Editor's Note: This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency.
Japan's upgrade of the Fukushima accident severity, though long overdue, has been greeted with some perplexity by reactor safety experts, as the Financial Times notes in a report today. After all, said one, there's been no sudden change in the state of the three affected reactors and four spent fuel ponds.
Had the upgrade been to 6 rather than 7 on the IAEA scale, it probably would not have encountered the same skepticism. From Day 2 of the unfolding accident, when the first explosion destroyed an outer containment building, the situation was plainly worse than Three Mile Island, which was rated Level 5. But has it been as bad as the Chernobyl Level 7 catastrophe?
Hidehiko Nishiyama, deputy director general of Japan's nuclear regulator, stated the essential difference admirably, showing that the upgrade decision must have been carefully considered. "Mr. Nishiyama," reported the New York Times, "stressed that unlike Chernobyl, where the reactor itself exploded and fire fanned the release of radioactive material, the containments [that is, the steel reactor vessels] at the four Fukushima reactors remained intact over all."
The difference between Fukushima and Chernobyl deserves to be underlined. Both hydrogen and steam explosions can occur in the standard light water reactor used around the world, and such explosions can take place in the inner reactor vessel, damaging the core (though at Fukushima, so far at least, they have occurred only in outer containment buildings). But at Chernobyl, the reactor core itself exploded, sending radioactive materials 9 kilometers high into the atmosphere, to be carried across Eastern Europe to Scandinavia, where they were first detected.
So why are the Japanese now classifying Fukushima as Level 7? It's because, as my fellow blogger Eliza Strickland has explained, the accident appears to meet the IEAE criteria for environmental release of radioactive materials into the environment. Though it's improbable that the total releases will ever exceed Chernobyl's, it's still possible, as the situation remains grim and the reactors not fully stabilized. Japanese authorities have said releases to date are about 10 percent of Chernobyl's, though Strickland makes a good case they may in fact be well under 5 percent.*
* The New York Times has a report today that sheds considerable light on the discrepancies in Japan's reported ejection of radioactive materials. The authorities are saying now that releases of radioactive cesium and iodine were much higher in the first days immediately after the accident than previously believed.