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Correction: An earlier version of this article incorrectly reported that Japan’s Nuclear Safety Commission raised its rating of the severity of the nuclear crisis. Instead, the country’s Nuclear and Industrial Safety Agency raised the rating. This version has been corrected.

April 12, 2011

Japanese authorities on Tuesday raised the severity rating of the nuclear crisis at the Fukushima Daiichi power plant to the highest level on an international scale, on a par with the 1986 Chernobyl disaster.

Officials from Japan’s nuclear agency reclassified the ongoing emergency from Level 5, an “accident with off-site risk,” to Level 7, a “major accident.” The reassessment comes at a time when the International Atomic Energy Agency says the plant is showing “early signs of recovery” but is still in critical condition.

At a news conference in Tokyo, Hidehiko Nishiyama, the chief of Japan’s Nuclear and Industrial Safety Agency, emphasized that radiation released from Fukushima amounted to one-tenth the total released from Chernobyl. But the plant continues to spew radiation, and at a separate news conference, an official from the Tokyo Electric Power Co. said that “our concern is that the amount of leakage could eventually reach that of Chernobyl or exceed it.”

The stark assessment reinforced the sense that this nuclear emergency ultimately will cause problems that exceed those first predicted by the government, which has downplayed long-term safety concerns and only Monday expanded its mandated 12-mile radius evacuation zone.

Still, the upgraded severity reading does not reflect a recent deterioration at the plant. Rather, it suggests Japan’s evolving understanding of the damage that occurred there one month ago — and the contamination that has been leaking ever since.

“We are taking this extremely seriously,” Tokyo Electric said in a statement signed by its president, Masataka Shimizu. “We deeply apologize for tremendous concerns and inconvenience we are causing the residents in the neighboring areas of the power plant as well as people of [Fukushima] prefecture, and further, to the people of” Japan.

A Level 7 accident, according to the International Nuclear and Radiological Event Scale (INES), is typified by a “major release of radioactive material with widespread health and environmental effects.” That measuring scale was established by the IAEA some 21 years ago, but its guidelines leave plenty of room for interpretations, nuclear experts say.

According to the Kyodo News agency, Japan’s Nuclear Safety Commission issued a recent report claiming that the Fukushima plant, at one unspecified point after the March 11 earthquake and tsunami, had been releasing 10,000 terabecquerels of radioactivity per hour. A release of tens of thousands of terabecquerels per hour corresponds with the leakage level that the IAEA recommends as a minimum benchmark for a Level 7 accident.

“This corresponds to a large fraction of the core inventory of a power reactor, typically involving a mixture of short- and long-lived radionuclides,” an IAEA document says. “With such a release, stochastic health effects over a wide area, perhaps involving more than one country, are expected.”

Most radiation readings around Fukushima have been decreasing for several weeks now, but the plant still faces numerous risks. Thousands of tons of contaminated water has flooded key buildings adjacent to the reactors. Nitrogen gas is being injected into one unit to prevent another explosion.

In the meantime, the plant faces the constant threat of aftershocks, and on Tuesday a 6.2-magnitude temblor caused a brief fire at a building near Daiichi’s No. 4 reactor. Tokyo Electric said the aftershock did not interrupt the critical injection process used to cool hot fuel rods -- but there had been a 50-minute interruption one day earlier, the result of a 6.6-magntiude quake with an epicenter just 42 miles from the plant.

“Right now, the situation of the nuclear reactors at the Fukushima plant has been stabilizing step by step,” Prime Minister Naoto Kan said in a televised address to the nation. “The amount of radiation leaks is on the decline. But we are not at the stage yet where we can let our guards down.”

Though the Fukushima crisis now stands alongside Chernobyl on the INES event rating, experts note several important distinctions. No deaths have yet resulted from radiation leaked at Fukushima. At Chernobyl, a single reactor exploded, and a massive cloud of cesium, plutonium and strontium spread across Belarus, Ukraine, Russia and Europe. At Fukushima, workers have struggled to stabilize seven reactors and spent fuel pools, but so far the radioactive releases have come as byproduct of emergency cooling efforts, with the venting of radioactive steam and the leaking of contaminated water.

Technically, Japan’s reassessed severity rating applies to only three of Fukushima’s six units — Nos. 1, 2 and 3, which have all sustained core damage. Each of those units, on March 18, had been initially given a Level 5 rating. At the same time last month Japan gave a Level 3 rating to unit 4; that remains unchanged. The IAEA cautioned that Japan could still change its ratings as more information becomes available.

The latest reevaluation underscores the difficulty of measuring the amount of contamination released and the danger it poses. Rather than creating a hard “no-go” circle around the plant, the government has instead singled out five particular towns between 12 and 19 miles from the plant for mandatory evacuation. Residents there should leave within a month, the government said.

Those who stay are at risk of receiving more than 20 millisieverts of radiation in a year — the government’s baseline for evacuations. Exposure at that level doesn’t cause immediate sickness, but it raises risk for cancer and other long-term health problems.

Japan’s various nuclear regulatory groups have come up with widely differing numbers regarding the total contamination released into the environment.

The Nuclear and Industrial Safety Agency, for instance, says that 6,000 terabecquerels of cesium-137 have been released. Japan’s Nuclear Safety Commission, meanwhile, quotes a number twice that. Much of that contamination, though, was released in the first week after an earthquake and tsunami knocked out Fukushima Daiichi’s primary and back-up power supplies, stopping the cooling of the reactors’ cores.

“Monitoring data available shows that, in my view, the government probably knew around March 16, 17 or 18 that it would reach Level 7,” said Hironobu Unesaki, a professor at the Kyoto University Research Reactor Institute. “I just don’t understand why it took them so long to raise the level to 7. They were completely slow. ... Their response has been extremely regrettable. The government is being very careful not to cause unnecessary panic, but they are being too cautious.”

Special correspondent Akiko Yamamoto contributed to this report.

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