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 northeastern Japan should be ranked Level 6 on the scale.