Japan’s nuclear regulators said the plant has likely released so much radiation into the environment that it must boost the accident’s severity rating on the International Nuclear Event scale to a 7 from 5 currently. That’s the highest level by international standards—a level only conferred so far on the Chernobyl accident in the former Soviet Union, which struck almost exactly 25 years ago, on April 26, 1986…

A 7 ranking labels this is “a major accident,” the most serious on the international scale. It means high levels of radiation have been released, and that the amount of time needed to bring the plant under control. But not all “major accidents” are equal in severity…

Japanese nuclear regulators determined that in the early hours after the accident, the plant had likely been releasing tens of thousands of terabecquerals—or a mind-boggling tens of thousands of trillions of bequerals—of radiation per hour in the immediate area, Japanese media reported Tuesday morning. Those figures couldn’t be immediately confirmed. That is a level that has been recorded only during the Chernobyl accident, the reports said.