5 Worst Nuclear Disasters in the World

The catastrophic failure of the Unit 4 reactor at the nuclear power plant in Chernobyl was by far the worst nuclear disaster in history. In fact, Chernobyl remains the only accident at a commercial nuclear power facility to result in deaths from acute radiation poisoning.

The disaster at Chernobyl was the result of human error and a faulty reactor design. The plant's operators decided to run a safety test while the Unit 4 reactor was temporarily offline. The test was supposed to determine if the reactor's cooling system could operate during a loss of electrical power. Instead, poor communication resulted in a sudden power surge that caused a massive explosion and fire at the reactor.

The 30 deaths at Chernobyl included two workers killed in the explosion and 28 fire fighters and emergency responders who were exposed to massive levels of radiation (one died of a heart attack). Wind-blown plumes of radiation from the fire were detected across the Northern Hemisphere in the weeks that followed.

Hundreds of thousands of people were evacuated from the region surrounding the damaged plant, known as the "Exclusion Zone." It's been estimated that 7,000 children and adolescents exposed to radiation from Chernobyl had developed thyroid cancer over the past 35 years. Fortunately, most were not fatal.

On March 11, 2011, a 9.0-magnitude earthquake off the northeastern shore of Japan triggered a deadly tsunami with waves reaching heights of nearly 100 feet (30 meters). The devastating event, known as the Tohoku Earthquake and Tsunami, killed more than 19,000 people in Japanese coastal cities and towns.

The Fukushima Daiichi nuclear power plant was directly in the path of the killer waves and its backup generators were destroyed by the tsunami. Without power, the plant's cooling systems were inoperable, causing overheating and meltdowns in three of the plant's four reactors. There were also hydrogen explosions at the plant.

Fearing a massive radiation leak, Japanese authorities implemented a forced evacuation of 160,000 in the immediate area, including frail and elderly individuals in hospitals and nursing homes. The rushed evacuation has been blamed for more than 2,300 deaths.

Thankfully, no deaths or increased cancer diagnoses have been directly attributed to radiation exposure from the Fukushima Daiichi disaster, according to the U.N. Scientific Committee on the Effects of Atomic Radiation.

This explosion at an experimental nuclear power reactor in Idaho remains the only fatal nuclear disaster in American history.

The SL-1 reactor was one of 52 small reactors being tested by the military as electrical generators in this remote part of Idaho. On Jan. 3, the three engineers who operated the SL-1 reactor returned to work after an 11-day shutdown. While attempting to restart the reactor, 22-year-old Army Specialist John A. Byrnes accidentally pulled out a control rod too far, causing the reactor to surge to more than 6,000 times its rated power level.

The heat generated by the power surge instantly vaporized the water in the generator and created a steam explosion that lifted the entire reactor 9 feet (3 meters) off the ground. Sadly, the explosion killed all three men on duty. The victims, in addition to Byrnes, were Navy Seabee Richard C. Legg and Army Specialist Richard Leroy McKinley. The three men were buried in lead-lined caskets to avoid radiation contamination.

To recover the men's bodies and clean up the site, 790 workers were exposed to radiation, but no adverse health effects were reported. And given the remote setting, the public was also spared.

After World War II, the British government strove to join the ranks of the United States and the U.S.S.R. as nuclear powers, so the U.K. began constructing facilities to build its own atomic bombs. One of those was a nuclear reactor called Windscale for manufacturing weapons-grade plutonium.

Since this was a military reactor, the public was largely ignorant of what was happening at Windscale, which was why no evacuation was ordered in October 1957 when a reactor failure triggered a massive fire that burned for 16 hours. Luckily, prevailing winds blew most of the radiation out to the Irish Sea rather than inland toward populated areas.

In the wake of the accident, the only safety precaution taken by the British government was to monitor radiation levels in local milk and throw away batches that were contaminated. Given the amount of radiation released during the fire, scientists estimate that 240 U.K. cancer deaths should be attributed to the disaster, sometimes called "Britain's Chernobyl."

The near-meltdown at Three Mile Island is the most notorious nuclear incident in American history, although thankfully there were no deaths and no adverse health effects from radiation exposure. The psychological damage was significant, though, and the narrowly averted disaster at Three Mile Island set back support for nuclear power in the U.S. for decades.

The accident at Three Mile Island was caused by both mechanical failures and human error. It started with an electrical failure at 4 a.m. that prevented proper cooling of the reactor core. When the core began to overheat, a pressure release valve automatically opened, but then failed to close. Not only was radioactive steam being released into the air, but there wasn't enough coolant in the system.

Because of faulty and misleading indicator lights, plant operators didn't realize any of this was happening. And once they did, they took steps that only compounded the problems. At one point, the reactor core reached 4,300 degrees Fahrenheit (2,371 C) dangerously close to a full meltdown.

As news of the unfolding incident leaked to the press, the Pennsylvania governor advised all "pregnant women and school-age children" to evacuate the area within a 5-mile (8-kilometer) radius of Three Mile Island. Thousands of panic-stricken residents fled the greater Harrisburg area fearing a nuclear disaster.

Ultimately, plant operators averted a meltdown and no significant amount of radiation was released into the air. Decades of studies and reports have confirmed that nearby residents were not exposed to dangerous levels of radiation in the air or through contaminated food.