How bad is disaster? Chernobyl was worse

The International Atomic Energy Agency (IAEA) on Tuesday rated the seriousness of the nuclear emergency at the Fukushima Daiichi reactors as 6 on a scale of 7.

Three Mile Island was rated as 5/7, while Chernobyl was rated 7/7.

Here's a comparison of the three nuclear emergencies:

Chernobyl

Where: Near Kiev, Ukraine

When: April 26, 1986

Scale of accident: 7 out of 7

Cause: Human error. Workers conducting a test had switched off important control systems at the plant's reactor 4 and allowed it to reach unstable, low-power conditions, according to a United Nations report.

What happened: Explosions blew the roof off the reactor and vaporized its radioactive fuel, unleashing a radiation cloud that swept across Europe and around the world. The blasts occurred while the reactor was operating at full temperature, adding energy to the explosion. Far more radioactive material was blasted, and dangerous levels were found as far as 600 miles away.

Casualties: Estimates vary widely. While fewer than 100 deaths were directly attributed to the disaster as of 2009, Ukraine's health ministry estimated 3.5 million people have suffered some illness as a result of the contamination, and the Ukraine Radiological Institute suggested there were 2,500 deaths. Some environmental groups say up to 500,000 people have died from the effects of radiation.

Government reaction: The disaster was not revealed by Soviet authorities until two days later, after nuclear technicians in Sweden discovered the radiation.

Aftermath: Officials say it could be up to 100 years before the station is completely decommissioned. A temporary "sarcophagus" covering the reactor is showing signs of decay. A 19-mile exclusion zone is in place round the site.

Three Mile Island

Where: near Harrisburg, Pa.

When: March 28, 1979

Scale of accident: 5 out of 7

Cause: A combination of personnel error, design deficiencies and component failures.

What happened: Operators trying to unclog some piping in the steam-generating water circulation system accidentally blocked the flow of water, causing heat to build in the reactor. A gradual loss of water to cool the reactor's core led to partial melting of the fuel-rod cladding and the uranium fuel, and a small amount of radioactive gas was released.

Casualties: No deaths or illnesses.

Government reaction: Sweeping changes were made, including more emergency-response planning, increased operator training and heightened regulatory oversight.

Fukushima Daiichi

Where: Japan (135 miles north of Tokyo)

When: March 11, 2011

Scale of accident: 6 out of 7

Cause: Earthquake and tsunami cut power to the plant, crippling cooling systems needed to keep nuclear fuel from melting down.

What happened: Three reactor units suffered explosions. Some radiation was released, and officials believe a partial meltdown has occurred in at least two units.

Casualties: Unknown.

Government reaction: About 185,000 residents within a 12-mile radius of the plant were told to evacuate the area, and about 140,000 residents living from 12 to 19 miles from the plant were warned to seal themselves indoors.

Sources: International Atomic Energy Agency, BBC, Time and Seattle Times news services