Tokyo - Japan has raised 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, in an acknowledgement that the human and environmental consequences of the nuclear crisis could be dire and long-lasting.
The decision to raise the alert level to 7 from 5 on the scale, overseen by the International Atomic Energy Agency, is based on new estimates by Japanese authorities that suggest that the total amount of radioactive materials released so far from Fukushima Daiichi since the beginning of the crisis had reached that threshold. Hidehiko Nishiyama, deputy director general of Japan’s nuclear regulator, the Nuclear and Industrial Safety Agency, said that the total amount of radioactive materials released so far from Fukushima Daiichi equaled about 10 percent of that released in the Chernobyl disaster.
But at a separate news conference, an official from the plant’s operator, Tokyo Electric Power Company, said that the radiation release from Fukushima could, in time, surpass levels seen in 1986.
“The radiation leak has not stopped completely, and our concern is that it could eventually exceed Chernobyl,” said Junichi Matsumoto, a nuclear executive for the company.
Nevertheless, officials agreed that tens of thousands of terabecquerels of radioactive materials have escaped the plant, far higher than acknowledged so far by Japanese authorities. Some in the nuclear industry had been saying for weeks that the accident had released far larger amounts of radiation, but Japanese officials had until now played down this possibility. (A terabecquerel is a unit of measure representing a trillion small nuclear disintegrations per second.)
A member of a government commission that oversees the nuclear regulator, NISA, said that those levels could be in error by a factor of two to three. That means the amount of radiation released could be up to three times lower, or higher, than government estimates.
Much of the radiation release happened within a week of the March 11 earthquake and tsunami, which ravaged the Fukushima plant, raising questions about the delay in acknowledging the severity of the crisis.
Mr. Nishiyama stressed that unlike the disaster 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. Far less radiation has been released so far, he said, though he admitted that radiation leaks continue from Fukushima Daiichi as workers struggle to bring four of the plant’s six reactors under control.
Mr. Nishiyama also said that the radiation release at Chernobyl forced workers to abandon the facility at one point. At Fukushima, workers have remained on site and continue efforts to contain the crisis, he said.
“We are trying to keep leaks of radioactive materials into the environment to a minimum, but they do continue to a certain extent,” he said.
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, developed by the I.A.E.A. and countries that use nuclear energy, leave it to 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 Ukraine.
“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.”
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.
Prime Minister Naoto Kan of Japan called in a nationally televised speech on Tuesday evening for Japan to rebuild. While taking note of the decision to raise the severity of the nuclear accident at Fukushima to the worst level on the international scale, he took pains to say that the reactors were being stabilized and to emphasize that releases of radioactive material are now declining.
The prime minister said that he had ordered Tokyo Electric to present its plans and expectations for the stricken nuclear power plant. He also expressed concern about the economic consequences of the accident, calling on people across Japan to continue buying products from the affected areas of the country’s northeast.
Mr. Kan defended the government’s record in releasing information.
“What I can say for the information I obtained — of course the government is very large, so I don’t have all the information — is that no information was ever suppressed or hidden after the accident,” he said.
According to Japan’s Nuclear Safety Commission, a government panel, 370,000 to 630,000 terabecquerels of radioactive materials have been released into the air from the Nos. 1 to 3 reactors of the plant.
Seiji Shiroya, a commissioner and the former director of the Research Reactor Institute at Kyoto University, said that a much of the radioactivity escaped the suppression chamber at Reactor No. 2, which is thought to have suffered a hydrogen explosion on March 15, four days after the quake.
“It went up with a bang with what could have been a hydrogen explosion at Reactor No. 2’s suppression chamber, and the radioactive release has continued until now,” Mr. Shiroya said at a briefing. After several hours of a high rate of release, he said, the release slowed significantly.
Mr. Shiroya also suggested that by alluding to the possibility that amounts of radiation released from Fukushima Daiichi could approach or even surpass that to Chernobyl, Tokyo Electric was probably touching on the extremely unlikely possibility that all of the radioactive inventory at the Fukushima plant might be released into the environment.
“If everything inside the reactor came out, obviously that would surpass Chernobyl. There was only one troubled reactor there, while we have three or more, so simply speaking, that’s three times as worse,” he said. But at Fukushima, he said, most of the reactors’ radioactive elements remained within the reactor. “That is a big difference,” he said.
The commissioner also provided an unexpected hint of a possible motive for the government to have initially minimized the severity of the nuclear accident. The Japanese government had only a vague grasp of the amounts of radiation released in initial weeks, and last week had the amounts down to an error margin within several digits, he said.
“Some foreigners fled the country even when there appeared to be little risk,” he said. “If we immediately decided to label the situation as Level 7, we could have triggered a panicked reaction.”
In terms of health effects, the Chernobyl accident is expected to remain far worse than the one in Fukushima.
In Chernobyl, at least 31 people died in the accident and in the immediate aftermath, mainly those trying to fight the fires. So far at Fukushima, about 20 workers have been injured.
There were about 6,000 excess cases of thyroid cancer in Chernobyl, according to a recent commentary by Dr. Robert Peter Gale, who led the international medical team responding to that accident.
In his article, which was published in the April 1 edition of The Cancer Letter, a Washington newsletter, Dr. Gale said that if the Fukushima accident got no worse, there might be few if any cases of thyroid cancer and about 200 to 1,500 cases of leukemia and other cancers combined over the next 50 years. During that interval, he wrote, 20 million Japanese will die of cancer unrelated to Fukushima.
Dr. Gale could not immediately be reached for comment to see if his analysis had changed in the last several days.
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 I.A.E.A. 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 that 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 the epicenter of a magnitude 6.6 temblor that struck the area on Monday, knocking out power to the plant for almost an hour and stopping vital cooling work.
Ken Ijichi and Moshe Komata contributed reporting.