Almost four years after the Fukushima disaster, Japan is about to restart its nuclear industry.

The 2011 meltdowns at the tsunami-hit plant prompted a shutdown of all Japan's nuclear power stations and saw the power station's operator TEPCO accused of cover-ups and gross negligence.

TEPCO says radiation levels at Fukushima have significantly decreased and major steps have been taken to decommission the molten reactors.

Others say the plant cannot be fixed and thousands of people will never be allowed to return to their homes because of high radiation.

With vacuum-sealed protection gear and special breathing apparatus, TEPCO gave the ABC an exclusive tour of the crippled plant.

TEPCO was keen to show the progress that had been made, first with a visit to reactor 4.

Inside, empty pools could be seen where nuclear fuel rods once were.

Reactor 4 was offline when the tsunami hit, but a powerful hydrogen explosion left the rods in a precarious situation.

Now they have been moved to a secure location.

TEPCO guide Kenichiro Matsui said the project took about a year to complete.

"On one side is the reactor and [on the] other is the pool where we removed 1,500 spent fuel assemblies," he said.

"It finished last December and was a success."

Entry to closed areas would lead to instant death

Reactors 1, 2 and 3 were strictly off limits, and looking from about 500 metres away the area was clearly deserted, with cars and equipment abandoned.

Removing the molten fuel from these reactors will be an enormous challenge as workers cannot enter because it would lead to instant death.

Mr Matsui admitted they did not know the extent of the problems.

"We do not know [the] exact situation in detail," he said.

"Fuel has been melted down but nobody has seen it ... We need to develop robotic technology with help from around the world to know the real situation."

The only way TEPCO can control the meltdowns in 1, 2 and 3 is to pump water in to cool them, but the water becomes highly radioactive and mixes with the massive amount of groundwater that flows into the reactors from the surrounding hills.

This is TEPCO's most urgent problem - every day it has to deal with more than 500,000 litres of radioactive water.

Standing in a high position, the whole 3.5-kilometre site could be seen filling up with massive storage tanks.

The looming crisis is lack of space and where to put the water, and TEPCO is scrambling for solutions.

Officials want to build an ice wall around the crippled reactors to stop the groundwater entering, but that is untested technology.

Locals distrust TEPCO, say future is 'hopeless'

During the visit radiation levels were recorded of 40 microsieverts per hour and more - about 1,000 times the rate of Tokyo.

Mr Matsui showed the TEPCO's advanced liquid processing system which he said could treat the radioactive water.

"We can get rid of 62 kinds nuclear substances and can make it to safe levels so only tritium remains," he said.

But it cannot cope with the sheer volume of water and technical problems have meant TEPCO has had to employ three different systems, and it still missing projected deadlines.

Ultimately TEPCO will probably have to release the water into the sea, and the company has been given the all clear by Japan's nuclear authority to discharge decontaminated water.

This is making many people very nervous.

Fourth-generation fisherman Hisashi Yoshida said any trust with TEPCO was broken a long time ago.

"The fishermen attended meetings but don't believe what TEPCO says," he said.

"Although they said it was treated and OK, we regard it as contaminated."

Four years after the disaster, 120,000 people are still living in temporary accommodation, unable to return to their homes around the plant.

"We escaped with nothing but the clothes on our backs and I know we won't be going home," said Shinichi Kumadah, a Fukushima evacuee.

"When I think of the future I can't think of anything. It's hopeless."