Japan’s nuclear crisis deepened today with the severity level at the crippled Fukushima Daiichi nuclear plant being pegged at a maximum seven, bringing it on par with the 1986 Chernobyl catastrophe as another strong earthquake jolted the northeast.

The decision to raise the threat level was made after radiation of 10,000 terabequerels per hour had been estimated at the stricken plant for several hours, Kyodo News Agency said.

Japan’s Nuclear and Industry Safety Agency (NISA) used the International Nuclear and Radiological Event Scale or INES to gauge the level. The scale was designed by an international group of experts to indicate the significance of nuclear events with ratings of 0 to 7.

On March 18, one week after the massive quake, the agency declared the Fukushima trouble a level 5 incident, the same as the accident at Three Mile Island in the United States in 1979.

Until now, Level 7 has only been applied to the Chernobyl accident in the former Soviet Union in 1986 when hundreds of thousands of terabecquerels of radioactive iodine-131 were released into the air.

The Chernobyl accident was caused by rupture of a reactor vessel followed by a series of explosions leading to a radioactive fallout that claimed the lives of over 4,000 people.

However, NISA said radiation emissions from the plant, whose cooling system was hit by a massive earthquake and tsunami on March 11, were a tenth of that from the Chernobyl accident.

“We have upgraded the severity level to seven as the impact of radiation leaks has been widespread from the air, vegetables, tap water and the ocean,” said Mr Minoru Oogoda of NISA.

“In terms of volume of radioactive materials released, our estimate shows it is about 10 per cent of what was released by Chernobyl,” he said.

The level seven signifies a “major accident” with “wider consequences” than the previous level, an official at NISA said.

The assessment comes a day after the Government announced it was extending the exclusion zone around the nuclear plant.

Meanwhile, a strong 6.3-magnitude earthquake jolted Chiba prefecture, 77 km east of Tokyo, this morning, a day after an aftershock of 7.1 rocked Japan’s northeast, the country’s Meteorological Agency said.

There was no immediate report of any damage to life or property in the aftermath of the quake which occurred at 8.08 am (local time). Residents in Tokyo also felt the tremors as buildings in the Japanese capital shook for several seconds.

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