The Yomiuri Shimbun
The Nuclear and Industrial Safety Agency raised the provisional severity level of the crisis at the Fukushima No. 1 nuclear power plant on Tuesday to the worst rating of seven on an international scale, equivalent to that of the 1986 Chernobyl crisis.
The agency had previously rated the accident as a five.
The agency, under the Economy, Trade and Industry Ministry, said the amount of radioactive material, calculated based on the reactors' estimated condition, reached "more than several tens of thousands of terabecquerels."
A terabecquerel equals 1 trillion becquerels.
The level is defined as a "major accident" under the International Nuclear and Radiological Event Scale (INES), or the highest level on its scale from zero to seven.
According to the agency, the total amount of iodine-131 and cesium-137 emitted between March 11 and at 11 a.m. Tuesday reached 370,000 terabecquerels according to the reactors' estimated condition. Within this assessment, cesium levels were converted to their equivalent in iodine-131 levels.
Cabinet Office's Nuclear Safety Commission, meanwhile, announced Tuesday that the total amount of iodine and cesium emitted between March 11 and April 5 was 630,000 terabecquerels (again, with cesium levels converted to the iodine equivalent), calculated according to the amount of radiation observed around the facility.
"The total amount of radioactive materials emitted thus far is equal to about 10 percent of that released in the Chernobyl accident. The amount of radiation exposure is small," said Agency spokesman Hidehiko Nishiyama.
According to the commission, the current volume of radioactivity being emitted is about one-ten thousandth of that monitored at its peak.
The agency decided to raise the INES level not only because of the calculated radiative material released into the atmosphere but also because of the widespread ramifications of the accident.
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Effect on health, environment
By Kyoichi Sasazawa / Yomiuri Shimbun Staff Writer
For an incident to be rated on the International Nuclear and Radiological Event Scale (INES), it is assessed by the nuclear supervisory authorities in the nation where it occurred and is then reported to the International Atomic Energy Agency (IAEA).
In Japan, the Nuclear and Industrial Safety Agency is in charge of the assessment.
Last month, the nuclear safety agency provisionally rated the Fukushima crisis at level 5. However, domestic and international experts argued the provisional level should be 6 or higher based on the facts that high levels of radiation were measured around the facility and significant amounts of radioactive material had escaped.
In the Chernobyl crisis, about 5.2 million terabecquerels of radioactive material was emitted into the air in the space of 10 days.
The amount of radioactive material aerially emitted from the Fukushima No. 1 plant right after the accident was about 10 percent of that of Chernobyl. But the scale of the Fukushima accident still stands out among other nuclear power accidents that have happened around the world.
Also, a large volume of radioactive material has been dumped into the sea in the current incident. Since work to bring the situation under control has run into one complication after another, an end to the crisis could be months or even a year away, with the amount of nuclear leakage increasing.
The assessed INES level reflects negative impacts on health and the environment. It will be important to carefully monitor the incidence of leukemia and cancer as well as soil contamination. The crisis also could strongly affect the international image of Japan.
(Apr. 13, 2011)
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