Japan Nuclear Disaster Put on Par With Chernobyl

According to 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.”

Japan’s previous 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.

Japan’s Nuclear and Industrial Safety Agency said at a news conference Tuesday morning that the rating resulted from new estimates by Japan’s 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 was preparing to urge more residents around a crippled nuclear power plant to evacuate, because of concerns over long-term exposure to radiation.

Also on Monday, tens of thousands of people bowed their heads in silence at 2:46 p.m., exactly one month since the 9.0-magnitude earthquake and ensuing tsunami brought widespread destruction to Japan’s northeast coast.

The mourning was punctuated by another strong aftershock near Japan’s Pacific coast, which briefly set off a tsunami warning, killed at least one person and knocked out cooling at the severely damaged Fukushima Daiichi Nuclear Power Station for almost an hour, underscoring the vulnerability of the plant’s reactors to continuing seismic activity.

On Tuesday morning, there was another strong aftershock, which shook Tokyo. 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 18 miles.

The government’s decision to expand the zone came in response to high readings of radiation 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 18-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.

Yukio Edano, the government’s chief cabinet secretary, said that the government would order Iitate and four other towns and villages to prepare to evacuate.

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. Evacuation orders will come within a month for Katsurao, Namie, Iitate and parts of Minamisoma and Kawamata, Mr. Edano said.

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 had 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.

But the Soviet Union used a lower threshold — five millisieverts per year — in eventually offering resettlement to people who lived near the Chernobyl reactors in 1986.

Mr. Friedlander, the former nuclear plant operator , who is a specialist in emergency responses to nuclear accidents, said that the Japanese decision to evacuate more communities made sense not just to protect people, but also to make the eventual decontamination of farms and communities easier.

Allowing people and nonemergency vehicles to continue moving through both radiation-contaminated areas and safer areas farther from the Fukushima reactors runs the risk of spreading radioactively contaminated particles, which could result in more square miles of territory ultimately being contaminated. “Unless you gain control, it will be like trying to mop your kitchen floor with the kids running in and out of the house,” Mr. Friedlander said.

Masataka Shimizu, the president of Tokyo Electric, visited the tsunami-stricken area on Monday for the first time since the crisis began. He called on the governor of Fukushima Prefecture, Yuhei Sato, but was refused a meeting. He left his business card instead.