The Yomiuri Shimbun
The government upgraded the severity of the crisis at the Fukushima No. 1 nuclear power plant to Level 7. But what exactly does this rating mean? And was the decision made too slowly?
The Economy, Trade and Industry Ministry's Nuclear and Industrial Safety Agency raised its provisional evaluation of the seriousness of the accident to the maximum of Level 7 in the International Nuclear Event Scale (INES) because of the large quantity of radioactive substances that have been discharged from the plant's damaged reactors.
INES is designed to show how serious a nuclear accident is in an easily understandable way.
The scale was introduced in 1992 by the International Atomic Energy Agency and the Organization for Economic Cooperation and Development in the wake of the 1979 accident at the Three Mile Island nuclear plant and the 1986 Chernobyl disaster.
The scale is from zero to seven and takes into account three factors:
-- Degree of damage to functions that contain radioactive material.
-- Effects inside nuclear facilities, such as workers exposed to radiation.
-- Amount of radioactive material discharged into the atmosphere.
In accidents Level 5 or higher, the amount of radioactive material released plays a major role in the evaluation process. In a Level 7 accident, the quantity is at least several tens of thousands of terabecquerels.
The IAEA defines Level 7 as a "major accident" that is a "major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures."
Level 6, classified as a "serious accident," involves a "significant release of radioactive material likely to require implementation of planned countermeasures." Level 5 is an accident with wide consequences and is a "limited release of radioactive material likely to require implementation of some planned countermeasures" with "several deaths from radiation."
The seriousness, therefore, of a Level 7 accident is incredibly high.
The nuclear safety agency said the amount of radioactive material released from the Fukushima plant is about 10 percent of that discharged during the Chernobyl accident in the former Soviet Union, which is the only other nuclear accident to be rated Level 7.
But the quantity of radiation released in the Chernobyl accident took place in the 10 days it took for the radiation leak to be contained.
According to the Cabinet Office's Nuclear Safety Commission, about one terabecquerel per hour of radiation is currently being discharged into the air. If this situation continues for months, thousands of terabecquerels will be released, putting the Fukushima crisis on par with the world's only other major nuclear accident.
Junichi Matsumoto, acting head of Tokyo Electric Power Co.'s Nuclear Power & Plant Siting Division, acknowledged the seriousness of the Fukushima accident at a press conference Tuesday.
"Although the details of the [Chernobyl and Fukushima] accidents are different, from the standpoint of how much radiation has been released, [Fukushima] is equal to or more serious than Chernobyl."Taichi Maki, guest professor of climatic environmental studies at the University of Tsukuba, said,
"It's significant the discharge is still going on. As long as the reactors are unstable, we have to worry about an increase in the quantity of radiation released. I assume the increase to Level 7 took that into consideration."The nuclear safety agency and the Nuclear Safety Commission on Tuesday announced separately their calculations of how much radioactive material has been released from the Fukushima plant.
The commission, which is in charge of measuring radiation, made its calculations based on air measurements taken at 33 locations near the plant and data from the System for Prediction of Environmental Emergency Dose Information, which forecasts the spread of radioactive material. The commission concluded the total amount released was 630,000 terabecquerels of iodine-131 and cesium-137, which was converted into iodine-131 to make the calculations.
The agency, which monitors nuclear reactors, calculated the total quantity of radioactive material that was in the Nos. 1-3 reactors when the crisis began and how much leaked when steam in the containment vessels was released into the air. The agency concluded that 370,000 terabecquerels of radioactive iodine and cesium have been released.
According to TEPCO, at the time of the March 11 earthquake, 660 million terabecquerels of radioactive material was in the Nos. 1-4 reactors.
Comparing this figure with the calculation made by the nuclear safety commission and agency, about one-500th of the reactors' radioactive material has been released. This means most of the radioactive material likely is still in the reactors.
But where did the discharged radioactive substances go?
Atsushi Kasai, former senior official of the Japan Atomic Energy Research Institute, said the material has three major destinations--air, soil and seawater.
Of the radioactive material discharged into the atmosphere, including the large quantity released at the beginning of the accident, Kasai said more than half was probably still floating around in the air.
"Though these substances don't pose a hazard to human health in the short term, if radioactive material keeps being discharged, the likelihood of it entering people's bodies through the food chain will increase," Kasai said.
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Was the nuclear crisis deliberately downplayed?
The government may have been trying to downplay the crisis at the Fukushima No. 1 nuclear power plant by setting its initial severity rating at Level 4 on an international scale, according to specialists.
The initial assessment was made immediately after the emergency erupted on March 12. A week later, the rating was raised to Level 5--and eventually to Level 7 on Tuesday, more than 20 days after the second assessment.
According to the Cabinet Office's Nuclear Safety Commission (NSC), most of the radioactive substances released from the nuclear complex had escaped by around March 15 after a series of hydrogen explosions and fires at the Nos. 1-4 reactors.
The amount of radioactive materials being emitted into the air skyrocketed on March 15 and 16. The NSC believes the increase was due to a massive discharge of radioactivity after the pressure suppression chamber at the No. 2 reactor was damaged.
In mid-March, the French Nuclear Safety Authority rated the Fukushima crisis as Level 6, while the U.S.-based Institute for Science and International Security (ISIS) said it was Level 6 or 7. But the Nuclear and Industrial Safety Agency left its rating unchanged, saying, "The crisis will not harm people's health."
But the agency hiked the crisis to Level 5 on March 18, seeming to cave to international opinion. According to agency spokesman Hidehiko Nishiyama, "It was difficult to make an evaluation because the pressure and temperature [in the reactors] was changing drastically."
Up until Tuesday, Nishiyama insisted it was "too early to raise it to Level 6."
The agency announced radiation forecasts by the System for Prediction of Environmental Emergency Dose Information (SPEEDI) only once on March 23. These results serve as the basis for severity assessments. The data showed 30,000 to 1,100,000 terabecquerels of radiation were emitted over the 12 days immediately following the disaster.
In a press conference Tuesday, NSC member Seiji Shiroya distanced himself from the decision to raise the severity level. "I thought it might've been a Level 7 at the time, but it's the agency that makes the call," he said.
Hirotada Hirose, a disaster expert and former professor at Tokyo Woman's Christian University, said, "I wonder if the government treated the [nuclear crisis] as a political matter just to try to prevent causing panic and further problems. It's possible political considerations drove them to downplay the situation in the early stages."
"The government might've wanted to ease people into the harshness of the reality. But if reassuring information is the main priority, people won't be able to properly prepare for a disaster and distrust in the government will grow," he said.
(Apr. 14, 2011)
東電と保安院の解析結果の違い
