For many people, the meltdown in Japan was inconceiveable. "Before the disaster, I believed that no serious nuclear accident could take place in Japan as our technology was very advanced," then-Prime Minister of Japan, Naoto Kan, told DW in an exclusive interview. "Such colossal damage usually occurs only after a crushing defeat in war."

But the nuclear disaster could have been even worse. In addition to the nuclear meltdowns in reactors 1 to 3, and the hydrogen explosions in reactors 1 to 4, reactor 4 was also in danger of causing an even greater release of radioactivity.

The holding basin for the radioactive fuel rods was located on the upper floors of the reactor. The hydrogen explosions and the earthquake threatened the tank's stability. Moreover, active fuel rods had been stacked to cool at reactor 4.

If the tank had collapsed and the fuel rods had caught fire due to a lack of cooling water, the radioactive contamination could have been 10 times greater.

The Japanese government had considered evacuating the greater area, to a radius of 250 kilometers. That would have included the entire metropolitan area of Tokyo. As Kan said, they would have had to evacuate 50 million people.

213 mikrosieverts measured near Fukushima reactors 2 and 3 mean that short visits are possible here

580 Hiroshimas

Radioactive contamination was significant in the area - and later in other places around the world - after the disaster.

According to calculations by Christian Küppers, head of nuclear safety at the German Öko-Institut, the quantity of radioactive cesium-137 released in the Fukushima accident was about 580 times the amount than the single in the atomic bomb dropped in Hiroshima.

A large part of the fission product cesium-137 entered into the atmosphere, another part of it went directly to the sea.

Cesium-137 has a half-life of about 30 years, and since it is a salt, it is water soluble. As it enters the food chain, it becomes deposited in living tissues. According to Küppers, Fukushima made up 5 percent of radioactive contamination in oceans in 2015.

Above-ground nuclear testing accounts for the most radioactive contamination over the last century - that amounts to 86 percent. According to Küppers' calculations, 30 years after the nuclear disaster, Chernobyl - the biggest nuclear accident in history - is responsible for 3 percent of cesium-137 on the oceans. Radioactive waste from nuclear fuel manufacturing accounts for the remaining 6 percent.

At reactor 3, which remains in ruins, radioactivity is still too high for people

Continued lack of knowledge

According to the energy company Tepco, the power plant area has been "stabilized." Decommissioning work has been completed "to around 10 percent," said Tepco's manager Akira Ono.

Ono estimates the complete dismantling will take around 30 or 40 years. Other experts calculate that it will take longer. "The consequences will last for centuries," said Heinz Smital, a nuclear physicist with Greenpeace.

On the positive side, the fuel rods were successfully relocated out of the holding basin in one of the reactors - that process was completed in 2014. Had this failed, a new big nuclear disaster could have occurred at Fukushima.

In upcoming years, experts want to implement the same procedure in the three other reactors. A medium-intensity earthquake could damage the holding basins and lead to further release of radioactive material.

The melted nuclear fuel and the destroyed pressure vessel in the nuclear reactors 1 to 3 continue to be major problems. Field control has not been possible due to extremely high radioactivity for people, and the use of robots has not yet been successful.

"So far, nobody knows what exactly happened in there and how to solve it," Smital told DW. "Until now, there is no solution to recover the melted fuel rods from the reactors."

"There is a lack of global knowledge on how to handle such a disaster," Smital concluded.

Tanks holding radioactive water sit in front of damaged reactors

Radioactive ocean waters

Around 7,000 employees from Tepco are working on the nuclear plant area, in addition to other contractors. Over the last five years, much of the debris has been removed, the destroyed reactor building has been partly stabilized, and new coverings have been built.

The radioactive contamination of water continues to be a problem. Several hundred tons of groundwater flow through the area every day, entering the basement of the destroyed reactors. There, it gets mixed with the water that Tepco uses to cool the hot reactors.

Part of the radioactive material will be separated from the water - which continues to be radioactive. But the storage of nearly 800,000 tons of water in the tanks faces space crunch.

To reduce the quantity of water, according to own data Tepco has disposed of more than 50 tons of radioactively contaminated water into the sea since September 2015, and further releases are planned for the future.

In order to slow the flow of groundwater through the contaminated areas, Tepco intends to erect an underground wall of ice - but this was supposed to have been completed one year ago.

Currently, the energy company is awaiting government approval to begin the ice construction. For now, 1,500 tubes have been inserted into the earth.

Whether the ice wall will solve the contaminated water problem or not is still unclear. Effectiveness of this costly measure is controversial.

Contaminated earth is stored in sacks - as these break apart, the contamination can spread

Lessons learned

New earthquakes and tsunamis represent a risk for the Fukushima nuclear plant, said Ono. Tepco emphasized in an interview with DW that to avoid accidents in reactors still functioning in Japan, it has implemented new security measures and learned lessons from the nuclear accident.

"Multiple layers of safety and protection against earthquake and tsunamis, as well as against loss of power or cooling functions, has been installed," wrote Tepco in a message to DW.

Former Japan Prime Minister Kan draws a different conclusion: "The accident radically changed my perspective. I now consider nuclear energy to be the most dangerous form of energy, and the risks associated with it are too great for us to continue generating atomic power."

Experts warn that an accident at an existing plant can have 10 or even 100 times greater consequences than Fukushima.

"The big risk comes from the still-functioning reactors," Smital said. In light of ongoing problems with existing nuclear facilities in Europe, "a serious accident can also be in the offing."

"All reactors still operating are a potential risk," Smital concluded.