It was the worst disaster in post-war Japanese history. Four years ago, a massive 9.3-magnitude undersea earthquake erupted off the coast of the Tohoku region, triggering a tsunami that devastated the northeastern coast of Japan, taking the lives of at least 15,880 people and leaving another 2,694 missing. Some 6,135 people were injured.

But the natural disaster turned into a manmade one when a 13-meter (43-foot) tsunami hit the Fukushima Daiichi nuclear power plant. The plant's cooling systems failed, leading to overheating in three reactor cores, and radiation leaks. About 20,000 people were evacuated, while some 80,000 additional cancer cases could surface due to radiation exposure. Cleanup could take 30 years.

The Fukushima disaster was not without precedent. In 1979, the Three Mile Island nuclear plant near Middletown, Pennsylvania, underwent a partial nuclear meltdown. Feedwater pumps stopped sending water to the steam generator that cooled the reactor core, and a malfunctioning valve allowed cooling water to pour out. Some 140,000 children and pregnant women were evacuated from the area.

Until Fukushima, the Chernobyl disaster was the worst nuclear accident in history. In 1986, a sudden power surge at Unit 4 of the Chernobyl plant in Ukraine destroyed the reactor, releasing a radioactive cloud that spread over Russia and Europe. A 30-kilometer area was sealed off and some 335,000 people were evacuated, while at least 30 people died as a result.

Finishing touches are being completed at the Watts Bar Unit 2 plant in Tennessee, after a long delay due to low regional power demand. Its sister plant, Watts Bar Unit 1, was the latest nuclear plant to go online in the United States, in 1996. Further new nuclear plants are planned for the US - which sees atomic energy as a viable alternative to fossil fuels.

Even in Germany, with its strong anti-nuclear movement, Chancellor Angela Merkel's center-right coalition originally sought to delay the country's nuclear phase-out date from 2022 to 2034. The 2022 goal had been set by Merkel's center-left predecessor, Chancellor Gerhard Schröder. Merkel's coalition justified the delay by claiming that it was a buffer in the transition toward renewables.

But after the Fukushima disaster in March 2011, Berlin reacted quickly by permanently shutting down eight nuclear plants. Merkel's coalition then decided to completely phase out atomic power by 2022, readopting the date originally set by Schröder. Germany has set a goal of 80 percent renewables by 2050 - the country recently reached the 27 percent mark in renewable energy production.

Like Germany, Italy also has a long history of anti-nuclear activism. After the Chernobyl disaster, Italians voted in 1987 to ban nuclear power. But in 2011, then-Prime Minister Silvio Berlusconi sought to reintroduce atomic energy. The question was put to Italians in a referendum, who again voted down atomic energy.

In the United Kingdom, the Conservative-Liberal coalition is also seeking to promote nuclear to reduce greenhouse gas emissions. But the approved Hinkley Point C plant in Somerset (pictured above) - which would be the newest since 1996 - faces a legal challenge over use of state aid for construction. A recent estimate put the total cost of Hinkley C at 24.5 billion pounds (34.4 billion euros).

New Dehli is planning to quadruple its nuclear capacity by 2020, relying on atomic energy to supply 25 percent of its electricity. But the plan has sparked fierce opposition. Demonstrators have repeatedly interrupted construction work at the Kudankulam Nuclear Power Plant, which was built with Russian support. Russia has also offered to build a dozen more nuclear power reactors in India.

Beijing is seeking a more modest rise in its nuclear capacity. The People's Republic plans to generate 6 percent of its electricity through atomic power by 2020, compared to just 2 percent currently. The Changjiang Nuclear Power Plant, pictured above, is currently under construction in Hainan province. Nuclear power could reduce China's dependence on heavily polluting coal plants.

France depends on atomic energy for 75 percent of its electricity. Although President Francois Hollande had pledged to reduced the country's dependency on nuclear, only one power station - an aging plant on the German border (pictured above, with protest banner) - has been earmarked for closure. In the meantime, France's 20 nuclear reactors continue to hum away - at Europe's core.