Huge radiation release led to severity hike

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 per cent 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.

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