TOKYO—The Japanese government raised its assessment of the monthlong crisis at its Fukushima Daiichi nuclear power plant to the highest severity level by international standards—a rating only conferred so far upon the Chernobyl accident. 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 is the same level reached by the Chernobyl nuclear disaster in the former Soviet Union, which struck almost exactly 25 years ago, on April 26, 1986. "Based on the cumulative...
