The decision reflects the entire publication of radiation on the damaged Fukushima Daiichi power plant, which is in progress, rather than a sudden deterioration.
Level seven previously only on the Chernobyl disaster in 1986, where emits 10 times more radiation was applied.
Japanese Prime Minister Naoto Kan said radiation leaks at the plant were declining.
To the Tokyo Electric Power Co. (Tepco), which would provide for operators of the plant, soon a schedule getting it under control, he said at a press conference.
"Step by step, the reactors at the power plant of Fukushima Daiichi move towards stability," he said.
There were no fatalities through the holes in Fukushima, and risks to human health are as low enough.
In the meantime a 6.0 earthquake on Tuesday asked the operator of the installation, your staff to spaces. There was no other damage.
The aftershocks come a month after a huge Quake and North-East Japan tsunami, so that missing 13,228 dead and 14,529. More than 150,000 people have been made homeless.
Effects of leakageThe nuclear safety Commission by Japan announced in a statement, increasing the crisis at the site of Fukushima Daiichi was, adding that it was a preliminary assessment the further technical evaluation of specialists required.
Continue reading the most important story
Richard Black environment correspondent of BBC NewsWhy the expansion? What does this, is that things in the work have worsened. Rather has Japan's nuclear and Agency for industrial (NISA) re-analysed data from the incident and decided to share the releases of radioactivity mean it seven in a level categorization slots.
Measured radioactivity is in Bequerels (Bq); a Terabequerel (TBq) is a million million.
Fukushima is event above the threshold to be considered as a level seven, although published an order of magnitude lower than the 5.2 million TBq from Chernobyl.
But telling that you nothing about the danger to people. Bequerels are a measure of the rate of radioactive decay - a nucleus per second.
On the other hand, Sieverts measure the expected medical effects of radiation which an individual is exposed to. And a variety of Bequerels not automatically translated into a huge amount of Sieverts.
The level seven means a "serious accident" with "larger follow" than the previous level, say officials."We the severity to seven have updated as the effects of radiation leaks from the air, vegetables, tap water and the ocean, has been widely used", said Minoru Oogoda of Japan's nuclear and industrial Agency (NISA), the Government of nuclear watchdog.
Reporting the decision of the Commission, said the IAEA previous level five reviews had separated in workplace accidents reactors 1, 2 and 3 was offered but now as a single event had been combined. Another affected unit, reactor 4, which has kept three rating level, it said.
An official of Tepco said that radiation leaks not completely stopped and not all can finally Chernobyl news agency Reuters reported.
But a nuclear safety said to Agency spokesman reporters that the leaks were still small in comparison in the plant in Ukraine, then part of the Soviet Union.
"In terms of volume of radioactive substances released, it shows our estimate is about 10% of Chernobyl, which appeared", he said.
The decision, which was to increase threat had estimated after radiation by up to 630,000 Terabequerels on the affected plants.
The main story level 7 read on: Chernobyl, Ukraine, 1986 - explosion and fire in the operating reactor, fallout over thousands of square kilometers, possible 4,000 cancer CasesLevel 7: Fukushima, 2011 - tsunami and earthquake may have caused damage of seismic activity on plant design. Long-term effects unknown level 6: Kyshtym, Russia, 1957 - explosion in which hundreds of cancer, contamination of hundreds of square KilometresLevel 5 waste water tank: Windscale, UK, 1957 - fire in the operating reactor, release of contamination of local area, possible 240 cancer CasesLevel 5: Three mile Iceland, United States, 1979 - instrument fault leads to major meltdown, the crisis on level seven on the international nuclear and radiological event scale (INES) would classify serious damage to reactor CoreThat.It was not clear when it reaches level. The level reported for less than a Terabequerel one hour, then discarded.
In comparison, the Japanese Government, which was 5.2 million Terabecquerels release from Chernobyl said.
These measurements must not necessarily no effect on the likely medical impact on people measured in Sieverts.
The severity of Japan's nuclear crisis had previously to five, same as, established in 1979 by the accident in three mile Iceland in the United States.
The Fukushima Daiichi nuclear power plant cooling systems have been damaged disaster last month and have workers fight to prevent several reactors from overheating.
Officials have warned that it several months before the situation at the nuclear facility is fully brought under control.
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