Kyodo - The Nuclear Safety Commission of Japan released a preliminary calculation Monday saying that the crippled 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.
The disclosure prompted 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 has only been applied to the 1986 Chernobyl catastrophe.
The current provisional evaluation of 5 is at the same level as the Three Mile Island accident in the United States in 1979.
According to an evaluation by the 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.
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 says the release has since come down to under 1 terabecquerel per hour and said that it is still examining the total amount of radioactive materials released.
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, and some areas including the town of Hirono in the prefecture.
Within a 20-km exclusion zone set by the government, the amount varied from under 1 millisieverts to 100 millisieverts or more, and in the 20-30 km radius ring where residents are asked to stay indoors, it came to under 50 millisieverts.
The commission used the System for Prediction of Environmental Emergency Dose Information to calculate the spread of radiation.