Fukushima has maxed out the nuclear rating scale, only the second accident to rate 7. So does that really mean that this event is as bad as Chernobyl?
First, a quick explanation of the INES scale. Technically, it's the International Nuclear and Radiological Event Scale, and was developed by a group of international nuclear experts who were convened by the International Atomic Energy Agency (IAEA) back in 1989. IAEA explains that it is meant as "a means for promptly communicating to the public in consistent terms the safety significance of events reported at nuclear installations."
There are all kinds of technical details that go into the ratings (which you can read about on the IAEA site), but they generally fall into three main criteria: offsite radiological effects, onsite radiological effects, impairment of safety measures. When we're talking about accidents above a Level 4, we're mostly looking at the radiological effects outside of the plant. In other words: how bad is the radiation threat to normal civilians living nearby?
To prompt a Level 7 rating, the radiation releases from the plant have to add up to more than 10,000 terabequerels of iodine-131 equivalents.
An event resulting in an environmental release corresponding to a quantity of radioactivity radiologically equivalent to a release to the atmosphere of more than several tens of thousands of terabequerels of I-131. -INES User's Manual, 2008 Edition (PDF)
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Which, translated to English, means: "Major Release: Widespread health and environmental effects."
So why did Fukushima suddenly jump two levels? According to the IAEA, the new rating "considers the accidents that occurred at Units 1, 2 and 3 as a single event on INES and uses estimated total release to the atmosphere as a justification. Previously, separate provisional INES Level 5 ratings had been applied for Units 1, 2 and 3." So, the earlier Level 5 ratings were for the individual reactors. The new rating reflects the total release.
Finally, if Fukushima is now a Level 7, is it really as bad as Chernobyl? Not really.
By most measures, Fukushima has "only" released about 10 percent of the total radiation released 25 years ago at Chernobyl. Then there's the longer time frame that the Japanese disaster has played out over, as
When Chernobyl's reactor number 4 exploded in 1986, it scattered debris over a wide area and sent radioactive fallout high into the atmosphere. Entire villages near the reactor had to be evacuated in a matter of hours, and many residents had to leave personal effects behind. A fire burned at the site until 5 May, spewing tones of radioactive material over 200,000 square kilometres...In the short period following the explosion, the accident spewed some 14 million terabecquerels of radiation into the environment.
The Fukushima accident has unfolded much more slowly. The damaged reactors exploded over a period of days, and after a modest initial release, radiation has fallen off. So far, the reactors have spread about half-a-million terabecquerels into the air.
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So Fukushima has released much less radiation, and much more slowly. Because of this—and the obvious cultural and political contrasts between the Japanese and the Soviets—local residents have had fair warning and, hopefully, enough time to escape the worst effects of radiation.
How can two accidents with radiation releases that differ by an order of magnitude share the same rating? The only thing that is clear about this entire Fukushima rating upgrade is that the INES scale desperately needs to be updated.
Special hat tip to Michael Graham Richard of Treehugger, for the graphic inspiration. His coverage of Fukushima has been phenomenal. Bookmark him.
Photo (cc) adapted from original by Digital Globe on Wikimedia Commons. Chart from IAEA
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