe.
The change doesn't come as a result of new releases, but rather reviews of data about initial releases of radiation in the early days of the accident events and also current measurements.
The current provisional evaluation of 5 is at the same level as the Three Mile Island accident in the United States in 1979.
INES level 7 accidents correspond with a release into the external environment radioactive materials equal to more than tens of thousands terabecquerels of radioactive iodine 131. One terabecquerel equals 1 trillion becquerels.
The measurement reports how much radioactive material was emitted, not the dose absorbed by people, plants, or animals.
Radioactive iodine has a half-life of about eight days. The wire service report did not identify cesium releases or numbers.
(Note to readers: See this web page at MIT for an explanation of radiation measurements.)
Kyodo news reported that Haruki Madarame, chairman of the commission, which is a government panel, said it has estimated that the release of 10,000 terabecquerels of radioactive materials per hour continued for several hours.
The commission also released a preliminary calculation for the cumulative amount of external exposure to radiation, saying it exceeded the yearly limit of 1 millisieverts in areas extending more than 60 kilometers to the northwest of the plant and about 40 km to the south-southwest of the plant.
It encompasses the cities of Fukushima, Date, Soma, Minamisoma, and Iwaki, which are all in Fukushima Prefecture.
The commission used the System for Prediction of Environmental Emergency Dose Information to calculate the spread of radiation.
Reuters reported that a spokesman for Japan's Nuclear and Industrial Safety Agency said on April 12 that the level of the Fukushima incident was still a 5 and that he was unaware of any move by the government to raise the level.
CNN reports that Chief Cabinet Secretary Yukio Edano, the Japanese government's leading spokesman on the crisis, made distinctions between their crisis and Soviet experience at Chernobyl.
"The change in the level reminds us the accident is very big. What's different here from the Chernobyl accident is that we have not yet seen a direct impact on the health of the people as a result of the nuclear accident. The accident itself is big, but we will make, as our first priority, our utmost effort to avoid any health impact on the people."
Separately, media reports indicate the Japanese government may order permanent evacuations of some of the areas affected by the radiation. The Global Security Newswire reported that while the evacuation order is not mandatory, it does expand the evacuation zone from 13 to 18 miles. According to other media reports, the population of the new evacuation zone is 115,000 people.
World Nuclear News has a map of the new radiation levels which shows the location of hot spots.
Update April 12, 2011: Full text of NISA press release in English.
In other developments the government clarified that the expansion of the evacuation zone is for specific hot spots and not a blanket enlargement of an arbitrary distance around Fukushima. Also, the evacuation order isn't mandatory.
For humanitarian reasons, the Japanese government has been encouraging people in the earthquake and tsunami damaged areas to move to government run shelters so that delivery of food, medicine, and the necessities of daily life can be brought to them.
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