A massive undersea earthquake on March 11, 2011 sent a tsunami barrelling into Japan's northeast coast, leaving more than 18,000 people dead or missing and sparking the Fukushima crisis, the worst nuclear accident since Chernobyl in 1986.

Initially, visitors to Fukushima Daiichi Nuclear Power plant were strictly limited to a handful of nuclear experts, lawmakers, government officials and selected media.

Visitor numbers have gradually increased as levels of radiation in most of the compound have dropped low enough to allow workers to operate without special protective equipment.

Tokyo Electric Power Co. (TEPCO), which runs the plant, is now accepting requests for tours from groups of local residents, embassy officials and school students, although it has yet to accept individual applications.

The number of visitors for the fiscal year to March last year rose to around 10,000 -- a figure the operator aims to double to 20,000 in 2020 when Tokyo hosts the Summer Games, said Takahiro Kimoto, a TEPCO official.

"Our objective is not to send a message saying 'It's safe. It's secure'," Kimoto told AFP.

"It is more important for us to have people watch what's really going on... without a prejudiced eye," he said.

"The inspections will help revitalise the region and reduce reputational damage," Kimoto said, adding that the company would be happy to show around International Olympic Committee officials.

- Next era -

Fukushima is expected to be in the spotlight during the Games as it will stage Olympic baseball and softball matches as part of Japan's effort to regenerate the area.

TEPCO also hopes that a football training centre used as a base for the plant's workers after the disaster will host teams competing in the 2019 Rugby World Cup.

Kimoto stressed that the company is responsible for not only reviving the region but also conveying bitter lessons to future generations.

Decontamination work is under way inside the plant, with thousands of workers enjoying hot meals, taking showers and buying sweets at a convenience store.

However, levels of radiation in areas around the three melted-down reactors remain extremely high, hampering the plant's decommissioning process, which is expected to take decades.

The scars of the catastrophe remain visible -- steel frames are gnarled and walls are missing, ripped off by the tsunami and hydrogen explosions.

- 'Strictly controlled' -

With the seventh anniversary of the disaster looming, AFP journalists given exclusive access to the roof of the plant's No. 3 reactor saw stagnant water stored inside a deep pool under which lay more than 560 fuel rods.

Each worker is required to wear a protective suit, three sets of gloves and a heavy-duty mask and carry a dosimeter, used to measure exposure to radiation.

Workers only stay a maximum of two hours per day on the roof where electric gauges showing current radiation levels hang on every corner.

A gigantic steel dome is now being built on the roof to prevent radiation leaking when the fuel rods are transferred from the pool to remote storage later this year.

As the initial stages of decommissioning the plant draw to a close, the biggest challenge is a protracted battle against high radiation, said Daisuke Hirose, a plant official.

"We have to lower radiation exposure to workers, but this prevents them from working for a long time up there," Hirose said.

"We want them to work under strictly controlled exposure plans. That's the big difference from working conditions at ordinary sites," he said.

The total costs for decommissioning, decontamination and compensation are estimated to reach 21.5 trillion yen ($194 billion) and TEPCO aims to dismantle the plant in three to four decades.