TOKYO — Japan has decided to raise its assessment of the accident at the crippled Fukushima Daiichi nuclear power plant to the worst rating on an international scale, putting the disaster on par with the 1986 Chernobyl explosion, the Japanese nuclear regulatory agency said on Tuesday.
The decision to raise the alert level to 7 from 5 on the scale amounts to an admission that the accident at the nuclear facility, brought on by the March 11 earthquake and tsunami, is likely to have substantial and long-lasting consequences for health and for the environment. Some in the nuclear industry have been saying for weeks that the accident released large amounts of radiation, but Japanese officials had played down this possibility.
The new estimates by Japanese authorities suggest that the total amount of radioactive materials released so far is equal to about 10 percent of that released in the Chernobyl accident, said Hidehiko Nishiyama, deputy director general of Japan’s nuclear regulator, the Nuclear and Industrial Safety Agency.
Mr. Nishiyama stressed that unlike at Chernobyl, where the reactor itself exploded and fire fanned the release of radioactive material, the containments at the four troubled reactors at Fukushima remained intact over all.
But at a separate news conference, an official from the plant’s operator, Tokyo Electric Power Company, said, “The radiation leak has not stopped completely and our concern is that it could eventually exceed Chernobyl.”
On the International Nuclear Event Scale, a Level 7 nuclear accident involves “widespread health and environmental effects” and the “external release of a significant fraction of the reactor core inventory.” The scale, which was developed by the International Atomic Energy Agency and countries that use nuclear energy, leaves it to the nuclear agency of the country where the accident occurs to calculate a rating based on complicated criteria.
Japan’s previous rating of 5 placed the Fukushima accident at the same level as the Three Mile Island accident in Pennsylvania in 1979. Level 7 has been applied only to the disaster at Chernobyl, in the former Soviet Union.
“This is an admission by the Japanese government that the amount of radiation released into the environment has reached a new order of magnitude,” said Tetsuo Iguchi, a professor in the department of quantum engineering at Nagoya University. “The fact that we have now confirmed the world’s second-ever level 7 accident will have huge consequences for the global nuclear industry. It shows that current safety standards are woefully inadequate.”
Mr. Nishiyama said “tens of thousands of terabecquerels” of radiation per hour have been released from the plant. (The measurement refers to how much radioactive material was emitted, not the dose absorbed by living organisms.) The scale of the radiation leak has since dropped to under one terabecquerel per hour, the Kyodo news agency said, citing government officials.
The announcement came as Japan was preparing to urge more residents around the crippled nuclear plant to evacuate, because of concerns over long-term exposure to radiation.
The authorities have already ordered people living within a 12-mile radius of the plant to evacuate, and recommended that people remain indoors or avoid an area within a radius of about 19 miles.
The government’s decision to expand the zone came in response to radiation readings that would be worrisome over months in certain communities beyond those areas, underscoring how difficult it has been to predict the ways radiation spreads from the damaged plant.
Unlike the previous definitions of the areas to be evacuated, this time the government designated specific communities that should be evacuated, instead of a radius expressed in miles.
The radiation has not spread evenly from the reactors, but instead has been directed to some areas and not others by weather patterns and the terrain. Iitate, one of the communities told on Monday to prepare for evacuation, lies well beyond the 19-mile radius, but the winds over the last month have tended to blow northwest from the Fukushima plant toward Iitate, which may explain why high readings were detected there.
Officials are concerned that people in these communities are being exposed to radiation equivalent to at least 20 millisieverts a year, he said, which could be harmful to human health over the long term. In addition to Iitate, evacuation orders will come within a month for Katsurao, Namie and parts of Minamisoma and Kawamata, said Yukio Edano, the chief cabinet secretary.
People in five other areas may also be told to evacuate if the conditions at the Fukushima Daiichi plant grow worse, Mr. Edano said. Those areas are Hirono, Naraha, Kawauchi, Tamura and other sections of Minamisoma.
“This measure is not an order for you to evacuate or take actions immediately,” he said. “We arrived at this decision by taking into account the risks of remaining in the area in the long term.” He appealed for calm and said that the chance of a large-scale radiation leak from the Fukushima Daiichi plant had, in fact, decreased.
Mr. Edano also said that pregnant women, children and hospital patients should stay out of the area within 19 miles of the reactors and that schools in that zone would remain closed.
Until now, the Japanese government had refused to expand the evacuation zone, despite urging from the International Atomic Energy Agency. The United States and Australia have advised their citizens to stay at least 50 miles away from the plant.
The international agency, which is based in Vienna, said Sunday that its team measured radiation on Saturday of 0.4 to 3.7 microsieverts per hour at distances of 20 to 40 miles from the damaged plant — well outside the initial evacuation zone. At that rate of accumulation, it would take 225 days to 5.7 years to reach the Japanese government’s threshold level for evacuations: radiation accumulating at a rate of at least 20 millisieverts per year.
In other words, only the areas with the highest readings would qualify for the new evacuation ordered by the government.
Aftershocks have continued in northeastern Japan since the March 11 quake and tsunami. The latest occurred Tuesday at 2 p.m. local time when a shock measuring magnitude 6.0 struck off the Fukushima coast at a relatively shallow depth of about six miles under the seabed, according to the United States Geological Survey. Officials at Tokyo Electric said that workers were moved to safer areas within the Fukushima Daiichi plant, but there appeared to be no damage to the power supply and no disruption to pumps sending cooling water into the plant’s four most severely damaged reactors.
The aftershock appeared to be centered very close to a magnitude 6.6 temblor that struck the area on Monday, which knocked out power to the plant for almost an hour, stopping vital coo