Japan has raised the severity rating for its tsunami-stricken nuclear reactor from Level 5 to Level 7, the highest grade, equal to the 1986 disaster at Chornobyl in the former Soviet Union.

The country's Nuclear and Industrial Safety Agency made the decision Monday, saying that while the radiation emission rate at the Fukushima Daiichi plant is about 10 per cent of that at Chornobyl, the crippled Japanese facility has emitted a huge amount of radioactive substances that pose a risk over a large area.

okuma, japanLocation of Fukushima Daiichi plant

"We have upgraded the severity level to 7 as the impact of radiation leaks has been widespread from the air, vegetables, tap water and the ocean," agency spokesman Minoru Oogoda said Tuesday. 

Agency officials said one of the factors behind the decision was that the total amount of radioactive particles released into the atmosphere since the incident had reached levels that apply to a Level 7 disaster.

The Chornobyl disaster is the only previous time that an event has been rated Level 7 — or "major accident" — on the international and radiological events scale, which starts at zero and is designed to express the severity of nuclear mishaps.

The Fukushima crisis was previously rated Level 5, equalling the 1979 Three Mile Island incident in Pennsylvania. Each level higher is considered to be roughly 10 times more severe.

A small fire broke out at the Fukushima nuclear facility on Tuesday in a box that holds batteries, but it was quickly extinguished. Plant operator Tokyo Electric Power Co. said the flames had no effect on radiation levels or any critical systems, and the fire had nothing to do with the decision to hike the incident rating at the plant.

2 more aftershocks

The news came as two new earthquakes rattled Japan's east and northeast, and as the government urged more people living near the Fukushima Daiichi generating station to leave, citing concerns about long-term health risks from radiation.

A magnitude 6.3 aftershock struck an area neighbouring Tokyo just after 8 a.m. local time Tuesday, the country's meteorological agency reported, though there were no initial reports of injuries or damage. No tsunami warning was issued.

An earlier magnitude 7.0 aftershock on Monday trapped some people in collapsed homes, and came just hours after residents bowed their heads and wept in ceremonies to mark a month since the massive earthquake and tsunami killed up to 25,000 people and set off radiation leaks at the nuclear plant by knocking out its cooling systems.

"Even after a month, I still cry when I watch the news," said Marina Seito, 19, a student at a junior college who recalled being in a basement restaurant in Sendai when the original 9.0 magnitude earthquake hit on March 11. Plates fell and parts of the ceiling crashed down around her.

The epicentre of Monday's aftershock was just inland and about 160 kilometres north of Tokyo. A warning was issued for a one-metre tsunami, but it was subsequently lifted.

In Iwaki, a city close to the epicentre of Monday's tremor, a landslide brought down three houses, trapping up to seven people. Four were rescued alive, but one of those — a 16-year-old girl — died at the hospital, a police official said. He would not give his name, citing policy.

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Kiyoko Takahashi, 81, holds a moment of silence on a burial ground at 2:46 p.m. Monday, exactly a month after a massive earthquake struck the area in Higashimatsushima in northeastern Japan. ((Yomiuru Shimbun/Kota Kawasaki/Associated Press))

The 9.0 magnitude earthquake and the tsunami it generated on March 11 are believed to have caused as much as $310 billion in damage. The nuclear power plant they disabled has been spewing radiation since, and even a month on, officials say they don't know how long it will take to cool reactors there.

In a move unrelated to Monday's aftershock, the Japanese government is expanding the evacuation zone around the plant to 30 kilometres from 20, citing the risks of cumulative radiation exposure.

"This isn't an immediate evacuation," CBC's Craig Dale reported from Tokyo. "Over a month's period they're hoping that people living 30 kilometres away from Fukushima Daiichi will leave those areas, or at least stay indoors."

The latest calculations from Japan's Nuclear Safety Commission found that an area stretching 60 kilometres north of the nuclear plant and 40 kilometres south have, in the month since the earthquake, already been exposed to radiation equivalent to the annual dosage limit. Within the old 20-kilometre evacuation zone, radiation exposure has reached up to 100 times the annual limit.  

At-risk groups prioritized

People being targeted in the evacuation are children, pregnant women and patients in hospitals. Dale said the evacuation zone is not a perfect circle and stretches up to 40 kilometres from the plant in at least one place.   

A nuclear safety official said the repeated strong aftershocks were slowing work at the plant, and said that if one of them were to spawn a tsunami, the complex would be just as vulnerable as on March 11.

"At the moment, no tsunami resistance has been added to the plant. At the moment, there is nothing we can do about it," the Nuclear and Industrial Safety Agency's Hidehiko Nishiyama said.

Monday's aftershock briefly forced Tokyo's main international airport to close both of its runways. There were no new reports of damage.

'My chest has been ripped open by the suffering and pain that this disaster has caused.' —Yuhei Sato, governor of Fukushima

Aftershocks have repeatedly rattled the disaster-weary region, but there is little left in the northeast to ruin. Last Thursday's 7.1 magnitude aftershock, which had been the strongest tremor since the day the original quake hit, did sink hundreds of thousands more households into darkness, however. Electricity has been restored to most of those homes.

With workers still far from bringing the nuclear plant under control, the bodies of thousands of tsunami victims yet to be found and more than 150,000 people living in shelters, there was little time for reflection on Japan's worst disaster since the Second World War.

People in hard-hit towns gathered for ceremonies at 2:46 p.m., the exact moment of the massive quake a month earlier.

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Participants offer a silent of prayer during a charity concert in Tokyo on Monday to raise funds for the victims of the earthquake and tsunami. ((Shuji Kajiyama/Associated Press))

"My chest has been ripped open by the suffering and pain that this disaster has caused the people of our prefecture," said Yuhei Sato, the governor of Fukushima, which saw its coastal areas devastated by the tsunami and is home to the damaged plant at the centre of the nuclear crisis. "I have no words to express my sorrow."

Japan's government marked the one-month period by putting an ad in newspapers in China, South Korea, Russia, France, the United Kingdom and the United States — a letter from Prime Minister Naoto Kan thanking people for the outpouring of support that followed the tsunami. The Red Cross alone said it has collected $102 million Cdn from overseas.

Kan described the outpouring as "kizuna," the bond of friendship.

"We deeply appreciate the kizuna our friends from around the world have shown and I want to thank every nation, entity, and you personally, from the bottom of my heart."

With files from The Associated Press