Japans current assessment of the accident puts it at level 5 on the scale, the same level as the Three Mile Island accident in the United States in 1979. The level 7 assessment has been applied only to the disaster at Chernobyl in the former Soviet Union.
The scale, which was developed by the International Atomic Energy Agency and countries that use nuclear energy, requires that the nuclear agency of the country where the accident occurs calculate a rating based on complicated criteria.
Japans Nuclear and Industrial Safety Agency said at a news conference Tuesday morning that the rating resulted from new estimates by Japans Nuclear Safety Commission that suggest some 10,000 terabecquerels of radiation per hour was released from the plant into the environment for several hours in the aftermath of the March 11 earthquake and tsunami. (The measurement refers to how much radioactive material was emitted, not the dose absorbed by living things. )
The scale of the radiation leak has since dropped to under 1 terabecquerel per hour, the Kyodo news agency said, citing the commission. Commission officials in Tokyo said they could not immediately comment.
Michael Friedlander, a former senior nuclear power plant operator for 13 years in the United States, said that the biggest surprise in the Japanese reassessment was that it took a month for public confirmation that so much radiation had been released.
Some in the nuclear industry have been saying for weeks that the nuclear accident released large amounts of radiation, but Japanese officials had consistently played down this possibility.
The announcement came as Japan is preparing to urge more residents around a crippled nuclear power plant to evacuate, because of concerns over long-term exposure to radiation.