The Japanese government on Tuesday raised the rating of the crisis at the Fukushima Dai-ichi power plant to a level 7. That's the highest possible level on the international scale used to evaluate the seriousness of nuclear incidents. The only other nuclear incident to earn that rating: the 1986 accident at the Chernobyl nuclear plant in Ukraine.
The decision to bump up the rating from 5 to 7 was prompted by new data on the amount of radiation released at the plant in the early days of the crisis — not by any recent change in the plant's status.
Some 370,000 terabecquerels of radioactive iodine and cesium have been released at Fukushima — more than officials originally thought. (A terabecquerel equals 1 trillion becquerels, a measure for radiation emissions.) Most of it spewed into the atmosphere in the early days of the crisis, and radiation levels have generally been declining.
Though Fukushima and Chernobyl are both level 7 nuclear accidents, the health consequences in Japan to date are much less severe. In part, that's because far more radiation was released at Chernobyl. So far, Fukushima Dai-ichi has released about one-tenth of the amount of radioactive material that escaped Chernobyl, according to an official from the International Atomic Energy Agency.
At Chernobyl, an entire reactor exploded, sending up a massive fire and radioactive plume that dispersed radiation over a wide area. The reactor at the Soviet plant was not surrounded by any containment structure, so radiation escaped freely.
People near Chernobyl were not warned against drinking contaminated milk, and many residents later developed thyroid cancer. Two Chernobyl plant workers died on the night of the accident, and 28 more people died within a few weeks from radiation poisoning. Over the long term, several thousand more people were put at risk for cancer.
Fukushima has also experienced explosions and fires, and some of the reactors' containment vessels may be damaged, but the highly radioactive cores remain largely protected. Much of the radioactive material already released in Japan has been carried out to sea away from populated areas, thanks to prevailing winds. And the government moved quickly to evacuate people from risky areas and to keep contaminated food out of the stores.
Some workers at Fukushima have been exposed to high levels of radiation since the crisis began. The long-term health effects of that exposure remain unknown, but there have been no known deaths associated with the crisis to date.
The World Health Organization confirmed Tuesday there was "very little" public health risk outside the 18-mile evacuation zone around the plant.
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