(This January 29 story corrects details about putting water back into environment in para 15)

Reuters journalist Aaron Sheldrick wearing a protective suit, visits the Tokyo Electric Power Co's (TEPCO) tsunami-crippled Fukushima Daiichi nuclear power plant in Okuma town, Fukushima prefecture, Japanm January 15, 2020. TEPCO/Handout via REUTERS

OKUMA, Japan (Reuters) - Reuters was recently given exclusive access to Japan’s Fukushima nuclear plant, where three reactors melted down in 2011 after a powerful earthquake and tsunami overwhelmed the seaside facility.

It was my fourth visit to the plant since the disaster to report on a massive clean-up. Work to dismantle the plant has taken nearly a decade so far, but with Tokyo due to host the Olympics this summer - including some events less than 60 kilometers (38 miles) from the power station - there has been renewed focus on safeguarding the venues.

Nearly 10 years into the decades-long clean-up some progress has been made, with potentially dangerous spent fuel removed from the top of one damaged reactor building and removal underway from another.

Graphic: Fukushima's "ice wall" -

But the melted fuel inside the reactors has yet to be extracted and areas around the station remain closed to residents. Some towns have been reopened further away but not all residents have returned.

This time I was taken to the site’s water treatment building, a cavernous hall where huge machines called Advanced Liquid Processing Systems (ALPS) are used to filter water contaminated by the reactors.

On my first visit in 2012 I had to wear full protective gear put on at an operations base located in a sports facility about 20 kilometers south of Fukushima Daiichi called J-Village, where the Olympic torch relay will start in March, then taken to the site by bus.

This time I was driven by van from a railway station in Tomioka, a town re-opened in 2017, about 9 kilometers away with no precautions. More than 90% of the plant is deemed to have so little radioactivity that few precautions are needed. Nevertheless, reporting from there was not easy.

Before entering the plant itself, which is about the size of 400 football fields, I was asked to take off my shoes and socks, given a dosimeter - a device that measures radiation levels - three pairs of blue socks, a pair of cloth gloves, a simple face mask, a cotton cap, a helmet and a white vest with clear panels to carry my equipment and display my pass.

I put on all three pairs of socks and the rest of the gear given to me, later including rubber boots. I was to change in and out of different pairs of these boots many times - I lost count - color coded according to the zone we passed through, each time putting them in plastic bags that would be discarded after use.

After reaching the ALPS building in a small bus, I was decked out in protective equipment, a full-body Du Pont Tyvek suit along with two sets of heavy surgeon-like latex gloves that were taped fast to the outfit.

I also had to put on a full-face mask after taking off my spectacles since it would not fit otherwise and told to speak as loudly as possible due to the muffling effect of the gear.

“Will you be able to see?” asked one official from Tokyo Electric Power Co, or Tepco, the plant’s operator. I nodded with as much conviction as I could muster and we entered the building which was quite dark, making it even harder to see.

In the ALPS building I was taken up and down metal stairways that passed around piping, machinery, testing stations, changing in and out of the rubber boots as we crossed yellow and black demarcations, warnings signs everywhere for areas that could not be entered.

As well as being dark, it was surprisingly quiet given the machinery. My dosimeter alarm kept going off as the radiation levels rose.

Tepco officials later showed me containers of crystal clear water that had been through ALPS. They said it would be safe to release the liquid into the environment after mixing it with fresh water to meet regulatory standards.

About 4,000 workers are tackling the cleanup at Fukushima, including dismantling the reactors. Many wear protective gear for entering areas with higher radiation.

The plant resembles a huge construction site strewn in areas with twisted steel and crumpled concrete, along with cars that can no longer be used, while huge tanks to hold water contaminated by contact with the melted fuel in the reactors increasingly crowd the site.

Some wreckage is still so contaminated it is left in place or moved to a designated area for the radiation to decay while the important work on the reactor buildings is underway.

As we moved back into the so-called green zone we passed through a building where I was to take off the protective gear in precise order in stages with each piece going into a particular waste basket for each item. Gloves were first, then the facemask, after which the suit and socks were taken off at different locations till I was left with one pair for passing back through the various security cordons.

I was then given my external dosimeter reading, which was 20 microsierverts, about two dental x-rays worth.