We know that the nuclear situation in Japan is critical. But how bad is it? Japan’s nuclear safety agency on Friday raised the rating of the crisis at Fukushima from 4 to 5 on the 7-step international scale.

On one hand, this is scary: another Level 5 nuclear crisis was at Three Mile Island in 1979. (Chernobyl was a 7.) On the other hand, as a scientist friend pointed out to me this morning, the new rating means that the situation in Fukushima is 1/100 as bad as the Chernobyl accident.

An explanation: The 7-step international scale was created by the International Atomic Energy Agency (I.A.E.A.) to measure the intensity of nuclear disasters. It is a logarithmic scale; an increase of one level means it is 10 times as intense, as the agency explains on a fact sheet. The Richter scale is also a logarithmic scale. But because it is much easier to systematically measure the shaking caused by an earthquake than to gauge a nuclear crisis, the I.A.E.A.’s scale is necessarily more interpretive. For example, no one died during the Three Mile Island meltdown, so saying that it was even 1 percent as bad as Chernobyl seems like an overestimation. And given Japan’s history, the psychological effects of Fukushima will most likely be much greater than those of Three Mile Island, so rating them both the same may also be misleading. But despite its flaws, the I.A.E.A. scale is the only international measure we’ve got.

There is the horrifying possibility that the rating at Fukushima will rise. If the 1970s movie “The China Syndrome” is your best reference for what a nuclear meltdown — the worst-case scenario — might look like, it is worth checking out an informative M.I.T. blog. Students at the nuclear science and engineering department do a sober job of explaining the science behind what is unfolding in Japan. A mission statement of sorts from the blog:

The blog has received a great number of questions surrounding worst-case scenarios. This is not surprising given that such scenarios, with varying degrees of scientific merit, have been advanced in the media. The intent of this blog is to educate, using our best available information, and so we intend to refrain from making predictions of our own. We do, however, want to review some of the terminology used in these predictions, and describe the methods used by government agencies and scientific organizations to determine what actions must be taken to inform the public.

ProPublica also has a good explainer on why Fukushima is unlikely to turn into another Chernobyl. NYT

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