The hallmarks of a Level 5 emergency are severe damage to a reactor core, release of large quantities of radiation with a high probability of "significant" public exposure or several deaths from radiation.

Japan’s nuclear safety agency raised the severity rating of the country’s nuclear crisis on Friday from Level 4 to Level 5 on a seven—level international scale, putting it on par with the Three Mile Island accident in Pennsylvania in 1979.

Ryohei Shiomi, a spokesman for the nuclear safety agency, said on Friday that the agency raised the rating of the Fukushima Dai—ichi nuclear crisis on the International Nuclear Event Scale. The scale defines a Level 4 incident as having local consequences and a Level 5 incident as having wider consequences.

The hallmarks of a Level 5 emergency are severe damage to a reactor core, release of large quantities of radiation with a high probability of “significant” public exposure or several deaths from radiation.

A partial meltdown at Three Mile Island also was ranked a Level 5. The Chernobyl accident of 1986, which killed at least 31 people with radiation sickness, raised long—term cancer rates, and spewed radiation for hundreds of miles (kilometers), was ranked a Level 7.

France’s Nuclear Safety Authority has been saying since Tuesday that the crisis in north-eastern Japan should be ranked Level 6 on the scale.

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