TOKYO — Japan ranked its nuclear crisis at the highest possible severity on an international scale — the same level as the 1986 Chernobyl disaster — even as it insisted Tuesday that radiation leaks are declining at its tsunami-crippled nuclear plant.
The higher rating is an open acknowledgement of what was widely understood already: The nuclear accident at the Fukushima Dai-ichi plant is the second-worst in history. It does not signal a worsening of the plant’s status in recent days or any new health dangers.
Still, people living nearby who have endured a month of spewing radiation and frequent earthquakes said the change in status added to their unease despite government efforts to play down any notion that the crisis poses immediate health risks.
Miyuki Ichisawa closed her coffee shop this week when the government added her community, Iitate village, and four others to places people should leave to avoid long-term radiation exposure. The additions expanded the 12-mile zone where people already had been ordered to evacuate soon after the March 11 tsunami swamped the plant.
“And now the government is officially telling us this accident is at the same level of Chernobyl,” Ichisawa said. “It’s very shocking to me.”
Japanese nuclear regulators said the severity rating was raised from 5 to 7 on an international scale overseen by the International Atomic Energy Agency because of new assessments of the overall radiation leaks from the Fukushima Dai-ichi plant.
According to the Vienna-based atomic energy agency, the new ranking signifies a major accident that includes widespread effects on the environment and people’s health. The scale, designed by experts convened by the IAEA and other groups in 1989, is meant to help the public, the technical community and the media understand the public safety implications of nuclear events.
The upgraded status did not mean radiation from the plant was worsening, but rather reflected concern about long-term health risks as it continues to spew into the air, soil and seawater. Most radiation exposures around the region haven’t been high enough yet to raise significant health concerns.
Workers are still trying to restore disabled cooling systems at the plant, and radioactive isotopes have been detected in tap water, fish and vegetables.
Iitate’s town government decided Tuesday to ban planting of all farm products, including rice and vegetables, expanding the national government’s prohibition on growing rice there.
Japan’s prime minister, Naoto Kan, went on national television and urged people not to panic.
“Right now, the situation of the nuclear reactors at the Fukushima plant has been stabilizing step by step. The amount of radiation leaks is on the decline,” he said. “But we are not at the stage yet where we can let our guard down.”
Japanese officials said the leaks from the Fukushima plant so far amount to a tenth of the radiation emitted from Chernobyl, but about 10 times the amount needed to reach the level 7 threshold. They acknowledged the emissions eventually could exceed Chernobyl’s, but said the chance that will happen is very small. However, regulators also have acknowledged that a more severe nuclear accident is a distinct possibility until regular cooling systems are restored — a process likely to take months.
“Although the Fukushima accident is now at the equal level as Chernobyl, we should not consider the two incidents as the same,” said Hiroshi Horiike, professor of nuclear engineering at Osaka University. “Fukushima is not a Chernobyl.”
In Chernobyl, in what is now Ukraine, a reactor exploded on April 26, 1986, spewing a cloud of radiation over much of the Northern Hemisphere. A zone about 19 miles around the plant was declared uninhabitable.
Thirty-one men died mostly from being exposed to very high levels of radiation trying to contain the accident. There is no agreement on how many people are likely to die of cancers caused by its radiation.
No radiation exposure deaths have been blamed on the leaks at Fukushima Dai-ichi. Two plant workers were treated for burns after walking in heavily contaminated water in a building there.
The tsunami, spawned by a 9.0 magnitude earthquake, knocked out cooling systems and backup diesel generators, leading to hydrogen explosions at three reactors and a fire at a fourth that was undergoing regular maintenance and was empty of fuel. Workers have been improvising for weeks with everything from helicopter drops to fire hoses to supply cooling water to the plant.
Hidehiko Nishiyama, a spokesman for Japan’s Nuclear and Industrial Safety Agency, noted that unlike in Chernobyl there have been no explosions of reactor cores, which are more serious than hydrogen explosions.
“In that sense, this situation is totally different from Chernobyl,” he said.
NISA officials said they raised the incident level because of the cumulative amount of radioactive particles released into the atmosphere. Other factors included damage to the plant’s buildings and accumulated radiation levels for its workers.
The revision was based on cross-checking and assessments of data on leaks of radioactive iodine-131 and cesium-137. Officials did not say why they skipped level 6 or when exactly the radiation level exceeded the level 7 threshold.
Based on government estimates, the equivalent of 500,000 terabecquerels of radiation from iodine-131 has been released into the atmosphere since the crisis began, well above the several tens of thousands of terabecquerels needed to reach level 7. A terabecquerel equals a trillion becquerels, a measure of radiation emissions. The Chernobyl incident released 5.2 million terabecquerels into the air.
“We have refrained from making announcements until we have reliable data,” Nishiyama said. He also emphasized that no more major leaks are expected from the reactors, though he acknowledged more work is needed to keep the reactors stable.
Work to stabilize the plant has been impeded by continued aftershocks, the latest a 6.3 magnitude quake Tuesday that prompted plant operator Tokyo Electric Power Co., or TEPCO, to pull back workers temporarily. Work removing highly radioactive water, a necessary step before cooling systems can be restored, finally resumed around 7:30 p.m.
In his televised address, Kan gave the nation a pep talk, telling people to focus on recovering from the disasters that are believed to have killed 25,000 people.
“Let’s live normally without falling into excessive self-restraint,” he said. “We should eat and drink products from the quake-hit areas as a form of support.”
Many of the more than 14,500 people still listed as missing from the quake and tsunami are thought to have been swept out to sea. A month after the disaster, more than 145,000 people are still living in shelters.
Among them is Kenichi Yomogita, a plumbing contract worker at Fukushima Dai-ichi who was off work the day of the tsunami and has not returned. His hometown of Tomioka is in the evacuation zone, and he thinks it will be at least three years before he can return. For now he is living at a shelter in Koriyama, and said the upgraded crisis level has not improved his hopes.
“At first the reality of this situation didn’t sink in,” he said, “but this news shows how serious it is.”
AP writers Yuri Kageyama, Mari Yamaguchi, Mayumi Saito and Malcolm J. Foster and Noriko Kitano in Tokyo and photographer Hiro Komae contributed to this report.