World's worst nuclear accidents

The most serious nuclear accident in United States history occurred at Three Mile Island in Pennsylvania on March 28, 1979. About 140,000 people were forced to evacuate their homes after a combination of equipment malfunctions, problems with the design of the reactor and human error led to a partial meltdown of the TMI 2 reactor core.

Although the partial meltdown resulted in some contamination within the plant, nobody died or got injured. However, the other casualty was the nuclear energy industry itself. The accident stirred public protest and subsequently led the U.S. Nuclear Regulatory Commission to tighten its regulatory oversight over the industry. It also contributed to a freeze in the construction of new nuclear plants in the U.S. over the next three decades.

On 10 October 1957, an undetermined amount of contamination got released into the air after the graphite core of Britain's first nuclear reactor, a site at Windscale, Cumberland, caught fire. The incident, known as the Windscale fire, was the worst nuclear accident in England's history. Five decades later, medical researchers reported mortality and cancer rates for workers involved in the 1957 fire "does not reveal any measurable effect of the fire upon their health." (You can read the report of a British board of inquiry into the accident here. The former Windscale power station was shut down and decommissioned.

In this Nov. 10, 2000 file photo, the control room with its damaged machinery is seen inside reactor No. 4 in the Chernobyl nuclear power plant. Geiger counters registered about 80,000 microroentgens an hour, 16,000 times the safe limit. Reactor No. 4 at the Chernobyl nuclear power plant in Ukraine, then part of the Soviet Union, exploded on April 26, 1986, sending a cloud of radiation over much of Europe.

Around 200 people died after a ruptured reactor vessel and fire caused an explosion, resulting in the release into the environment of radioactive materials.

Researchers tracking an increase in the number of thyroid cancer cases in the region believe the rise is related to the Chernobyl accident. However, the long-term health impact on local residents is still unclear and experts believe it will take years to quantify.

A fire and subsequent cover-up that sparked public anger led to a 14-year shutdown of the Monju Prototype Fast Breeder Reactor in Tsuruga, Fukui prefecture, west of Tokyo. Some 278 people were affected by four successive leaks of radioactivity. The leaks, which also led to the widespread evacuation of local residents, resulted in fall-out levels equivalent to that of more than 200 atomic bombs of the type dropped on Hiroshima at the end of World War 2. The official charged with investigating the cover-up would later commit suicide by jumping from the roof of a Tokyo hotel. He was not involved in the cover-up, but reports at the time said he was distressed by the evidence that came across his desk.

In April 1993, the Soviets reported an explosion at a secret nuclear reprocessing plant in Siberia near the town of Tomsk. The reactor was believed to have been involved in the production of weapons-grade nuclear fuel but authorities kept a tight lid on the release of information about the accident. The number of casualties is unknown. Despite the end of the Cold War, the area remains closed to non-residents, and visitors must present entry documents at checkpoints, such as the one in the accompanying photo.

The Japanese town of Tokaimura was the scene of the world's worst nuclear accident since the 1986 Chernobyl reactor explosion. On September 30, 1999, an accident at the uranium processing plant killed two workers and more than 600 people got exposed to moderate levels of radiation. A subsequent investigation turned up evidence of fraud and safety lapses.

Will this one be next? The Fukushima Daiichi power plant's Unit 1 is seen in Okumamachi, Fukushima prefecture, Japan, on March 11, 2011. In the aftermath of a massive earthquake that devastated Japan, the plant suffered an explosion that led to the release of large amounts of radiation into the atmosphere and the evacuation of residents with a 12-mile radius. The quake was reported to have interrupted the plant's cooling system. That led to a buildup of pressure to the point that it blew out the concrete walls surrounding the plant's nuclear reactor. In the immediate aftermath of the explosion, officials described the release of radioactive matter as small. Officials confirmed three people were exposed to radiation, but said they showed no ill effects.