What's the News: Japan raised its assessment of the severely damaged Fukushima Daiichi nuclear power plant to Level 7, "Major Accident," the highest ranking on the International Atomic Energy Agency's International Nuclear and Radiological Event Scale. The explosion at Chernobyl in 1986 is the only other nuclear accident to be ranked at Level 7. Both accidents were extremely severe, the two largest nuclear power accidents ever---but there are some big, important differences between them. What's Similar:
A Level 7 accident is a "major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures," according to the IAEA. Both plants clearly meet these criteria: Fukushima will require an extensive clean-up effort, and the international community is still working to make the area near Chernobyl safe.
The situation at Fukushima also qualifies as Level 7 by the numbers. Japanese officials estimate the reactors have released between 370,000 and 630,000 terabecquerels (or, between 370 and 630 quadrillion becquerels) of radioactive material, far more than the equivalent of tens of thousands of terabequerels iodine-131 that is the cutoff for a Level 7 accident.
A spokesman for Japan's Industrial and Nuclear Safety agency said in a press conference that the Fukushima reactors are still releasing radiation, and total levels could eventually exceed those released by Chernobyl.
The Fukushima reactors have containment structures, an extra safety layer that has helped limit the spread of radioactivity; Chernobyl had no containment structures.
The two accidents happened under very different conditions: Fukushima's reactors shut down after the March earthquake, then overheated as a result of later cooling system malfunctions. The reactor at Chernobyl, on the other hand, was still running when it exploded, causing a much larger release of heat.
The accident at Chernobyl unfolded much more quickly. The explosion there spewed debris and radioactive materials over a wide distance, and sparked a fire that burned for days. The surroundings had to be evacuated within hours. At Fukushima, the reactors have been releasing radiation at a much slower rate.
To date, the Fukushima plant has released much less radiation: only one-tenth as much as Chernobyl, according to Japan’s Nuclear and Industrial Safety Agency.
Dr. Robert Peter Gale, leader of the medical team that responded to the Chernobyl accident, estimated that, if the situation doesn't worsen, radiation from the Fukushima accident would cause fewer excess cancer cases an Chernobyl did. Chernobyl resulted in 6,000 additional cases of thyroid cancer, says Gale, while Fukushima would lead to few additional thyroid cancer cases, and 200 to 1,500 total additional cancer cases over the next half-century. (The actual number of cancers and deaths that resulted from Chernobyl is a scientifically and politically tricky question.)
Precautions taken by Japan---including monitoring of food and water supplies and timely distribution of potassium iodide tablets---may lessen the severity of human health effects of the Fukushima accident, according to World Nuclear News.
What's the Context:
The raising of Fukushima to a Level 7 accident doesn't mean that the situation at Fukushima is worsening; in fact, the reactors' condition seems increasingly stable.