Biggest nuclear disasters
Biggest nuclear accidents
Today marks the 25th anniversary of the nuclear accident at Chernobyl. In light of that, and the story continuing to unfold at Japan's Fukishima plant, we'll look at three of the worst nuclear plant disasters. First, to give some context, here's how the International Nuclear and Radiological Event Scale (INES) classifies each type of event:

Level 7 disaster
Major release of radioactive material; widespread health and environmental effects

Level 6 accident
Significant release; requires planned countermeasures

Level 5 accident
Limited release; several deaths from radiation

Level 4 accident
Minor release

Level 3 incident
Exposure in excess of 10 times the annual limit for workers; possible burns from exposure

Level 2 incident
Exposure of statutory annual limits

Level 1: Anomaly

Date: April 26, 1986

Level: 7 on INES

Comparison: TheChernobyl plant accident (map it) released significantly more radiation into the atmosphere than the Hiroshima bombing during World War II. (How much more radiation?) How does that compare with the Fukishima event?

What happened:

* Plant operators carrying out technical experiment switched off key safety systems
* Explosion blew off the reactor's 2,000-ton steel and concrete lid
* No secondary containment vessel
* Casualties
* Videos of the disaster


* Reactor later entombed (see photos); since has developed cracks and is being replaced by new structure
* An 18-mile (30-km) exclusion zone remains in force (map)
* You can visit the Chernobyl Museum in Kiev; tours are also offered inside the exclusion zone

Note: At the time of the accident, Chernobyl was located in the Soviet Union
Three Mile Island
Date: March 28, 1979

Level: 5 on INES

Comparison: Three Mile Island (map it) is said to have released a very low level of radiation. Some figures say the average exposure was equivalent to a common medical procedure (which one?).

What happened:

* Pump failed to circulate cooling water
* Efforts to ease pressure on reactor backfired when an open valve caused more coolant to pour out
* Rising temperatures caused hydrogen explosion in reactor building; 45 percent of the nuclear fuel rods melted
* After more than two hours, backup valve was shut, stopping leak; later reactor pump turned back on, temperature lowered
* Casualties – none
* Watch videos about the event


* One of the plant's reactors was permanently shut down; the other is licensed to operate well into the future
Date: March 11, 2011

Level: Initially 4 on INES, upgraded to level 7 on April 12

Comparison: According to Japanese officials, the radiation levels near the plant (map it) rose to roughly twice those that constitute an emergency situation.

What happened:

* 9.0-magnitude earthquake and following tsunami  caused three of the six reactors at Fukushima plant to shut down
* Tsunami destroyed backup diesel generators, cut off pumps that circulate coolant around reactor cores
* Reactors overheated, causing partial meltdowns; explosions have damaged outer buildings
* Casualties


* The latest on the disaster
* The evacuation zone has been recently extended