The Japan nuclear disaster rates between Three Mile Island in Pennsylvania and Chernobyl.

THE GIST

The nuclear danger in Japan has been upgraded on the International Nuclear Event Scale.

The situation at Japan's Fukushima plant is now rated a Level 6, or a "serious accident."

The April 26, 1986 explosion at the Soviet nuclear power plant in Chernobyl -- rated a maximum Level 7 -- was the world's worst nuclear disaster.

The accident at Japan's Fukushima plant now rates a Level 6 on an international scale of gravity for nuclear accidents, the head of France's Nuclear Safety Authority (ASN) said Tuesday.

The 1979 accident at Three Mile Island in Pennsylvania rates Level 5 on an international scale of Level 0 to Level 7, while Chernobyl is put at Level 7, the highest.

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Japan's nuclear safety agency has estimated the accident at Fukushima at Level 4.

"The incident has taken on a completely different dimension compared to yesterday. It is clear that we are at Level 6," Andre-Claude Lacoste told a press conference.

"The order of gravity has changed," he said.

Level 3 indicates a "serious incident" according to the International Atomic Energy Agency (IAEA) scale, while Level 4 means there has been an "accident with local consequences."

The March 28, 1979 accident at Three Mile Island was a partial reactor meltdown that led to "very small" releases of radioactivity, according to the US Nuclear Regulatory Commission (NRC).

It caused no casualties, but was rated five on the scale, corresponding to "an accident with wider consequences."

Level 6 is a "serious accident".

The April 26, 1986 explosion at the Soviet nuclear power plant in Chernobyl -- rated a maximum Level 7 -- was the world's worst nuclear disaster.

Unleashed by an unauthorized technical experiment, it spewed radioactive dust over swathes of Ukraine, Belarus, Russia and western Europe. The death toll ranges from a UN 2005 estimate of 4,000 to tens or even hundreds of thousands, proposed by non-governmental groups.