The 19 worst nuclear accidents

The International Atomic Energy Agency ranks accidents and on a scale of zero to seven, with seven being the most severe.

Dec. 12, 1952

Chalk River Ontario, Canada A shutoff rod fails to stop the reaction. Built-up hydrogen gas explodes, damaging the reactor core. Among those who work to clean up the facility: U.S. Navy Lt. Jimmy Carter. Rating: 5

March 25, 1955

Sellafield Seascale, England Highly radioactive liquid is spilled during transport to a military nuclear facility. Several workers are contaminated. Rating: 5

Sept. 29, 1957

Kyshtym Mayak, Russia Cooling system fails at a military nuclear waste processing facility. An explosion blasts 80 tons of radioactive material into the surrounding area, contaminating 310 square miles. Rating: 6

Oct. 10, 1957

Windscale Seascale, England A Plutonium production reactor catches fire and burns for three days. Radioactive iodine is spread across the U.K. and northern Europe. Rating: 5

Jan. 3, 1961

SL-1 Idaho Falls, Idaho Operators of an experimental military reactor mistakenly withdraw a central control rod, allowing the reactor to melt down and causing a steam explosion. All three are killed. Rating: 4

May 1968

Sellafield Seascale, England Radioactive material is released into the air when a filter is accidentally bypassed. The incident is discovered later during a review of data. Rating: 5

Jan. 21, 1969

Lucens Switzerland Fuel elements in the reactor core become corroded, blocking the flow of coolant. The fuel catches fire and coolant leaks from the reactor, contaminating the containment building. Rating: 5

Oct. 12, 1969

Sellafield Seascale, England Radioactive material is released into the air. Rating: 5

Oct. 17, 1969

SaintLaurent France The day after a new reactor is started up, a combination of technical glitches and human error cause a loss of coolant. Two fuel elements melt down. Rating: 4

Sept. 26, 1973

Sellafield Seascale, England Waste zirconium reacts with solvents in an area where oxide fuels are reprocessed. The release of radiation exposes 35 workers. Rating: 5

Feb. 22, 1977

KS 150 Jaslovské Bohunice, Czechoslovakia An operator changing fuel rods fails to remove silica gel packs that keep the fuel dry during shipping. The reactor overheats, filling the containment building with radioactive carbon dioxide. Rating: 4

March 28, 1979

Three Mile Island Harrisburg, Pa. Operators fail to notice a coolant valve has stuck open. The reactor core partially melts down but a feared explosion of built-up hydrogen gas never happens. Rating: 5

Sept. 11, 1979

Sellafield Seascale, England A tank overflows during a routine transfer of radioactive liquid. Plutonium is released into the atmosphere. Rating: 5

March 13, 1980

Saint-Laurent France A plate protecting instruments becomes stuck in a channel that carries coolant to the reactor core. Three fuel elements melt down in the sister reactor to the one involved in the 1969 incident. Rating: 4

Sept. 23, 1983

RA-2 Buenos Aires, Argentina While rearranging fuel rods in a research reactor, an inexperienced operator causes the reactor to go critical. He dies two days later and eight more employees are irradiated. Rating: 4

April 26, 1986

Chernobyl Present-day Ukraine A routine test goes wrong when the reactor core overheats and blows off the top of its containment structure. Radioactive dust was blown over much of Europe. 28 died from radiation poisoning. Rating: 7

Sept. 13, 1987

Goiânia Brazil Radioactive cesium chloride used in radiotherapy is left in an abandoned hospital. Scavengers steal the container, hoping to sell it as scrap. At least 249 people are irradiated and four die, including the six-year-old niece of the thief. Rating: 5

Sept. 30, 1999

Tokaimura Tokai, Japan Inexperienced operators at a uranium reprocessing facility accidentally cause a chain reaction in a mixing tank. Thirty-nine employees are irradiated and two die of their injuries a few weeks later. Rating: 4

March 11, 2011

Fukushima Daiichi Okuma, Japan A tsunami knocks backup equipment offline. Three of the plant’s six reactors melt down, releasing radioactive material into the surrounding area. Residents within a 6.2-mile radius are evacuated. Rating: 7