A filter mask covering the mouth and nose, a headscarf, a helmet, gloves and two layers of socks – they constitute the protective gear that must be worn by any ordinary visitor to the Fukushima nuclear power station.

Only a few workers now have to wear face masks and hazmat suits, since most of the ground at the site has been sealed with concrete.

"The radiation is now as low as in the Tokyo's Ginza shopping district," Tokyo Electric Power Co. (TEPCO) manager Yuichi Okamura assured a group of journalists during their recent visit to the plant.

But the illusion of normality evaporates as soon as the visitors get off their bus and stand within sight of the reactors, with dosimeters indicating radiation levels of around 160 to 170 microsieverts per hour - nearly 2,000 times above what is considered safe.

"We cannot stay here for long," warns Okamura.

On the surface, it appears that much has changed in Fukushima since the disaster struck six years ago. The clean-up work has evidently made progress.

But the sight of skeletal steel frames, torn walls and broken pipes immediately reminds one of the 17-meter-high tsunami which flooded the facility six years ago and brought its reactors to a complete standstill.

It's expected to take 30 to 40 years to completely clean up the Fukushima Daiichi plant, which was hit by the world's worst nuclear disaster since Chernobyl following a magnitude-9 earthquake and the subsequent tsunami. The operation is likely to carry a hefty price tag, with Japanese officials recently estimating it to cost around $189 billion in total.

Today, with 6,000 workers employed, the nuclear power plant is Japan's largest and most expensive construction site - and it will remain so for decades. "We're struggling with four problems," says TEPCO manager Okamura: "Reducing the radiation at the site, stopping the influx of groundwater, retrieving the spent fuel rods and removing the molten nuclear fuel."

Black lumps in the reactor containment

Progress in these areas, however, is slow. For instance, workers are erecting scaffolding around the collapsed roof of reactor No 1, but it will likely take four more years for the debris there to be cleared away. Only then can the almost 400 old fuel rods be retrieved from the reactor's holding basin.

In the adjacent reactor No 2, the blue exterior still remains intact. Workers in hazmat suits can be seen walking on a new metal platform halfway up the reactor building. But behind the wall lies a nuclear nightmare. A robot sent into the reactor in January found highly dangerous black lumps of leaked fuel on a platform in the outer reactor containment.

"There is now fatally high radiation in that part," says Okamura.

The engineer quickly turns to reactor No 3, where the progress is more obvious. A hydrogen explosion had turned the reactor's roof into a tangle of bent metal. It took years of work to dismantle this steel scrap and remove the rubble. "Now we're building a new roof with an integrated hoisting crane," says Okamura proudly.

"From next year, we would finally be able to close in on the nearly 600 burnt fuel rods," he noted. But unlike in reactor No 4, the clean-up must be undertaken remotely as the radiation is so strong that people can only stay there for a few minutes. As a result, the construction of the lifting device has already been delayed by several years.

Unclear conditions

The situation at the reactors raises doubts about the optimism shared by Japanese officials with regard to the orderly decommissioning of the plant. At the next stop, Okamura shows the control center of the underground ice wall that was built to prevent groundwater from leaking into the reactor basements and mixing with radioactive coolant water.

Since its construction, it has managed to reduce the amount of groundwater flowing into the reactor basements. But five sections of the wall have had to be kept open to prevent water inside the reactor basements from rising and flowing out too rapidly.

Despite all these adversities, the Japanese government and TEPCO are planning to decide as early as this summer how to remove the molten nuclear fuel from the reactors.

Even Shunji Uchida, the Fukushima Daiichi plant manager, couldn't hide his skepticism from the visiting journalists. "Robots and cameras have already provided us with valuable pictures," says Uchida, adding: "But it is still unclear what is really going on inside."