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Nuclear power has something of a checkered history. Although it does provide carbon-free energy at reasonable prices, it has exposed its dangerous side with near meltdowns and leaked radiation. The International Atomic Energy Agency -- an intergovernmental agency for scientific co-operation in the nuclear field -- judges nuclear accidents based on the International Nuclear Event Scale, which ranges from 1 to 7. The most serious events are classified as a 7, referred to as a "major accident," while a 1 is considered a minor "anomoly."

"It's very important that we have an effective way to communicate with the public the seriousness of a nuclear accident," Nuclear Regulatory Commission spokesman Scott Burnell told Discovery News.

That rating system, along with reports and information from the regulatory commission and the United States Department of Energy, helped develop this list of the five most dangerous nuclear accidents in the world.

Chernobyl, Soviet Union (now Ukraine)
April 26, 1986
INES Rating: 7
Image credit: Wikimedia Commons

The Chernobyl nuclear accident is widely regarded as the worst accident in the history of nuclear power. It is the only nuclear accident that has been classified a "major accident" by the International Atomic Energy Agency.

During a routine test, the plant's safety systems were turned off to prevent any interruptions of power to the reactor. The reactor was supposed to be powered down to 25 percent of capacity, but this is when the problems began. The reactor's power fell to less than one percent, and so the power had to be slowly increased to 25 percent. Just a few seconds after facility operators began the test, however, the power surged unexpectedly and the reactor's emergency shutdown failed. What followed was a full-blown nuclear meltdown.

The reactor's fuel elements ruptured and there was a violent explosion. The fuel rods melted after reaching a temperature over 3,600 degrees Fahrenheit. The graphite covering the reactor then ignited and burned for over a week, spewing huge amounts of radiation into the environment.

About 200,000 people had to be permanently relocated after the disaster. IAEA reported in 2005 that 56 deaths could be linked directly to the accident. Forty-seven of those were plant workers and nine were children who died of thyroid cancer. The report went on to estimate that up to 4,000 people may die from long-term diseases related to the accident. Those numbers are a subject of debate, however, as the Soviet Union did much to cover up the extent of the damage. The World Health Organization reported the actual number of deaths related to Chernobyl was about 9,000.

Kyshtym, Soviet Union (now Russia)
Sept. 29, 1957
INES Rating: 6

The Soviet Union was also home to the second-most disastrous nuclear accident, at the Mayak Nuclear Power Plant near the city of Kyshtym. IAEA classified the event as a Level 6 Disaster, which is a "serious accident."

Soviet scientists were frantically trying to catch up to the Americans after World War II when they began construction of the Mayak nuclear facility. Soviet nuclear knowledge had many holes, so it was impossible to know whether some decisions made in the construction were safe. As it turned out, many of those decisions seriously compromised the plant's facility.

Initially, the plant's operators simply dumped the nuclear waste into a nearby river, before a storage facility for that waste opened in 1953. The storage facility began to overheat, and a cooler was soon added, but it was poorly constructed.

In September 1957, the cooling system in a tank containing about 70 tons of radioactive waste failed, and the temperature started to rise. This caused a non-nuclear explosion of dried waste. There were no immediate casualties as a result of the explosion, but the IAEA found there had been a significant release of radioactive material into the environment. The radioactive cloud spread out for hundreds of miles to the northeast.

The Soviet government released little information about the accident, but was forced to evacuate 10,000 people in the affected area after reports surfaced of people's skin literally falling off. The radiation is estimated to have directly caused the deaths of 200 people due to cancer.

Windscale Fire, Great Britain
Oct. 10, 1957
INES Rating: 5
Image credit: Getty Images

Great Britain's first foray into nuclear energy had been successful for several years before the Windscale fire occurred in 1957. Operators noticed that the reactor's temperature was steadily rising when it should have been decreasing. They originally suspected the equipment was malfunctioning, so two plant workers went to inspect the reactor. When they reached the reactor, they discovered it was engulfed in flames.

At first, they did not use water, because plant operators were worried the flames were so hot the water would break down instantly, and the hydrogen in the water would cause an explosion. But their other methods to put out the fire did not work, and so they turned on the hoses. The water was able to put the fire out without an explosion.

It is estimated that 200 people in Britain developed cancer because of Windscale, half of them fatal. The exact number of fatalities is hard to come by because the British government attempted to cover up how serious the fire had been. Prime Minister Harold Macmillan worried the incident would embarrass the British government and erode public support for nuclear projects. It's also difficult to put an exact number on the deaths because radiation from Windscale spread hundreds of miles across northern Europe.

Three Mile Island, United States
March 28, 1979
INES Rating: 5
Image credit: Wikimedia Commons

The United States' most disastrous nuclear accident took place at the Three Mile Island Plant near Harrisburg, Penn., the state's capitol.

It all began with a simple plumbing break down. A small valve opened to relieve pressure in the reactor, but it malfunctioned and failed to close. This caused cooling water to drain, and the core began to overheat. The machines monitoring conditions inside the nuclear core provided false information, so plant operators shut down the very emergency water that would have cooled the nuclear core and solved the problem. The core began to overheat, and reached 4,300 degrees Fahrenheit. The water nearly reached the fuel rods, which would have caused a full meltdown of the core. But the nuclear plant's designers were finally able to reach the plant operators several hours later to instruct them to turn the water back on, and conditions stabilized.

"Not only were there issues with training of operators, but management for both the plant and NRC did not know how to approach this kind of emergency and to communicate with the public," said Burnell.
The NRC determined that no one had died of causes related to the incident at Three Mile Island, but found there might be one excessive cancer death over a 30-year period as a result of radiation. Only one person outside of the nuclear plant was found to have any radiation in his system after the incident.

Three Mile Island had a profound impact on the public's attitude toward nuclear energy. In the 30 years since Three Mile Island, not a single nuclear power plant has been approved for development.

Tokaimura, Japan
Sept. 30, 1999
INES Rating: 4

Japan's most disastrous nuclear accident took place over a decade ago just outside Tokyo.

A batch of highly-enriched uranium was prepared for a nuclear reactor that had not been used in more than three years. The operators had not been trained in how to handle uranium that was so highly enriched. They put far more uranium into the solution in a precipitation tank than is allowed. The tank was not designed for this type of uranium.

Only when the tank was drained of the solution could the critical reaction be stopped, but by then, it was too late for two of the three operators working with the uranium, as they died of radiation.

Less than a hundred workers and people who lived nearby were hospitalized for exposure to radiation, and 161 people who lived within 1,000 feet of the plant were evacuated, according to the World Nuclear Association.

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