TOKYO: Japan put its nuclear calamity on par with the world's worst nuclear disaster, Chernobyl, on Tuesday after new data showed that more radiation had leaked from its earthquake-crippled power plant in the early days of the crisis than first thought.

Officials said it had taken time to measure radiation from the Fukushima Daiichi facility after it was smashed by March 11's massive quake and tsunami, and the upgrade in its severity rating to the highest level on a globally recognised scale did not mean the situation had suddenly become more critical.

“Our preparations for how to measure (the radiation leakage) when such a tsunami and earthquake occurred were insufficient and, as a result, we were late in disseminating information internationally,” said a senior official in Prime Minister Naoto Kan's office.

Hidehiko Nishiyama, a deputy director-general of the Nuclear and Industrial Safety Agency (NISA), said the decision to raise the severity of the incident from level 5 to 7 – the same as the Chernobyl disaster in Ukraine in 1986 – was based on cumulative quantities of radiation released.

“Even before this, we had considered this a very serious incident so, in that sense, there will be no big change in the way we deal with it just because it has been designated level 7,” an agency official said.

As another major aftershock rattled the earthquake-ravaged east of the country, a fire broke out at the plant, but engineers later extinguished the blaze.

However, the operator of the stricken facility appears to be no closer to restoring cooling systems at the reactors, critical to lowering the temperature of overheated nuclear fuel rods.

The official in Kan's official said that, at a news conference expected later on Tuesday, the prime minister would instruct plant operator Tokyo Electric Power (TEPCO) to set target dates for when it would halt the radiation leakage as well as restore the cooling systems.

No radiation-linked deaths have been reported since the earthquake struck, and only 21 plant workers have been affected by minor radiation sickness, according to Chief Cabinet Secretary Yukio Edano.

“Nowhere near Chernobyl”

A level 7 incident means a major release of radiation with a widespread health and environmental impact, while a 5 level is a limited release of radioactive material, with several deaths, according to the International Atomic Energy Agency (IAEA).

Several experts said the new rating exaggerated the severity of the crisis, and that the Chernobyl disaster was far worse.

“It's nowhere near that level. Chernobyl was terrible – it blew and they had no containment, and they were stuck,” said nuclear industry specialist Murray Jennex, an associate professor at San Diego State University in California.

“Their (Japan's) containment has been holding, the only thing that hasn't is the fuel pool that caught fire.”

The blast at Chernobyl blew the roof off a reactor and sent large amounts of radiation wafting across Europe. The accident contaminated vast areas, particularly in Ukraine and neighbouring Belarus, led to the evacuation of well over 100,000 and affected livestock as far away as Scandinavia and Britain.

Nevertheless, the increase in the severity level heightens the risk of diplomatic tension with Japan's neighbours over radioactive fallout. China and South Korea have already been critical of the operator's decision to pump radioactive water into the sea, a process it has now stopped.

“Raising the level to a 7 has serious diplomatic implications. It is telling people that the accident has the potential to cause trouble to our neighbours,” said Kenji Sumita, a nuclear expert at Osaka University.

Huge economic damage

The March earthquake and tsunami killed up to 28,000 lives and the estimated cost stands at $300 billion, making it the world's most expensive disaster.

Japan's economics minister warned the economic damage was likely to be worse than first thought as power shortages will cut factory output and disrupt supply chains.

The Bank of Japan governor said the economy was in a “severe state”, while central bankers were uncertain when efforts to rebuild the northeast would boost growth, according to minutes from a meeting held three days after the earthquake struck.

NISA said the amount of radiation released into the atmosphere from the plant, 240 km (150 miles) north of Tokyo, was around 10 per cent that of Chernobyl.

“Radiation released into the atmosphere peaked from March 15 to 16. Radiation is still being released, but the amount now has fallen considerably,” said NISA's Nishiyama. – Reuters