The dire rating, which was not a suprise to many experts, follows on announcements by Japan’s Chief Cabinet Secretary Yukio Edano that the government proscribed 20-kilometre evacuation radius around the plant would beginning to include communities located between 20 and 30 kilometers from the plant over the course of the next month.

The new rating, which tops the International Nuclear Event Scale (INES) comes almost exactly 25 years after the 1986 Chernobyl explosion on April 26. Partial and total core meltdowns and massive released of radiation were calculated under the Administration of US President Dwight Eisenhower, and his famous “Atoms for Peace” drive in the early years of nuclear power, to have a chance of occurring only once every 17,000 years.

Such events have now happened three times in a quarter of a century: a partial meltdown at Pennsylvania’s Three Mile Island in 1979, which ranked 5 on the INES scale, Chernobyl and now Fukushima Daiichi, both weighing in at the top of the scale.

The decision to raise the alert level to 7 from 5 at Fukushima Daiichi on the scale – which was rated at a 5 – amounts to an admission that the accident at the nuclear facility, brought on by the March 11 earthquake and tsunami, is likely to have substantial and long-lasting consequences for health and for the environment.

A Level 7 nuclear accident involves “widespread health and environmental effects” and the “external release of a significant fraction of the reactor core inventory.” The INES scale, which was developed by the International Atomic Energy Agency and countries that use nuclear energy, leaves it to the nuclear agency of the country where the accident occurs to calculate a rating based on complicated criteria.

Some in the nuclear industry have been saying for weeks that the accident released large amounts of radiation, but Japanese officials had played down this possibility.

The government, however, came under pressure to raise the level at the plant after Japan’s nuclear safety commission estimated the amount of radioactive material released from its stricken reactors reached 10,000 terabecquerels per hour for several hours following the earthquake and tsunami that devastated the country’s northeast coast on 11 March. That level of radiation constitutes a major accident, according to the INES scale.

Hidehiko Nishiyama, deputy director general of Japan’s nuclear regulator, the Nuclear and Industrial Safety Agency (NISA) told reporters that these new estimated mean that

the total amount of radioactive materials released so far is equal to about 10 percent of that released in the Chernobyl accident – something environmental groups have asserted for nearly a month.

The French Institute for Radiation Protection and Nuclear Safety told Vladimir Slivyak, co-chair of the Russian environmental group Ecodefence, on March 18th that the radiation released up to that point was equal to one-tenth of that released by Chernobyl. According to Sliyak’s calculations in his  blog on the crisis, the Japanese radiation release, therefore, equal 5 million Curies.

So, Nishiyama’s admission that radiation releases from Fukushima Daiichi amount to 10 percent of that released from Chernobyl have been aloft for a number of weeks.

“[Raising the level on the INES scale] is another sign of the seriousness of the situation,” said Bellona physicist Nils Bøhmer. “Japanese officials have observed the radiation situation over the month and have assigned the accident the level it deserves.”

Tetsuo Iguchi, a professor in the department of quantum engineering at Nagoya University, agreed, saying: “The fact that we have now confirmed the world’s second-ever level 7 accident will have huge consequences for the global nuclear industry. It shows that current safety standards are woefully inadequate.”

Bøhmer said that it was curious that Japan had not come forth with a new rating sooner, however, saying the French and US officials have known what Japanese authorities have known for some time.

“It is strange that France and the US didn’t urge them to come forward – they have been sitting on this information for some time,” said Bøhmer. “Evidently they now believe it is time to issue full disclosure to the international comminute, and now the situation is even more serious than we believed it to be even yesterday.”

Bøhmer said that Edano’s approach to targeting specific communities in the 20 kilometre to 30 kilometre area surrounding the plant over the next month was a good approach.

“They are getting fuller information about which areas will be effected because of weather patterns, and hence have a better idea of who to evacuated” he said, adding that he expects even more evacuation orders to be issued soon.

NISA’ Nishiyama stressed that unlike at Chernobyl, where the reactor itself exploded and fire fanned the release of radioactive material, the containments at the four troubled reactors at Fukushima remained intact over all.

But these reactors have been rocked by several hydrogen explosions, a fire in reactor No 4 spend nuclear fuel storage pool and other spikes in radiation at reactor No 3, which burns uranium and plutonium oxide, or MOX fuel, to leaks of highly contaminated water into the ocean from reactor No 2. That leak was finally plugged last Wednesday.

This water measured 1000 millisiverts of radiation – 330 times the amount of radiation that someone living in an industrial country is exposed to over a year.

All of the conditions, said Bøhmer, amount to a serious core breach at reactor No 2 and 3.

Reactor Nos 1, 2, 3 and 4 have been inundated with water from water cannons, back up pumps and helicopters to bring temperatures of exposed fuel rods at reactor No 2 down. Reactor No 2 had, until last Wednesday, been gushing into the Pacific.

However the polymer plug installed in the leaking shaft at reactor No 2 was said by Bøhmer and other leading experts to be only a temporary fix, and that leaks from that reactor would be a continued headache.

Indeed, on the day the plug was fixed, technicians observed a rise in the water in the leaking shaft of 2 centimeters. Two days later, that rise had increased to 10 centimeters

Japan has also dumped into the sea some 11,500 tons of water used to cool the reactor in massive air and ground dumps to cool reactors and their spent nuclear fuel storage pools, but that water became contaminated as it passed through the wreckage of the reactors.

The bulk of the dumped water reported by Tokyo Electrical Power Co, (TEPCO) was said to have come from Fukushima Daiichi’s central waste treatment facility in order to make room to store far more radioactive water technicians hope to remove from reactor No 2.

Japan had yesterday promised to stop the dumping of irradiated water into the sea, as China and South Korean had raised protests. It is unclear if the water dumps, whose stoppage has delayed been since Friday, had actually ceased. Later in the afternoon yesterday, the powerful aftershocks rocked Fukushima Daiichi, which led to an hour-long cessation in reactor cooling operations.