Japan's prime minister is urging the public not to panic after the government boosted the severity level of the crisis at a tsunami-damaged nuclear plant to the highest rating — on par with the 1986 Chornobyl disaster.

Prime Minister Naoto Kan urged people in a televised address to focus on recovering from the country's disasters.

"Right now, the situation of the nuclear reactors at the Fukushima plant has been stabilizing step by step," he said. "The amount of radiation leaks is on the decline. But we are not at the stage yet where we can let our guards down."

Understanding the scale

The severity level of nuclear accidents on the International Nuclear and Radiological Event Scale is based on a number of factors, including the scale of disaster, the amount of radioactive material released and the number of people affected.

The international scale has seven levels, with Level 7 being the most severe. Levels 1, 2 and 3 are called "incidents" and levels four through seven are "accidents."

The scale is designed so that the severity of an event at each level is roughly 10 times greater for each new level on the scale. Events without safety significance are called "deviations" and are classified as "below scale" or level zero.

Level 7 disasters, of which there have only been two, are described as a "major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures."

Japanese regulators said they raised the rating from five to seven after new assessments of radiation leaks from the Fukushima Daiichi plant.

The country's nuclear watchdog said that while the radiation emission rate at the stricken plant is only about 10 per cent of that released at Chornobyl, the crippled Japanese facility has still emitted a huge amount of radioactive substances that pose a risk over a large area.

Until now, the Chornobyl disaster in Ukraine was the only event rated Level 7. The Fukushima crisis was previously rated Level 5, the same as the 1979 Three Mile Island incident in Pennsylvania.

Although the upgrade was dramatic, the two disasters — Fukushima and Chornobyl — are not all that similar, experts say.

In Chornobyl, it was the reactor core itself that exploded, releasing a huge amount of radioactive material in a very short time. Fukushima experienced a less critical hydrogen explosion. And the total amount of radioactive particles released so far is believed to be only a small fraction of that seen in Ukraine.

Japan nuclear agency official Hidehiko Nishiyama said 29 people died of acute radiation exposure at Chornobyl, but there have been no fatal radiation casualties at Fukushima.

The upgrade came as at least two new earthquakes with a magnitude of more than 6.0 hit Japan's northeast on Monday night and Tuesday morning. The area has been rocked by numerous aftershocks since then. Until the subducted plate settles back into position, any quake within the rupture zone — in this case an area 300 kilometres long by 150 kilometres wide off the Japanese coast — that has less force than the initial quake is considered an aftershock.

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Workers operate a modified Putzmeister 70Z, the world's largest concrete pump mounted on a truck, to pump contaminated water from Unit 4 at the Fukushima Daiichi nuclear power plant Tuesday. ((Tokyo Electric Power Co./Associated Press))

The 9.0 magnitude earthquake on March 11 and the tsunami it generated  are believed to have caused as much as $310 billion in damage. Japanese officials have updated the death toll from the disaster to 13,219 people. More than 14,000 others are still missing and more than 145,000 people are living in evacuation centres across the country.

The Fukushima Daiichi plant has been spewing radiation since the two disasters, and even a month on, officials say they don't know how long it will take to cool reactors there.

Chinese Premier Wen Jiabao expressed concern Tuesday about radiation reaching the ocean. He told his Japanese counterpart the country must seriously examine the effects on the marine environment and on Japan's neighbours. 

He also reminded Japan's prime minister that he must strictly abide by international law and issue comprehensive reports to China.

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.

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.

Work resumes

At the plant itself, the Tokyo Electric Power Co., known as TEPCO, has resumed efforts to contain the plant's radioactive leak, after a one-day delay because of strong aftershocks.

TEPCO will begin pumping contaminated water from the No. 2 reactor Tuesday and transfer it to a condenser, after checking the safety of equipment.

The radioactive water has been hampering work to restore cooling functions in the damaged reactors. TEPCO says it also resumed injecting nitrogen into the containment vessel of the No.1 reactor late on Monday night. That's aimed at preventing further hydrogen explosions.

With files from The Associated Press